Species Account and Population Assessment for the Northern Pacific Rattlesnake in Canada

Posted August 15, 2013 | Categories : 77,Issues,Management,Rare Species,Reports,Research |

 Prepared By:

Jared Hobbs, M.Sc. / RPBio

 August 15, 2013


You can access the complete report in PDF form here: CROR sps account

This account is intended to provide a complete synthesis of over a decade of research and inventory on the Northern Pacific Rattlesnake (Crotalus oreganus oreganus) throughout the species’ Canadian range. This synthesis also rattlesnake2includes an analysis of covariates on a collaborative snake den database that was initiated by Mike Sarell and subsequently (and collaboratively) expanded, refined and maintained by both M. Sarell and J.Hobbs. In addition relevant learning’s from my MSc thesis work are included as this research on the species’ thermal ecology adds clarity and provides important information regarding our understanding of appropriate survey timing windows for this species in BC. The section on den ecology presents a summary of den site characteristics favored by the species in BC. The “Summary of Existing Information” section provides the first (and only) province wide assessment of various influential covariates including den elevation, distance to road, tenure and provincial distribution (within each of five provincial populations). The “Discussion” and “Conservation Recommendations” sections provide a context-based perspective with insights on how these covariates are suspected to affect vital rates for rattlesnakes in BC. In  addition, the BC snake den database was also used to develop new, more accurate and current range maps for gopher snakes, rattlesnakes and racers in BC. A revised and more accurate range map for the species, in BC, is presented here.


The results presented here suggest there is strong cause for conservation concern regarding the continued persistence of the Western Rattlesnake in BC. The species has been extirpated from several large areas of its former range in both the central portion of the Okanagan population and from large areas along the south side of the Thompson Valley from Chase to Ashcroft. More localized extirpations are also evident in the Midway and Grand Forks populations. Indeed, there are only five meta-populations (see Discussion) in BC that remain relatively un-impacted by negative anthropogenic influences. Stronger and more effective legal protection for this species is likely required to arrest or reverse apparent range wide declines of rattlesnakes in BC.

Table of Contents

Abstract. 2

Species Information. 4

Current Taxonomic Status. 4

Current Conservation Status in BC. 4

Distribution in British
. 5

Habitat Description. 7

Den Ecology:9

Geo-physical Attributes of
Den Features
. 9

Timing of Den Ingress and
. 11

Den Ecology:  Thermal Considerations. 11

Summary of Known Den Information in Canada. 13

Population Size. 14

Impacts of Roads. 14

Elevation. 15

Summary of Legal Protection. 16

Discussion. 18

Conservation Recommendations. 22

Literature Cited. 24


Species Information

Current Taxonomic Status

The Northern PacificRattlesnake was formerly recognized as one of nine sub-species (formerly: C. viridis oreganus) of the widely distributed Prairie Rattlesnake (C. viridis). Previous genetic work by Ashton and de Queiroz (2001) split C. viridis into two clades (IUCN. 2012). The Eastern clade (C.viridis) contains the nominal subspecies viridis (and nuntius) and includes populations from east and south of the Rockies; the western clade (C.oreganus) includes all five other former subspecies of C.viridis that occur west of the Rocky Mountains, including:

1.Northern Pacific Rattlesnake (C.o. oreganus) (nominal species: Holbrook,
1840). From east of the Cascades in BC, continuing on both sides of the
Cascades into Washington, Idaho, Oregon and east of the Cascades in California.

2.Grand Canyon Rattlesnake (C.o.abyssus). Restricted to the Grand
Canyon, Utah.

3.Midget Faded Rattlesnake (C.o.concolor). Utah, Colorado, Wyoming
east of the Rockies.

4.Southern Pacific Rattlesnake (C.o.helleri). Southern California,
including Santa Catalina Island.

5.Great Basin Rattlesnake (C.o.lutosus). East of the Cascades
including Southeastern Oregon, southern Idaho, eastern California, Nevada, Utah
and northern Arizona.

This new polytypic species is now referred to by its common name, the Western Rattlesnake (C. oreganus), with five recognized subspecies (Rubio. 2010).

In North America, the Western Rattlesnake (Crotalus oreganus) is the most northerly distributed species of the Crotalinae sub-family. The nominate subspecies, C. o. oreganus, retains its previous common name (the Northern Pacific Rattlesnake) and extends further north than any of the other four sub-species of Western Rattlesnake as it extends north into BC. Within Canada, the Northern Pacific Rattlesnake is the only extant subspecies of the Western Rattlesnake clade1. Because Canada is home to onlya single sub-species of the Western Rattlesnake (i.e. the Northern Pacific Rattlesnake), this sub-species can be referred to by either the full species name (Western Rattlesnake) or the sub-species name (Northern Pacific Rattlesnake) when discussed in a Canadian context. The remainder of this report is focused on the Northern Pacific Rattlesnake as a sub-species of the polytypic Western Rattlesnake.

1: Two other species of rattlesnake occur in Canada – the Prairie Rattlesnake (C. viridis viridis) occurs in Alberta and the Eastern Massasauga Rattlesnake (Sistrurus catenatus catenatus) occurs in Ontario.

Current Conservation Status in BC

In BC, the Northern Pacific Rattlesnake (Crotalus oreganus oreganus) occupies portions of the dry southern interior grasslands of BC and is designated as a ‘blue listed’ species by the BC Conservation Data Centre (BC Conservation Data Centre 2012) and as ‘threatened’ by the Committee on the Status of Endangered Wildlife In Canada (COSEWIC 2012). Restricted geographic range within BC, threats (habitat loss and persecution), and reported declines in the BC population (due to localized extirpations and habitat loss) make this species a cause for conservation concern (Charland et al. 1993). As such, the Northern Pacific Rattlesnake is also listed on the federal Species at Risk Act (SARA) under Schedule One; its residences (including den (hibernacula) features and mating sites) are legally required to receive both federal and provincial protection under SARA (on federal land) and under the Bilateral Agreement (on provincial crown land and on private land).

Distribution in British Columbia

In North America, the Northern Pacific Rattlesnake occurs on the east and west sides of the Cascades from Oregon north to western Washington along the US portion of the Okanogan River Valley with extensions into Canada. In Canada, the northernmost population of the Western Rattlesnake occurs along the Thompson-Nicola (and Fraser) river drainages and is geographically disjunct from the three BC populations that occur along the international border. This northernmost population has likely been disjunct since the hypsithermal periods (~9,000-5,000 ans)1. This is suspected to be the northernmost extent of the range of the Crotalus genus in North America2. The three southern BC populations include the Grand Forks, Midway and Okanagan-Similkameen Populations. All three southern populations of C.o.oreganus are geographically contiguous south of the international border.

In BC, the distribution of the Northern Pacific Rattlesnake is restricted to four discrete extant populations on the east side of the Cascade mountain range as described below:

  1. The Thompson-Nicola
    likely occurs east to Chase along the South Thompson River (Hobbs.
    2007) and north to Westsyde along the North Thompson River. The population
    continues along Kamloops Lake and the Thompson River to the Thompson River
    – Fraser River confluence at Lytton. The population also extends for
    20 km up Bonaparte Creek to Hart Ridge and along the Nicola River Valley
    to Skahun Creek (J. Hobbs, pers. obs), although isolated records exist
    further east along the Nicola Valley to Spirit Creek (B. Davis, pers.
    com.). Along the Fraser River canyon historic records exist as far south
    as Boston Bar (COSEWIC 2004). Recent records (Pers obs 2004, 2010 &
    2011; F.Iredale. 2011) confirm that the population extends north of Lytton
    towards Lillooet for ~30 km to McGillivray Creek. There is only one
    confirmed record on the west side of the Fraser River at Kwoiek Creek
    (confirmed from photo). It is suspected that this snake drifted across the
    Fraser after escaping from the fire at Mt. Arthur Seat in 2008. There is
    no evidence of an established population or den at this site.
  2. The Okanagan –
    Similkameen & Vernon populations
    extend from the international border at
    Osoyoos and continue north along the Okanagan valley to Vernon.
    Connectivity between the extant population in Vernon and the known extant
    population further south in the Okanagan is compromised by extensive urban
    development. The species has been extirpated from much of its former
    habitat in Kelowna and the two populations are now likely disjunct. The
    southern portion of the population continues south from Okanagan Mountain
    Park to the international border and east along the Similkameen River
    through Richter Pass and Yellow Lake Pass, from Chopaka (at the
    International Border) northwest to Bromley Rock.
  3. The Midway population occurs from
    Rock Creek and continues west along the West Kettle River Valley to
    Midway. From Midway, this population extends north to Kerr Creek (~4 km
    northeast of Midway) and continues south of Midway (south of the
    International Border) along the Kettle River. The Kettle River re-enters
    BC at Grand Forks, connecting the West Kettle
    population to the East Kettle population.
  4. Grand Forks population occurs from Grand
    Forks and continues north up the Granby River to Niagara. This population
    extends east along the East Kettle River where it continues north along
    the Christina Lake valley. To the south, this population is connected with
    the population in the United States (south of the International Border)
    below Christina Creek along the Kettle River Valley to where it joins the
    Columbia River. (Note: Historical accounts exist for Castlegar but no
    recent records are available for confirmation. This sub-population is
    likely extirpated (M. Sarell, pers. com.) but may have once extended along
    the Columbia River north to Montrose, Trail and Castlegar.)

The total number of dens for each population is summarized in Table 1.

It should be noted that there is a potential fifth population, along the west shore of the Columbia River, near Trail, BC. A single historic record exists for the area however current confirmation of continued occurrence of an extant population in this area is required. The species is currently confirmed near the town of Northport, Washington approximately 13km south of the international border near Trail.

 1: The Holocene
Climate Optimum
(HCO) was a
warm period during roughly the interval9,000 to 5,000 years before present. This event has also been known by many other names, including: Hypsithermal, Altithermal, Climatic Optimum, Holocene Optimum, Holocene Thermal Maximum, and Holocene Megathermal.

2: The population of C.viridis in Brooks, Alberta potentially extends further north but current records suggest that the most northerly den in Dinosaur NP is 8.6km further south than BC’s most northern den (Abandonment Den) near Cache Creek.


Figure 1: Distribution of the Northern Pacific Rattlesnake in BC showing all four populations. Red shaded areas represent the current confirmed range of the species in BC. The grey line indicates the international border. The potential population, near Trail, is not shown on the map.


Table 1: Number of confirmed (C1), suspected (C2), probable (C3) and extirpated snake dens (all
) in BC.
(Note: numbers are inclusive of all dens, regardless of confirmation of rattlesnake use; rattlesnakes
have been confirmed at 380 of the 459 known dens reported below


Confirmed (C1) Dens

C2 Dens

C3 Dens

Extirpated Dens






Okanagan-Similkameen & Vernon










Grand Forks






355 (318 with rattlesnake)




Habitat Description

Within BC, the Northern Pacific Rattlesnake is restricted to grasslands characterized by bluebunch wheatgrass (Agropyron
) and big sagebrush (Artemisia tridentata); and open parkland forests characterized by Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa). These vegetation types are recognized as the Bunchgrass (BG), Ponderosa Pine (PP) and Interior Douglas-fir (IDF) Biogeoclimatic Zones (BEC) and occur primarily within the Thompson-Okanagan Plateau Eco-region. In BC, the Northern Pacific Rattlesnake has been confirmed at elevations ranging from 152 m to ~1,430 m ASL (L. Gomez., however reports exist for at least two populations (Cathedral Mountain) above 1,750 m (M. Sarell, pers. com.). Elevation of known rattlesnake dens in BC (n=318) is from 292-975m ASL (BC Snake Den Database-2012).

The population of the Northern Pacific Rattlesnake near Kamloops and along the Thompson River, Nicola River and Fraser
River canyons is the northern extent of the species’ range in North America. This population is unique in BC because, unlike the other BC populations, it is disjunct from the larger US population to the south. Habitat connectivity was likely lost after the Hypsithermal Period (see P.7) approximately ~6,000-9,000 years ago (Cannings and Cannings 1996). As such, continued persistence of this population is completely dependent on survival of snakes within this sub-population. Over the past decade there has been significant effort devoted to inventory of den sites, within the Thompson-Nicola sub-population of rattlesnakes (data available from MFLNRO). This allocation of effort was intended to address a relative ‘gap’ in our provincial understanding of the species’ ecology. We currently have 76 documented/confirmed rattlesnake dens in this population as a direct result of these efforts.


Figure 2: Map depicting the current range extent of the Thompson-Nicola population.

The South Okanagan-Similkameen rattlesnake population represents the northern extent of the larger contiguous US range of the Northern Pacific Rattlesnake; figure one shows the recent (post European contact) break in connectivity of the Vernon
population with the Okanagan-Similkameen populations. Further south, connectivity is maintained across the international border into Washington along corridors of suitable habitat that follow the undeveloped valley slopes that span the International Border. The South Okanagan-Similkameen population encompasses portions of the Okanagan and Similkameen Valleys. Known dens range in elevation from 356-975m ASL and occur within the BGxh1, PPxh1 and the IDFxh1 BEC variants (Meidinger and Pojar 1991). Dominant vegetation at the den sites is typical of desert and grassland shrub-steppe habitat and includes antelope-brush (Purshia tridentata); big sagebrush, rabbit brush (Chrysothamnus nauseosus) and Bluebunch wheatgrass plant communities. Den substrates mainly occur within granite and gneiss rock outcrops.


Figure 3: Map depicting the four southern populations in BC, note the fifth suspected/potential population near Trail, BC is not shown.

Den Ecology: 

Aggregation to survive the hibernation period is perhaps one of the most interesting behavioural attributes of snakes.
Information on thermal characteristics of den sites and of over-wintering Northern Pacific Rattlesnakes will improve our understanding of the species’ ability to overcome the thermal challenges that it faces at the extreme northern limits of its range. As well, this information will help managers identify important habitats and develop guidelines for the conservation of rattlesnakes in British Columbia. The importance of over-wintering dens (i.e. hibernacula) to the ecology of snakes in temperate regions, coupled with the high fidelity of snakes to these sites (Klauber 1972) suggests that the conservation of these sites is likely required to ensure snakes continue to persist on the local landscape.

Geo-physical Attributes of Den Features

Four main components are suspected to influence the suitability of a site for bedrock denning. It is useful to be aware of these attributes when attempting to objectively assess the denning potential at a site. Dens can also occur in fluvial material (Bertram et al. 2001) although these “earth dens” are suspected to be rare as significant retreats that penetrate below the frost line are more typically associated with rock material, especially in the Cariboo where there are few burrowing rodents.

1. THERMAL MOMENTUM: The first, and perhaps most significant component, is best described as “thermal momentum”. This term describes the ability of the den entrance area to absorb and retain sufficient thermal radiation during warm, sunny periods to permit cool-weather basking in early spring and late fall, thereby extending their growing period. There are several factors that may influence the “thermal momentum” of the denning entrance and nearby cover objects, as follows:

Aspect: ‘warm’ aspects (south/southwest aspects between 170º-240º) receive
a greater proportion of solar radiation (relative to ‘cool’ aspects).

Slope: slope also influences the amount of incident solar radiation
received by an area. Steep slopes, that are oriented perpendicularly to the
sun, will receive the most solar radiation.

Mass: the mass of a body of rock will influence its capacity to retain
absorbed heat. Heat retention will increase as the mass of the absorptive body

Position: The position of a den (i.e. Exposure) can have a strong influence
on the absorptive capacity of the denning material. Dens positioned on an
exposed ridge top will receive more incident thermal radiation than dens
positioned in a shaded ravine.

Surface Albedo:
The amount of radiation that is reflected off a surface rather than being
absorbed. Dark surfaces may have an albedo as low as 10% (absorbing most
visible light). This absorbed light is converted to thermal energy, so low
albedo surfaces tend to emit more heat.


2. FRACTURING: This component also has significant influence of the suitability of a site’s potential for denning and is weighted equally to thermal momentum in its importance. Fracturing refers to the level of fracturing that is evident in the rock or denning material. Certain types of rock are more prone to severe fracturing than others (e.g. basalt and gneiss). Fractures must be deep enough to penetrate the denning material well below the frost line and access geothermal heat. Internal den temperatures and den probe measurements collected from 15 dens suggest that depths between 0.86-3.00 m may be sufficient to facilitate hibernation by rattlesnakes in BC (Hobbs 2007). Typically, these deep fractures occur in rock bodies between geomorphic layering events (e.g. separate volcanic deposits) or at severe stress points (faults) in the rock face. Deep talus may also provide deep fractures suitable for denning.


3. HUMIDITY: Although there is little supporting evidence for this second factor, the humidity in the den may also influence winter survivorship. In BC, there have been no previous attempts to quantitatively measure the effect of relative humidity on survivorship. However, Macartney (1995) suggested that most of the weight loss that occurred in snakes (measured prior to entering the den and upon emergence from the den in the spring) occurred due to water loss (desiccation) and was not due to fat loss (starvation). In the Okanagan, many dens have been found at the base of cliffs. This position is often associated with a higher level of moisture as surface precipitation drains down the impermeable cliff face to reach the toe of the cliff. The relatively higher moisture levels at these sites are often indicated by the presence of plant species typically associated with more mesic sites than the surrounding xeric habitats. Species such as rose (Rosa spp.), sumac (Rhus glabra), poison ivy (Toxicodendron rydbergii) and chokecherry (Prunus virginiana) find a niche in these areas because of the higher moisture content present in the “drip zone” of these cliffs. The increased moisture levels associated with these sites may increase the humidity levels within dens located at these sites and may be of benefit to over-wintering snake populations.


4. COVER: Upon spring emergence from the den and fall retreat into the den, snakes tend to spend a disproportionate amount of time at or near the den entrance engaged in thermoregulatory activities (i.e. basking). During denning, snakes enter a type of torpor and their body temperature falls significantly (Rubio 1998). During torpor, digestive processes are halted and any food matter remaining in the digestive system may putrefy and cause poisoning (Macartney. 1996). Effective thermoregulation may be particularly important prior to entering the den in the fall to aid completion of any digestive processes that are occurring. Upon emergence in the spring, thermoregulation may also be important in restoring metabolic processes when reviving from torpor. In many species of Thamnophis, mating also occurs at the den entrance upon spring emergence. Due to the disproportionate amount of time spent at or near the den entrance engaged in this type of thermoregulatory behaviour, available cover (at or near the den entrance) may have a direct influence on predation levels. Most dens (personal obs) appear to feature components of cover that may influence survivorship. These areas are referred to as solariums and may be provided by vegetative matter or rock material. Coarse talus, bushes, boulders (with cavities underneath) and other similar objects near the den are all heavily used by snakes as cover objects during the spring and fall emergence and retreat and may be of significant importance.

Cover must also be available along movement corridors near the den. Cover along these movement corridors may be provided by vegetation, which, as discussed previously, often increases in diversity and density along the drip line of significant cliffs.
Snake movement corridors are often found in these areas and are evidenced by snakes and snake tracks in the vegetation along the base of cliffs near den sites. Talus is also often used to provide a covered approach along movement corridors between the den and the summer range and may be an important component in poorly vegetated rocky areas. Bands of coarse talus extending below a cliff towards foraging habitat can provide effective security cover (and foraging opportunities) along subterranean movement corridors.

An evaluation of these four broad components may provide insight to the suitability of a site for denning. However, caution should be used when eliminating sites as den candidates as none of these factors works in isolation and several of these factors are
difficult to visually estimate. For example, a den may receive a very low rating of thermal momentum (due to poor positioning and/or aspect) but may still provide ideal denning habitat if fracturing is deep enough to extend well below the frost line (Sarell 1993, Hobbs 2001a).

Timing of Den Ingress and Egress

Each fall, in temperate latitudes, snakes return to den features to avoid lethal surface temperatures. In BC the timing of the fall return movement is somewhat variable as it differs at different elevations and it varies geographically (depending on site specific local conditions (e.g. aspect)). In general, in BC, the fall movement back to the den (ingress) is correlated with the onset of cooler
nights when temperatures drop below 9°C. This typically occurs during September through to mid-October although some snakes have been observed traveling back to dens as late as early November in the South Okanagan (M.Sarell. pers com).

In 2006 I attempted to more accurately and precisely define the timing of den return movements by monitoring snake body temperature and snake movement (using concurrent telemetry data contributed by L. Gomez). These results confirmed that, on
average, rattlesnakes returned to the den commencing September 17 and that all monitored snakes had returned by October 13th. Median return date was September 22nd (Hobbs 2007).  A larger sample size would likely show that ingress, or return movements to the den, generally occur in mid to late September with snakes being reliably seen at dens when evening temperatures in the area fall below 9C° (pers obs). This finding is consistent with prior and subsequent repeated observation and research by the author and others.


Each spring, sometime in early to mid-April, rattlesnakes leave their den sites to return to summer foraging areas. In BC, rattlesnakes have a relatively short emergence, or egress, period. During this period they frequently bask at the den entrance for
several days prior to leaving the vicinity of the den to forage for the summer (Hobbs. 2006).
In one study in southern BC, over the three-year duration, dispersal from the den occurred between April 23 and May 30 (Shewchuk. 1996).


Similarly, in 2006, I attempted to more accurately and precisely define the timing of the spring den egress (i.e. when snakes leave the den). My research refined earlier estimates for spring egress. Emergence from dens began in late March or early April (dates of first emergence ranged from March 22 to April 4). Upon emergence, snakes remained at, or near, the den entrance exiting to bask on the surface during warm or sunny days when surface temperature exceeded 10°C and air temperatures exceeded 15°C. This period of den emergence typically lasts until mid to late April. All monitored snakes (n=8) left the den between April 20-24th.
Some snakes may remain at the den beyond this period but the number of snakes visible (or remaining) dwindles rapidly by late April and, as a consequence, den detectability diminishes rapidly.

Den Ecology:  Thermal Considerations

Two studies have been conducted on overwinter thermal ecology of rattlesnake in BC. The first was near Kalamalka Lake (near Vernon, BC) by Macartney (1996). This research suggested a critical body temperature (Tb) (between 4-9°C) must be maintained while the snakes are in the den. More recent studies (Hobbs 2007) on overwintering thermal ecology of rattlesnakes in BC (including dens from the Thompson-Nicola and Okanagan-Similkameen and Vernon populations) showed that mean weekly Tb
declined towards the end of the active period (in September) from 23.2°C to 15.9°C immediately before den entry. Once Tb fell below 15°C rattlesnakes would enter the den and most snakes remained inside the den for the duration of the denning
period (although at least some snakes showed a tendency to exit the den periodically, throughout the winter denning period, to bask on the surface (see Figure 4)). A graphic example of internal body temperature readings, collected during my research in 2006/7, illustrates these patterns (Figure 4 & 5).

figure 4

Figure 4: Body temperature readings were collected four times each 24 hour period,
from eight snakes, between July 24, 2006 to April 24, 2007. The graph below
indicates mid-winter basking, seen as ‘spikes’ in body temperature that
occurred when the snake exited the den to bask on the surface during the winter
(Tb exceeded internal den temperature).


figure5Figure 5:  In contrast, there was no evidence of mid-winter basking observed for most dens monitored, as evidenced by the relatively stable (i.e. lacking spikes) body temperature observed for
most snakes.

After ingress in September and October 2005, Tb declined at ~0.5°C/week over winter to reach a mean weekly Tb of 6.4°C
by February-March before exiting the den again in the spring. During the denning period, mean Tb was 9.6°C (n=8).

Summary of Known Den Information in Canada

Ectotherms are unable to generate sufficient warmth internally (for prolonged periods) to maintain body temperatures above ambient surface temperatures. Hence northern latitudes, characterized by prolonged sub-zero winter temperatures, pose a
thermo-regulatory challenge to the survival of ectotherms, including snakes. The survival of snakes, during cold periods, depends on the thermal characteristics of their hibernation sites (Brown 1982, Marion and Sexton 1984). Snakes must select sites, for over-winter denning, which maintain temperatures consistently above freezing. In BC, they persist by taking advantage of geo-thermal characteristics that occur naturally, in limited availability, in the landscapes that they occupy. Snakes at northern latitudes, including the Northern Pacific Rattlesnake, gather each fall at communal winter den sites, or hibernacula, to overwinter. Many individuals of several snake species commonly share these dens sites in BC including the Great Basin Gopher
Snake (Pituophis catanifer), Racer (Coluber constrictor), Common Garter Snake (Thamnophis sirtalis) and the Western Terrestrial Garter Snake (T. elegans) (Klauber 1972, Macartney 1985, Sarell 1993, Hobbs 2001). Snake populations at some of these dens have been estimated at over 500 individuals (Hobbs 2001).


Mike Sarell began building and maintaining a database of snake dens in the early 1990’s. In 2001 J.Hobbs and M.Sarell collaboratively compiled all known BC den data to build and maintain a shared database of den location information for BC. Due to the confidential nature of the dens on private land and on IR are not submitted to the BC government however
summary level data, as presented here, are available (from the author) for conservation and planning purposes. The database contains coordinate and information (including species observed, approximate den population size (based on head-counts), land tenure, jurisdiction (MOE region) and basic habitat information (including distance to nearest paved road, BEC, elevation, slope, aspect & eco-section).

Including the results from this project (2012 field season) the BC den database contains:

355 Confirmed One (C1) dens:
Multiple snakes observed at or near the den entrance.

  • 82 (Suspected) Confirmed Two (C2) dens: Single snake observed at or near a suspected den entrance
    with only a single season of observation; further visits are required to
    confirm denning with absolute certainty.
  • 11 (Probable) Confirmed Three (C3) dens: Evidence of
    snakes (tracks, sheds, feces) at or near a  suspected den entrance)
  • 12 Extirpated (E) dens : The den is suspected to be extirpated based on sign, rumour
    of intentional persecution or repeated zero counts under ideal
  • 191 Candidate Dens: No
    evidence of denning, but denning likely based on geo-physical assessment
    of the potential den structure).


Conservation and protection of all confirmed (C1 status) dens is required as these sites are used, by multiple snakes over multiple generations, for critical habitat, or shelter, during the winter period. All suspected (C2) and probable (C3) sites should be re-surveyed in an attempt to elevate the status of these ‘suspected/probable’ dens to confirmed (C1) status. Candidate den sites should be resurveyed, under appropriate denning conditions, if there is any potential threat of change to these habitat features in an area.

Population Size

The BC snake den database can also be used to provide a very approximate estimate of the provincial population size
(absolute abundance) based on known counts (summarized into size classes) at
each den. When collecting den counts we consistently count snakes based on
‘head-counts’ only; we then assign it to a size class as follows:

  • Size
    Class 1: 1-9 snakes
  • Size
    Class 2: 10-25 snakes
  • Size
    Class 3: 26-50 snakes
  • Size
    Class 4: 51-75 snakes
  • Size
    Class 5: 75-100 snakes
  • Size
    Class 26: >100 snakes

A cumulative analysis of the “Size Class” of all dens in the BC Snake Den Database (Sarell and Hobbs data-2012) is
summarized in Table 2.

Table 2: Cumulative analysis of population size based on confirmed (C1) rattlesnake
dens in the BC snake den database.

# of Dens

Size Class

Number of snakes (max count)

Total estimated rattlesnake population

Minimum estimated

Maximum estimated



1-9 Snakes





10-25 Snakes





26-50 Snakes





51-75 Snakes





76-100 Snakes





> 100 Snakes





No count available







These numbers may be used as a minimum estimate, based on head counts, of the BC population of Northern Pacific Rattlesnakes however caution is required when applying this conservative and approximate absolute abundance estimate of the BC population size. The actual population size is likely larger as den counts will underestimate (by a suggested factor of up to five times) the number of snakes actually using the den. In addition, there are likely many dens that exist but are not documented (i.e. not yet
found). Conversely, this suspected underestimate is offset by likely declining trends that are suspected for most (>75%) of the known snake dens remaining in BC. Regardless of the limitations of these estimates they remain the most comprehensive surrogate estimate of the total provincial rattlesnake population available.

Impacts of Roads

In addition, a GIS analysis of den location in proximity to “nearest paved road” was conducted to provide some insight into habitat conditions. Road-related mortality is regarded as a severe stressor on the snake populations (Hubbard and Chalfoun. 2012). A review of road-related mortality, by Andrusiak and Sarell (COSEWIC 2004) found supporting literature that confirms depletion effects. A cited study suggested that snake populations were reduced by >50% within 450m of moderately used roads. Depletion effect was also still evident at distances ≥850m. A second cited study confirmed that the loss of only three adult female Black Ratsnakes (Elaphe obsoleta) in Ontario increased the probability of extinction to over 90% over 500 years. The life history of rattlesnakes, like other BC snake species, include traits that make a species’ population vulnerable (low fecundity, late maturity, long natural adult survivorship and seasonal migrations). As such, the cumulative effects of population depletion from road mortality may have serious effects on the genetic diversity of snake populations (Jackson and Fahrig 2011).

Every attempt has been made to survey all habitats, with a bias against survey of areas near paved roads. This analysis shows the challenge faced in achieving this objective. Despite survey bias to avoid surveys near roads almost all suitable rattlesnake habitats in BC are bisected by roads.

Ideally an analysis of population trend, at known dens or within known snake populations, is warranted however this
analysis is severely confounded by detection bias associated with sampling conditions. The data in the provincial snake den database is not robust enough to afford any insight to population trends provincially. An alternative analysis was attempted, to examine the population size (based on den count data) relative to (paved) road proximity to allow relative comparison of population sizes with differing road proximities however this afforded no additional insight. Unfortunately, the den count data in the database does not provide an accurate estimate of actual population size and as such population size patterns are ‘masked’ by variability in the available den counts. In summary, it is only pertinent to provide a summary of paved road proximity for 355 confirmed dens. This summary is provided here (refer to Table 3).

Table3: Summary of confirmed (C1) rattlesnake dens (n=318) relative to proximity to

Proximity to Road (m)

# of C1 Dens

% C1 Dens























Finally, an analysis of known confirmed rattlesnake den locations (n=318) in the Kamloops and Okanagan MOE regions was conducted to describe known elevation distribution patterns for rattlesnakes in BC (see Table 4). In summary, snakes show a strong tendency to den at low valley-bottom elevations, where anthropogenic influence is most pronounced, throughout their entire range in BC 

Table 4: Summary of confirmed (C1) rattlesnake dens (n=318), by elevation
range, for Region 8 & 3.

Elevation Range

Number of known (C1)
rattlesnake dens

Region 8 (%)

Region 3 (%)


9 (4%)

4 (5%)


126 (52%)

39 (51%)


82 (34%)

31 (41%)


21 (9%)

2 (3%)


3 (1%)

0 (0%)


1 (0%)

0 (0%)





Summary of Legal Protection

In October 1996, the governments of all Canadian provinces and territories, and the Federal Government, signed the national Accord for the Protection of Species at Risk. All signatories committed to establishing complementary legislation to provide effective legal protection for threatened or endangered species, including individuals and their habitats, within Canada. On June 1, 2004 Canada enacted the Species at Risk Act (SARA). SARA prohibits the destruction of residences of listed species and defines residences as, “…a place or area in, or a natural feature of, the habitat of the species at risk that is habitually occupied or used as a dwelling place by one or more individuals of the species at risk, or considered as being necessary for that occupation or use” (BC Ministry of Environment 2007b).

Rattlesnake hibernacula meet these criteria, and as such, the conservation and protection of rattlesnake hibernacula (i.e. residences) and critical habitat are federally mandated, within Canada, by the Species at Risk Act (SARA). In addition, in  2005, the BC provincial government signed a species at risk bi-lateral agreement with the federal government (the Canada-British Columbia Agreement on Species at Risk) (BC Ministry of Environment 2007b). This agreement clarified the roles and responsibilities of British Columbia and Canada as related to species at risk, with the purpose of coordinating federal-provincial efforts for species at risk conservation within BC. Eight years have passed and the development of effective legal protective mechanisms to ensure preservation of snake hibernacula is still required.

An analysis of known rattlesnake den locations (n=318) in the BC den database, by tenure type, was conducted to assess conservation options available under the current regulatory framework in BC (see Table 5).

Table 5: Summary of confirmed (C1) rattlesnake dens (n=318), by region, for each tenure type.

Tenure Type

Known Dens-Rgn 8 (%)

Known Dens-Rgn 3 (%)

Land (WHA)

74 (7) (46%)

61 (81%)


25 (9%)

0 (0%)


51 (19%)

4 (5%)


44 (9%)

9 (12)


39 (14%)

5 (6%)

Land (private)

16 (6%)

0 (0%)





Current mechanisms to protect rattlesnakes and rattlesnake hibernacula differ according to land tenure and are summarized for each tenure type as follows:

  1. Private Land (including lands held privately for
    conservation purposes): The BC Provincial Wildlife Act affords protection
    to the individual snake but does not provide any protection to the den
    feature. The Northern Pacific Rattlesnake is recognized as a “Schedule A” species
    under the Wildlife Act and as such it is an offence to harass, harm,
    capture or kill an individual unless the snake poses a direct threat to a
    person or to property. There is currently no protection afforded to snake
    hibernacula that occur on privately held lands in BC. Stewardship
    activities could be encouraged through land-owner contact programs, public
    outreach and education. Property tax reductions could also be explored as
    a possible incentive to encourage citizens to establish conservation covenants
    on privately held lands.
  2. Federal Lands (including First Nations Reserves, Federal
    parks and Federal Wildlife Management Areas): Under SARA, snake
    hibernacula should be categorized as a “Residences” for three of BC’s
    resident snake species (including the Northern Pacific Rattlesnake). As
    such, den sites for these species are afforded legal protection by the Act
    where they occur on federal lands, including both First Nations Reserves
    and federal conservation areas. It is a punishable offence to destroy a
    snake den; however continued inventory is required to identify den sites
    on federal lands in British Columbia. To meet federal objectives for den
    site conservation a monitoring program should be developed, in
    co-operation with First Nations, the Canadian Wildlife Service and the BC
    Ministry of Environment, to ensure managers are informed of snake den
    locations and to provide guidance and recommendations to land managers to
    aid in the conservation of these features on federal lands.
  3. Parks and Protected Areas: Existing Parks and protected areas have
    been established to conserve biodiversity values and habitat within
    portions of the rattlesnake’s range in British Columbia, however many
    potentially detrimental activities are still permitted within some of these
    parks (i.e. grazing, development of hiking trials etc.). Species specific
    management guidelines could be developed for den features within parks and
    protected areas to ensure these activities do not adversely affect
    resident snake populations. Communication should also be encouraged
    between park managers and wildlife managers within the BC MOE to ensure
    park’s staff are kept informed of den locations and are provided
    information to enable effective conservation and management of snakes and
    snake dens.
  4. Crown Land: the Forest and Range Practices Act (FRPA)
    affords limited protection to rattlesnake dens. Under FRPA, the Identified
    Wildlife Management Strategy (IWMS) (BC Ministry of Environment 2007a)
    enables the designation of Wildlife Habitat Areas (WHAs) at known den
    sites that occur on provincial crown land. Once an area is designated as a
    WHA, specific attributes of the habitat that could potentially be
    adversely affected by forest and range management activities are
    specifically regulated to maintain habitat values for rattlesnakes.
    Although this mechanism restricts harmful activities that may result from
    forest and range tenured activities, it cannot regulate recreational
    activities (i.e. off-road vehicle use), mining activities or
    transportation infrastructure development (i.e. road building and
    quarrying activity). The provincial government has not currently provided
    any other regulation, on Crown Land, to apply similar conservation
    objectives to these other potentially adverse activities.


Responsible and appropriate conservation and management of this species, and of the landscape features that are critical to the species’ long-term survival in Canada, is consistent with federal expectations of the BC Provincial Government, and is consistent with stated Provincial Government commitments under the 1996 national Accord for the Protection of Species at Risk (BC Ministry of Environment 2007b).


This report provides a complete synthesis of over a decade of research and inventory on the Northern Pacific Rattlesnake throughout the species’ Canadian range. This synthesis also includes an analysis of covariates on a collaborative snake den database that was initiated by Mike Sarell and subsequently (and collaboratively) expanded, refined and maintained by both M. Sarell and J.Hobbs. In addition relevant learning’s from Hobbs’ MSc thesis work are included as this research on the species’ thermal ecology adds clarity and provides important information regarding our understanding of appropriate survey timing windows for this species in BC.

To put our current understanding in context; the current compilation of 318 known rattlesnake dens in BC represents a massive undertaking, by multiple experienced individuals, in amassing such a comprehensive understanding of rattlesnakes in BC. For this
report I analyzed all confirmed (known) extant rattlesnake dens in BC to assess several key covariates, including: distance to road, den (population) size, den elevation distribution, known den distribution and abundance within each
meta-population and land tenure (of den sites). The results of this analysis are presented in the section “Summary of Known Den Information” in this report. Some key conclusions can be drawn from this analysis, as follows:

1) Impacts of roads:
Road-related mortality is regarded as a severe stressor on the snake populations (Hubbard and Chalfoun. 2012). A review of road-related mortality, by Andrusiak and Sarell (COSEWIC 2005) found supporting literature that confirms depletion effects. A cited study suggested that snake populations were reduced by >50% within 450m of moderately used roads. Depletion effect was also still evident at distances ≥850m. A second cited study confirmed that the loss of only three adult female Black Ratsnakes (Elaphe obsoleta) per year increased the probability of extinction to over 90% over 500 years. The life history of rattlesnakes, like other BC snake species, include traits that make a species’ population vulnerable (low fecundity, late maturity, long natural adult survivorship and seasonal migrations). As such, the cumulative effects of population depletion from road mortality may have serious effects on the genetic diversity of snake populations (Jackson and Fahrig 2011). To estimate the potential impact of road
mortality on rattlesnakes, within BC, I analyzed our BC database to assess “proximity to road” data for all 318 confirmed (C1) rattlesnake dens.
In this analysis I only included paved road surfaces; the inclusion of gravel (non-paved) roads would be even more alarming. Table 3 (see section “Summary of Known Information”) illustrates that 49% of known rattlesnake dens occur within <1km of a paved road surface; 86% are within 2km of the nearest paved road. Alarmingly, my den search efforts are biased against finding dens that are proximal to roads. I have intentionally under-sampled snake habitats within 1km of roads as my primary objective has consistently been to find dens that harbor large populations of rattlesnakes in order to maximize conservation gain that may result
from den-focused management and protection. In summary, this statistic has alarming implications that are consistent with my experience with rattlesnakes in BC. It suggests that a very high proportion of our snake population is currently being severely impacted by roads. It is strongly suspected that this single source of mortality is resulting in unsustainable population depletion
within most areas of the species range in BC; this opinion is widely held by many herpetologists in the BC scientific community and supported by the fact that rattlesnakes have been eliminated from several large areas of their historic range in BC (e.g. along the south side of the Thompson River Valley, from Chase to Savona).


2) Elevation
Distribution: Rattlesnakes are known to forage, in BC, at elevations up to 1,434m ASL (L. Gomez. pers comm 2006) however, thermal requirements are thought to constrain denning to lower elevations within all known BC populations. In BC
rattlesnakes tend to aggregate (often with other snake species) for the denning period; this ecological trait makes this species extremely vulnerable to mortality sources, at or near dens, as these have a particularly pronounced effect on the entire range of the population of individuals using the den. Considering that adult male rattlesnakes have been documented foraging (during the active season) as far as 8km from their den site in BC (L. Gomez (2006) & J. Gosling (2010). pers comm) point source mortality has a far reaching potential influence. For example, if a snake population is influenced by point-source mortality (i.e. roadkill, persecution) at a den site the resultant effect is a reduction in population density that may be felt up to 8km away from the den. An analysis of the distribution of known rattlesnake dens (n=318) illustrated that 56% of known rattlesnake dens occur below 600m ASL; 92% occur below 800m ASL. In summary the majority of our known BC population of rattlesnakes aggregate, each winter, in valley bottoms where human activity (e.g. agriculture, recreation, urban development) and road density are at their highest. This statistic has obvious and profound conservation significance.


3) Population Size:
There have been no previous attempts to provide an accurate quantitative estimate of BC’s resident extant snake population; as such, I attempted to address this using the provincial snake den database. To do this I assigned all known rattlesnake dens (n=318) to a size class based on the highest number of snakes observed at each den (see section on “Population Size”). In some cases these “den counts” were based on multiple visits however in many cases the value was collected during a single visit. Each size class encapsulated a range of values for the “number of snakes observed” (see section “Summary of Known Information”). A simple calculation of the number of dens within each size class yielded a range-value for the current BC population (based on den counts) of rattlesnakes at known dens. The estimated known population (based on den counts) ranged from a minimum of ~3,900 individuals to a maximum of ~8,000 individuals. These numbers may be used as a minimum estimate only as they are based on observed
rattlesnake den counts. As such, caution is required when applying this conservative and approximate absolute abundance estimate of the BC population size. The actual population size is likely larger as den counts will underestimate (by a suggested factor of up to five times) the number of snakes actually using the den. In addition, there are likely many dens that exist but are not documented (i.e. not yet found). Conversely, this suspected underestimate is offset by likely declining trends that are suspected for most (>75%) of the known snake dens remaining in BC. Regardless of the limitations of these estimates they remain the most comprehensive surrogate estimate of the total provincial rattlesnake population available for BC.

Any attempt to derive more information from den counts was confounded by multiple variables. Ideally we need to improve our understanding of the rate of population change, and population size, for each sub-population in BC. If population size information
was available a comparative assessment could be made to more accurately quantify an already widely held supposition: “population size and density is negatively dependent on proximity to roads”. I attempted to do this using “den size” (based on maximum number of snakes observed/recorded at each den) as a surrogate indicator of snake population size however I could
not discern any evident pattern with this approach. The supposition is undoubtedly accurate but this approach to analysis is confounded because den counts are a meaningless indication of population size for the following

  • Den counts are highly influenced by timing relative to emergence
    and retreat “start/end” dates
  • Den counts are highly influenced by sampling conditions, not just
    on the day of the count but also by several days preceding the count. As
    such, standardization of sampling conditions is virtually impossible.
  • Den counts are highly influenced by availability of suitable
    denning structures. If the local landscape has a high relative
    availability of denning options then the population of snakes using each
    den option will be diminished (i.e. the resident snake population (as
    limited by summer foraging habitat supply) will be more dispersed during
    the denning period).
  • Den counts are highly influenced by observability (e.g. at talus
    dens snakes have more concealment cover and, as such, are more difficult
    to see relative to dens with earth (closed surface) areas at the den
  • Den counts are highly influenced by observer skill. Snakes are
    notoriously difficult to see as they use concealment cover. In addition,
    inexperienced observers often fail to move discretely when counting
    snakes; as such, many snakes will retreat, unobserved, resulting in lower
    ‘counts’ relative to the actual number of snakes that were present.

As a result of this, there is no apparent significant correlation between distance to road and “den count”. This unclear result is due to the fact that den counts do not provide an accurate estimate of the size of the population of snakes actually using the
den. I suspect that there is likely a very strong correlation between distance to road relative to actual population size, actual rate of population change and actual population density within a given meta-population.

In the absence of more accurate quantitative information on population size, density and rate of change, perhaps the most obvious indication of the threats, and the concomitant perceived resulting decline in BC’s resident snake population, is an anecdotal (experience based) assessment of meta-population health. Our accuracy and understanding of the species’ current provincial range has improved dramatically in the last decade. This species has been the focal taxa of six masters’ research projects in BC (including M. McCartney, J. Brown, L. Gomez, J.Hobbs, J. Gosling and E. Lomas). In addition, there is now over two decades of inventory information, collected by several key researchers (M. Sarell, J.Hobbs, W. Alcock & F.Iredale), over the entire BC range of this species. This improved collective understanding provides consistent and alarming insights regarding the ‘health’ (or suspected rate of population change) of each meta-population in BC. Concerns regarding anthropogenic influences on the long-term persistence of this species have been iterated by each of these individuals.

This report presents a geographically broad field-based perspective for this species in BC. Based on this information it is suspected that declining trends are ongoing within >75% of the snake populations within the species range in BC. There are currently only five areas that remain relatively un-impacted by roads (or other anthropogenic activity) in BC. Four of these areas (described below) are in the Thompson-Nicola population; one is located in the Okanagan-Similkameen population and there are no un-impacted areas remaining in the Vernon, Midway or Grand Forks populations. Descriptions of each un-impacted area are provided below.

  1. Nicola South Side: The valley and shrub-steppe habitat
    along the south side of the Nicola River likely harbors near historic
    populations of rattlesnakes. There are no roads and no current evidence of
    grazing use from Skahun Creek downstream to Rattlesnake Bridge (along the
    south side of the Nicola valley) along this section of the valley. Much of
    this area presents very limited denning opportunities as the entire area
    is north east in aspect; there are available outcroppings of fractured
    rock but most are characterized by very low solar exposure. In this area
    all of the known dens occur along the top of the valley ridge or along the
    edge of incising ravines that have east aspect slopes. As the valley
    climbs the outcrops along the ridge-top receive relatively more solar
    exposure; all of the known dens occur within these relatively more exposed
    features. As such, dens in this area tend to be more significant as they
    provide shelter for a larger number of snakes (i.e. where features are
    limited denning aggregations tend to be larger (pers. obs)).
  2. North Shore of Kamloops Lake (including the Dewdrop): This area
    includes the shrub-steppe habitat (including grassland and forested
    ecosystems in the BG, PP and IDF ecosystems along the north shore of
    Kamloops Lake, between Tranquille Creek and Savona. Denning opportunities
    are abundant and road access is limited; this is likely some of the
    highest population density rattlesnake habitat remaining in BC. There is a
    rail-road running along the lake shore along the entire extent of this
    area, and I have observed associated mortality (adult rattlesnake trapped
    in exposed grease at a ‘lube station’ on the tracks), however impacts are
    suspected to be relatively low compared to mortality associated with paved
    roads. There are many known dens, including two of the largest dens in the
    Thompson population (n=>80), within this area of relatively pristine
    snake habitat.
  3. Thompson (West side) from Spence’s Bridge
    to Lytton
    : This area of
    the Thompson River canyon is rugged and steep; the grassland habitat
    climbs quickly into forested (foraging) habitat in this area. As such,
    denning opportunities are relatively restricted so dens in this area are
    likely to harbor large populations of rattlesnakes. Indeed, although only
    a single day of den searching has been conducted in this area (J.Hobbs and
    F.Iredale) a very large population of snakes (n=>80) was observed
    during a single visit at the only den (Epic Den) found that day.
    This ~50km linear section of snake habitat is impacted by a rail-road
    running along the entire extent of the canyon and I have observed
    associated mortality (adult rattlesnake trapped in exposed grease at a
    ‘lube station’ on the tracks), however impacts are suspected to be
    relatively low compared to mortality associated with paved roads. No den
    surveys have ever been conducted further downstream than Epic Den
    (approximately 25km downriver from Spence’s Bridge). It should be noted
    that there is no (or very limited) snake habitat along the canyon past
    this point as the canyon wall from ~35km South of Spence’s Bridge and
    continuing to Lytton are comprised of tall sheer limestone cliffs.
  4. Thompson (East side) from Sundance Ranch
    to the Nicola River
    : The
    portion of the Thompson River canyon, from Sundance Range (~10km south of
    Ashcroft) continuing downstream (south) to the Nicola River (near Spence’s
    Bridge) contains highly suitable snake habitat. The gentle valley slopes
    transition gradually into forested hillsides that contain high quality
    snake foraging habitat; denning opportunities along the southwest (prime)
    aspect valley side are numerous. I have surveyed this area, on several
    occasions, and have located several very large dens (e.g.; Pizza Box)
    but there are undoubtedly more dens waiting to be discovered in this area.
    Most of this area is contained within IR tenure; permission is required to
    access this portion of excellent snake habitat. There is a single dirt
    track running the length of the canyon along this section of the Thompson
    River valley however vehicle use is very light; I have never observed a
    vehicle on this section of the road in over a decade of working in this
  5. Okanagan Mountain Park: This is a relatively small area but
    warrants mention as it is the only area of snake habitat remaining in the
    entire Okanagan-Similkameen population that is not impacted by roads.
    Immediately north of the park the low elevation (valley bottom) snake
    habitat is so severely impacted, along both sides of the Okanagan Valley,
    that rattlesnakes have largely been eliminated (with the exception of a
    single very isolated (and likely declining) population at Mt. Boucherie).
    The majority of this area has never been surveyed for snakes. In 2012, I
    led surveys (with four very capable technicians) into the southern edge of
    this area and two dens were detected. Most of this area is inaccessible by
    road, creating ideal conditions for snakes, and denning opportunities are
    abundant. The area was severely impacted by fire in 2006 but the impact on
    the health of the resident snake population in the area is unclear.


Unfortunately all remaining areas of snake habitat, in the BG, PP and IDF BEC zones, within the range of all five currently disjunct BC populations are impacted, to varying extents, by anthropogenic influence. Throughout all remaining snake habitat in BC roads, urban development, intensive agriculture, railway right-of-ways and intensive recreational use continue to have a strong negative influence on the quality of snake habitat with a concomitant negative effect on adult survivorship of snakes (including racer, gopher snake, rubber boa, night snake and even garter snakes (T.sirtalis and T.elegans). The resultant influence on the current snake populations within many of these impacted areas (~75% of the species’ BC range) is typically quite severe. Despite difficulties associated with accurate quantification extensive field observation suggests that snake populations have (and will continue to), declined sharply within these areas. This supposition is strongly supported by the fact that the species has already been eliminated from several large areas of its former range.

In summary, as a direct result of these collective efforts we have improved our understanding of the species current distribution, current and historic range, timing of seasonal movements, population size and population health in BC. We have also gained important inferential (anecdotal) insights into vital rates including survival, fecundity and rate of population change. Although these observations are not statistically robust, they are still valid and important as they
represent the best available range-wide information for this species in Canada. This collective understanding, gained from over a decade of inventory and research on this species by multiple individuals, indicates strongly that there is obvious cause for concern regarding the likelihood of the continued persistence of rattlesnakes in BC.

Conservation Recommendations

Communal denning behaviour increases snake populations vulnerability. Point source mortality, especially when concentrated near snake hibernacula, results in population depletion that affects the local population at a broader landscape level for snakes with large territories (including racer, gopher snake and perhaps even garter snakes (T.sirtalis and T.elegans). Sources that exacerbate point source mortality for snakes (all species) include urban development, intensive agriculture, railway right-of-ways and intensive recreational use. These factors are estimated to negatively influence snakes in BC within approximately 75% of the species’ current BC range and the influence is generally thought to be quite severe despite difficulties associated with quantification. In addition, loss, through intentional or accidental destruction of snake hibernacula has occurred (and will likely continue to occur) in BC (pers obs). Den destruction also results in the elimination of many individuals and possibly entire populations of snakes from an area (Klauber 1972).

Naturally occurring features that facilitate over-winter survival in BC are limited in availability on the landscape. The importance of over-wintering denning sites (hibernacula) to the ecology of snakes in temperate regions, coupled with the high fidelity of snakes to these sites (Klauber 1972), suggests that the conservation of both den sites and habitat surrounding den sites is likely required to ensure snakes continue to persist. For these reasons, conservation of snake hibernacula is viewed as critically important for the conservation of entire snake populations in BC.

In BC, the only current conservation mechanism available for the management of snake populations is the Identified Wildlife Management Strategy (IWMS) under the Forest and Range Management Practices Act. In order to conserve habitat for identified species (as listed on the Category of Species at Risk) Wildlife Habitat Areas (WHAs) can be established at known den sites on provincial crown land. Measures are applied within established WHAs to ensure that the den site and the surrounding habitat are protected from potentially detrimental forestry or range practices. Although rattlesnakes, racers and gopher snakes have been designated as Identified Wildlife this enactment affords very limited effective protection to snake populations as the most intensive threats (road mortality and urban/agricultural development) are not addressed under this legislation. In addition, WHAs can only be designated on provincial crown lands. An analysis of all known rattlesnake dens demonstrates that only 42% of the known rattlesnake den sites (n=318) in BC occur on crown land and application of this conservation mechanism is thus restricted to less than half of the known dens in the province. Although IWMS is a positive start for addressing snake conservation in BC this mechanism alone is insufficient for conservation of rattlesnakes at the landscape scale.

In 2004, the federal Species at Risk Act (SARA) was assented and in 2006 the provincial government publicly committed (in a federal-provincial bilateral agreement) to develop legislation that would afford SARA listed species equivalent protection to ensure parody with SARA. As of 2013, this commitment still has not been met by the province, despite the bilateral agreement, for application on provincial crown land. Where SARA does currently apply (i.e. on federal lands) protection is still inadequate as a formal residence description (e.g. snake dens and maternity sites) have still not been accepted and critical habitat has not been defined. More effective and comprehensive legal protection is obviously still required to ensure that rattlesnakes, gopher snakes and racers have a reasonable chance of persistence in BC.

In the interim, continued survey for snake dens is recommended to build upon our collective understanding of species distribution and important habitats. The use of aerial assessment methods has proven to be an extremely cost effective method to refine and focus search efforts. The application of this method (as described in this report and in Hobbs. 2010) should be expanded to other areas. In addition, the use of infra-red aerial assessment methods should be explored, beginning with a pilot project in the Kamloops Region, to improve predictive aerial assessment methods even further.

Finally, a long-term monitoring project should be initiated at several known den sites. Baseline data already exists for Kalamalka Provincial Park so all 21 dens in this area are logical candidates for a longer term demographic study (to assess vital rates including fecundity, survivorship and rate of population changes). If a successful (i.e. non-detrimental and effective) method for population monitoring can be developed this method should be selectively applied in other portions, and at other dens, within the range of the species in BC.

Literature Cited

BC Conservation Framework. 2009.
June 18, 2010).

BC Ministry of Water, Land and Air Protection.
2004a. Appendix 5 in Accounts and Measures for Managing Identified Wildlife
– Accounts V. 2004. B.C. Ministry of Water, Land and Air Protection,
Victoria, B.C. Available:

procedures.html (accessed April 15, 2010).

COSEWIC 2004. COSEWIC assessment and status
report on the western rattlesnake Crotalus oreganus in Canada. Committee on the
Status of Endangered Wildlife in Canada. Ottawa. vi + 26 p.

COSEWIC 2013 (draft). COSEWIC status report on
Great Basin Gophersnake in Canada. Prepared for the Committee on the Status of
Endangered Wildlife in Canada. Ottawa. vi + 69 p.

Gomez, L. 2008. Personal communication

Gosling, J.2012. Personal communication

Hobbs, J. 2007. Thermal Factors in Relation to
the Denning Ecology of Northern Pacific Rattlesnakes in British Columbia.
Thesis. SFU/RRU. 107 p.

Hubbard, Kaylan and Chalfoun, Anna. 2012. An
experimental evaluation of Potential scavenger effects on snake road mortality
detections. Herpetologoical Conservation and Biology 7(2). September 2012.

IUCN Taxonomy Tool:

Jackson, N., and L. Fahrig. 2011. Relative
effects of road mortality and decreased connectivity on population genetic
diversity. Biological Conservation 144:3143–3148.

Klauber, L. M. 1972. Rattlesnakes: their
habits, life histories and influence on mankind. Reprint of second edition.
University of California Press, Berkeley, CA.

Lomas, E. 2013. Personal communication

Macartney, J. M.
1985. The ecology of the northern Pacific rattlesnake, Crotalus viridis
oreganus, in British Columbia. M. Sc. thesis, University of Victoria, Victoria,

Macartney, J. M., K. W. Larsen, and P. T.
Gregory. 1987. Body temperatures and movements of hibernating snakes (Crotalus
and Thamnophis) and thermal gradients of natural hibernacula. Canadian Journal
of Zoology 67:108-114.

Meidinger, D., and J. Pojar (eds.). 1993.
Ecosystems of British Columbia. Special Report Series 6. BC Ministry of
Forests, Victoria, British Columbia.

Sarell, M. 2013. Personal communication.

Rubio, Manny. Rattlesnakes of the United States
and Canada. ECO Herpetological Publishing and Distriubtion. 2010.