Northwestern cellar spider (Psilochorus hesperus): COSEWIC assessment and status report 2014

 

Document Information

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:

COSEWIC. 2014. COSEWIC assessment and status report on the Northwestern Cellar Spider Psilochorus hesperus name in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. x + 42 pp (Species at Risk Public Registry).

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3

Tel.: 819-953-3215
Fax: 819-994-3684
COSEWIC E-mail
COSEWIC web site

Également disponible en français sous le titre Ếvaluation et Rapport de situation du COSEPAC sur le Pholcide de l'Ouest (Psilochorus hesperus) au Canada.

Cover illustration/photo:
Northwestern Cellar Spider -- Photo provided by Darren Copley.

©Her Majesty the Queen in Right of Canada, 2014.

Catalogue No. CW69-14/693-2014E-PDF
ISBN 978-1-100-21531-0

COSEWIC Assessment Summary

COSEWIC Executive Summary

Northwestern Cellar Spider
Psilochorus hesperus

Southern Mountain population
Prairie population

Wildlife Species Description and Significance

Psilochorus hesperus (Northwestern Cellar Spider) is a cellar spider (Family Pholcidae) characterized by specialized copulatory structures, basally fused jaws and six of their eyes arranged in a pair of distinctive triads. The species has long and spindly legs, a small body size, Y-shaped groove on the head, distinctive spurs on the male jaws and bovine-udder-like female genitalia. This spider has limited dispersal abilities.

Cellar spiders derive their name from the human structures in which some of the species in this family are commonly found: undisturbed dark crevices within buildings, homes, basements and cellars. However, only a fraction of the 1350 named species occur within anthropogenic habitats. Most species live in natural habitats that include caves, under rocks and in abandoned mammal burrows. Psilochorus hesperus has not been found in basements or cellars.

Distribution

Psilochorus hesperus ranges from eastern California through Nevada, Idaho, Utah, Washington to southern interior British Columbia. In Canada the spider is restricted to the Western Interior Basin within British Columbia in a small area of the lower Similkameen and south Okanagan Valleys. The species is one of only two native pholcid spiders in Canada. An additional non-native pholcid species ranges in BC.

Habitat

Psilochorus hesperus inhabits low-elevation valley bottoms within the hot dry bunchgrass, Douglas-fir and Ponderosa Pine ecosystems of the southern interior of British Columbia. The species’ microhabitat includes the undersurfaces of relatively large, stable rocks, which provide some protection from temperature extremes. Occupied rocky areas are typically those situated in cooler habitats that are north- or east-facing and have a component of open forest or shrub which provides shading. The species likely wanders from its rocky residences in the cooler night temperatures and thus requires vegetation or other unqualified habitat elements adjacent to occupied rocky areas.

Biology

Psilochorus hesperus has a one-year life cycle. Individuals spend their lives within small territorial spaces under the surface of suitable rocks. Adult females and juveniles overwinter although adult females are present year-round (various age classes) and some may overwinter twice; adult males and new adult females mature in early spring; adult males are unlikely to be encountered after late summer. Mated females may produce several egg sacs, one at a time. Each egg sac is a loosely bound ball of up to two dozen eggs carried in the mother’s jaws. The extended egg production period results in a series of juvenile age classes within a population. Dispersal to new rock habitats is by nocturnal wandering for relatively short distances from areas of occupied rock habitat.

Population Sizes and Trends

Population size and trends are unknown.

Threats and Limiting Factors

The primary threat is the conversion of natural habitat to urban and agricultural development, and road construction and maintenance activities along transportation and service corridors. A combination of wildfires, water management activities, landslides, and pesticides also threaten the species. Limiting factors include small population sizes at occupied sites and isolation from other populations; widely separated areas of unsuitable habitat; and limited dispersal abilities that may prevent recolonization of new areas.

There are at least 16 locations: eight in the lower Similkameen Valley and eight in the south Okanagan Valley. One site at Summerland (Okanagan Valley) is situated in an artificial rock wall within a cemetery and considered a result of movement of material by humans. This site is still considered extant and included in the number of locations. An additional site in the Okanagan Valley was converted to agricultural use and is presumed extirpated, although the area has not been re-surveyed. There is a small possibility of unrecorded populations in adjacent habitats.

Habitat Protection and Ownership

Psilochorus hesperus has been recorded within provincially owned protected areas including the South Okanagan Grasslands Protected Area (East Chopaka site) and Haynes Ecological Reserve. One site is on a roadway adjacent to a private conservation property owned by The Nature Trust, and the spider is likely also present within this protected area. Most sites are on unprotected provincial Crown (e.g., roadsides or forestry land) or First Nations land.

Psilochorus hesperus is not protected by federal or provincial legislation. The species does not have a global rank (GNR). The Canadian and British Columbia General Status ranks of “2” (may be at risk) applied. The draft national status rank is N2 (imperilled) and the provincial is S2 as well.

Technical Summary

COSEWIC History

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.

COSEWIC Mandate

The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2014)

Wildlife Species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.

Wildlife Species Description and Significance

Name and Classification

Class:

Arachnida

Order:

Araneae

Subordinal:

Opisthothelae: Araneomorphae: Haplogynae

Family:

Pholcidae

Subfamily:

Modisiminae

Genus

Psilochorus Simon 1893

Species

Psilochorus hesperus Gertsch and Ivie 1936

English Common Name

Northwestern Cellar Spider
Northwestern Longlegged Pholcid

French Common Name

Pholcide de l'Ouest

The genus Psilochorus is part of a New World clade of pholcid (cellar) spiders (Huber 2005, 2011a) and includes over 40 described species (Platnick 2012). The majority of these species have small and restricted ranges within the Nearctic region. Simon (1893) described the genus; Gertsch and Ivie (1936) described Psilochorus hesperus; and Slowik (2009) revised the Nearctic species occurring north of Mexico. Huber (2000, 2011a) has analyzed the global phylogeny of pholcids. No subspecies of P. hesperus are described.

Although Psilochorus hesperus is called a cellar spider, the name is due to taxonomic history rather than habitat association. The first genus described in the family was its namesake taxon, the European Pholcus (in 1805). Because of its close association with human habitation, Pholcus phalangioides has accidentally been introduced around the world where they are found in undisturbed areas such as basements and cellars. It is for this reason the family is often referred to as cellar spiders, despite the fact that most of the 1300+ named species occur in natural habitats that include caves, under rocks, and in abandoned mammal burrows. In Canada, the only pholcid recorded in homes is this non-native pholcid, Pholcus phalangioides. There are no records of P. hesperus or the other native pholcid species occurring in association with human habitation.

Morphological Description

Pholcid spiders lack a cribellum (a broad, flat silk production organ located in front of the spinnerets in a variety of spiders) and have haplogyne genitalia (male genitalia feature an exposed genital bulb) on the tarsal (last) segment of each pedipalp (paired leg-like structures in front of the first pair of legs of all spiders, modified for copulation in males, unmodified in females, Fig 1). Female genitalia have a simple plate ventrally on the abdomen and lack the complex copulatory apparatus typical of most female spiders. In addition, pholcids are distinguished by the possession of basally fused chelicerae (jaws) and eyes arranged in three groups with the anterior median eyes occurring as a pair (or absent in some pholcids; specimens of Psilochorus have a full complement of eight eyes) and the other six eyes as distinctive triads.

Psilochorus have long and spindly legs (length of each tibia of the first pair of legs > body length [versus < body length]; Figure 1 and 2), small body size (< 4 mm [versus > 4 mm]; 3), carapace with a Y-shaped dorsal median groove and each male pedipalp with a pointed structure on the tip of a slender femur (versus pointed structure lacking and femur enlarged) and a simple slender procursus (a basal outgrowth on the tip of the pedipalp) (versus a much larger and complex procursus).

Males have a sharply pointed, inwardly curved, pair of elongate spurs projecting from the bases of the fused jaws and resembling steer horns; and females have a pair of closely spaced humps on a large prominence projecting ventrally from the copulatory region of the abdomen and resembling a bovine udder.

The following description of P. hesperus is summarized from Slowik (2009). Specimens are small (body length 2.5 mm [males, Figure 1] to 3.3 mm [females; Figure 2]). Colouration ranges from pale (almost translucent) yellow to dark brown with the abdomen darker than the carapace; most specimens are pale. Y-shaped mid-carapace groove makes the eye region appear somewhat elevated. Clypeus (between the jaws and eyes) projects strongly forward. Abdomen dorsally with lightly contrasting pattern of dark “heart mark” (central elongated dark mark anteriorly on abdomen) bracketed by several pairs of darkened spots laterally against a paler background. Abdominal pattern may be indistinct or absent. Further morphological details are provided in Gertsch and Ivie (1936), Huber (2000), and Slowik (2009).

Two native pholcids occur in Canada. Pholcophora americana is common and widespread in the southern interior of British Columbia (BC) and a small portion of southern Alberta (AB). Psilochorus hesperus occurs only in south central BC (see Distribution). A third non-native pholcid, Pholcus phalangioides (Fuesslin), ranges throughout southern BC within anthropogenic structures and buildings. The morphological characters separating these three species are clear. Psilochorus hesperus can be distinguished from other cellar spiders in this genus by its distribution. It is the only species occurring north of central Oregon in western North America (Figure 4).

Population Spatial Structure and Variability

No population structure or genetic studies have been conducted on P. hesperus. There is good potential for the species to exist in discrete, genetically distinct populations (e.g., see Keith and Hedin 2012).

Designatable Units

Psilochorus hesperus is being assessed as one designatable unit, in the absence of information on discreteness or evolutionary significance among populations. The species occurs within the COSEWIC (2011) Southern Mountain Ecological Area.

Special Significance

Pholcidae is a large family of spiders (1300+ species) (Platnick 2012) characterized by a high degree of habitat specialization. The majority of species have restricted ranges ( e.g., Huber 2000, 2001, 2011b; Slowik 2009) within cooler and humid tropical climates.

Psilochorus hesperus is one of two native pholcids in Canada. The majority of the species’ global range is within the Great Basin (Slowik 2009), a hot and dry ecosystem that extends through west-central North America, the northernmost extent within the Western Interior Basin of BC. The species is adapted to cool, humid microhabitats provided by the undersurface of large rocks within an otherwise hot, dry environment.

The range of P. hesperus within the Okanagan and Similkameen areas of BC overlaps with at least 204 additional species at risk (British Columbia Conservation Data Centre 2013), of which at least 65 have been assessed by COSEWIC (COSEWIC 2013).

The species is not known to have cultural significance to First Nations in the region (Jones pers. comm. 2012).

Distribution

Global Distribution

The global range of Psilochorus hesperus is primarily within North America’s hot, dry Great Basin, and stretches from extreme southcentral BC through central Washington (WA) to eastern California and southwestern Idaho (Figure 3) (Slowik 2009). The species is recorded through the arid interior of WA east of the Cascade Range, and narrows in the north to a single drainage, the Okanogan River Valley (Okanogan County, WA). At Oroville the distribution follows both the Similkameen and Okanagan River drainages north into BC. The approximate global range extent is less than 700 000 km².

Canadien Distribution

In Canada, P. hesperus is at the northern edge of its range in south-central BC (Slowik 2009). The speciesis recorded from 17 sites (sixteen extant and one presumed extirpated; Table 2) within lower elevations (280 – 760 metres elevation above sea level [asl]) of the Similkameen and Okanagan valleys (Figure 4). Less than 5% of the species’ global range occurs in Canada.

In the Similkameen Valley (Figure 4, Table 2), P. hesperus is recorded from eight sites within a 75 km corridor of the Similkameen River from the vicinity of the Hayes Creek drainage (~10 km east of Princeton, site 2) to the international boundary in the South Okanagan Grasslands Protected Area East Chopaka (site 5).

In the Okanagan Valley (Figure 4), the species is recorded at nine sites from Summerland (site 12, northernmost site) to Osoyoos (site 11, southernmost site). The Summerland site is within a cemetery and may be an outlier (see Special Significance and Dispersal and Migration).

The Similkameen and Okanagan valley sites may represent disjunct populations. These drainages are tributaries of the Columbia River system and join approximately 12 km south of the international border at Oroville, WA. The spiderhas been recorded in WA near Oroville (Table 2) and sites approximately 30 km southward to the Columbia River Valley (see Figure 3).

Extent of Occurrence and Area of Occupancy

Based on all records (17 sites) the extent of occurrence (EO) in Canada is 2376 km² (Filion pers. comm. 2012). If the cemetery site (site 12 Summerland) and a site now presumed extirpated from agricultural development (site 11 Osoyoos Indian Reserve 1) are excluded the EO is reduced to 1429 km².

The index of area of occupancy (IAO), based on a 2 X 2 km grid for all 17 sites, is 68 km² (Filion pers. comm. 2012). Excluding the presumed extirpated site the IAO is 64 km². The IAO is likely an underestimate; there are potential sites on the east side of the Okanagan Valley between Oliver and Osoyoos and in the Similkameen Valley, particularly on First Nations land between Hedley and the international border. Using Google Earth imagery (Google Inc. 2011), and “rock/talus/sandbar” mapping in Vegetation and Habitat Types in Public Utility District No. 1 of Okanogan County (2009), an estimated 2 km² of potentially suitable specific rock habitat is available (Bennett pers. comm. 2012).

Search Effort

Targeted spider sampling (in general) began in the 1970s. Pholcid spiders were unrecorded in BC before the 1980s (see West et al. 1984, 1988). From 2001 to 2012 Pholcid surveys across southern BC (Table 3; Figure 5) targeted habitats known to have P. hesperus, including grasslands, open Douglas-fir (Pseudotsuga menziesii)and Ponderosa Pine (Pinus ponderosa) woodlands, and rocky areas in relatively xeric landscapes (Table 1). Most survey sites involved hiking within suitable habitats or were adjacent to or within a few hundred metres of roadways.

Table 1. Table summarizing general data on xeric areas of British Columbia surveyed for pholcid spiders from 2001 to 2012. Information includes drainage name, waterway name, and general area surveyed.

Canada’s native pholcids are small, inconspicuous spiders that move quickly and therefore require focused search effort and specific sampling techniques. Psilochorus hesperus surveysare done through a combination of hand-collection (physical examination of habitat) and pitfall trapping. Hand-collection is the best method for determining pholcid presence and is the primary method in pholcid surveys (Bennett unpubl. data). Pitfall trapping is a passive method that involves burying a cup flush with the ground, filling the cup with preservative and leaving the cup for up to a month in order to record arthropod presence. This method is effective for long-term pholcid monitoring and detection of unrecorded populations.

More than 195 hours of hand searching for P. hesperus has accumulated in the past fifteen years, most of this search effort within the past few years by one to three spider specialists who specifically targeted habitats for this species. Pitfall trapping in suitable habitats (within at least 12 sites) spanning at least three field seasons (1991, 1994 and 1995) has also revealed some results (Table 2 and 3).

Table 2. Table summarizing Northwestern Cellar Spider records in southern British Columbia. Information includes site number, collection date, whether site was searched in 2012, site location, geographic coordinates, habitat description, collector's name, and search effort.

Table summarizing search effort within Northwestern Cellar Spider habitat where no specimens were found during targeted surveys. Information includes date range, site searched, geographic coordinates, elevation, remarks, surveyor(s), and search effort.

Table 4. Table identifying threats to Northwestern Cellar Spider sites (17 sites) in Canada based on the IUCN-CMP threats assessment calculator.

Surveys confirm P. hesperus is restricted to rocky habitats in a narrow low-elevation band along the Similkameen Valley and at scattered sites in the southern Okanagan Valley (Figure 5). Surveys have been done in urban environments in the region and the species has not been recorded in cellars or human-made structures within urban areas historically, or in recent, dedicated surveys.

To date surveys in the lower Similkameen Valley have focused on the northeastern side of the valley, including the Ashnola Valley. The distribution of P. hesperus is not known in the First Nations lands (Lower Similkameen Indian Band) on the southwestern side of the valley (more than 17,000 ha from Hedley to Keremeos to Cawston and the international border [Statistics Canada 2012a]). However, suitable rock habitat may be widely separated south of the Cawston area (see “Other” habitat types, in Public Utility District No. 1 of Okanogan County [2009]).

Surveys in the Okanagan Valley have focused on known sites and habitats where permission to survey was obtained. Large portions of habitat on the east side of the valley (e.g., approximately 13 000 ha of First Nations land around Oliver, Osoyoos, and Osoyoos Lake areas [Statistics Canada 2012a]) may have potential habitat for P. hesperus, based on previous collection records (e.g., Osoyoos Indian Reserve 1).

Loose talus habitats are considered unsuitable and the majority of rock habitat in the Okanagan and Similkameen Valleys that is talus does not appear suitable. The Similkameen Valley in WA has not been well surveyed (Crawford pers. comm. 2012). However there is also much unsuitable habitat, such as the alkali flats south of Chopaka/Nighthawk international border crossing, open sagebrush grassland and eroded silt slopes between the community of Nighthawk (international border) and Shanker’s Bend (United States) (see Vegetation and Habitat Types in Public Utility District No. 1 of Okanogan County [2009]).

Conversely, the more common and broadly distributed Pholcophora americana is recorded widely in southern BC (Figure 5). Since 2006 there has been substantial search effort that has specifically targeted spiders throughout BC (Copley pers. comm. 2013) (Figure 5). Each dot shown in Figure 5 represents search effort of at least 1.5 hours per site since 2008.

Habitat

Habitat Requirements

Psilochorus hesperusis associated with hot, dry and arid environments characteristic of the Great Basin ecosystem. In BC ecosystems are described as biogeoclimatic zones, a vegetative classification system developed by the BC Ministry of Forests (2009). Psilochorus hesperus habitat is within the Bunchgrass (sagebrush-steppe) and Ponderosa Pine (open parkland) biogeoclimatic zones of the Southern Interior Ecoprovince in the southernmost parts of the Similkameen and Okanagan valleys (Demarchi 2011; Figure 4). The species occupies xeric sites of the driest variant of the Bunchgrass zone (BGxh1 Very Hot Dry Bunchgrass) and margins of the Ponderosa Pine zone (PPxh1subzone Very Hot Dry Ponderosa Pine) (BC Ministry of Forests 2009).

Psilochorus hesperusis associated with specific rock undersurfaces that provide stable, cool and humid microclimatic conditions within an otherwise hot external environment (Figure 7 – 12 and 14) (Bennett unpubl. data; Crawford pers. comm. 2013). Spatially, rocky undersurfaces have open space for territorial and individual movement and are not flush with the underlying substrate (Bennett unpubl. data). Occupied rocks are stable and unlikely to be dislodged (i.e., rocks in active talus slopes are unsuitable habitat). The dimensions of suitable rock habitat are difficult to generalize. However, individual females appear to require a territorial area of about 10-15 cm diameter (Bennett unpubl. data).

Rocks occupied by P. hesperus are typically protected from full-day sun exposure (Figure 6-9) (Bennett unpubl. data) provided by open forest cover dominated by Douglas-fir (Pseudotsuga menziesii), Ponderosa Pine [Pinus ponderosa]), shrubs such as Oceanspray (Holodiscus discolor) and other vegetation characteristic of these ecosystems. The spider is less commonly recorded from similar rocky areas within open bunch grass vegetation with no forest cover. In the absence of shading factors, rocks on north- and east-facing slopes will host P. hesperus while similar rocks on south- or west-facing slopes will be less likely to.

Psilochorus hesperusrecords from woody substrates are considered anomalous (Bennett unpubl. data; R. Crawford pers. comm. 2013).

Habitat Trends

Substantial historical habitat loss has been documented within the lower-elevation grassland ecosystems of the Okanagan and Similkameen valleys (see Lea 2008). The habitat trends within the range of P. hesperus that apply specifically to the rock habitat (for residence) and suitable wandering/dispersal habitat needed to link these rocky sites are difficult to quantify. The current and historic use of rocky outcrops and other rock habitats for road construction materials and other resource extraction purposes is not well documented for the Okanagan and Similkameen valleys.

However, some general habitat trends can be inferred. The majority of the Canadian range of P. hesperus overlaps with the Okanagan-Similkameen Regional District, which has a current human population of 81,000 that is projected to increase 34% to 108,000 by 2031 (Regional District of Okanagan-Similkameen 2012; Statistics Canada 2012b). With this human population increase comes the need for further urban, rural and agricultural development. Historic ecosystem mapping for some of the Antelope-brush bunchgrass ecosystems shows a 67.4% loss to development (Iverson 2012).

Agriculture is one of the major economic drivers in the range of P. hesperus. For example, the lower Similkameen and south Okanagan Valleys have the largest concentration of vineyards in Canada (Regional District of Okanagan-Similkameen 2012). Vineyard area (ha) in BC increased between 1989 (445 ha) and 2011 (3946 ha) by almost 900% (British Columbia Ministry of Agriculture, Food and Fisheries 2004; Lea 2008; Simms 2012). The majority of vineyard development has been in the south Okanagan and lower Similkameen Valleys (see Threats).

Biology

Information on P. hesperus is summarized from Huber (2005) and Slowik (2009) and from unpubl. data (Bennett pers. comm. 2012; Huber pers. comm. 2012; Slowik pers. comm. 2012).

Life Cycle and Reproduction

Pholcid life cycles are poorly understood. Collection and search effort data (see Table 2 and 3) suggest P. hesperus has a one-year life span, although adult females are generally present year-round. Adult females and juveniles overwinter; adult males and new adult females mature in early spring; adult males are unlikely to be encountered after late summer (Bennett unpubl. data; Slowik 2009).

Psilochorus spiders live in small webs composed of a few lines of silk (Bennett unpubl. data; Huber 2005; Slowik 2009). Mating has not been observed. As in most spider species, a mature male spins a small sperm web, deposits a drop of sperm and transfers it to the genital bulbs of his palpi prior to courtship of females and mating. Mated female pholcid spiders generally produce and guard a series of egg sacs over several months resulting in a series of juvenile age classes (Huber pers. comm. 2012). Female P. hesperus produce and guard one egg sac at a time and each sac is a loosely bound ball of up to about two dozen eggs carried in the mother’s jaws.

Physiology and Adaptability

There is no information on the physiology or adaptability of P. hesperus. Artificial objects such as cement blocks (see Figure 9) may mimic microclimatic conditions and spatial territory requirements (e.g., site 12 Summerland site).

The general public as well as arachnologists regularly and routinely sample non-natural habitats throughout the world. While this search effort is not quantifiable, it is significant and assumed synanthropic records of P. hesperus would be recorded during such widespread searches. Within its Canadian range (as well as in the Okanogan Valley in WA) the only synanthropic record for the species is Peach Orchard Cemetery in Summerland (site 12). This population may be the result of passive anthropogenic dispersal via artificial substrate (concrete landscaping blocks, origin unknown). The cemetery is the northernmost P. hesperus site, surrounded by unsuitable habitat which has been searched (Copley pers. comm. 2013), and the nearest extant site is more than 35 km to the south.

Dispersal and Migration

Psilochorus hesperusis known to disperse short distances from occupied sites by wandering overland (Bennett unpubl. data). Pitfall trapping data (Site 6, 8, 9, 10 and 11, Table 2) suggest adult males, females and juveniles wander from rock substrates at night during the summer months. Dispersal is likely in response to crowding (which leads to cannibalism) and/or a lack of suitable prey; both of which are known behaviours for spiders (Duffey 1998; Foelix 2011). Mature males wander in search of receptive mature females (Foelix 2011).

Several species of pholcids, including at least one species of Psilochorus, are well-known synanthropes and easily dispersed by humans (Bennett unpubl. data; Huber 1994, 2005, pers. comm.; Slowik 2009, pers. comm.). Psilochorus hesperus may be dispersed passively through the transportation of rocks or artificial substrates (Bennett unpubl. data; J. Slowik pers. comm. 2013) (see Physiology and Adaptability). Synanthropic dispersal of P. hesperus is an unlikely occurrence in northern parts of its range, mainly because of widely separated areas of occupied habitat and overall small populations.

The ability of P. hesperus to recolonize large rocks within disturbed habitats has not been studied. Within BC, at least 10 of the 17 sites (59%) are adjacent to roadsides, suggesting the species may have an ability to slowly recolonize the undersurfaces of large suitable rocks within disturbed habitats, from adjacent natural habitats. In WA, P. hesperus has been recorded from under rocks within disturbed habitats such as roadside and railroad right-of-ways, at nine out of 82 collection sites (approximately 11% of sites). It is assumed the species colonized these disturbed habitats from adjacent natural habitats.

Aerial ballooning is a common dispersal mechanism in many spider families (Greenstone et al. 1987; Bennett unpubl. data 2003; Foelix 2011) but has not been observed nor is likely in pholcids (Huber pers. comm. 2012; Bennett unpubl. data).

Interspecific Interactions

Interspecific interactions for P. hesperus have not been well documented. Psilochorus spiders are often found in close association with the webs of other spiders (Gertsch 1979), especially cobweb weavers (Family Theridiidae), which may be common in the same rocky habitats. For example, Slowik (2009) documented P. imitatus living within the web of a species of Steatoda. P. hesperus has also been recorded living in the webs of Western Black Widows (Latrodectus hesperus) (R. Bennett (unpubl. data).

Psilochorus hesperusare generalist predators and consume insects and other spiders. Spider wasps (Hymenoptera: Pompilidae) are significant predators of spiders (Gertsch 1979; Foelix 2011). Other predators of spiders include insects, other spider species, frogs, birds and small mammals such as shrews. Fungal infections and nematodes often affect spiders (Gertsch 1979; Foelix 2011). Specimens of P. hesperus dispersing from the protection of their rock habitats at night are especially vulnerable to predation by bats and other nocturnal species.

Population Sizes and Trends

Sampling Effort and Methods

There are no studies on population sizes and trends. Targeted P. hesperus sampling to date (2012) has primarily been to record the species’ presence and range in BC (Table 2).

Abundance

Abundance estimates are not possible to calculate based on existing data.

Fluctuations and Trends

Population fluctuation and trends are unknown. However, inferences from presence/not detected data (Table 2 and 3) are available. Sites in the Similkameen Valley have remained occupied between 2001 and 2012. However, populations at four sites in the Okanagan Valley were not detected in 2012: Site 7 Kilpoola Lake (present 2007, not recorded 2012); Site 9a Haynes Lease (present 1992, not recorded 2012); Site 9b Haynes Lease “The Throne” (present 2001 and 2009, not recorded 2012); Site 8 Kennedy Flats (present 1995, not recorded 2009); Site 6 East shore Skaha Lake (present 1996, not recorded 2012). Similarly Site 13 McIntyre Road (undetected 2001 – 2011)was not recorded until 2012. Known populations of P. hesperus in the Okanagan Valley are in open, exposed rocky habitats (e.g., Figure 6, 8) while those in the Similkameen are in rocky habitats with some vegetative protection (Figure 7). These factors may contribute to presence/not-detected records at Okanagan valley sites.

Rescue Effect

The Okanagan and Similkameen valleys span the US border and join at Oroville to become Okanogan Valley in WA. Psilochorus hesperus was caught by pitfall trapping in 1995, less than 1.5 km south of the international border (Table 2). The next nearest P. hesperus site in WA is 35 km south near Tonasket (Crawford unpubl. data) (see Distribution). Potential habitat in the Okanogan Valley from the international boundary south of Tonasket is agricultural (Google Inc. 2011). However, that corridor and its tributaries have not been well surveyed, especially in the US portions of the Similkameen Valley between Oroville and the international boundary. This area does contain a small amount of potential habitat that could provide important linkages between populations in the US (see Vegetation and Habitat Types in Public Utility District No. 1 of Okanogan County [2009]).

Threats And Limiting Factors

Background

The International Union for Conservation of Nature-Conservation Measures Partnership (2006) (IUCN-CMP) threats calculator was used to classify and list threats to P. hesperus (Salafsky et al. 2008; Master et al. 2009). Results of the calculator suggest an overall low threat impact (Table 5). Threats applicable to P. hesperus are discussed below.

Table 5-a. Level 1 Threat Impact Counts for Southern Mountain Population

Table 5-B. Matrix summarizing the results of IUCN-CMP threats calculator applied to the Northwestern Cellar Spider.

Residential or Commercial Development (Threat 1)

1.1 Housing and Urban Areas.

Population growth in the Okanagan-Similkameen Regional District from 2011- 2031 is projected at more than 30% (Regional District of Okanagan-Similkameen 2012) (see Habitat Trends). The Regional Growth Strategy aims to protect sensitive habitats; however, this region has some of Canada’s most desirable real estate and there is constant pressure to build ( e.g., see http://www.regalridge.com/). Although trend data are not available for specific rocky areas, residential development continues on open slopes (dispersal habitat) and rocky outcrops (nest habitat).

This threat applies to potential habitat. For example, unsurveyed rock habitat on the western slopes of Anarchist Mountain has been converted to residential development. Blasting and filling of rock outcrops occurred on the east side of Skaha Lake (Dyer pers. comm. 2013).

Agriculture et Aquaculture (Threat 2)

2.1 Annual and Perennial Non-Timber Crops

Agricultural development (specifically vineyard development) has increased substantially from 1989 and 2011 (see Habitat Trends) (British Columbia Ministry of Agriculture, Food and Fisheries 2004; Lea 2008; Simms 2012). During this development, significant dispersal habitat for P. hesperus was also converted. For example, low elevation bunchgrass grasslands have decreased by about 70% since the 1800s (see Lea 2008) particularly along the east side of the Okanagan Valley from Vaseux Lake to Osoyoos (see Figures 8, 9 in Lea 2008). This area also corresponds to all recorded sites for P. hesperus in this valley. Valley bottom areas of the lower Similkameen Valley between Keremeos and the international boundary have also been converted to agricultural purposes (Google Earth 2011).

This threat applies to potential habitat. In the past, agricultural development eliminated site 11 (Osoyoos Indian Reserve 1).

Energy Production and Mining (Threat 3)

3.2 Mining and Quarrying

Sites with proposed mining and gravel extraction within the range of P. hesperus are currently under review, and government staff are regularly asked to review potential applications for conservation values. Staff are most likely not aware of this species and the importance of conservation of rocky habitats for the species.

This threat applies to sites adjacent to roadways, where sites could be designated as a source of gravel, rock or road materials that are necessary for road maintenance in the area. This threat also applies to potential habitat.

Transportation and Service Corridors (Threat 4)

4.1 Roads and railroads.

The major highways of the lower Similkameen Valley (Highway 3), Okanagan Valley around Skaha Lake, Vaseux Lake and McIntyre Bluffs (Highway 97) and at Osoyoos (Highway 3 and 97) are dispersal barriers for P. hesperus. Highway 3 traverses known or potential habitat from Princeton to the west side of Richter Pass and crosses the Okanagan Valley between the west side of Richter Pass on Mount Kobau to Anarchist Mountain. In the Okanagan Valley, particularly on the approaches to Anarchist Mountain west of Osoyoos, the highway traverses P. hesperus sites. Highway 97 at Skaha Lake/Vaseux Lake/McIntyre Bluffs for P. hesperus sites and potential habitat.

Highway maintenance and expansion may destroy roadside habitats. For example, about 6 km upstream from Hedley, P. hesperus occupies habitat immediately adjacent to the Similkameen River bridge. Replacement of this bridge and the associated rerouting of Highway 3 in the 1990s likely destroyed rocky habitat.

This threat applies to most sites adjacent to roadways or along transportation corridors (11 sites). At some sites, the occupied rock habitat is not likely suitable but adjacent dispersal and surveyed areas (where no specimens have been recorded) may be impacted in the future.

4.2 Utility and Service Lines.

Pipeline construction and maintenance can directly impact P. hesperus habitat. A natural gas pipeline crosses the Okanagan Valley at Manuel Flats/Oliver, enters the lower Similkameen Valley at Keremeos, and follows two routes north through the valley (Fortis BC 2011). One route crosses the mouth of the Ashnola Valley and travels overland via the Paul Creek drainage to rejoin the Similkameen Valley near Princeton. The other route follows the valley bottom on the northeast side to Princeton, parallel with Highway 3 and the Old Hedley Road. This route travels through all the Old Hedley Road sites occupied by P. hesperus (site 1, 2, 3, 4, 14, 15, 16). For example, the Oliver to Ashnola River/Paul Creek pathway is included in 161 km of pipeline upgrading scheduled to commence in late 2014 (FortisBC 2011). This will involve extensive excavation and other forms of significant habitat disruption in or adjacent to P. hesperus habitat, especially in the vicinity of the mouth of the Ashnola River (site 17). This threat is likely to impact eight sites, although slow natural colonization may occur if the species inhabits suitable adjacent and undisturbed habitats.

A hydroelectric transmission corridor travels through P. hesperus habitat between Keremeos and Princeton and maintenance activities may impact sites.

Where undisturbed P. hesperus habitat occurs adjacent to the pipeline and hydroelectric corridors, observational data suggest the species is able to disperse into and occupy suitable new habitat in the corridors following construction/maintenance disturbance events and sufficient time has passed to allow the disturbed sites to stabilize (Bennett unpubl. data). For example, P. hesperus is recorded in stabilized rocks adjacent to pipeline and hydroelectric corridor access roads at sites 1 and 2 along the Old Hedley Road.

Natural System Modifications (Threat 7)

7.1 Fire and Fire suppression

Wildfires occur in the southern third of the province and consume large tracts of forest and grasslands annually in the interior including the Similkameen-Okanagan Regional District (see BC Ministry of Forests and Range 2012c). For example, the 2500 wildfires recorded in BC in 2003 were concentrated in the southern interior of the province and burned large amounts of habitat in the south Okanagan Valley including important P. hesperus habitats in the vicinity of Vaseux Lake (British Columbia Ministry of Forests and Range 2012b), McIntyre Road (British Columbia Ministry of Forests and Range 2012a) and Haynes Lease in the mid-1990s. Fires in dispersal habitat destroy individual spiders as well as the cover objects and other habitat that provide protective retreats for them during dispersal. Hot ground fires destroy spiders in their residences.

7.2 Dams and Water Management/Use and 7.3 Other Ecosystem Modifications

Historically, the Okanagan River and many of its tributaries were altered through dam construction, stream bed alteration, and other water management activities. Today approximately 7% of the Okanagan River remains in undisturbed condition (Lea 2008).

It is not possible to quantify the impact of historic water management activities on P. hesperus. Within the Similkameen Valley, water storage and hydroelectric dam projects are under consideration at Shanker’s Bend (upstream from Oroville in WA) and Copper Mountain (upstream from Princeton in BC). The Copper Mountain site (Garstin 2012) will not impact P. hesperus habitat. Shanker’s Bend has been a proposed dam site since the 1920s and could have significant impact on P. hesperus habitat in Canada. In 2006 the Okanogan Public Utility District began steps towards a dam (Boyer 2009; Washington State Department of Ecology 2011). However, plans for these dam options were shelved in 2011 due to widespread opposition (Washington State Department of Ecology 2011).

Pollution (Threat 9)

9.3 Agricultural and forestry effluents

The application of pesticides in the McIntyre Bluff and Osoyoos corridor are ongoing. The study of pesticide impacts on spiders began only recently; however, the negative impacts are now well documented ( e.g., see review in Maloney et al. 2003). The combination of land conversion and operational activities likely renders agricultural areas unusable as dispersal habitat and prevents the migration of individuals and establishment of populations in previously unoccupied suitable habitat.

Geological Events (Threat 10)

10.3 Avalanches/Landslides.

Psilochorus hesperus does not occupy unstable rocky habitats, such as steep talus slopes or areas with frequent falling rock. Most known sites occur in stabilized habitats on the margins of active rock habitat. Summerland Peach Orchard Cemetery is within a steep silt slope surrounded by some residential development and at risk from silt washouts or slumping.

Limiting Factors

Psilochorus hesperushas limited natural dispersal abilities, small population sizes, may be isolated due to the rocky habitat associations, and in crowded populations may exhibit cannibalism.

Numbers of Locations

There are at least 16 locations of P. hesperus in Canada, corresponding to the 16 extant sites: eight in the lower Similkameen Valley and eight in the south Okanagan Valley. One location at Summerland (site 12) is situated in an artificial rock wall within a cemetery and likely a result of human movement. This site is still considered extant and included in the number of locations. An additional site in the Okanagan Valley was converted to agricultural use, and the species is presumed extirpated. This site has not been resurveyed; however, there is a possibility the species could remain within small rocky areas surrounding the agricultural area that have not been converted. Based on search effort data and areas of potential habitat, there are an estimated 5 to 10 additional locations.

Protection, Status, and Ranks

Legal Protection and Status

Psilochorus hesperusis not protected under provincial or federal legislation. Invertebrates assessed by COSEWIC as Threatened, Endangered or Extirpated will be protected through theBritish Columbia Wildlife Act and Wildlife Amendment Act once the regulations listing these species are completed. The British Columbia Parks Act and Ecological Reserves Act protect species at risk within these areas, and when P. hesperus is assigned a conservation status rank the species will be protected under these acts. The species is a potential Identified Wildlife candidate under the provincial Forest and Range Practices Act due to the possibility of rock substrate extraction for road construction.

Non-Legal Status and Ranks

Psilochorus hesperusis provisionally assigned a conservation status rank of N2S2 (nationally and provincially imperilled) in Canada (NatureServe Canada 2010). The species has not been assigned a global conservation status rank. The 2010 Canada and BC General Status rank for P. hesperus is “2” (may be at risk) (Wild Species 2012). The species has not been assigned a global (Natureserve 2013) or provincial conservation status rank (British Columbia Conservation Data Centre 2013). The species has not been assigned a conservation status rank at the national or subnational level in the United States. There are currently no international laws that protect this spider in other parts of its global range.

Habitat Protection and Ownership

Psilochorus hesperusis recorded within three provincial protected areas: Haynes Lease Ecological Reserve, Bromley Rock Provincial Park and South Okanagan Grasslands Protected Area (East Chopaka Site); one private conservation area (Skaha Conservation Area owned by The Nature Conservancy); and one federal protected area (the Northeast and Southeast Uplands Units of the Vaseux-Bighorn National Wildlife Area).

Sites on provincial Crown land are managed under the British Columbia Forest and Range Practices Act; however, the species is not listed as Identified Wildlife under this act and thus not protected (e.g., Old Hedley Road sites 3 and 4 [other than at Bromley Rock], Kilpoola Lake [site 7], and Ashnola River [site 17]).

A few sites are managed by First Nations (Site 17 Ashnola River and Site 11 Osoyoos Indian Reserve 1) or are private property ( e.g., Site 10 Osoyoos East Bench and Site 12 Summerland Peach Orchard Cemetery).

A Regional Growth Strategy (RGS) has been developed (Regional District of Okanagan-Similkameen 2012) that provides guidelines and policies for the protection of sensitive ecosystems. The document focuses on wetland protection and includes caveats such as “the right to farm”. However, the conservation value of the lower Similkameen and south Okanagan Valleys as a national biodiversity hotspot is well known, and the Strategy Steering Committee includes representation from Environment Canada, Natural Resources Canada, and the BC Ministry of Environment.

Acknowledgements and Authorities Contacted

The following people are gratefully thanked for their contributions to the preparation of this status report in the form of background information, logistical support, shared data, mapping, expert opinion, and/or field assistance. From the British Columbia provincial government – Orville Dyer (Ecosystems Biologist, Ministry of Forests, Lands and Natural Resource Operations, Penticton, BC), Dave Fraser (Scientific Authority Assessment, Ministry of Environment, Victoria, BC), Jennifer Heron (Invertebrates Specialist, Ministry of Environment, Vancouver, BC), and Leah Ramsay (Program Zoologist, Conservation Data Centre, Victoria, BC); from the COSEWIC Secretariat, Gatineau, QC – Angele Cyr (Scientific Project Officer), Wendy Dunford (Recovery Management), Alain Filion (Scientific and GIS Project Officer, mapping support), Monique Goit (Scientific Project Officer), Neil Jones (Scientific Project Officer and ATK Coordinator), Sonia Schnobb (Program Support Specialist), and Shirley Sheppard (Administrative Specialist); from the Royal British Columbia Museum, Victoria, BC – Rob Cannings (Curator of Entomology), Claudia Copley (Collections Manager), and Darren Copley; from the University of British Columbia – Launi Lucas (Department of Zoology), Karen Needham (Collections Manager, Spencer Entomological Museum), and Geoff Scudder (Professor Emeritus, Department of Zoology).

From elsewhere – Paul Catling (COSEWIC Arthropods Specialists Subcommittee, Ottawa, ON), J. Coddington and D. DeRoche (Entomology, Smithsonian Institution, National Museum of Natural History, Washington, DC), R.C. Crawford (Pacific Northwest regional spider expert, Burke Museum of Natural History and Culture, Seattle, WA), B. Huber (world Pholcidae expert, Zoologisches Forschungsmuseum Alexander Koenig, Bonn, Germany), L. Leibensperger (Invertebrate Zoology, Museum of Comparative Zoology, Harvard, MA), N. Platnick and L. Sorkin (Entomology, American Museum of Natural History, New York, NY), J. Slowik (Nearctic Psilochorus expert, University of Alaska Museum of the North, Fairbanks, AK), and D. Ubick (California Academy of Sciences, San Francisco, CA). All spider photographs by Darren Copley.

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Biographical Summary of Report Writer(s)

Robb Bennett holds MSc and PhD degrees earned from the study of spider taxonomy and systematics. He is a Fellow of the Entomological Society of Canada and currently works as a research associate at the Royal British Columbia Museum, and an entomology/arachnology and invasive species consultant. Previously he worked as a forest insect management specialist, mentored over two dozen graduate and an undergraduate student, served on a variety of professional administrative committees and boards of directors, and was a member of the COSEWIC Arthropod Specialists Subcommittee and Editor-in-Chief of The Canadian Entomologist.

Collections Examined

American Museum of Natural History. E-mail correspondence between R. Bennett and N. Platnick and L. Sorkin, October 2012 (no Canadian specimens).

California Academy of Sciences. E-mail correspondence between R. Bennett and D. Ubick, October 2012 (no Canadian specimens).

Canadian National Collection of Insects and Arachnids (Ottawa, ON). Collections examined by R. Bennett, May 2012.

Museum of Comparative Zoology. E-mail correspondence between R. Bennett and L. Leibensperger, October 2012 (no Canadian specimens).

Royal British Columbia Museum, Victoria, British Columbia. Collections examined by R. Bennett and C. Copley, September 2012.

Smithsonian Institution. E-mail correspondence between R. Bennett and J. Coddington and D. DeRoche, October 2012 (no Canadian specimens).

Spencer Entomological Museum. Collections examined by R. Bennett and D. Copley, August 2010 (no specimens).

University of Washington Burke Museum. E-mail correspondence between R. Bennett and R. Crawford, September 2012 (no Canadian specimens).