Spider Conservation in the United States

By: Kevin L. Skerl

Spiders, like many other invertebrates, have traditionally suffered a lack of attention from conservation professionals and the general public. As more information becomes available, however, scientists are gaining a better understanding of spiders' integral role in natural systems and of the need to improve protection efforts.

Approximately 3,500 spider species, with an additional 350 yet undescribed, exist in the United States and Canada (Roth 1993). They are abundant predators in many terrestrial ecosystems, with estimates of populations in field habitats approaching one million individuals per hectare (Bristowe 1971). Almost all spiders are generalist predators, mainly eating insects and secondarily other spiders (Wise 1993); a few have become specialists (Nentwig 1986). Some larger species may even occasionally feed on small mice, birds, and lizards.

Individual spider species do not possess the characteristics of successful natural control species, since most are generalists and have long generation times in comparison to prey species (Riechert and Lockley 1984). However, when viewed as an assemblage, spiders may play an important role in stabilizing or regulating insect populations because they are one of the most numerous insectivores and exhibit a wide variety of lifestyles and foraging strategies (Nyffeler et al. 1994; for reviews see Riechert and Lockley 1984; Nyffeler and Benz 1987; Wise 1993). Spiders possess the characteristics of predators that can contribute to density-independent limitation of prey, including self-damping, high levels of polyphagy, and life cycles that are asynchronous to those of prey species (Riechert and Bishop 1990). While biological control by spiders has not been clearly demonstrated in natural systems, evidence in agro-ecosystems has been found in several studies (Riechert and Bishop 1990; Breene et al. 1993), and benefits to primary producers have been measured (Carter and Rypstra 1995).

Additionally, spiders are an important food source for birds, lizards, wasps and other animals. In a study of trunk arthropods, spiders provided a relatively constant food source throughout the year for bark-gleaning birds (Peterson et al. 1989). Hogstad (1984) demonstrated that spiders were a primary winter food source for goldcrests (Regulus regulus). Also, spider silk is important to bird species for nest building; 24 of 42 families of passerine birds and nearly all species of hummingbird depend on silk from spiders and caterpillars for nest construction (Hansel 1993).

Several species of spiders have been recognized as rare or worthy of concern on three different lists of threatened species: the Endangered Species Act (ESA), IUCN Red Lists, and lists compiled by The Nature Conservancy and Natural Heritage Programs.

Only two spiders have been listed under the ESA, the Tooth Cave spider (Neoleptoneta myopica) of Texas and the spruce-fir moss spider (Microhexura montivaga) of Appalachia. Two species, Warton's cave spider (Cicurina wartoni) of Texas, and the Kauai cave wolf spider (Adelocosa anops) of Hawaii are considered candidates for listing; seventeen additional species were listed as C2 candidate species before this category was dropped.

The 1996 IUCN Red List of Threatened Animals includes eight U.S. spiders. One spider is listed as Endangered, three are considered Vulnerable, and five are listed as Data Deficient (without sufficient information to select an appropriate category).

The Nature Conservancy, in cooperation with the Network of Natural Heritage Programs (NHP) and Conservation Data Centers, maintains one of the most comprehensive biological databases in the Western hemisphere. This database includes global, national and state conservation priority ranks (Master 1991). Only 114 spider species are being tracked in these databases, with 57 assigned priority ranks, clearly illustrating the lack of compiled information on the status of spiders. Of these, 40 species are considered of national concern, with 29 species considered imperiled or critically imperiled (The Nature Conservancy 1997). The other 11 species are considered rare and not necessarily imperiled, but six of these have some uncertainty regarding their status.

The level of protection afforded to spider species is by no means fully documented or comprehensive. While only the two species listed under the ESA receive direct federal protection, several species that are considered rare or threatened are indirectly protected by other means. Some rare cave species in West Virginia, such as the Appalachian cave spider (Porrhomma cavernicolum), receive protection by virtue of being located in caves with the federally protected Virginia big-eared bats (Plecotus townsendii virginianus) (S. Blackburn, West Virginia NHP, personal communication 1996). Other species have populations within protected federal lands (e.g., Microhexura montivaga in Great Smoky Mountains National Park) (Harp 1992), lands managed by conservation organizations (e.g., McCrone's burrowing wolf spider, Geolycosa xera, in Lake Apthorpe Nature Conservancy Preserve, Florida) (Edwards 1992b), and other privately owned lands (e.g., the Rosemary wolf spider, Hogna ericeticola, in the University of Florida Ordway Preserve) (Reiskind 1987).

Habitat loss and degradation

Spiders, like most terrestrial invertebrates, are affected by habitat alteration such as deforestation, agriculture, grazing, and urbanization (Wells et al. 1983). For example, clear-cutting of forests reduces spider abundance and changes spider community composition drastically (Coyle 1981; McIver et al. 1992).

Habitat loss and degradation threaten the Kauai cave wolf spider, which lives in the lava-tube habitats of Hawaii (Howarth 1983b). Resort development and agricultural activities harm the species by polluting the groundwater and burying lava caves. The destruction of surface vegetation removes plant roots, which is an important food source for the cave fauna.

Cave invertebrates in Texas, including the endangered Tooth Cave spider, face similar threats as a result of development, such as the collapse or filling in of caves, flooding due to sewer overflow, and vandalism (Chambers and Jahrsdoerfer 1988). These are concerns for other, often rare, cave species (e.g., Nesticus spp. in Appalachia and Cicurina spp. in Texas) as well. Many Florida spiders, such as the Lake Placid funnel wolf spider (Sossipus placidus), McCrone's burrowing wolf spider, the red widow (Latrodectus bishopi), and the Escambia burrowing wolf spider (Geolycosa escambensis) are threatened by urban development and encroachment by citrus plantations (Edwards 1992a, b, c; Marshall 1992).

Evidence also suggests that paved roads and railway lines may act as linear barriers to dispersal, isolating some cursorial spider species into fragments of habitat (Mader et al. 1990). The magnitude of this effect depends upon other dispersal abilities (e.g., ballooning) of the species.

Alien species

The introduction of alien species can have serious direct and indirect consequences for native species. Alien ant species are perhaps one of the most invasive exotic species, with many reports of effects on native invertebrate species (New 1995). For example, Gillespie and Reimer (1993) demonstrated that endemic Hawaiian spider species (Tetragnatha spp.) are extremely susceptible to alien ant species attack and may be restricted from natural habitat by their presence. Other predatory species, such as sow bugs, cockroaches, and fire ants, which often accompany human development, may also be threats to the Tooth Cave spider and other endangered Texas cave fauna (Chambers and Jahrsdoerfer 1988; Stanford and Shull 1993).

Species introductions can have indirect effects on spider populations as well. The exotic balsam wooly adelgid (Adelges piceae) is decimating the spruce-fir forest in which the endangered spruce-fir moss spider exists, decreasing the forest canopy, which provides vital cover for the spider's sensitive moss mat habitat (Fridell 1995).

Pesticides and pollution

Pesticide use has decreased spider populations in agro-ecosystems, affecting the ability of spiders to control pest species (Riechert and Lockley 1984; Clausen 1990; Young and Edwards 1990). Fertilizers may change spider community composition and activity as well (Kajak 1978). Chemical contamination of groundwater can have especially deleterious effects on cave spiders, and has been cited as a threat to several endangered arthropods, including the Tooth Cave spider (Chambers and Jahrsdoerfer 1988; Stanford and Shull 1993).

Acid rain has been proposed as a factor contributing to the decline of the Appalachian spruce-fir forests, thus affecting the spruce-fir moss spider (Fridell 1995). Also, needle loss in spruce forests, due to air pollution, impacts spider species composition on spruce-living species (Gunnarson 1988).

Federal protection efforts currently focus on the two ESA-listed spiders. The conservation strategies and actions implemented may be representative of those necessary for protection of other spider species and illustrate some challenges in invertebrate conservation.

Tooth cave spider

The Tooth Cave spider is a small (1.6 mm length), whitish spider found in two (and possibly an additional two) caves in Travis County, Texas (O'Donnell et al. 1994). Imminent development in the vicinity of this cave and karst system prompted the listing of the spider as endangered along with four other Texas cave invertebrates in September 1988; a Recovery Plan for these invertebrates has been approved.

The secretive nature of most cave species makes assessing population size and distributions difficult, since many individuals may reside in humanly inaccessible spaces (Howarth 1983a). The special needs of cave and karst species, including high humidity (nearly 100%), stable temperatures, and a dependency on surface communities and inputs, create a complex conservation task (O'Donnell et al. 1994).

In addition to continued surveying to perhaps uncover additional populations, numerous other conservation measures are planned to protect the Tooth Cave spider and associated fauna (O'Donnell et al. 1994). Habitat protection efforts are planned in areas around the inhabited caves in hopes of limiting contamination and nutrient depletion. A locked gate has been installed on Tooth Cave (the first cave in which the spider was found) in order to prevent vandalism. Several techniques to control the alien fire ant populations have been examined, including hot water and several chemicals, with additional research needed to determine effectiveness and any adverse effects on the spider. The development of these conservation techniques will hopefully aid in the future protection of numerous rare and threatened cave species.

Spruce-fir moss spider

The spruce-fir moss spider is a tiny (3-5 mm adult size) mygalomorph spider (a group of spiders commonly called 'tarantulas') that is restricted to moist but well-drained moss mat habitats in high-elevation spruce-fir forests of southern Appalachia (Harp 1992). The spider was listed as endangered in February 1995, after exhaustive assessments of population size and distribution revealed only four sites that harbored the species with only one relatively stable population identified (Fridell 1995). Another population has recently been discovered by a group led by Dr. F. Coyle of Western Carolina University (J. Harp, personal communication 1996). A Recovery Plan has yet to be completed, and major challenges to conservation include the complex threats to the spider's fragile habitat and limited information on the natural history, ecology, and genetics of the species.

To assess the threats to the spider's habitat by the balsam wooly adelgid, a monitoring study of the infestation was initiated on the site of the one viable spruce-fir moss spider population. Early results of this study indicate that the level of infestation may require a quicker management response than had been earlier thought (J. Thompson, NC Nature Conservancy, personal communication 1996). Possible responses to this threat include experimental techniques such as insecticidal soaping of trees and transplanting of spider populations to uninfested sites. The effectiveness and viability of these new techniques remains unknown, however.

Due to its secretive nature, little information has been collected on feeding and breeding habits, life span, or dispersal ability of spruce-fir moss spiders. This lack of knowledge contributes to the difficulty in relocation and pursuing another conservation technique: captive breeding. A captive breeding program was initiated in 1992 at the Louisville Zoological Park, and while techniques in maintaining the species in captivity have advanced, successful long-term captivity and reproduction has not yet occurred (J. Harp, personal communication 1996). Proper techniques might need to be learned through the use of a related, non-endangered species, M. idahoana.

Education

A major obstacle for spider conservation is a lack of public support, possibly due to fears and ignorance. For example, in a survey asking whether participants would favor the protection of an endangered spider if it meant increased costs to an energy development project, only 34% of those surveyed said yes, while a bird, cougar, crocodile, plant, and snake fared more favorably (Kellert 1986). Fears can be addressed through pointing out that only a few easily identifiable species pose a threat to humans. By stressing beauty and interesting behaviors and qualities, attitudes towards these invertebrates may change (Robinson 1991).

Several public exhibits, for example the Louisville Zoo's Arachnid Exhibit and the traveling Smithsonian exhibit, "Spiders!," are helping to promote public interest and understanding. In addition, the Terrestrial Invertebrate Taxonomic Advisory Group of the American Zoo and Aquarium Association has an Arachnid Specialist Group which is developing both education programs and conservation plans for arachnids in North American zoos (Wolfe and Mason 1995).

Key development and systematics

A lack of keys for the identification of U.S. spiders remains a major impediment for conservation. Kaston (1978) provided a key to 223 genera, but no key to species. A key to the 515 genera in the U.S. has been completed by Roth (1993). A key to species by Kaston (1981) attempts to identify all species in the New England region. Information for other regions and species are buried in hundreds of technical taxonomic literature, much of which is unobtainable or rare with existing keys to species geographically limited, outdated or unreliable (Coddington et al. 1990). Development of identification keys will allow non-specialists to conduct spider inventories.

Riechert et al. (1985) and Coddington et al. (1990) both report that fewer than two dozen competent arachnid taxonomists are available for identification services of arachnids other than mites and ticks, few comprehensive museum collections exist, and little or no funding for systematic studies is available. For example, many Texas cave species that may be threatened and in need of conservation attention await taxonomic studies and names before actions can be taken; little funding is available to do this work (J. Cokendolpher, personal communication 1996).

Inventories and habitat protection

Without appropriate occurrence information, species in need of conservation may be overlooked. While most U.S. states have some type of spider species checklist, many lists are either outdated, difficult to obtain, or unreliable. Additionally, most of the checklists provide vague collection locations, little information on habitat use, and no discussion of abundance. Species are often known from only one or few localities. For example, of 621 species recorded in a checklist of Utah spiders, 265 (42.7%) were known from only one site, and 498 species (80.2%) were known from fewer than five localities (Allred and Kaston 1983).

While many states are conducting invertebrate surveys in their state, often including butterflies, dragonflies, beetles and mollusks, Ohio is specifically organizing a habitat-specific spider survey (R. Bradley, Ohio State University, personal communication 1996). This survey includes plans to sample a wide variety of habitats over ten years, including reconstructed prairies, oak savanna, and several forest types. Other states (e.g., Washington and North Carolina) have completed some habitat-specific inventories, but these are often the independent work of one scientist who is contracted for specific inventory projects (e.g., Crawford 1994). Even biotic inventories of existing protected areas, such as U.S. national parks, are severely inadequate (Stohlgren et al. 1995). In many cases focusing inventories on rare or threatened habitats could be most useful as it might provide data on associated rare and threatened species in need of attention.

Protection of rare or threatened ecosystems is arguably the best way to preserve the biodiversity that remains in them. Without the necessary inventories, however, it will be difficult to develop management regimes that benefit the most species. A good example of the approach necessary for proper protection is the work being done in Illinois, Iowa, Wisconsin, Minnesota, Ohio, and Indiana, which are each surveying arthropods to develop baseline information for management of midwest prairie ecosystems (K. Methven, Illinois NHP, personal communication 1996).

More specifically, spider inventories in these habitats are necessary for determining spider management needs. The recent studies on spiders in Appalachian caves (Dellinger and Hedin 1994), Florida scrub (Carrel 1995; Marshall 1995), mature and old-growth forests in Washington (Crawford 1994), fens in Missouri (Baltman 1992), and rare sand prairie in Illinois (Landes et al. 1995) are extremely important. Through continued spider inventories in rare and endangered habitats, efforts to preserve biodiversity through ecosystem protection can only be enhanced.

Appropriate ESA listings

Listings for invertebrates under the ESA are becoming more common, but this type of single-species approach to the conservation of such a diverse group of organisms results in protecting only a fraction of the fauna (Franklin 1993). The structure and implementation of the ESA are not well suited for invertebrates because of the strict and vertebrate-biased taxonomic requirements and the general lack of public support for such listings (Murphy 1991). Recently, the U.S. Fish and Wildlife Service expressed an explicit commitment to pursue a "multi-species, ecosystem approach" to listings (Glitzenstein 1993), which should result in more comprehensive and less expensive conservation.

This type of effort would benefit spiders. The first ESA listing of a spider was with four other cave invertebrates (Chambers and Jahrsdoerfer 1988), and this successful approach has been proposed for other cave communities (Ekis and Opler 1978; Stanford and Shull 1993). One possibility would be to list threatened spiders associated with Florida scrub habitats (e.g., Geolycosa xera) together with vertebrate species such as the threatened Florida scrub jay (Aphelocoma coerulescens coerulescens). This would increase public awareness of biotic communities and the specific needs of some of the smaller residents of these habitats. Due in part to limited dispersal abilities, many of these spiders may persist on smaller habitat fragments than vertebrates (Marshall 1995), resulting in increased ecosystem protection.

Captive Breeding

Undoubtedly, spiders are best conserved through habitat protection. However, in some extreme cases, species conservation may be accomplished only through the aid of ex situ breeding programs. This conservation tool remains largely unexplored for invertebrates, especially spiders. Invertebrates generally can be housed, maintained and bred relatively inexpensively in comparison to vertebrate species (Wilson 1987). Existing coordinated captive breeding programs for spiders have been small and have focused primarily on the CITES-listed Mexican red-kneed tarantula (Brachypelma smithi). The London Zoo has pioneered a program that developed preliminary breeding techniques and genetic management protocols for this spider (Clarke 1991). A similar program involving four U.S. zoos has also been developed (D. Hodge, Louisville Zoological Park, personal communication 1995). These two programs may serve as models for others involving endangered spider species.

While few coordinated captive breeding programs for spiders exist, information is available on general breeding techniques (Frye 1992) and nutritional ecology (Riechert and Harp 1986). Successful techniques may also be learned from amateur breeders, venom suppliers, and academic researchers. In fact, cooperation between dedicated amateurs and zoo professionals is an integral part of the London Zoo program (Clarke 1991).

Additional information is needed for successful breeding programs. The effects of long-term captive breeding of invertebrates remain largely unknown (Drummond 1995). Research on specific aspects of the physiological ecology of spiders and its effects on phenotype is in its infancy (Reichling 1995). While some studies in conservation genetics have been completed (e.g. Ramirez and Froelig 1997), research on genetic management is generally nonexistent. Also, since spiders may have high rates of reproduction in captivity, the problem of surplus individuals needs to be addressed (Clarke 1991).

Future conservation efforts for spiders and other arachnids will depend on increased cooperation and communication between arachnologists and conservation professionals. Little information is available to the conservation community regarding the status and distribution of spiders, due to the general difficulty in finding reliable or appropriate sources and an unfamiliarity with the taxa. Similarly, conservationists should make known information needs to researchers in order to encourage information exchange and promote scientific study.

Specifically, through continued public education, appropriately targeted inventories and habitat protection, and additional research in management needs and techniques, spider conservation efforts may be implemented with greater vigor. The incorporation of spiders in all levels of protection efforts, including focused ecosystem-based multi-species ESA listings, can result in better habitat protection. Ecosystem conservation can only be enhanced by focusing some energy on the needs of these diverse and fascinating creatures.

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Kevin L. Skerl studied spider conservation issues while earning his M.S. in Conservation Biology at the University of Maryland. He works for The Nature Conservancy, Latin America and Caribbean Division, 1815 N. Lynn Street, Arlington, VA 22209.