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Importance of Spiders

Spiders are abundant and ecologically important in almost every terrestrial and semi-terrestrial (wetland) habitat on Earth, from cold tundra and alpine ecosystems to tropical rainforests and deserts. Some species live on the shores of the ocean and some dive into ponds and streams in search of prey-members of one Eurasian spider family (Argyronetidae) e Spider Web ven live underwater in silken "diving bells" which they fill with air brought from the surface clinging to their hairy abdomens. Spiders are found high up on Mount Everest, and some of the most northerly animals are spiders, as well (although there are no spider species indigenous to Antarctica, some undoubtedly share quarters with another "exotic" Antarctic inhabitant.humans).

Because they are usually so abundant, spiders are among the most important predators in many ecosystems. Numbers often exceed 100 per square meter, which equates to 1 million spiders per hectare (Coleman and Crossley, 1996). Although spiders are relatively small (body length from 1 mm to several cm), their combined weight in a habitat often exceeds that of the larger, but more rare, vertebrates such as birds.

All spiders are predators, and because the main item in most spider diets is insects, humans benefit enormously from spiders. Spiders are important in controlling insect pests in most agricultural crops. By controlling insect population sizes, they keep most potential insect populations too small to be economically damaging, most of the time. But because spiders cannot reproduce as quickly as their insect prey, they cannot control an insect population once it has grown to "outbreak" proportions. For this reason, the importance of spiders to agriculture has generally been underestimated. After their spider enemies have been killed by the same insecticides, the few surviving insect pests are often able to reproduce and attain larger population sizes than before the insecticide was applied, a phenomenon known as "resurgence".

Spiders also eat many insects that bother humans, such as mosquitoes and cockroaches. Since a single spider may eat many mosquitoes in one day, over the course of its lifetime a spider may prevent hundreds of mosquitoes from surviving to produce even more mosquito offspring. Similarly, a spider in your house "pays rent" by ridding the premises of many cockroaches or other insects. With fewer spiders, the world would be a much "buggier" place.

Additionally, spiders are an important food source for birds, lizards, wasps and other animals. Ground-dwelling spiders may be important in transferring energy directly from the below-ground detritus food web to the above-ground terrestrial food webs of familiar birds, reptiles, amphibians, and mammals (Johnston, 2000). Spiders are an important source of food for many birds, especially in the winter (Peterson et al. 1989; Hogstad, 1984, in Skerl 1997). 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, in Skerl 1997).

Many people fear spiders, but actually the majority of spiders are too small to bite humans. Any spider smaller than about 8 mm, or one-quarter inch, in body length cannot break human skin with their tiny fangs, and most spider species are smaller than this. Of those spiders large enough to bite humans, only a few have bites which are dangerous to humans. In the U.S. Great Lakes region, only black widows (Latrodectus species) and the brown recluse spider (Loxosceles reclusa) have potentially dangerous bites, and these are much more abundant in regions farther south. Spiders only bite humans as a last resort when threatened (as when they are accidentally pressed between a glove they've been living in and a hand). Spiders simply do not "attack" humans.

Spider Silk

Spider Silk Many types of spiders construct specific types of silken web to trap prey. These come in a variety of forms, from the familiar orb web and cobweb to sheets, funnels, tubes, ladders, and even a single thread. A single spider may produce more than a half-dozen different kinds of silk for different purposes, some sticky, some not. One kind of silk, for example, is used by many web-spinning spiders to wrap up prey after it initially becomes trapped in the web. This immobilizes the prey and makes it safer for the spider to approach and bite the potential prey. Ogre spiders (family Deinopidae) hold a small web in their front legs, and use it as a net to snare insects. Bolas spiders (Mastophora species, in the family Araneidae) have abandoned the "traditional" web in favor of a silk line with a ball of sticky glue at the end. These spiders exude a chemical which mimic certain female moths by secreting pheromones similar to the ones the moths use to attract mates. The male moths are drawn to the spider, which throws the glue ball at the moth and "reels" its victim in like an angler hauling in a fish.

Spiders use silk in mating activities, too. Male spiders construct a "sperm web" in which they secrete sperm from their gonads, so that it can be sucked up into the pedipalps. These modified mouthparts are not directly connected with the abdominal gonads. Males of many species seem to locate females by following draglines laid down by the females. These draglines seem to contain chemical signals telling the males that an appropriate female laid down the silk. Some males also use silk to tie up the females as part of a courtship ritual. Mating can be dangerous for male spiders because females are often larger and may consider the male a better meal than a mate. These silk "nuptial veils" may give the male a little extra time to escape after mating.

All female spiders wrap their eggs in silk egg sacs, often using several different kinds of silk, some leathery for protection from the outside environment, some soft and fluffy to cushion the eggs. Most spiders constantly lay down a "dragline" as they walk, which acts as a safety line if they need to jump or drop away from a potential predator, for example. Some species hang their egg sacs in or near their webs, while others carry their egg sacs around with them.

Many larger spiders find shelter in burrows they dig in the soil. These are usually lined with silk and may be capped with lids called "trap doors" which are made of silk and other materials such as soil. Many smaller spiders construct various retreats and "sleeping bags" out of silk, as well.

Small spiders can move through the air (Usually only a few feet; sometimes for hundreds of miles) by "ballooning," in which a long strand of silk attached to the spider lifts the spider up in rising air currents. By this method, spiders are able to disperse to new habitats, even to remote islands, and habitats can be recolonized rapidly after a disturbance, such as a fire that might kill most spiders living in a prairie. Spiders can only balloon if they are small (less than about 5-8 mm long), so many spiders balloon only as small immatures. Some small spiders such as sheet-web spiders (Linyphiids), though, can balloon as adults, too. Silk is clearly one big reason for the great success of spiders in so many habitats.

References

Buckle, D. J., D. Carroll, R. L. Crawford, and V. D. Roth. 1993. Linyphiidae of America north of Mexico: Checklists, synonymy, and literature. Version 2.1, July 1994. Available on disk from third author, Washington State Museum, Univ. Washington, Seattle, WA 98198.

Coddington, J. A. and H. W. Levi. 1991. Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics 22: 565-592.

Coleman, D. C., and D. A. Crossley, Jr. 1996. Fundamentals of Soil Ecology. Academic Press, NY.

Hansel, M. 1993. Secondhand silk. Natural History 102: 40-46.

Hogstad, O. 1984. Variation in numbers, territoriality and flock size of a goldcrest Regulus regulus population in winter. Ibis 126: 296-306.

Johnston, J. M. 2000. The contribution of microarthropods to aboveground food webs: A review and model of belowground transfer in a coniferous forest. American Midland Naturalist 143: 226-238.

Peterson, A. T., D. R. Osborne, and D. H. Taylor. 1989. Tree trunk arthropod faunas as food resources for birds. Ohio Journal of Science 89(1): 23-25.

Roth, V. D. 1993. Spider genera of North America. Third edition. American Arachnological Society, Gainesville, Florida.

Schaefer, C. W. and M. Kosztarab. 1991. Systematics of insects and arachnids-Status, problems and needs in North America.

Skerl, K. L. 1997. Spider conservation in the United States. Endangered Species Update 14. 11 pp.

Biodiversity Topics: Introduction . Plants . Animals . Mammals . Birds . Reptiles & Amphibians . Arthropods . Spiders . Insects