Their capability to survive under extreme conditions and to disperse by ballooning through the atmosphere from place to place on silken threads allowed spiders to colonize a wide variety of different terrestrial habitats. Spiders have efficient survival mechanisms given that they are so numerous and widespread. Very rarely nonarthropod prey and sometimes even plant materials are consumed as a supplement to the arthropod diet (Symondson et al. All spiders are carnivores, feeding predominantly on insects/collembolans and to a lesser extent on other spiders (Nyffeler 1999 Birkhofer and Wolters 2012 Pekár and Toft 2015). Barth ( 1997) partially attributes the evolutionary success of spiders to the fact that they are equipped with highly developed sensory systems providing individuals with detailed information about potential predators and prey in their surroundings. At the present time, >45,000 species of spiders are described and those exhibit a very diverse range of lifestyles and foraging behaviors (Wise 1993 Platnick 2014). Under favorable conditions, spiders can reach peak densities of up to 1000 individuals m −2 (Ellenberg et al. For instance, Turnbull ( 1973) calculated an overall mean density of 131 spiders m −2 based on assessments from many different areas of the globe, and Nyffeler ( 2000) found an overall mean density of 152 spiders m −2 for a large variety of grassland habitats. Spiders, which evolved from an arachnid ancestor during the Devonian period around 400 million years ago, are among the most common and abundant predators in terrestrial ecosystems (Turnbull 1973 Coddington and Levi 1991 Selden et al. The presented estimates of the global annual prey kill and the relative contribution of spider predation in different biomes improve the general understanding of spider ecology and provide a first assessment of the global impact of this very important predator group. Our estimates are supported by the published results of exclusion experiments, showing that the number of herbivorous/detritivorous insects and collembolans increased significantly after spider removal from experimental plots. This, however, can be partly explained by the fact that annual crop fields are “disturbed habitats” with a low buildup of spider biomass and that agrobiont spiders often only kill prey over short time periods in a year. The spider communities associated with annual crops contribute less than 2% to the global annual prey kill. Spiders associated with forests and grasslands account for >95% of the annual prey kill of the global spider community, whereas spiders in other habitats are rather insignificant contributors over a full year. This equals approximately 1‰ of the global terrestrial net primary production. Our estimates assessed with two different methods suggest that the annual prey kill of the global spider community is in the range of 400–800 million metric tons (fresh weight), with insects and collembolans composing >90% of the captured prey. To document the impact of the global spider community as insect predators, we present estimates of the biomass of annually killed insect prey. Spiders have been suspected to be one of the most important groups of natural enemies of insects worldwide.
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