An estimated 400–800 million tons of prey are annually killed by the global spider community

Abstract

Spiders have been suspected to be one of the most important groups of natural enemies of insects worldwide. To document the impact of the global spider community as insect predators, we present estimates of the biomass of annually killed insect prey. 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. This equals approximately 1‰ of the global terrestrial net primary production. 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. The spider communities associated with annual crops contribute less than 2% to the global annual prey kill. 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. 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. 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.

Insectivorous birds consume an estimated 400–500 million tons of prey annually

“…Forests cover a large portion of the global terrestrial surface area (41.6 million km 2 ; Saugier et al 2001), and in these productive and vegetatively complex habitats, birds usually reach higher diversities (Willson 1974) and numbers ha −1 compared to non-forested areas (Gaston et al 2003). A similar trend of highest predation impact occurring in forested areas has been reported for spiders (Nyffeler and Birkhofer 2017). Forest birds feed frequently on potentially harmful caterpillar and beetle pests (Holmes et al 1979; Fayt et al 2005; Moorman et al 2007).…”

“…Because it is assumed that ≈ 90% of all land bird individuals in the temperate, boreal, and arctic zones and ≈ 60% in the tropics are insectivorous foragers (see Assumption 1, “Methods” section), it follows that the standing biomass of the global community of insectivorous birds might be on the order of ≈ 3 million tons (Table 1). This value is a small fraction of the global standing biomass of other predaceous animal taxa such as spiders (≈ 25 million tons; Nyffeler and Birkhofer 2017), ants (≈ 280 million tons; Hölldobler and Wilson 1994), or whales (16–103 million tons; Pershing et al 2010). The comparatively low value of the global standing biomass of wild birds is partially explained by the fact that birds have a very low production efficiency (i.e., low P/A-ratio).…”

“…Spiders are an important food source for birds because of their high content of taurine, an amino acid that plays a vital role in the early development of many types of passerine birds (Ramsay and Houston 2003; Arnold et al 2007). The propensity for birds to feed on spiders can reduce some positive economic impact of avian insectivory because spiders themselves are highly beneficial natural enemies of insects (Nyffeler 2000; Nyffeler and Birkhofer 2017). The same is true when birds feed on large numbers of predaceous ants or odonates, as is sometimes the case in purple martins ( Progne subis ) and house martins ( Delichon urbicum ) (Kelly et al 2013; Orłowski et al 2014; Helms et al 2016).…”

“…We used a simple model involving few assumptions as is advised in cases where a field of study is still largely undeveloped (Weathers 1983; Nyffeler and Birkhofer 2017). Our estimate is based on mean values of prey consumption ha −1  year −1 in the various biome types, which subsequently were extrapolated on a global scale.…”

“…Furthermore, we present an estimate of the standing biomass of the global population of insectivorous birds. This study is intended as a continuation of the papers by Nyffeler (2000) and Nyffeler and Birkhofer (2017)—who were studying spiders—to get a better understanding of the global extent to which potentially harmful herbivorous insects are suppressed by major natural enemies.…”