Estimate of the standing biomass of the global spider community

A total of 65 values of spider biomass m−2 were gathered from the literature. The data were assigned to the following seven groups of terrestrial biomes: (1) tropical forests, (2) temperate and boreal forests, (3) tropical grasslands and savannas, (4) temperate grasslands (incl. old fields, permanent pastures, mown meadows) and Mediterranean shrublands, (5) annual cropland, (6) deserts, and (7) Arctic tundra. To retrieve comparable data, all values were converted to fresh weight m−2 taking into account an average water content of the spider body of ≈75% (Pulz 1987). The data were pooled by computing an average biomass value (g m−2) for each biome type. By extrapolation—using the global land cover data from Saugier et al. (2001)—the standing biomass of the global spider community was then computed (Table 1).

Table 1 Estimated standing biomass of the global spider community based on grams per square meter values ( \( \overline{\mathrm{x}} \) ± SE, all values expressed as fresh weight) Full size table

Estimate of the annual prey kill by the global spider community

We used simple models involving few assumptions as is advised in cases where a field of study is still largely undeveloped (Weathers and Weathers 1983). Two different approaches were taken to estimate the annual prey kill of the global spider community. In the case of method I, the estimate is based on the spiders’ food requirements per unit body weight known from the literature in combination with spider biomass m−2 values (data for various biome types being taken from the literature), whereas method II is based on complete assessments of the spiders’ annual prey kill (e.g., prey censuses in the field combined with web density estimates) in selected biome types published in the literature. The two estimation methods are based on different sets of studies (with zero overlap of data between the two methods).

Prey items that are killed in webs but remain uneaten are considered prey as well (Nentwig 1987). This issue is playing a role when dealing with spider communities dominated by large-sized orb-weaving spiders known to often kill prey in excess (“wasteful killing”). Accordingly, authors who conducted prey censuses of large-sized orb weavers usually have taken into account the “prey killed in webs but uneaten” in their assessments (see Robinson and Robinson 1974; Kajak et al. 1971; Nyffeler 1976, 1982; Nyffeler and Benz 1978, 1979, 1989; Malt 1996). In other studies dealing predominantly with cursorial hunters and/or small-sized web-building spiders which rarely catch prey in excess under natural conditions (Nyffeler, pers. obs.), this issue was disregarded.

Method I : Based on the spider biomass m−2 values from Table 1 and an assumptive food intake rate (mg prey per mg spider body mass day−1), the prey kill m−2 day−1 of spider communities for each of the seven biome types was computed. These values were extrapolated to prey kill m−2 year−1 for each biome type, considering the length of the spiders’ feeding season (in days, see assumptions below). By multiplying the prey kill m−2 year−1 with the corresponding area size of a particular biome type (based on Saugier et al. 2001), a prey kill subtotal was derived for each biome type. Summing up the seven subtotals produced an estimate of the annual prey kill of the global spider community (Table 2). The estimate derived by method I was based on the following assumptions:

Table 2 Estimated annual prey kill (fresh weight) of the global spider community assessed with method I Full size table

- Assumption 1: Spiders have pulsed feeding patterns, with periods of excessive feeding (when food is very abundant) alternating with episodes of starvation (when prey gets scarce or spiders are inactive) (Turnbull 1973; Anderson 1974). During periods of high feeding activity, the spiders store surplus energy in their body’s interstitial tissue as lipid or glycogen (Foelix 2011). The spiders depend on these stored energy reserves during periods of starvation (e.g., on rainy days). To estimate the annual prey kill of the global spider community, we proceed from an overall mean food intake which is intermediate between pronounced high or low daily food consumption. After an extensive literature survey on spider feeding, we propose an average daily food ingestion rate of ≈0.1 mg per milligram spider body mass which is the equivalent to ≈10% of a spider’s body weight (all values expressed as fresh weight). A daily food intake in this order of magnitude appears to be typical of most species of free-living araneomorph spiders in forests, grasslands, and agroecosystems (see Edgar 1970; Robinson and Robinson 1970; Van Hook 1971; Foelix 2011; Nyffeler, pers. obs.) except a few rare cases of extraordinarily low food intake (e.g., Santana et al. 1990; Henschel 1997). In the case of desert spiders, a daily food ingestion rate of 0.01–0.04 mg per milligram spider body mass was used (see Lubin and Henschel 1996; Henschel 1997). Incidents of wasteful killing (and coupled with it “partial consumption”) in cursorial spiders apparently occur very rarely under natural conditions (Nyffeler, pers. obs.), and this issue has therefore not been taken into account in this study.

- Assumption 2: Spiders ingest, on average, ≈80% of the biomass of a killed prey (Edgar 1971; Moulder and Reichle 1972). Hence, we proceed with the assumption that the daily prey kill equals the daily amount of food ingested multiplied by the factor 1.25.

- Assumption 3: We assume that spiders forage on 365 days year−1 in tropical forests, on 240 days year−1 in deserts, on 180 days year−1 in temperate forests as well as tropical and temperate grasslands, on 120 days year−1 in the arctic tundra, and on 60–130 days year−1 in annual cropland (Kajak et al. 1971; Robinson and Robinson 1970, 1973, 1974; Breymeyer 1978; Shook 1978; Nyffeler 1982; Byzova et al. 1995). The contribution of winter-active spiders in terms of prey kill in temperate and cold climates (see Aitchison 1984) is considered to be very low and has therefore been neglected.

- Assumption 4: Spider biomass in forests has in most cases been assessed with the Berlese-Tullgren funnel method. This technique is limited to the investigation of spiders on the forest floor, and the calculated biomass values underestimate true biomasses. In temperate forests, at least 20% of the spider biomass are found in the canopy and understory (see Turnbull 1960; Reichle and Crossley 1967; Moulder and Reichle 1972; Zitnanska 1981). This pattern seems to hold for tropical forests (see Basset et al. 1992; Silva 1996; Yanoviak et al. 2003; Ellwood and Foster 2004; Dial et al. 2006). By multiplying the litter spider biomass values with a correction factor of 1.25, estimates for the total spider biomass in temperate, boreal, and tropical forests were obtained (Table 1).

- Assumption 5: For biomass m−2 of spiders associated with Mediterranean shrublands, no data are available. We arbitrarily placed this biome type in the category “Temperate grasslands (old fields, permanent pastures, mown meadows)”. The area size of Mediterranean shrublands is small (2.8 × 1012 m2) relative to the global terrestrial area, and a possible error resulting from insufficient data can be considered to be negligible.

Method II : The second approach is based on published studies of the annual prey kill of spider communities in various biome types (see Kirchner 1964; Reichle and Crossley 1967; Kajak et al. 1971; Van Hook 1971; Moulder and Reichle 1972; Robinson and Robinson 1974; Luczak 1975; Nyffeler 1976, 1982; Nyffeler and Benz 1978, 1979, 1988a,b, 1989; Schaefer 1990; Ysnel 1993; Jmhasly and Nentwig 1995; Malt 1996). For purposes of comparison, all prey kill values (including those expressed in terms of energy flow) were converted to grams of fresh weight m−2 year−1. Values were converted taking into account a prey water content of ≈75% (see Hagstrum 1970; Edgar 1971) and a caloric equivalent of prey of ≈23.5 kJ g−1 dry weight (mean value from literature data, see Hagstrum 1970; Moulder and Reichle 1972). Thus, 1 g fresh weight prey biomass equals ≈5.875 kJ. By means of extrapolation, the annual prey kill of the global spider community was computed, taking into account the global coverage of the different biome types. The global annual prey kill assessment with method II was based on the following assumptions:

- Assumption 1: Assessments of the annual prey kill of spider communities in tropical forests are currently unavailable. In lieu thereof, a study by Robinson and Robinson (1974) on the prey kill by the web-building spider community of an insecticide-free coffee plantation in New Guinea was used as a surrogate. We were operating on the assumption that the spider communities of tropical insecticide-free coffee plantations are to some degree comparable to those of tropical forests, given that coffee plantations are inhabited to a large extent by tropical woodland spiders (e.g., Nephila maculata) (Robinson and Robinson 1973, 1974; Robinson et al. 1974; Lubin 1978). Robinson and Robinson (1974) came to the conclusion that the web-building spider community in their study killed 16 g insect prey m−2 year−1. These authors suggested that the annual prey kill may even have been twice as high (≈32 g insect prey m−2 year−1) if hunting spiders would have been considered. The spider density in this coffee plantation (5.8 individuals m−2) was higher than the reported densities in the understory of tropical rain forests (3.3–3.6 individuals m−2; Rypstra 1986; Reagan and Waide 1996). However, we have to take into account that coffee plants reach a height of only 3–3.5 m, whereas tropical forest trees grow to a height of up to 55 m (Silva 1996). The canopy of tropical forests is inhabited by an abundant spider fauna (Basset et al. 1992; Russell-Smith and Stork 1994; Silva 1996; Ellwood and Foster 2004), and it is to be expected that those spiders kill considerable numbers of insects in addition to the insects killed by the spiders of the understory. Thus, it is well possible that the annual prey kill by spiders in tropical forests does exceed the conservative estimate of 16 g insect prey m−2 year−1.

- Assumption 2: Annual prey kill values for temperate forests appear to vary widely. Kirchner (1964) estimated an annual prey kill of ≈10 g m−2 year−1 for a semi-natural temperate forest in Central Europe, whereas lower values were reported for managed temperate forests. The annual prey kill in managed temperate deciduous forests in North America and Central Europe was estimated at ≈2 g m−2 year−1 (calculated by combining data for the spiders of the forest floor, understory, and canopy [Reichle and Crossley 1967; Moulder and Reichle 1972; Schaefer 1990]).

- Assumption 3: Annual prey kill values for unmanaged grasslands vary widely from ≈2 g m−2 year−1 (Ysnel 1993) up to >10 g m−2 year−1 (Kajak et al. 1971; Nyffeler and Benz 1989). In order to avoid overestimation, we used a conservative annual prey kill range of 2–10 g m−2 year−1 for grasslands and savannas (Table 3). The split in global coverage between unmanaged grassland/savannas (13.7 × 1012 m2) vs. permanent pastures/mown meadows (28.9 × 1012 m2) was based on SAGE/GTAP data http://www.agter.org/images/merlet_c2a_cultivablelands_G1.png

Table 3 Estimated annual prey kill by the global spider community (expressed as fresh weight g year−1) assessed with method II Full size table

- Assumption 4: Published prey kill records for desert spider or arctic tundra communities are not available. Nevertheless, based on published natural history data (Polis 1991; Henschel 1997), we conclude that the annual prey kill in deserts is most likely very low. One might compare deserts to some degree to urban environments where the annual prey kill by spiders is also very low (0.2 g m−2 year−1; Nyffeler 1976). Supposing that the annual prey kill in deserts might be equally low as in urban areas, we arbitrarily assigned an assumptive value of ≈0.2 g m−2 year−1 to the spider communities in desert areas in order to be able to compute the global annual prey kill with method II. Due to many similarities between tundra and agricultural habitats (regarding population densities, body size composition and faunistic composition), we assume that the annual prey kill in arctic tundra sites might be comparable in magnitude to field crops in Europe (≈0.1–1 g m−2 year−1; Table 3).

Summing up the estimated prey kill subtotals for the seven biome types produced a second estimate of the annual prey kill of the global spider community.