Utilization of prey from the decomposer system by generalist predators of grassland

Abstract

We investigated the linkage between the detrital subsystem and generalist predators of meadow ecosystems by manipulating prey availability in two different ways: we increased resource availability for the decomposer subsystem and thereby decomposer prey by adding mulch materials (detritus enhancement), and we added fruitflies (Drosophila melanogaster, Diptera; prey enhancement) to fenced plots. Both supplemented materials significantly differed in their (13)C/(12)C and (15)N/(14)N ratios from those of the natural litter. We measured density responses of detritivorous, herbivorous and predaceous arthropods to the increased resource supply. We used ratios of natural stable isotopes of N and C in arthropod tissues to trace the flux from the added resources to consumers and to relate density responses of consumers to changes in resource supply. Effects of resource enhancement propagated through at least two trophic levels, resulting in higher densities of major decomposer and predator taxa. Effects of detritus enhancement were much stronger than those of prey enhancement. Signatures of delta(13)C proved density responses of Collembola taxa to be related to the added mulch materials. Among generalist predators, densities of juvenile wolf spiders (Lycosidae) responded more to detritus-enhancement than to prey-enhancement treatments. In contrast, the density of the web-building linyphiid and the non-web gnaphosid spiders remained unaffected. Each spider taxon, including those which did not respond numerically, was significantly enriched in (13)C in detritus-enhancement treatments, suggesting that they gain energy from the decomposer system. Numbers of herbivores-cicadellids and aphids-were similar in each of the treatments, indicating that they were unaffected by changes in predator density. Our results indicate that the lack of a numerical response to resource supplementation is not necessarily due to the absence of a trophic linkage, but may be caused by compensatory changes in mortality factors such as cannibalism and intraguild predation.

Opportunistic predator prefers habitat complexity that exposes prey while reducing cannibalism and intraguild encounters

“…That same analysis did not uncover effects of habitat structure on the density of herbivorous prey, but not all types of potential prey were incorporated into the analysis and, as aptly noted, there are cases where arthropod predators accumulate in response to high densities of detritivores (Chen and Wise 1999;McNabb et al 2001;Halaj and Wise 2002;Oelbermann et al 2008). Indeed, other studies have shown that enhancing the decomposer community plays a role in the establishment of robust populations of predators living up in the vegetation (Scheu 2001;Rypstra and Marshall 2005).…”

Differential response of ants to nutrient addition in a tropical Brown Food Web

“…Moreover, the absence of numerical response to resource supplementation is not necessarily due to the absence of either trophic linkage or bottom-up control (Oelbermann et al, 2008). Indeed, any increase in microbial activity leads to the destruction of the litter habitat as detritus is converted into microbial biomass and CO 2 (Kaspari et al, 2008a;).…”

Long-term resource addition to a detrital food web yields a pattern of responses more complex than pervasive bottom-up control

“…In addition, increased cannibalism and intraguild predation could have contributed to the absence of an effect on total spiders. Oelbermann, Langel & Scheu (2008) directly manipulated energy input to a grassland detrital web in fenced 5-m 2 plots for five months. Many Collembola families responded positively as did total spider numbers, which were ca.…”

“…Such experiments in terrestrial ecosystems have been conducted in prairies and grasslands (Fountain et al, 2008; Hoekman et al, 2011; Oelbermann, Langel & Scheu, 2008; Patrick, Kershner & Fraser, 2012) and forests (Chen & Wise, 1999; David et al, 1991; Lessard et al, 2011; Maraun et al, 2001; Raub et al, 2014; Salamon et al, 2006; Scheu & Schaefer, 1998; Yang, 2006). Most experiments uncovered some evidence for bottom-up control, but the strength and pervasiveness across trophic levels of the responses varied substantially.…”

“…Among experiments conducted to date, densities of some trophic groups occasionally responded negatively to addition of detritus. Do these negative effects reflect differential responses by predators to resource-addition combined with intensification of top-down control processes along a subset of trophic pathways due to the indirect effects of trophic-level omnivory in a detritus-based food web (Polis & Strong, 1996; Halaj & Wise, 2002; Oelbermann, Langel & Scheu, 2008)? Were different pathways of top-down control processes modulating the strength of bottom-up control, producing different responses among trophic levels as the system moved towards a new equilibrium state?…”