Wild boar rooting and rural abandonment may alter food-chain length in arthropod assemblages in a European forest region

Abstract

Food-chain length, or the trophic level of an apex predator, is among the most important properties of food-webs with implications for community structure, ecosystem processes and pollutant accumulation in forests. Three main hypotheses (ecosystem-size, productivity, and disturbance) have been erected to explain variation in food chain length in freshwater ecosystems, yet the support for these hypotheses in less spatially restricted terrestrial ecosystems has not been extensively studied. Here, we used nitrogen (δ15N) and carbon (δ13C) stable isotopes to explore variation in the realized trophic positions (δ15N) of a beetle Carabus lineatus lateralis and a wasp Vespula vulgaris in 32 chestnut woodland patches in northwestern Spain, while accounting for the insects’ relative mo bility by using inferences based on δ13C. We used five potential predictors of ecosystem-size productivity, seven of disturbance and six covariates in anticipation that biological assemblages in woodlands might be influenced by ecosystem-size productivity, and from a change from the human management to a much-increased activity of ecosystem engineers, especially wild boar. Our results provided support for the disturbance hypothesis and suggested that the beetle FCL seems to be more affected by wild boar disturbance than by human-forest man agement, possibly due to increased forest cover and to rural abandonment in recent decades. Moreover, we found a negative association between the wasp FCL and the ecosystem-size productivity hypothesis, as indicated by the plant Ellengberg’s indicator value for nitrogen, which contrasts to the hypothesis that ecosystem-size productivity should increase FCL. Our findings are discussed in relation to: (1) differences in intensity and frequency between human- and wild boar-induced disturbances; (2) the diets and mobilities of the two pre dators; and (3) the near lack of hard ecological boundaries in terrestrial ecosystems, such as chestnut wood