Threshold extinction in food webs

Food web response to species loss has been investigated in several ways in the last years. In binary food webs species go secondarily extinct if no resource item remains to exploit. We considered that species can go extinct well before their remain without energy intake and we explore this issue by introducing thresholds of minimum energy requirement for species survival. According to this approach extinction would occur whenever an initial extinction event eliminates links so that certain nodes are left with an overall energy intake lower than the threshold value. We tested 18 real food webs by removing species from most to least connected and considering different scenarios defined by a progressively increasing extinction threshold. Increasing energy requirement threshold negatively affect food web robustness. We found that a very low increase of the energy requirement induces a significative increase in system fragility. In addition, above a certain value of energy requirement threshold we found no relationship between the robustness and the connectance of the web. Further, networks with more species showed higher level of fragility when energy threshold is more severe. Such discovery indicates that the shape of the robustnesscomplexity relationship of a web depends on the sensitivity of consumers to loss of prey