Invasion of a stream food web by a new top predator

1. A new top predator, the dragonfly Cordulegaster boltonii Donovan,'invaded' a stream with a well-described food web. 2. The pre-invasion web was species-poor but complex, with prevalent intraguild predation, cannibalism and omnivory. Such characteristics differ from expectations based upon the early food web literature, but are consistent with more recent empirical webs and theoretical developments. 3, Exhaustive sampling was necessary to describe web structure, with the gut contents of several hundred individuals being required to reach the asymptote of the total number of links for individual species. There was no single 'standard' sample size that was applicable for estimating the number of links: sampling 'x' guts gave a different fraction of the asymptotic value for different species. Smaller predators were more prone to underestimation of links than larger species higher in the web. 4, The number of feeding links, trophic status and the degree of omnivory increased progressively with predator body size, both within and among species. The diet of each predator species (or instar) was effectively a subset of the diet of the next largest predator. 5. The invader was extremely polyphagous and fed at all trophic levels. Mean chain length increased by half a link following the invasion. Web complexity, and omnivory in particular, also increased. Pre- and post-invasion webs displayed intervality and rigid circuitry. The resident predators were frequently eaten by the invader, but the only significant predators of C. boltonii were larger conspecifics. Although no species have yet been deleted, there has been a 21% increase in links for a 6% increase in species since the invasion, suggesting that the members of the web had become more tightly packed within niche space. Most prey species were eaten by every predator species (including C. boltonii), indicating the potential for strong apparent competition within the web.