Productivity and food web structure: association between productivity and link richness among top predators

Abstract

Summary 1A prime goal in ecology is to understand the consequences of different productivity levels on system structure and stability. The number of trophic connections per species is a main parameter of food web structure often associated with total number of species and disturbance but not with productivity. We analyse the association between number of trophic connections and productivity for top predators in a terrestrial ecosystem. 2Since 1987, the diets of the strigiforms Speotyto cunicularia (Burrowing owl), Bubo magellanicus (Magellan horned owl), Glaucidium nanum (Austral pygmy owl), Tyto alba (Barn owl), the falconiform Falco sparverius (American kestrel), and the canid Pseudalopex culpaeus (Culpeo fox) have been monitored in a semi-desert scrubland of central Chile. The collective sample yielded 18 707 regurgitated pellets, 6557 fox faeces and 87 698 individual prey identified over 15 years of study. 3Prey richness was standardized with a rarefaction procedure to a constant number of individual prey. The association between precipitation (a proxy of ecosystem productivity in semi-deserts) and mean number of trophic connections was analysed with polynomial multiple regression. The analysis was also done for each predator separately. 4The diet richness of all six predators varied idiosyncratically with precipitation, supporting the hypothesis that detailed food web structure varies dynamically with productivity. However, no association between these variables was detected across the entire predator assembly. Conservation of food web topology in spite of large variation in productivity, species composition, and individual species richness, supports the view that connection pattern is a key attribute of food web structure.