Negative intraguild interactions drive niche variation in arthropod predators

The niche variation hypothesis states that the population niche width expands with increasing interindividual differences in prey utilization (i.e., individual dietary specialization). The main ecological drivers forming this relationship include a) ecological opportunity, b) food limitation and exploitative competition, and c) intraguild interference. Only a limited number of empirical studies have tested the impact of these drivers on the niche variation–width relationship and focused only on vertebrates. Using molecular gut content analysis, we investigated how prey diversity (proxy for ecological opportunity), prey abundance (proxy for exploitative competition / food-limitation), and activity density of guild members (proxy for intraguild interference) affect the short-term individual dietary specialization and consequently the population niche width in local communities of 13 species of predatory beetles and spiders. The study took place in 10 spring barley fields in Sweden in 2011. We found that the niche variation and consequently the average population niche width of the species in the predator community decreased with prey abundance but increased with activity density of guild members. The results indicate that intraguild interference and exploitative competition / food limitation increased dietary variation. The increased diet variation led to the observed population diet expansion. Our results support the niche variation hypothesis and, in contrast to the traditional view, show that negative intraguild interactions may act as a diversifying force.