Predators and mutualists influence the exclusion of aphid species from natural communities

We investigated why two species of aphids (Aphis jacobaeae and Brachycaudus cardui) were very rare in a study site despite their abundance in the surr ounding area. The study site contained many common species of aphid and Mle tested the hypothesis that the community of aphid predators in the held ex cluded the missing species, Colonies of the two aphid species were artifici ally initiated in the experimental site and allocated to one of four treatm ents: control; ground predator exclusion; total predator exclusion, and pro vision of mutualist ants. Two measures of colony performance were analysed: longevity and cumulative aphid density. Colonies decline naturally in late summer but control colonies disappeared very quickly while colonies protec ted from all predators survived the longest. The performance of colonies pr otected from just ground predators was intermediate. We failed to persuade ants to tend A. jacobaeae. Colonies of B. cardui attended by ants performed better than controls and those with ground predators excluded, but not as well as those with all predators excluded. We conclude that the absence of the two species of aphid in the study site is influenced by the resident pr edator community, and by the availability of mutualists.