Geographic variation in ant defence of a widespread Australian euphorb

Abstract

Consequences of ant visitation to plants with extra-floral nectaries (EFNs) in the endemic Australian genus Adriana (Euphorbiaceae) were studied at two locations near Broome, WA and Toowoomba in south-eastern Queensland. At both localities, the prevailing weather conditions were very dry during the study, and ant and herbivore densities were low. The ant and herbivore faunas on adrianas differed between the Western Australian and Queensland sites. At Broome, sap-sucking insects were the most common herbivores seen on plants. Densities of these insects tended to increase when ants were experimentally excluded. At the Queensland study sites, leaf-chewing beetles were the most common herbivores. Although numbers of these insects did not increase significantly when ants were experimentally excluded from treatment branches, behavioural assays showed that the presence or absence of ants could significantly affect the beetles' residence time on plants. Attendance by an assemblage of several ant species apparently provides adrianas with a defence that can function in seasonally dry conditions, that acts primarily to protect young and developing tissues and that acts against a variety of insect herbivores.