On benefits of indirect defence: short- and long-term studies of antiherbivore protection via mutualistic ants

Many plants are defended indirectly by mutualistic animals. In this field s tudy, we investigated the efficacy of indirect antiherbivore defence in sym biotic and non-symbiotic ant-plant associations in three Macaranga species. We tested whether obligate interactions are more effective than facultativ e ones by comparing ant-free plants, or parts of plants, with untreated con trols. All three species gained significant protection from the ants' prese nce. The efficacy of defence was higher in the obligate associations repres ented by M. triloba and M. hosei than in the facultative interaction (M. ta narius). After 40 days of ant exclusion, missing leaf area amounted to 1.7% in M. hosei (compared to 0.2% in untreated, ant-defended controls), 2.6% i n M. triloba (controls 1.2%) and 4.2% in M. tanarius (controls 3.2%). In a long-term study of M, triloba and M. hosei, ant protection was orders of ma gnitude higher than in the short-term results. Short-term experiments obvio usly are unsuited to obtaining a realistic picture of the long-term efficac y of antiherbivore defence. Within 1 year, ant-free plants lost, on average , between 70% (M. hosei) and 80% (M. triloba) of their total leaf area. Bot h species appear to require their mutualistic ants for survival. Defence vi a symbiotic ants is obviously a very effective form of antiherbivore protec tion. Ants are highly mobile and defend preferentially young, vulnerable le aves and shoot tips, and they fulfil several functions which normally have to be provided by different chemical substances. This may be a general bene fit of indirect plant defence, which makes use of "animal-specific" traits rather than intrinsic plant properties.