Host specificity of arboreal and free‐living insect herbivores in rain forests

Available information about the host specificity of arboreal and free-living insect herbivores in rain forests is scarce, but suggests that polyphagy may be more widespread than previously thought. The study of the arthropod fauna associated with an overstorey tree in Australia, Argyrodendron actinophyllum (Sterculiaceae), supports this contention. A conservative 11% of herbivore species foraging within this tree appear to be specialists, whereas this proportion decreases further to 3.0–4.5% when non-herbivore species are included. Argyrodendron actinophyllum presents several features which may promote a polyphagous strategy among herbivores. These observations are discussed in the wider context of ecological and evolutionary pressures shaping feeding strategies of insect herbivores in rain forests. These selection pressures include principally plant biochemistry, predation pressure of generalist predators, and structural diversity and environmental unpredictability of rainforest environments. The strength of association between herbivores and rainforest host-trees is likely to depend on three factors, acting at both the tree and rainforest community level: chemical traits of foliage (including chemical defences and nutrient levels), young foliage availability and enemy-free space. Given the complex interactions possible among these factors, rainforest host-trees may sustain herbivore faunas ranging from highly specialized to highly generalist. SUMMARY Argyrodendron actinophyllum presents several features which may promote a polyphagous strategy among herbivores: low nutrient levels and high fibre content, apparent scarcity of qualitative defences, relatively unpredictable availability of young foliage and low predation-pressure from arboreal ants in its foliage. This situation may be different for other rainforest tree species, depending on their biological features. Without speculating on the relative importance of each factor, these observations suggest that the strength of association between communities of insect herbivores and their rainforest host-trees may depend on the following factors, acting at both the component and compound rainforest-community level: chemical traits of foliage (including chemical defences and nutrient levels), young foliage availability (including factors related to plant phenology, plant abundance and plant growth strategy) and enemy-free space. These factors, which may be interrelated (i.e. amount and type of chemical defences related to nutrient availability, host phenology related to nutrient availability and searching behaviour of generalist predators/ parasitoids related to host phenology and apparency), may generate complex sets of combinations, which are likely to differ between rainforest component communities. In these conditions, rainforest host-trees may sustain herbivore faunas ranging from highly specialized to highly generalist. These considerations demonstrate a requirement for the study of arboreal insect communities associated with host-trees of dissimilar features growing in similar rain forest types, as well as in different rainforest environments.