Local Communities of Arboreal Herbivores in Papua New Guinea: Predictors of Insect Variables

Abstract

As a contribution to enlarging and widening the array of studies available for examination of insect—plant relations, this study contrasts local and regional predictors of species richness and host specificity of leaf—chewing insects feeding on New Guinea trees. I tested the degree to which the variance in local insect variables (species richness, ratio of specialist to generalist chewers, abundance, and biomass) for leaf chewers associated with different species of tropical trees was accounted for by local and regional host—related variables, and by local abundance of potential enemies. Leaf—chewing insects feeding externally on ten species of forest trees were collected over a 1—yr field study in a submontane area in Papua New Guinea. The tree species were taxonomically distant, and the list included species of different successional status, different patterns of leaf production, and different heights. The affinity of chewing insects with particular tree species was ascertained by feeding trials in the laboratory. Local insect species richness varied greatly among tree species, by more than four—fold between the poorest (18 species) and the richest (94 species) tree species. The proportion of specialist insect species associated with particular tree species was significantly different among tree species and ranged from 16 to 72%. Trees supporting a rich fauna usually also supported a high proportion of specialists. The abundance and biomass of leaf—chewing insects per host species was difficult to predict. However, path analyses showed that most of the causal variance in insect species richness and in the ratio of specialist to generalist chewers was accounted for by five properties of the tree species: numbers of young leaves available throughout the year, ant (i.e., enemy) abundance, leaf palatability, leaf water content, and altitudinal range. Most of the variation in local species richness could be predicted from local processes (i.e., food resources and abundance of enemies), not from regional processes. This study and others suggest that in complex environments, such as tropical rain forests, local processes may be comparatively more important in maintaining the local species richness of insect herbivores than in less complex ones, such as temperate woodlands. The implications of these findings are evident: the effects of moderate habitat fragmentation and ecological stress may be more pronounced in tropical systems than in temperate systems and may result in a greater proportional loss of local biodiversity in the former.

Communities of insect herbivores foraging on saplings versus mature trees of Pourouma bicolor (Cecropiaceae) in Panama

“…Further, herbivore species richness on this host appeared to be rather low, as compared with other tropical hosts (e.g. Basset 1996;, and faunal differences may well be more pronounced on tree species supporting a diverse fauna of herbivores.…”

“…For leaf-feeding insects, the availability and predictability of young foliage is crucial (e.g. Strong 1977;Basset 1996). Many studies of rainforest insects have reported a higher abundance, activity or diversity of insect herbivores in the upper canopy than in the understorey (e.g.…”

Host specificity of insect herbivores in tropical forests

“…Usually, studies of herbivores in tropical forests sample only part of the local flora-typically less than 100 species (Marquis 1991;Basset 1996;Barone 1998;Ødegaard 2000a;Novotny et al 2002a). Often, the flora studied is structured according to plant phylogeny (e.g.…”

“…Often, the flora studied is structured according to plant phylogeny (e.g. congeneric species: Marquis 1991; allofamilial species : Basset 1996) or ecology (e.g. lianas versus trees: Ødegaard 2000a; early versus late successional species: Leps et al 2001) and selected from locally common species.…”

“…Rather surprisingly, such analyses are rare, which contrasts with the attention paid to a similar variable: the number of herbivores feeding on different plant species (Lewinsohn 1991;Basset 1996;Basset & Novotny 1999;Kelly & Southwood 1999).…”