Summary: 1. Species diversity of arboreal arthropods tends to increase during rainforest succession so that primary forest communities comprise more species than those from secondary vegetation, but it is not well understood why. Primary forests differ from secondary forests in a wide array of factors whose relative impacts on arthropod diversity have not yet been quantified.2. We assessed the effects of succession‐related determinants on a keystone ecological group, arboreal ants, by conducting a complete census of 1332 ant nests from all trees with diameter at breast height ≥ 5 cm occurring within two (unreplicated) 0·32‐ha plots, one in primary and one in secondary lowland forest in New Guinea. Specifically, we used a novel rarefaction‐based approach to match number, size distribution and taxonomic structure of trees in primary forest communities to those in secondary forest and compared the resulting numbers of ant species.3. In total, we recorded 80 nesting ant species from 389 trees in primary forest but only 42 species from 295 trees in secondary forest. The two habitats did not differ in the mean number of ant species per tree or in the relationship between ant diversity and tree size. However, the between‐tree similarity of ant communities was higher in secondary forest than in primary forest, as was the between‐tree nest site similarity, suggesting that secondary trees were more uniform in providing nesting microhabitats.4. Using our rarefaction method, the difference in ant species richness between two forest types was partitioned according to the effects of higher tree density (22·6%), larger tree size (15·5%) and higher taxonomic diversity of trees (14·3%) in primary than in secondary forest. The remaining difference (47·6%) was because of higher beta diversity of ant communities between primary forest trees. In contrast, difference in nest density was explained solely by difference in tree density.5. Our study shows that reduction in plant taxonomic diversity in secondary forests is not the main driver of the reduction in canopy ant species richness. We suggest that the majority of arboreal species losses in secondary tropical forests are attributable to simpler vegetation structure, combined with lower turnover of nesting microhabitats between trees.

Phylogenetic ecology has identified patterns of diversity in communities that may find explanation in trophic interactions, and yet there have been few attempts to directly relate such patterns among trophic levels. Density‐dependent processes involving pests and pathogens, for example, have been invoked to account for plant community phylogenetic patterns, but relatively little is known about how plant relatedness might affect community structure at other trophic levels. We examined the degree to which the abundance of herbivores in a rain forest community is explained by the phylogeny and functional traits of host plants. We destructively sampled all stems ≥5 cm diameter in two 1‐ha plots of New Guinea primary and secondary lowland forest to test predicted relationships between herbivore abundance and plant resources. We analyzed per‐tree caterpillar and leaf miner abundance, total leaf biomass (kg), percentage of immature foliage, specific leaf area (cm²/g), leaf nitrogen content (percentage of dry mass), and presence of exudates in the context of a plant community phylogeny estimated from DNA barcodes.Apart from nitrogen content and exudates, neither plant resources nor herbivore abundance showed evidence of phylogenetic signal in our community sample. The plant traits we measured could account for only 30% and 16% of variation in caterpillar and leaf miner abundance, respectively, among individual trees. Leaf biomass was a stronger predictor of herbivore abundance than either resource quality (leaf nitrogen content) or palatability (percentage of immature foliage, specific leaf area). The primary importance of resource quantity was also observed at the plant species level in analyses of species means and phylogenetic generalized least‐squares regression. Plant relatedness did not account for much variation in herbivore abundance, but significant effects of exudates and leaf nitrogen content on caterpillar abundance illustrate how conserved traits at one trophic level may influence community‐wide patterns at another.

Tropical canopies are known for their high abundance and diversity of ants. However, the factors which enable coexistence of so many species in trees, and in particular, the role of foragers in determining local diversity, are not well understood. We censused nesting and foraging arboreal ant communities in two 0.32 ha plots of primary and secondary lowland rainforest in New Guinea and explored their species diversity and composition. Null models were used to test if the records of species foraging (but not nesting) in a tree were dependent on the spatial distribution of nests in surrounding trees. In total, 102 ant species from 389 trees occurred in the primary plot compared with only 50 species from 295 trees in the secondary forest plot. However, there was only a small difference in mean ant richness per tree between primary and secondary forest (3.8 and 3.3 sp. respectively) and considerably lower richness per tree was found only when nests were considered (1.5 sp. in both forests). About half of foraging individuals collected in a tree belonged to species which were not nesting in that tree. Null models showed that the ants foraging but not nesting in a tree are more likely to nest in nearby trees than would be expected at random. The effects of both forest stage and tree size traits were similar regardless of whether only foragers, only nests, or both datasets combined were considered. However, relative abundance distributions of species differed between foraging and nesting communities. The primary forest plot was dominated by native ant species, whereas invasive species were common in secondary forest. This study demonstrates the high contribution of foragers to arboreal ant diversity, indicating an important role of connectivity between trees, and also highlights the importance of primary vegetation for the conservation of native ant communities.