A cross-taxon study in Thailand demonstrates significance of vertical stratification and seasonality in monitoring tropical forest insects

Abstract

In tropical rainforests, trees and associated plants create vertically heterogeneous habitats mediated by seasonal changes in climate and plant phenology. Despite extensive studies on the spatiotemporal dynamics of tropical forest ecosystems, insect monitoring programs often neglect the vertical and seasonal dimensions because the relative importance of muti-dimensional dynamics of insect diversity has not been well understood. In this study, we employed a spatially explicit sampling design to understand the distribution patterns of Coleoptera, Hymenoptera, and two families (Noctuidae and Nolidae) of Lepidoptera in the canopy and understory during the dry and wet seasons in two tropical rainforests located at lower and higher latitudes in Thailand. We compared the vertical stratification of gamma diversity, alpha diversity, total abundance, and beta turnover and nestedness. We also identified the insect species characteristic of certain vertical strata and/or seasons. Our samples resulted in a total of 4,452 insects (1,622 coleopterans, 1,763 hymenopterans, and 1,067 lepidopterans) representing 437 Coleoptera species, 694 Hymenoptera species, and 98 Lepidoptera species. All insect groups generally displayed moderately higher diversity in the canopy than in the understory, but the significance of this pattern varied among seasons, forests, and insect groups. Over 50% of significant habitat indicator species were restricted to certain vertical strata but were only found in certain seasons. Our findings demonstrate that spatiotemporal dynamics of insect diversity are highly context-dependent and are not as clearly discernible as we hypothesized in three major insect groups, suggesting the importance of monitoring insects across vertical strata and seasons, even in low-latitude tropical rainforests.