Responses of forest insects to environmental gradients

Abstract

Insects comprise the vast majority of global biodiversity, carry out important ecosystem functions such as decomposition, pollination and pest suppression, and are impacted by environmental change. We are currently experiencing accelerating environmental change, which will affect insect food web interactions, and their ability to carry out ecosystem functions. Ecologists have studied insects across environmental gradients to try to understand their response to environmental change, which has implications for ecosystem functioning under future climate change and forest regeneration. The relative importance of how different components of insect communities change across environmental gradients has not been resolved. Our current understating of how environmental change will alter insect communities or what consequences this will have for the wider ecosystem is limited. A clear understanding of how insect biodiversity and trophic interactions respond to environmental gradients is necessary to predict changes in ecosystem health and functioning following environmental change. In this thesis, I collected ecological data across elevational, vertical and forest age gradients to address questions concerning the community composition and trophic dynamics of insect food webs across environmental gradients. I used environmental assays of leaf nutrition and predation to evaluate ‘bottom-up’ and ‘top-down’ control of folivory by insect herbivores, respectively, in tropical and sub-tropical forest in Yunnan, China and Hong Kong. I demonstrated that distinct trophic levels respond differently to environmental gradients of elevation and forest succession. Next, I used stable isotope analysis (SIA) to construct food webs from artificial phytotelmata deployed across a forest successional gradient in Hong Kong and to compare the food webs that developed in young and old forest. I showed that in isolated forest with a long history of disturbance, forest stands of different ages may support ecologically similar insect food webs, possibly due to succession being arrested resulting from species dispersal limitation. Finally, I assessed the stratification of multiple insect taxa from tropical forest understorey and canopy in Daintree, Australia to explore how the vertical gradient influences insect communities. I demonstrated that distinct taxa differentially prefer separate canopy strata according to their niche requirements. This thesis shows that environmental gradients influence relationships in insect food webs as separate trophic levels and taxonomic groups have differing responses to environmental change. This suggests that predicting future structure and function of insect food webs will be difficult and must be nuanced and grounded in robust understanding of insect community interactions and ecology. This has implications for understanding insect communities under climate change, and restoration and conservation management.