How will increased drought affect herbivory-based insect communities in Australian tropical rainforests?

Climate change is predicted to increase the frequency, duration and severity of drought in many regions. Predicting how forest ecosystems will respond to increased drought is a key issue in forest planning and forest management. Invertebrates play a central role in forest health and diversity, yet our understanding of the impact of drought on invertebrate communities in forests is very poor. This PhD addresses the impacts of increased drought on herbivory-based insect communities in tropical rainforests. Most of our current knowledge on that topic consists of observations from around the world that outbreaks of leaf feeders and bark beetles are typically preceded with unusually warm, dry weather. While there have been many experimental drought studies on plants and insects living on them, most of them were small scale experiments limited to one or a few individual plants rather than a whole forest ecosystem, and often reported contradictory results. A framework linking drought severity, plant defence production, plant nutritional quality, and insect response among different insect feeding guilds could help understand why different insects seem to respond differently to drought. After reviewing the literature, I developed a framework that might predict how insects, and particularly herbivorous insects, may respond to prolonged drought. The framework incorporates the severity and longevity of drought and captures the plant physiological adjustments that follow moderate and severe drought and predicts the response of (i) different feeding guilds; (ii) flush feeders and senescence feeders; (iii) specialist and generalist insect herbivores; and (iv) temperate versus tropical forest communities. Intermittent and moderate drought may result in increases of carbon-based and nitrogenbased chemical defences, whereas long and severe drought events can result in decreases in plant secondary defence compounds. I predict that different herbivore feeding guilds will show different but predictable responses to drought events, with most feeding guilds being negatively affected by water stress, with the exception of wood borers and bark beetles during severe drought and sap-sucking insects and leaf miners during moderate and intermittent drought. Time of feeding and host specificity are important considerations. Some insects, regardless of feeding guild, prefer to feed on younger tissues from leaf flush, whereas others are adapted to feed on senescing tissues of severely stressed trees. I argue that moderate water stress could benefit specialist insect herbivores, while generalists might prefer severe drought conditions. In this thesis, I test some of these hypotheses using an experimentally droughted Australian rainforest site. In particular, I examined the effects of increased drought on i) intensity of wood-boring attack, ii) insect abundance and diversity and insect community composition, iii) insect seasonality, and iv) ant diets Rainforests in far North Queensland, Australia, experience a dry period with on average 5-6 months of less than 100mm rain per month. An experiment was established in the Daintree rainforest to simulate the effects of an increased and extended drought. Plastic panels were set up above the ground in half a hectare of rainforest to prevent rainfall from reaching the ground throughout the year. The experiment was implemented underneath the arc of a 47-m tall construction crane so that access could be made to the rainforest canopy for both the drought experiment and the juxtaposed area used as a control plot. Measurements of soil humidity from soil pits at the plots indicate that there was an overall reduction of soil humidity of at least 30% in the drought plot throughout the experiment. I carried out two full years of insect collection on the drought experiment site two years after experimental droughting had commenced as well as on the nearby control plot. Insects were collected bi-monthly by beating leaves at the top of the canopy and from foliage at ground level, with the aim of exploring the effect of drought on insect abundance, diversity and community composition. I used stable isotope techniques to determine if drought could cause any variation in ant diets. Finally, I established 36 line transects across the study site to test whether wood boring activity would be more prevalent in increased drought conditions. Data on weekly soil-water availability from eight soil pits (four at the drought plot and four at the control plot) were used to measure impact of drought. I found evidence that insect communities were deeply impacted by increased drought. First, I have found that saplings were significantly more affected by wood-borers under increased conditions. Second, I found that every feeding guild was less abundant on large trees under water stress than on control trees. On saplings, however, I found that some feeding guilds (sap-suckers and fungivores) were significantly more abundant with increased drought conditions. Insect diversity and community composition was also impacted by the drought experiment, as beetle species richness was significantly lower on drought-stressed large trees, and there was a great dissimilarity in ant and beetle assemblages on shrubs and saplings from the drought and control plot. In addition, I found that leaf-chewers did not show any strong seasonal patterns or peak in abundance during leaf flush when living in increased drought conditions. Overall, most hypotheses built in the framework proved to be correct, with the exception of sapsuckers and fungivores which were positively affected by drought on saplings, possibly because they might benefit from decreased levels of plant defences more than other feeding guilds. In addition, isotope analyses revealed that with increased drought, ants δ15N isotopic composition increased, which could indicate a shift from a diet relying on plant sources (extra-floral nectar, uptake of honeydew) towards a more predatory diet. However, the shift observed could also be due to the increase in δ15N in leaves in drought-stressed trees. The framework I developed and the results of the drought experiment suggest a number of important avenues for further research on the impact of drought on insect communities. These include i) further assessment of how drought-induced changes in various plant traits, such as secondary compound concentrations and leaf water potential, affect herbivores, (ii) food web implications of changes in insect feeding guild structure and composition and how this might affect those that feed on them, and (iii) interactions between the effects on insects of increasing drought and other forms of environmental change including rising temperatures and CO2 levels. Since this is the first examination of how large-scale droughting affects insect communities, there is clearly a need for further large, temperate and tropical forest-scale drought experiments to look at herbivorous insect responses and their role in tree death.