Hidden herbivory, precarious precipitation and punished pastures : Australia's grasslands under root herbivory and altered rainfall

Abstract

Climate change researchers are predicting more variable rainfall in the coming decades. This is important for grasslands because they are highly responsive to inter- and intrannual rainfall patterns. At the same time, root damage from soil-dwelling arthropods has the potential to exacerbate, or in some cases reverse, the effects of rainfall variability in grassland. Plant community changes are likely to affect aboveground arthropod communities as a result. Recent literature on the effects of rainfall on grassland ecosystems and soil-dwelling invertebrates are synthesized in chapters one and two, respectively. While reduced rainfall amounts are expected to result in lower soil water content, the effects of variable rainfall are unclear and depend highly on grassland type. Plant functional types within grassland communities are likely to play a role in determining these responses and will interact with above- and belowground arthropod communities. Soil-dwelling insects are known in Australia for their high densities during pest outbreaks, especially scarab larva. While these organisms are affected by soil water content, they are predicted to be less vulnerable than organisms aboveground due to physiological and behavioural adaptations. Therefore, they are likely to damage plant roots in grassland ecosystems, even under fluctuating soil moisture. In this thesis I aimed to better understand the responses of grassland organisms that were subjected to rainfall regimes predicted under climate change for SE Australia. I also sought to determine if root damage by local scarab beetles could change the productivity and diversity of the aforementioned grassland in response to rainfall alterations. As such, I attempted to answer the following questions: 1) how will varying water regimes and root damage impact the performance of grasslands at the individual species level; 2) how will changing the amount and frequency of rainfall affect soil water content and how soil water content changes impact grassland plant and invertebrate communities when coupled with increased root herbivore densities and; how are above- and belowground plant community traits relevant to arthropod herbivore performance affected by both extreme rainfall and increased root herbivore density . I hypothesized that rainfall alterations would lead to shifts in grassland productivity and composition, on both the individual and community level. Additionally, root damage would exacerbate the effects of water stress on grassland communities. Finally, grassland community plant traits (e.g. nutrient density, diversity, apparency, etc.) were hypothesized to shift in response to the most extreme forms of water limitation, affecting aboveground herbivore performance. In Chapter three, I examined plant growth, phenology, productivity and physiology to evaluate the effects of rainfall regime and root damage on plant performance. I found that simulated rainfall reduction had a much larger impact on plant productivity than root damage. However, the magnitude of reduction in biomass due to water stress was highly dependent on grass species. Water stress and larval activity interacted leading to increased biomass root:shoot ratios for two grass species. Plant growth and flowering time was affected by water treatments, but not larval activity. I did find, however, strong trends in the concentration of silicon in plant tissue in response to water stress. In Chapter four I measured the responses of grassland plots under altered rainfall and increased root herbivore densities. I found rainfall had a much stronger influence on grassland productivity and plant community composition than increased root herbivore densities. Where the plant community was affected by rainfall, there was evidence of plant-mediated effects on aboveground invertebrate community structure. In Chapter 5 I use the same experimental plots as above but with a focus on plant trait changes in response to extreme rainfall, as well as the other rainfall scenarios. I found that extreme drought can have severe and immediate effects on plant communities in terms of apparency and plant traits that are potentially important for aboveground herbivore performance. Interactions with root herbivory treatments are mixed; in some cases it had a stimulatory effect on plant traits and in others a depressive effect. Plant traits also shift with rainfall regime and, overall, summer drought and reduced frequency rainfall elicited plant trait responses counter to arthropod herbivore performance. Increased rainfall did not have as much of an effect as expected, possibly because of the high sand content and, thus, porosity of the soil at the site. The effects of watering regimes in this thesis were expected to be measurable and to interact with damage caused by root feeders. However, for the most part reduced amount and frequency watering regimes resulted in very similar response levels and root damage effects tended to be negligible. At the community level, responses of root damage and altered water regimes were even less pronounced and almost exclusively interacted in ambient (control) water treatments. It can therefore be assumed that plants use a stress hierarchy when responding to both stressors; here, available soil water seems to be at the top and moderate root damage a lesser concern. This work brings the field one step closer to understanding the complex relationships above- and belowground that are expected to shift in grasslands under climate change. Researchers should strive to understand ecological interactions, no matter how difficult they are to elucidate.

Date of Award	2016
Original language	English