Drought, deluge and diversity decline - How do root herbivores affect grassland resilience to predicted changes in rainfall patterns?

A project undertaken at the Hawksbury Institute for the Environment, University of Western Sydney, and supervised by Sally Power

Climate models predict more extreme rainfall patterns in the future, including reduced rainfall amounts, prolonged periods of severe drought and longer intervals between rainfall events. Given the already high level of variability in within- and between- year rainfall patterns in Australia and the strong links between productivity, diversity and climate in grassland ecosystems, future changes in rainfall regimes may have large negative impacts on the productivity and functioning of grasslands and rangelands across the country. At the ecosystem level, responses to rainfall extremes are likely to depend critically on the nature and strength of competition between plant species and interactions with other trophic groups, such as insect herbivores. Belowground herbivory is particularly significant in this context since the damage caused by root-feeding insects will greatly exacerbate the level of water and nutrient stress experienced by plants under altered rainfall regimes. Root herbivores are a ubiquitous feature of grassland ecosystems and, in addition to amplifying the effects of climate extremes for plants, are also likely to respond directly to changes in rainfall regimes themselves. Taken together, shifts in rainfall regimes and root herbivory therefore have the potential to drive very substantial change in the composition and diversity of grassland ecosystems, the magnitude and even direction of which may depend on the precise nature of future rainfall change.

We established a field scale rainfall manipulation experiment (Figures 1 & 2) in a former pasture grassland in western Sydney. The overall objective of the project is to evaluate the impacts of reductions in the size and frequency of rainfall events in terms of the composition and diversity of a grassland plant community, and the role of root herbivores in amplifying the effects of changing rainfall regimes. Specific aims are to:

  1. Quantify the effects of reductions in the size and frequency of rainfall events on plant species’ performance in a mixed grassland community;
  2. Determine the responses of root herbivores to changing rainfall regimes;
  3. Evaluate the extent to which root herbivory reduces grassland resilience to rainfall extremes and establish whether the combined effects of these biotic and abiotic stressors are additive or greater than additive.

Strong effects of rainfall treatment on plant productivity and community composition were apparent in all years, with the biggest impacts seen under summer drought. Summer drought was associated with growing season productivity declines of up to 72%, compared to ambient plots, and a reduction in the ratio of C3:C4 plant species. However, rapid recovery of productivity in these plots during the cool season, despite altered community composition, indicates a high functional resilience of this grassland.

Reducing either the size or frequency of rainfall inputs had relatively little impact on overall productivity, in contrast to studies elsewhere. Reducing rainfall frequency did, however, reduce species diversity and had a major impact on ecosystem carbon fluxes, turning the grassland from a modest carbon sink into a carbon source. This provides strong evidence that altering rainfall frequency can have highly significant impacts on not only the diversity but also the functioning of grassland ecosystems. Where the plant community was affected by rainfall, there was also evidence of plant-mediated effects on aboveground invertebrate community structure, highlighting the sensitivity of trophic interactions to altered rainfall patterns.

Overall, the effects of changing rainfall regimes were much greater than the effects of adding root herbivores and there was little evidence of herbivory increasing ecosystem sensitivity to altered rainfall. Root herbivory did nevertheless result in subtle shifts in plant community composition, with concurrent increases in the abundance of aboveground arthropods. Root herbivory also appears to be associated with induced plant defense (silicon) chemistry. The linkages between rainfall, plant and invertebrate communities – both above- and belowground – identified in this study highlight the importance of adopting an integrated, multi-trophic approach for evaluating ecosystem-level responses to future climate change.
Figure 1. Rainout shelters at a former pasture grassland in western Sydney

Figure 2. Rainfall treatments are applied automatically to achieve the following treatments: a) ambient, b) ambient-50%, c) reduced frequency (applied once every 3 weeks, with the total amount as for ambient)