Andrea Wilson

Effects of different management regimes on aquatic macroinvertebrate diversity in Australian rice fields

The maintenance of invertebrate diversity within agricultural environments can enhance a number of agronomically important processes, such as nutrient cycling and biological pest control. However, few Australian studies have been undertaken which specifically address the effects of commercial management regimes on rice field biodiversity. In this study, we compared aquatic macroinvertebrate communities within Australian rice fields cultivated under three commercial management regimes: conventional-aerial (agrochemicals applied, aerially sown), conventional-drill (agrochemicals applied, directly drill-sown) and organic-drill (agrochemical-free, directly drill-sown). These comparisons were undertaken using a combination of community assessment approaches, including morphospecies richness, abundance, diversity and community composition. In general, greater biodiversity existed within macroinvertebrate communities that developed under organic management regimes than under conventional regimes (i.e., higher morphospecies richness and Shannon diversity). Although there were significant differences in several parameters across management regimes early in the rice-growing season, as the growing season progressed the invertebrate communities that developed in the different management regimes became more similar. Only community composition analyses showed significant differences late in the growing season, with functional differences across aquatic faunal assemblages suggested by increased predator abundance in communities sampled from the organic management regime. In order to improve biodiversity within these aquatic environments, management techniques need to be examined individually and the most disruptive processes identified. Alternative management procedures can then be developed that minimise biodiversity loss whilst still delivering required agronomic outcomes.

Stable isotope analysis of aquatic invertebrate communities in irrigated rice fields cultivated under different management regimes

In this study we have used stable isotope analysis to identify major food resources driving food webs in commercial rice agroecosystems and to examine the effects of agricultural management practices on the trophic structure of these food webs. Potential carbon sources and aquatic macroinvertebrate consumers were collected from large-scale rice farms in south-eastern Australia cultivated under three different crop management regimes conventional-aerial (agrochemicals applied, aerially sown), conventional-sod (agrochemicals applied, directly sown) and organic-sod (agrochemical-free, directly sown). Evidence from stable isotope analysis demonstrated the importance of food sources, such as biofilm and detritus, as the principal energy sources driving aquatic food webs in rice agroecosystems. Despite the greater diversity of potential food sources collected from the organic-sod regime across all sampling occasions, the range of food resources directly assimilated by macroinvertebrate consumers did not differ substantially across management regimes. Trophic complexity of aquatic food webs, as evidenced by the number of trophic levels identified using δ15N data, differed across management regimes at the early season sampling. Sites with low or no agrochemical applications contained more than two trophic levels, but at the site with the highest pesticide application no primary or secondary consumers were found. Our data demonstrates that the choice of agricultural management regime has a season-long influence on aquatic food webs in rice crops, and highlights the importance of conserving non-rice food resources that drive these trophic networks.