From plantings to the paddock: ground-dwelling beetles in a dynamic agricultural landscape

Abstract

The intensification of agriculture for increased food production is leading to new challenges for biodiversity conservation, particularly managing complex changing landscapes for mutually beneficial outcomes for agriculture and the environment. My thesis aimed to understand the diversity and distribution of beetles (Coleoptera), and the mechanisms shaping beetle assemblages across a dynamic and fragmented agricultural landscape. I used a landscape-scale study in south-eastern Australia to examine beetle assemblages in remnant woodland patches and four types of adjoining farmlands: crop, fallow, plantings, and fine woody debris applied over harvested crop.

My thesis comprised four chapters written as journal articles. In Paper I, I examined seasonal differences in beetle assemblages between the woodland interior and four adjoining farmland uses. I found that overall species richness was significantly lower in woodlands than farmlands, although both habitats supported significantly different assemblages. Abundance responses were taxon-specific, and influenced by interactions between land-use and season. These results suggest the importance of maintaining farmland heterogeneity with a mix of low-intensity land-uses, with further agricultural intensification a likely threat to beetle diversity in the region.

In Paper II, I examined temporal patterns of edge responses and movement of beetle assemblages between woodlands and the four farmland uses. The use of directional pitfall traps allowed inference of cross-habitat movement. Farmland use and season interactively affected beetle abundance across farmland–woodland edges. Applying woody debris was a novel way of reducing seasonal fluctuations in edge responses and increasing permeability for cross-habitat movement. Edges likely provided resources for beetles in adjoining habitats, but seasonal movement of predators into edges might negatively affect prey assemblages. In Paper III, I quantified relationships between ground-layer structure, plant species richness and plant composition, and the diversity and composition of beetles from different habitats or seasons. Plant composition better predicted beetle composition than vegetation structure. Plant richness and vegetation structure both significantly affected beetle abundance and composition. The influence of these vegetation attributes often varied depending on habitat and season for all trophic groups. These dynamic plant–beetle relationships suggest a need for targeted ways of managing vegetation to improve beetle diversity in different parts of the landscape.

In Paper IV, I disentangled the effects of farmland use, edge effects and vegetation structure on the morphological traits of Carabidae species. Carabid body size increased across a distance from edges between woodlands and farmlands, and there were strong mediating effects of farmland use on this association. Vegetation structure was associated with traits relating to body size, flying ability and body shape, and helped explain some of the effects of farmland use and edge effects on body size. These results provide evidence of vegetation- and land-use-mediated filtering of traits as an important factor shaping carabid assemblages in human-modified landscapes.

My results indicate that farmlands can provide important habitat for many beetle species. However, spatio-temporal changes in farmland habitat strongly influence beetle assemblages across the landscape. Conservation strategies, therefore, need to take a whole-of-landscape approach, and exploit heterogeneity of mixed-farmlands over space and time to maximise outcomes for biodiversity.