Aggregation and temporal stability of carabid beetle distributions in field and hedgerow habitats.
Understanding the spatial dynamics of insect distributions in farmland can provide insights into their ecological requirements and potential for management. Identifying the scale, location and persistence of species' aggregations is an important step towards understanding the factors driving population distributions. This study examined how different carabid species were distributed in field and field margin habitats near Bristol, UK, and analysed their spatiotemporal dynamics. We sampled 156 locations in a grid of pitfall traps over approximately 1 ha, comprising a hedgerow and adjacent parts of two arable fields. Traps were monitored between June and November 1994 to provide data with high spatial and temporal resolution of the two-dimensional distributions of six carabid beetle taxa (Pterostichus melanarius, P. cupreus, Harpalus rufipes, Nebria brevicollis, Agonum dorsale and Amara spp.). The pattern of carabid activity-density over the experimental site was displayed in a series of contour plots. Spatial analyses by distance indices (SADIE) were used to provide aggregation indices (Ia) for the distributions of carabid species at different times. A second SADIE index (Im) of spatial association between two sets of counts was used to measure the spatial stability of species' aggregation patterns through time. Im was also used to measure association or dissociation between the distributions of different species, and the spatial association between a species' distribution and an environmental variable. Each taxon was aggregated most of the time, but different taxa aggregated in patches within the field and/or hedgerow to different and varying extents. The field boundaries were important for some species, either as the only habitat in which they occurred (Amara spp.), as the major focus of more widespread distribution (H. rufipes), or as a seasonal refuge (N. brevicollis). Within the crop habitat, different species also occupied different areas, providing some evidence to suggest species packing in space. Spatial distributions between consecutive samples were strongly and positively associated, indicating stability of patch location over short time-scales. The strength of the association in turn was correlated with activity-density. When time lags were introduced between sample distributions, some negative association indices indicated displacement of patches. In most cases, however, positive association was maintained over long periods, demonstrating patch stability. The spatial stability of patches suggests that future studies should focus on the detailed measurement of biotic and abiotic factors associated with patch location. Identifying the underlying causes of population distributions in farmland has important implications for managing habitat to enhance populations of beneficial predators, and targeting insecticide applications in precision agriculture to minimize their adverse side-effects.