A central goal of ecology is to understand how species’ densities relate to the spatial distribution of their habitats and resources. When habitats are patchily distributed, their size and edge-to-area ratios are likely to influence the immigration and emigration of their inhabitants and to contribute to different population densities among patches. Prior experiments, however, have not manipulated these patch traits independently despite their wide variation in both natural and human-influenced landscapes. Therefore, the influence of patch size per se on movement and density remains unclear and the potential for an interaction between patch size and edge-to-area ratios remains unexplored. We manipulated the size (4, 6, or 12 m2) and edge-to-area ratios (2:1 and 4:1 perimeter to area ratios) of soybean patches (Glycine max) in a 3x2 factorial design to investigate the individual and interactive effects of these patch attributes on the densities of Calosoma sayi, a predaceous beetle and naturally occurring biological control agent in row crops. We also marked and recaptured individual beetles to determine if their movement among patches contributed to the observed patterns of density.
Results/Conclusions
We found that patch size and edge-to-area ratios significantly interacted to influence density among patch types. Over 61 days we caught 186 C. sayi, with a median of 5 individuals per patch (IQR = 3 to 7). While patch size had little influence on the beetle density within low edge-to-area patches, beetle density increased significantly with patch size in high edge-to-area patches—with the largest patches having more than twice the density of the smallest patches on average. Analysis of recaptures (n = 31), which accounted for the spatial arrangement of patches, was consistent with observations of density; beetles were least likely to be recaptured in small patches with high edge-to-area ratios and most likely to be recaptured in large patches with high edge-to-area ratios. Together, our results provide experimental evidence that patch size and edge-to-area ratios can interact to influence the movement and density of patch inhabitants. This finding underscores the importance of considering both patch traits for the management and optimization of habitat for biological control.