Landscape complexity (diversity, composition, connectivity) drives natural enemy populations and biological control services in agricultural landscapes. More complex landscapes (e.g., greater land-use diversity, high connectivity) generally support a greater diversity of natural enemies through resource availability and dispersal facilitation. However, local management changes have stronger effects in simple landscapes (e.g., less land-use diversity, low connectivity) by increasing temporal and spatial resource availability. While such landscape-local-ecosystem service relationships in agricultural landscapes are increasingly understood, we lack an understanding of how local and landscape factors interact to affect natural enemies in urban landscapes. In this study, we used urban community gardens as a system to test the importance of local and landscape factors for supporting natural enemy permanence and movement. We conducted a natural enemy release experiment using a common ladybeetle species (Hippodamia convergens) in 7 gardens of different landscape types: low impervious, medium impervious, and high impervious cover. We assessed ladybeetle permanence within gardens (as a proxy for movement) while simultaneously measuring local habitat factors (e.g., vegetation complexity, groundcover composition, garden size) at each garden. We then analyzed what landscape and local factors were strong predictors of ladybeetle permanence.
Results/Conclusions
We found that permanence varied with landscape type. Ladybeetle permanence tended to increase in gardens surrounded by medium impervious cover, particularly gardens surrounded by more open managed lawns. However, local factors were more important for explaining permanence in some gardens. Permanence tended to be higher in smaller gardens. Further, permanence tended to decrease with increased mulch cover, but increase with increased herbaceous groundcover. Our results suggest that vegetation and connectivity at multiple scales – both landscape and garden – may be important for ladybeetles. First, small gardens may be habitat islands surrounded by land-uses that are not conducive to beetle dispersal across a landscape. Second, while increased herbaceous groundcover may facilitate connectivity and resource access throughout the garden, mulch cover may be difficult for ladybeetles to traverse and instead trigger dispersal out of gardens. Having more herbaceous vegetation in gardens to facilitate ladybeetle movement within, rather than outside of, gardens may may support natural enemy permanence in gardens, particularly those that are large, to provide natural biological control services to gardeners.