The effects of conspecific density on individual performance can be substantial, but are rarely straightforward. Most studies of local density (or group size) focus on short-term success, limiting our understanding of whether or how these effects directly contribute to lifetime fitness and population-level consequences. For example, trade-offs in the effects of density may be expected especially for organisms with complex life cycles as ecological interactions and the scale of movement change across development. Density-dependent interactions such as predator foraging or facilitative feeding can also occur at different spatial scales, so that capturing the influence of these processes while avoiding bias towards particular types of interactions requires measuring the effects of density at multiple scales simultaneously. In this study, we use a series of density manipulation experiments at three nested spatial scales (leaf, plant, and patch) to investigate how density affects vital rates during different life stages, across different interaction types (e.g., predation and competition), and at multiple spatial scales in the leaf beetle, Leptinotarsa juncta. We identify the stages and scales at which the effects on density on survival and growth are strongest, and look for trade-offs by comparing optimal density between developmental stages and across scales.
Results/Conclusions
The optimal density predicted at each spatial scale differed by life stage and interaction type. Egg survival depended on egg abundance within a patch, with greater survival at the highest leaf- and patch-level densities tested. Positive relationships between density and survival suggest a predation dilution effect, where the shared risk of consumption is lower at high densities. In contrast, growth of early and late instar larvae was more strongly affected by leaf- and plant-level density, respectively, implicating both physical and chemical plant defenses: physical defenses can be overcome by small-scale group feeding, while chemical defenses are often systemic. Growth was maximized at intermediate densities, which were much lower than optimal egg densities at the same scales. Decreases in optimal densities are consistent with observed patterns of dispersion through L. juncta’s life cycle. Our results demonstrate trade-offs in effects of density across life stages and suggest that spatial distributions may be adaptive with respect to these trade-offs. Additionally, our approach reveals likely mechanisms of density dependence and the scales at which they operate. Next, we will measure larval and pupal survival and use these density-dependent vital rates to model how the effects of density integrate across the entire life cycle.