Densities across Hierarchical Scales Influence Ecological and Evolutionary Processes

Density, or the number of individuals per unit area, is known to have substantial effects on individual organisms and on populations. In particular, densities at small spatial scales often affect species interactions, e.g., predation and resource competition. Because of this, variation in local densities within a population (e.g., when individuals are patchily distributed) may lead to differences among individuals in interaction strength and fitness. Previous work shows that variance in local density can influence populations by affecting population growth independently of total density or by selecting for traits that maximize density-dependent performance. Determining the consequences of local density dependence is therefore critical for understanding how interactions occurring among individuals translate into patterns of population growth and phenotypic change through time. However, our ability to predict the long-term or large-scale effects of local density is limited by many factors. Specifically, we lack empirical data describing the spatial scale(s) at which density dependence occurs, a mechanism for integrating the direct and indirect effects of local density across life stages, and studies that address how local densities affect both ecological and evolutionary processes. In this dissertation, I addressed these gaps using a combination of experiments and modeling. Specifically, I (1) investigated how densities at hierarchical scales affect individual performance across the life cycle, (2) determined how the effects of local density integrate across individuals to shape population-level patterns of selection, and (3) quantified genetic and environmental sources of variation in phenotypes that help to set the local densities experienced by individuals. These studies are based on a specialist leaf beetle, Leptinotarsa juncta, and its interactions with its host plant (Solanum carolinense) and a community of generalist predators. As is the case with many insects, populations of L. juncta tend to be patchy, with substantial spatial variation in local densities. For the first study, I manipulated the density of immature L. juncta at three nested spatial scales (patch, plant within a patch, and leaf within plant) to determine how local densities affect predator-mediated survival and resource-mediated growth across multiple life stages. My results showed that the effects of local L. juncta density on individual performance varied with spatial scale, with some consistency in the direction of these effects across life stages. For example, survival of both eggs and late-instar larvae was higher on plants with low L. juncta densities and in patches with high L. juncta densities. The greater final mass of larvae at low plant-scale densities additionally suggests that the reduction in plant-scale larval density due to predation is beneficial for survivors. These findings demonstrate the role of spatial scale in mediating the effects of density-dependent interactions and reveal the interdependence of predator- and resource-mediated effects of density across the life cycle. For the second study, I developed and parameterized a novel structured population model in which both life stage and conspecific density are predictors of individual vital rates. Using evolutionary analyses, I quantified how both the direct and indirect effects of particular density-dependent interactions affect fitness. I additionally demonstrated that local and population densities jointly affect the strength and shape of selection on traits such as clutch size and sensitivity to local density (via reduced growth), while evolutionarily stable clutch size depends on the factor that most limits oviposition (e.g., time constraints, egg availability, or both). For the third study, I used maternal families of L. juncta to examine how genetic and environmental factors affect the expression of traits that help determine local densities. Specifically, I measured clutch size, egg size, egg cannibalism, larval aggregation, and larval movement among host plants for several maternal families; I also characterized density-dependent reaction norms for a subset of these traits (egg cannibalism, larval aggregation, and larval movement). I found evidence of genetic variance in some, but not all, of these traits. In contrast, density-dependent plasticity was common. For all three traits measured, at least some of the maternal families exhibited plastic responses to local density. These findings establish the evolutionary potential of some density-determining traits, suggesting that phenotypic evolution may play a role in shaping patterns of local density within populations. Together, this research demonstrates the critical role of local density dependence in both ecological and evolutionary processes and suggests that feedbacks between individual traits and emergent spatial patterns are an important source of eco-evolutionary feedbacks in natural populations.