The positive relationship between species diversity and ecosystem function has received increasing attention in ecology. Recently, it has also become apparent that genetic diversity within-species can have similar benefits at the population level to those that species diversity has at the community level. Specifically, studies have shown that populations with higher genetic diversity are more stable and productive, as well as resistant to disturbance, disease and invasion than populations with lower genetic diversity. These widespread ecological consequences of genetic diversity manifest within a single generation, and have changed the way we view and manage populations. However, fundamental gaps in our understanding of genetic diversity effects remain. First, current efforts to investigate biodiversity effects, and in particular genetic diversity effects, suffer from taxonomic and ontogenetic biases. Second, a limited commitment to identifying the processes that underlie ecological genetic diversity effects had slowed the advancement of the field. Last, additive genetic variation in ecologically significant traits underlies ecological genetic diversity effects, however, it is also the raw material on which selection acts. Unfortunately, few studies consider these linkages between the evolutionary forces shaping the expression of additive genetic variation and ecological genetic diversity effects. Here, I present a series of experiments targeting these gaps in our understanding of genetic diversity effects. In the second and third Chapters, I examine the effects of genetic diversity in two sexually reproducing marine invertebrates in the field. In the fourth Chapter, I examine the effects of genetic diversity on the colonization of new sites by the larvae of four sessile marine invertebrates. In the fifth Chapter, I quantify the contribution of additive genetic variation to phenotypic variation in larval and adult survival of the solitary ascidian, Ciona intestinalis. Overall, I show that populations with greater genetic diversity had significantly greater survival, growth and reproduction than populations with lower genetic diversity, and that both selection and complementary effects contribute to the benefits of diversity. Additionally, I found that genetic diversity effects during the larval stages can have strong effects on colonization. Importantly, the effects of genetic diversity on colonization were species-specific, and thus have broad implications for the ecological properties of populations. Last, I demonstrate the existence of additive genetic variation in the survival of Ciona intestinalis, and provide evidence supporting the maintenance of additive genetic variation by negative genetic correlations among fitness components.
Biodiversity is a key component of ecosystems, responsible for driving variation in ecosystem productivity, stability and resilience. The ecological consequences of genetic diversity therefore, lie at the core of efforts to understand the links between biodiversity and ecosystem function. However, one feature that distinguishes genetic diversity effects from biodiversity effects at higher levels is that the source of ecological genetic diversity effects – additive genetic variation – is also the target of selection. In future studies, I believe a more synthetic approach, which integrates the evolutionary and the ecological outcomes of differences in genetic diversity among populations will allow for more accurate predictions of the forces structuring populations, and unveil the broader relevance of genetic diversity effects in nature.