This week’s BEACON Researchers at Work blog post is by Michigan State University graduate student Paige Howell.
Understanding the processes that influence the spatial distribution of diversity is a long-standing goal in ecology and evolutionary biology. It has also always been an area of interest for me, both in the classroom and in my own research. A major theme of my research is identifying the processes that are most influential to the spatial genetic structure of individual species and communities of co-distributed species. In most taxa, dispersal is correlated with gene flow and consequently, it is a major contributor to population genetic differentiation. Dispersal ability and behavior are largely determined by the variety and spatial configuration of landscapes. Consequently, to understand the processes influencing how populations, species and communities are genetically structured requires consideration of the landscape features in which individuals exist and through which they disperse.
The emergence of landscape genetics as a synthesis of population genetics and landscape ecology has provided important advancements to our understanding of how dispersal and spatial genetic patterns are influenced by environmental features. To date, most research has focused on identifying physical landscape features (e.g., habitat matrix) or barriers (e.g., roads, waterfalls) that are correlated with genetic discontinuities. Identifying the environmental factors that most strongly affect gene flow and the spatial genetic patterns of different organisms has implications for basic research as well as conservation and management. For example, because of the potential influence of gene flow on the evolution of adaptation, evolutionary biologists are often concerned with understanding the processes governing gene flow. The dispersal ability of individuals between local sites may also impact the persistence of populations and so is an important study area for managers.
Unfortunately, I have found that most landscape genetic studies have ignored the potential impact of species interactions (e.g., density of heterospecifics, competition for food resources) on patterns of spatial genetic structure. Co-distributed species may respond differently or at different scales to the structure and spatial arrangement of landscapes. However, the dynamics of each species within the greater community are linked via ecological processes such as competition or predation. Ecological processes that are influential to the structure and function of biological communities are dependent on the intrinsic properties of each species as well as underlying landscape characteristics. Thus, incorporating measures of species interactions and physical landscape features at multiple spatial scales is crucial to understanding the spatial distribution of genetic variation for single species and communities of co-distributed species. A better understanding of the factors contributing to patterns of genetic diversity will enhance our ability to predict the probability of population, species, and community persistence in the face of changing landscapes.
The community I’m working with now is a predator-prey system in the upper peninsula (UP) of Michigan composed of the American marten (Martes americana) and their small mammal prey (such as grey squirrels, voles). Previous research in my lab has developed genotypic data sets for American marten using neutral, microsatellite markers. Based on these data, three geographically distinct genetic clusters of American marten have been identified. Although individuals are continuously distributed across the landscape, the presence of genetic discontinuities suggests there are barriers to dispersal limiting the interaction between genetic groups. Based on all data from the UP, I am currently developing single species models of the associations between landscape (e.g., land cover, roads) and climatic (e.g., snow depth) features and measures of spatial genetic structure. In one such model, I tested whether a pattern of isolation by distance could explain the distribution of genetic variation in this species. Isolation by distance refers to the phenomenon that gene flow decreases with increasing geographic distance between groups and this results in higher genetic differentiation. Using a simple Mantel test to compare pair-wise geographic distances based on Euclidean distance and pairwise genetic distances based on inter-individual relatedness, I have detected a significant pattern of isolation by distance for marten over the entire study.
Because there exist a number of putative barriers to dispersal for marten in the UP, I was curious to see whether including any of these barriers in my model would improve my ability to explain genetic variation. The first barrier I have investigated is the presence of state roads and whether it contributes to the maintenance of genetic discontinuities within the population. Similar to models of isolation by distance, the spatial genetic structure of marten appears to be correlated with the presence of state roads as a factor influencing resistance to movement. However, the amount of genetic variation explained by either of these models is relatively low and incorporating other habitat features (e.g., landcover type, size of suitable habitat patches) may improve model fit.
While neutral markers like microsatellites serve as one measure of population genetic structure, I’m interested in looking at non-neutral markers as well. Certain phenotypic traits may be selected for in a population based on advantages they provide during dispersal through a complex mosaic of habitats. Phenotypic traits with high heritability may be used in addition to neutral markers as a proxy to evaluate the spatial distribution of genetic variation at non-neutral loci. Using a geometric morphometric approach, I will investigate whether spatial genetic structure in these animals is paralleled by morphological differences in skull shape.
As I mentioned earlier, I think it is critical to consider the species interactions that may be strongly contributing to the patterns of diversity of different species within a community. With marten, the diversity (e.g., beta-diversity in an ecological sense and intraspecific genetic variation) in their prey species may be one important factor. In some studies of community genetics, the genetic variation of one species has been found to predict the genetic structure of the other. The correlation of genetic variation in one species with another will depend on the strength of the ecological interactions linking each species and the shared responses of each species to environmental heterogeneity. At a more local scale where ecological processes, such as predation, are operating between marten and their prey, what factors are driving the observed patterns in spatial genetic structure? Is it the composition and configuration of habitat features, the spatial genetic structure of their prey, or a combination?
For more information about Paige’s work, you can contact her at howellp
at msu dot edu.