What does genetic structure of small locally distributed populations tell us about the overall geographic distribution of a species? The success of small local populations is vital for maintaining the overall success of larger units that comprise a hierarchy of populations across the entire geographic range of a species. Local survival and even recolonization is especially critical when appropriate habitat is naturally patchy, or if that habitat is subject to fragmentation and degradation due to human land-use practices. The Pacific Jumping Mouse (Zapus trinotatus) is distributed along the Pacific Coast in naturally patchy stream-side, wetland, and meadow habitats. This species provides a unique opportunity to examine how processes at the smallest population scale translate into patterns at higher levels. To reveal these connections this Burke research project has probed the genetic structure of populations at several scales, using DNA technology ("microsatellite markers") together with field observations of the movements of individual mice at a small-scale study site. These data are combined with studies over larger areas to demonstrate the importance of genetic diversity in the small local populations.
At the smallest scale we have found that individual mice move very little. This is caused by two main factors. First, an individual tends to stay within one area, or home range, for most of its life. Second, because these animals require a specific kind of habitat, their dispersal (movement away from the spot where they were born) is limited by habitat availability. As a result of this limited movement of individual animals, neighboring animals are closely related. Over time this can create a distinctive genetic structure in the population. In the Pacific Jumping Mouse we have found that because movement of individuals is limited to patches of their preferred habitat, they are distributed in clumps of closely related individuals. This clumping of related individuals results in a distinctive genetic structure of the population, varying from small to large scales.
When an animal's movement is limited by habitat availability, not only will it be restricted as to how far it can move, but where it can move, as dictated by the location of its required habitat. If you were to guess what limits the movement of an animal, you might first think of large topographic features such as mountains. Interestingly, we have found that this is not necessarily the case for jumping mice. Instead, the location of their preferred habitat seems to direct their patterns of movement. Past patterns of movement can be discerned by constructing a neighbor-joining dendrogram (branching "tree" diagram) which connects those groups of individuals (or sub-populations) that are more closely related. We have found that past and recent patterns of movement follow similar paths, indicating that animals have been following these routes for a long time.
Our study has revealed that the limitations placed on individual animals due to the availability of habitat has an effect on the species as a whole. In Pacific Jumping Mice, the patchy distribution of the preferred habitat has resulted in the development of genetic structure that characterizes groups of closely related individuals. It has also led to unexpected patterns of connectivity between groups, with the distribution of habitat being more important than large topographic barriers. Overall, our study indicates that it is important to understand what is occurring at small scales, because these processes can have a direct effect on the species at larger scales.
Sacha Vignieri is a Burke Museum graduate student and expects to complete her Ph.D. in Zoology in 2005.
Jim Kenagy is Curator of Mammals at the Burke Museum and Professor of Zoology in the Biology Department.
S. N. Vignieri. 2003. Isolation and characterization of eight highly variable microsatellite markers in the Pacific jumping mouse (Zapus trinotatus). Molecular Ecology Notes 3:638-640.
In patchily distributed populations it is possible to investigate gene flow and relatedness at multiple scales. Using an enrichment procedure, I developed a suite of eight microsatellite markers in such a heterogeneously distributed species, the Pacific jumping mouse.
S. N. Vignieri. 2005. Streams over mountains: influence of riparian connectivity on gene flow in the Pacific jumping mouse (Zapus trinotatus). Molecular Ecology 14:1925-1937.
In species affiliated with heterogeneous habitat, we expect gene flow to be restricted due to constraints placed on individual movement by habitat boundaries. This is likely to impact both individual dispersal and connectivity between populations. In this study, a GIS-based landscape genetics approach was used to infer patterns of dispersal and migration in the riparian-affiliated Pacific jumping mouse