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Abstract
The muskrat, Ondatra zibethicus, is a semiaquatic rodent native to North America that has become a highly successful invader across Europe, Asia, and South America. It can inflict ecological and economic damage on wetland systems outside of its native range. Anecdotal evidence suggests that, in the early 1900s, a population of muskrats was introduced to the Isles of Shoals archipelago, located within the Gulf of Maine, for the purposes of fur harvest. However, because muskrats are native to the northeastern coast of North America, their presence on the Isles of Shoals could be interpreted as part of the native range of the species, potentially obscuring management planning and biogeographic inferences. To investigate their introduced status and identify a historic source population, muskrats from Appledore Island of the Isles of Shoals, and from the adjacent mainland of Maine and New Hampshire, were compared for mitochondrial cytochrome b sequences and allele frequencies at eight microsatellite loci. Appledore Island muskrats consistently exhibited reduced genetic diversity compared with mainland populations, and displayed signatures of a historic bottleneck. The distribution of mitochondrial haplotypes is suggestive of a New Hampshire source population. The data presented here are consistent with a human-mediated introduction that took place in the early 1900s. This scenario is further supported by the zooarchaeological record and island biogeographic patterns. This is the first genetic study of an introduced muskrat population within US borders and of any island muskrat population, and provides an important contrast with other studies of introduced muskrat populations worldwide.
Highlights
Questions regarding the origin, distribution, and maintenance of insular fauna have been central to the study of evolutionary biology since the time of Charles Darwin and Alfred Russell Wallace
ME and New Hampshire (NH) samples were donated by furtrappers acting under valid state collection permits
Mismatch distributions plotted for mainland sequences (ME and NH were pooled, as microsatellite analyses revealed little population subdivision) corresponded to a model for expected values of demographic stability, as suggested by the small value of r = 0.06 (Figure 2A)
Summary
Distribution, and maintenance of insular fauna have been central to the study of evolutionary biology since the time of Charles Darwin and Alfred Russell Wallace. Attempts to delimit the biodiversity of island systems can be confounded by ‘‘cryptic’’ invasive species, i.e., a human-aided, otherwise nonnative species may be mistaken for a native island species [2]. This has the effect of inflating the number of endemic taxa recorded and can mislead biogeographic inferences (e.g., by distorting species-area curves) or conservation priorities. Because invasive species are a major cause of extinction in endemic insular fauna [3,4], determining whether a species is native or introduced to an island ecosystem is critical to informing its subsequent protection or eradication [5,6]. Because morphological evidence of endemism can be ambiguous [2,6], the combined use of microsatellite and mitochondrial DNA analyses has become a powerful approach for identifying past bottlenecks, source populations, and evaluating the native status of a potentially cryptically introduced species [e.g., 5]
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