Sika deer introduction, population genetics, and their competition with white-tailed deer

Date
2015
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University of Delaware
Abstract
The ecological problems caused by invasive species are not new. Over time these species have been labeled in several ways, however they are labeled; non-native, exotic, non-indigenous, or alien, species that establish populations outside of their native range cause changes in the balance of the natural ecosystem. The negative effects of an introduction can go unnoticed for generations, and once we realize the problem, it is often too late to control. In these situations, we as stewards to the natural environment must manage the new species. Proper management requires not only a knowledge of the species biology, ecology etc., but also a working understanding of how these species affect the native ecosystem. Ungulates are the most commonly introduced animal species in the world. Whether for controlled/contained hunting or the intentional introduction into wild populations, cervids (deer) are a popular non-native species worldwide (Dolman and Wäber 2008). While currently in several states (captive and free ranging), the first introduction of sika deer, was actually in Maryland. Sika deer were first introduced to the United States as an additional game species when our native white-tailed deer population numbers were at an all-time low. Their introduction was not noticed or was overlooked for 4 decades, until the 1950’s when they began to rapidly increase in number. The belief that sika deer could coexist with white-tailed deer was accepted, and applauded as some considered sika deer a replacement and/or additional game species for areas that no longer retained white-tailed deer. However, as sika deer continue to expand their range and increasing in number, this notion needs to be more thoroughly addressed. Until now the history of sika deer in Maryland has started at James Island, the point of original release into the wild. Here I investigate the complete history of how sika deer came to Maryland from their native Asia, and their spread to both shores of the Delmarva Peninsula. I trace the lineage of sika deer in Maryland back to stocks in the UK from a population in Japan. I also highlight the key players in multiple countries that have led to the establishment of sika deer on both the eastern and western sides of the Delmarva Peninsula. With an understanding of their stocking source I investigate the genetic fitness of these sika deer and implications that result from the founder events and natural changes in population. I compared the molecular variation of sika deer on the Delmarva Peninsula to those of the source population on Yakushima Island, Japan and to captive held sika deer in Delaware using 10 microsatellite markers. Through a series of founder events and population bottlenecks, condensed into a few generations sika deer in Maryland are depauperate in neutral allelic diversity. I observed 14 alleles (including two unique) in the Dorchester population, and 11 alleles in the Assateague population (none unique). The paucity of variation that does exist alludes to a history of hybridization or genetic admixture as well as a period of invasive lag once the deer were release in Maryland. The genetic diversity I observed support that limited variation that has existed in neutral alleles of sika deer since glacial periods of the Precambrian. The allelic richness and levels of heterozygosity are similar to those observed in other severely bottlenecked populations or populations founded from few individuals. I used the aforementioned pre-introduction information to create an approximate Bayesian computation (ABC) of the demographic history of Maryland’s sika deer, including estimations of current and historic effective breeding population sizes. ABC supported a single introduction to the Delmarva by way of a sika deer introduced to the UK from Japan. Multiple founding of a large population from few individuals in a short period of time has resulted in changes in genetic diversity, and rates of change within population growth. My results from ABC modeling support a rapid population expansion in the UK, then a slower growth in numbers, and longer generational periods once sika deer were brought to the USA. The change observed in growth rate show an invasive lag in sika deer population suggesting that sika deer numbers have the potential to expand more rapidly than the current rate. Despite the complicated introduction history and depleted level of genetic diversity, sika deer have proven to be healthy and highly successful deer on the Delmarva Peninsula currently in addition to native white-tailed deer. I also investigated the competition that exists between the introduced sika deer with our native white-tailed deer, based on dietary resource overlap. A large portion of both species diets were composed of woody browse in the winter, when resources are most limited. Most of the resources used by sika deer (60%) were resources that are part of white-tailed deer diet, and are critical resources for white-tailed deer (>75% white-tailed deer resource use). The degree of dietary overlap measured across several niche size estimators show that sika deer and white-tailed deer significantly (70%) overlap in diet thereby competing for similar resources. The degree of overlap creates a change in white-tailed deer diet, increasing their niche size by 108%, again demonstrating competition over resources. The presence of sika deer on the Delmarva Peninsula also altered the resources that are used by white-tailed deer. The change in resources used by white-tailed deer with sika deer present are of lower quality than those used by white-tailed deer while they are alone. The change in resource use, and the additional pressure from another ungulate on the landscape has the potential to have cascading effects through the trophic levels of the Delmarva, starting with the decline in white-tailed deer numbers. The continued increase in sika deer numbers on the Delmarva is a growing concern for the prolonged success of white-tailed deer and requires additional management considerations.
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