Sika deer introduction, population genetics, and their competition with white-tailed deer
Date
2015
Authors
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Journal ISSN
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Publisher
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.