Introduction
Low levels of genetic diversity can result from many factors, including small founder populations, stochastic events that reduce population size, and inbreeding (Markert et al. 2010; Trinkel, et al. 2010; Pekkala, et. al. 2012). Small and inbred populations can exhibit negative consequences for growth, disease resistance, survival, fertility, and development (Spielman 2004; Ruiz-Lopez et. al. 2012), which can further diminish population size over time, and lead to increased extinction risk (Reed and Frankham 2003). The deleterious effects of inbreeding depression may be more pronounced under stressful conditions to the population than in benign conditions, and result in conditionally expressed deleterious genes (Bouzat 2010, Fox et al. 2011). To increase genetic diversity, new individuals from outbred populations can be added to the population and increase genomic heterozygosity. This can be highly effective in reducing the deleterious effects of inbreeding to a population (Fredrickson et al. 2007; Heber et al. 2012).
Island populations are often highly isolated spatially and are more prone to losing genetic diversity through genetic drift, or by bottlenecks due to small population sizes at the population’s founding (Jamieson 2007). However, islands offer several advantages to examining genetic processes. Immigration and emigration are often minimal, and therefore selection and genetic drift become more dominant as the processes most likely to affect levels of genetic variation (Pemberton et al. 1996). In addition, island populations tend to be more tractable because they are restricted within the physical confines of the island perimeter. As a result, the genetic composition of an island population is highly influenced by the number of founding individuals, their genetic diversity, the population rate of increase over time, and the extent of gene flow within the population (Freeland 2005; Simpson et. al. 2013)
The deer of St. John, U.S. Virgin Islands have a unique genetic history. The first mention of white-tailed deer (Odocoileus virginianus)in the U.S. Virgin Islands was from a Danish ship log, where the captain mentions five white-tailed deer being released on St. Croix during or before 1790 (Heffelfinger 2011), most probably for hunting purposes. Deer were described as inhabiting the mountainous parts of St. Croix in 1840 (Seaman1966), and in 1854 some of the deer were moved to St. Thomas and subsequently swam to inhabit St. John (Heffelfinger 2011). Additional deer were brought to St. Thomas and St. John from Texas and the Carolinas in the 1950s as part of a USDA translocation program (Baker 1984). Reports of deer swimming between the islands of St. Thomas and St. John are quite consistent through time (Hefflefinger 2011). Before an extended drought in 2015, the population was estimated at approximately 2000 deer on St. John. The deer are protected from hunting within Virgin Islands National Park, and there are no natural predators on St. John. The deer are highly habituated to humans, and show very limited fear as they forage near popular tourist trails and beaches during daylight hours.
The size of the deer population on St. John changes in response to environmental conditions and food availability. An increase in of twinning is often a sign that food is abundant and that the population is increasing (DeYoung 2011). Signs of a stressed and a decreasing population include high levels of mange and tick infestation, as well as muscle atrophy and poor body condition (Nemeth et al. 2013; Nelson et al. 2017). More recently, the St. John deer have undergone the intense stress of two category 5 hurricanes, Irma and Maria, in the fall of 2017. The current population estimate of deer on St. John following the two recent hurricanes is unknown.
The objective of this study was to determine the level of inbreeding in this isolated population of white-tailed deer on St. John following a drought on the island. A formal study of the genetics of this group has not been previously conducted, and the level of heterozygosity for this population has yet to be described.