Hypothesis 3- A consistent level of immigration from St. Thomas may have resulted in genetic rescue to the St. John deer population.
Many species on islands or within small isolated populations that experience bottlenecks can exhibit the effects of inbreeding depression and the subsequent loss of genetic and allelic variation (Kekkonen et al. 2012). These populations often require genetic contributions from unrelated individuals to reduce their number of deleterious alleles, a process called genetic rescue (Tallmon et al. 2004; Fredrickson et al. 2007). Genetic rescue can have a significant effect on fitness, including increases to composite fitness, which combines fecundity and survival estimates (Frankham 2015). Additionally, the effects of genetic rescue tend to be most pronounced in animals living within stressful environments (Frankham 1998). Outbred individuals with increased genetic diversity demonstrate increased resilience through juvenile survival, sperm quality, and immunocompetence compared with inbred control individuals, even if if the genetic rescue donors were from another inbred population (Heber et al. 2012; Fredrickson et al. 2007) that contained low genetic variation and fixed deleterious alleles (Vila et al. 2003; Kekkonen et al. 2012). Genetic rescue is most successful within a population if the novel alleles continue in subsequent generations, and can potentially influence lifetime reproductive success for individuals within a population (Heber et al. 2012; Fredrickson et al. 2007).
The deer of St. John could have possibly benefitted from genetic rescue, resulting in their current level of heterozygosity. Despite the 6.4 km of open water and challenging currents, deer have been consistently described and observed swimming between the islands throughout their history, and most often from St. Thomas to St. John (Heffelfinger 2011). The deer on St. Thomas are considered agricultural pests, and have been actively hunted. Hunting may have resulted in different selective pressures that altered the genetic base of the St. Thomas deer. The introduction of new genes from St. Thomas, even though they share a similar history, may be enough to diversify the gene pool of the St. John deer and maintain healthy heterozygosity levels. It is currently not known what number of deer are immigrating from St. Thomas to St. John. Also, there has been no study of the genetics of the St. Thomas deer to know their current levels of allelic diversity. However, the allelic contribution of the St. Thomas deer to the St. John population may be significant over time, and may have acted as a steady infusion of new alleles to the population, even if the deer population of St. Thomas is not genetically very diverse.
Despite the unexpected genetic heterozygosity found in this study, there have still been changes to the deer population of St. John compared to mainland deer as a result of their isolation on the island of St. John for over 200 years. In the absence of predation, these changes appear to be largely environmentally induced. The individual and population changes observed in the St. John deer population include reduced physical stature of the deer (Webb and Nellis 1981; Heffelfinger 2011; Reuter and Nelson 2018), high levels of disease manifestation for ticks and mange (Nelson et al. 2017), acute die-offs resulting from epizootic hemorrhagic disease virus (EDHV) (Reuter and Nelson 2018), and reduced fecundity levels observed in the deer on island. Many of these changes may be multifaceted in origin. For example, reduced physical stature could be influenced by genetics (Webb and Nellis 1981), nutritional deficiencies (Hewitt 2011), food scarcity (Robbins 2012), phenotypic plasticity (Rozzi and Lomolino 2017), changes to climate (Gardner et al., 2011), or Foster’s rule, where large mammals become smaller on islands through time (Foster 1964; Millien 2011). The drought in 2015 resulted in significant food and water stress to the deer population of St. John, resulting in a diminished number of deer (Nelson et al. 2017). Although episodic, stressful events like drought and hurricanes could be acting as strong evolutionary forces to the population, and influence the genetic portrait of the population over time.
It is currently unknown which of the three proposed hypotheses explain the levels of heterozygosity found within the St. John deer population, or if the answer is a combination of several of the scenarios described. To identify the mechanism(s) responsible for preserving allelic diversity with more precision will require additional research, and should include a genetic analysis of the source populations, a better understanding of the St. Thomas deer genetic profile, and more detail on the history of deer introductions to both St. Thomas and St. John. Overall, the deer of St. John provide an engaging case study to examine complex themes within ecology, including island ecology, predator-free landscapes, isolated population dynamics, the founder effect, and the effects of episodic environmental stressors on both population dynamics and to individual animals.