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.