So far this blog has addressed
a number of anthropogenic threats facing Ontario snake species, including a
habitat loss, degradation, and fragmentation/road mortality, persecution, and
even climate change. All of these result in small, often isolated populations,
which is a major conservation concern as these types of populations are very
vulnerable to extinction. This is mainly a function of bottlenecks and
inbreeding depression, which often accompany or occur in small populations.
A
bottleneck is essentially a dramatic drop in population size, but because only
a fraction of the population persists only a small amount of genetic diversity
persists. This is a problem because high genetic diversity is essential for the
survival of populations (Frankham et al., 1999). Put another way, a population
that has experienced a bottleneck has low allelic variety, which impairs the
ability of a population to survive. Survival ability is impaired by low genetic
variation because it reduces a population’s evolutionary potential, its ability
to adapt to changes in the environment (Frankham et al., 1999).
Bottlenecks
also often result in inbreeding depression. When a population becomes very
small, related individuals will breed with each other, or inbreed. In animals
inbreeding often results in reduced fitness (the ability of an individual to
survive and reproduce), this phenomenon is termed inbreeding depression.
Inbreeding depression has serious negative consequences, it can result in
offspring with low birth weight, impaired disease and stress resistance and reduced
growth, all of which impair the survival and reproductive ability of
individuals (Keller and Waller, 2002). Like with bottlenecks inbreeding reduces
fitness by lowering genetic diversity, and consequently evolutionary potential,
but also by enhancing the expression of harmful alleles that are not a problem
in large populations because they are rare and recessive (Bijlsma et al., 2000).
Inbreeding depression significantly increases the likelihood of a population
going extinct, and this likelihood of extinction is enhanced in the presence of
environmental stress, such as anthropogenic threats (Bijlsma et al., 2000).
This
directly applies to snake populations in Ontario, take for example the eastern
hog-nosed snake. Xuereh et al., (2015) carried out genetic analysis on eastern
hog-nosed snakes in Ontario and found evidence of inbreeding in all four of the
genetically distinct subpopulations. Keep in mind the number and intensity of anthropogenic
threats facing this species, particularly habitat degradation and road
mortality. Inbreeding alone suggests the populations are experiencing reduced
fitness, but when you also consider the added impacts of human activities we
see that these populations are likley under serious threat of becoming endangered
and extinct.
By virtue of their low genetic diversity,
which impairs survival ability, small populations are at high risk of becoming
extinct (Allendorf, 1986; Briskie and Mackintosh, 2004). Human activity has
greatly reduced the ranges and population sizes of many Ontario snake species,
and because of the genetic consequences this means that besides killing snakes
directly (through persecution and road mortality) we are indirectly enhancing
the likelihood of entire snake population extirpations or extinctions.
References:
Allendorf, F. W. (1986). Genetic drift and the loss of alleles
versus heterozygosity. Zoo Biology 5 :181–190.
Bijlsma, R., Bundgaard, J., and Boerema, A. C. (2000). Does
inbreeding affect the extinction risk of small populations? Predictions from
Drosophila. Journal of Evolutionary Biology 12:1125–1137.
Briskie, J. V., and Mackintosh, M. (2004). Hatching failure increases
with severity of population bottlenecks in birds. Proceedings of the National
Academy of Sciences of the United States of America 101:558–561.
Frankham, R., Lees, K., Montgomery, M. E., England, P. R., Lowe,
E. H., and
Briscoe, D. A. (1999). Do population size bottlenecks reduce
evolutionary potential? Animal Conservation 2:255–260.
Keller, L. F., and Waller, D. M. (2002). Inbreeding effects in
wild populations. Trends in Ecology Evolution 17:19–23.
Xuereb, A. T. J., Rouse, J. D., Cunnington, G. and Lougheed, S. C.
(2015). Population genetic structure at the northern range limit
of the
threatened eastern hog-nosed snake (Heterodon platirhinos).
Conservation Genetics 16:1265–1276.
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