Comparing male densities and fertilization rates as potential Allee effects in Alaskan and Canadian Ursus maritimus populations Introduction Research suggests that our world today is in the midst of a 6th mass extinction where populations in a number of animal species are declining due to habitat encroachment, climate change, prey scarcity, and other threats (Ceballos and Ehrlich, 2002; Barnosky et. al, 2011). Population decline is a precursor to extinction. Thus, research focused on conspecific population decline and its potential effects is important to conservation efforts. Clyde Allee provided the first empirical research to suggest that under-crowding as opposed to competition within populations was the limiting factor of population growth (Allee, 1927). Termed Allee effects, several studies have focused on these concepts and providing evidence to support them. Recent research distinguishes between component and demographic Allee effects. Mechanisms leading to all Allee effects such as mate-scarcity or cooperative feeding may directly lead to component Allee effects (Drake & Kramer, 2011). Should these component Allee effects not be offset by negative density dependence in other measures of fitness, they may lead to demographic Allee effects, which are positive density dependence on the population level i.e. demographic Allee effects manifest at the level of total fitness and cannot exist without the presence of one or more component Allee effects (Stephens et. al. 1999). Though there have been some empirical studies done on Allee effects in insect, bird, fish, and mammal populations, the primary focus has not been on Allee effects as they apply to species conservation (Kramar et. al., 2009). One of the reasons for this is because it is difficult to collect empirical evidence to suggest that a component Allee
effect will manifest at the level of total fitness in a population. In order to effectively protect endangered species, it is important that we investigate these relationships and their role in declining populations. I propose that this study focus on polar bears (Ursus maritimus) as an organism increasingly impacted by climate change and hunting, leading to declining population sizes. In particular, this study will focus on polar bear dispersion and mating system and the potential Allee effects resulting from difficulties encountering mates. There have been no empirical data collected on the relationships between fertilization rates and male/female densities in any polar bear population (Molnar et. al, 2008). Polar bears are solitary and only pair up during the mating season, March-May with mating activity peaking in early April (Molnar et. al, 2008). This dispersion is important in considering the amount of time and energy required to find mates. Male polar bears are polygynous and often forgo hunting and feeding in order to pursue females to mate with (Ramsay and Stirling, 1988). The number of females a single male can fertilize is limited by the timing of the female oestrus period, which only lasts up to four weeks (Molnar et. al, 2008). Additionally, females provide a lot of care to their young, and are not receptive to mating while they have cubs less than 2.5 years. Potential Allee effects are especially relevant in animals, such as polar bears, which are subject to sport hunting. Allee effects are not only driven by population size, but sex ratios as well. Hunting practices and regulations have long targeted males, because the pair bond between female polar bears and their cubs renders them unable to mate every year. However, this hunting bias has lead to concerns regarding the depletion of males to the point where females are having difficulties encountering mates (Derocher
et. al., 1997). Mate-scarcity is a potential component Allee effect that could be dangerous to polar bear populations experiencing declining male densities. I will aim to determine the relationship between male densities and fertilization rates in polar bear populations. This will require collecting data on populations both exposed to male-biased hunting and populations that are not, and following both over the course of several years. Additionally, it is important to consider the other factors impacting polar bear survival and mating success. These factors include shrinking sea ice, which they rely on to hunt, and prey availability. Polar bears primary diet consists of ringed and bearded seals (Thiemann et. al., 2008). I hypothesize that declining male densities due to sex-biased hunting in an already relatively low-density population will lead to a component Allee effect of reduced fertilization rates in females. Additionally, I hypothesize that there will be a threshold male density below which there will be a rapid decline in mate-encounters and as a result, fertilization rates in female bears. Methods This study will be conducted on two polar bear populations: The Northern Beaufort Sea population, subject to sex-specific hunting, and the Chukchi Sea population, protected against hunting. Both populations will be systematically searched before mating season and every polar bear within the boundaries will be captured and tagged with tracking devices. The age, weight, and reproductive status of every captured bear will be recorded. Tracking devices will provide GPS location data, so that I can tell where these bears are in relation to others at any given time (important for tracking mateencounters). Additionally, the devices record GPS location at different time points,
essentially creating a map of where every bear has been that can be followed on a computer system. During mating season, distance and time spent searching for a mate, number of males encountered per female, and female fertility will be recorded for each population. These data will be continued every mating season for 10 years. Because prey availability and changing sea-ice levels also have potential impacts on polar bear populations, I will monitor the seal availability within Beaufort Sound and the Chukchi Sea using underwater sonar, similar to the way salmon are counted by wildlife biologists in many areas. I will monitor sea-ice extent yearly using satellite images taken by NSIDC (NSIDC, 2017). Expected Outcomes I expect the Northern Beaufort Sound population to have a lower density of male polar bears than the Chukchi Sea population. As a result, I expect that these lower male densities, caused by sex-biased hunting, will reduce fertilization rates in female bears. This effect could develop into a demographic Allee effect, leading to a decrease in overall population size in the Northern Beaufort Sound population. Additionally, I expect to see a point at which fertilization rates begin to decline rapidly in response to a specific male density in the Northern Beaufort Sound population. Because polar bears occupy home ranges, I expect that decreasing density of males in the population will lead to decreased mate encounter rates for females and thus cause lower fertility rates. In comparison, I expect the Chukchi Sea population to have higher male densities resulting in higher mating success (female fertility rates), therefore maintaining a stable population. In the event that results point to a decline in the Chukchi Sea population, it is
important to consider other variables affecting polar bear populations. Data collected on primary food availability and sea ice levels will provide insight into the extent of these other factors on polar bear densities and the significance of them in the conservation of this species. This study will provide data to determine a threshold above which we need to maintain male densities in polar bear populations to ensure their survival. This information is vital to the conservation of this species, ensuring that they do not reach a critical male density below which their population may not recover. References Barnosky et. al. (2011). Has the Earth s sixth mass extinction already arrived?. Nature. Vol.471 (51-57). Cevallos, G & Erlich, P. (2002). Mammal Population Losses and the Extinction Crisis. Science. Vol. 296. 5569. (904). http://science.sciencemag.org/content/296/5569/904 Dennis, Brian.(1989). Allee Effects: Population growth, critical density, and the chance of extinction. Natural Resource Modeling. 3:123-132. Derocher A.E, Stirling I, Calvert W. (1997). Male-biased harvesting of polar bears in western Hudson Bay. J. Wildl. Manage. 61, 1075 1082. Drake, J.M. & Kramer, A.M. (2011). Allee Effects. Nature Education Knowledge. vol. 3(10):2 Kramer, A.M., Dennis, B., Liebhold, A.M. et al. Popul Ecol. (2009). 51: 341. doi:10.1007/s10144-009-0152-6. http://link.springer.com/article/10.1007/s10144-009- 0152-6 Olsen EM, Heino M, Lilly R, Morgan, JM, Brattey, Ernande, Dieckmann. (2004). Maturation trends indicative of rapid evolution preceded the collapse of northern cod. Nature.Vol.428.932-935. Molnár PK, Derocher A, Lewis M, Taylor M. (2008). Modeling mating systems in Polar Bears. Proc. R. Soc. B 275 217-226; DOI: 10.1098/rspb.2007.1307. Published 22 January 2008
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