Genetic diversity of wildlife populations are an important consideration for long term conservation. Having a variety of genetic traits across a population is generally considered highly advantageous, as it allows a species a large toolbox of traits to respond to changes in environment, population size, mutation and other ecological, biological or social factors. Grizzly bears are no such exception and a closer look at bear reproductively and biology can yield some insight into how having genetic variability among individuals is an important thing for a healthy population.
Within wildlife populations the “depth” of a genetic pool can be a serious consideration. For example, inbreeding can have numerous consequences in wildlife populations. Inbreeding depression in wildlife populations can cause a reduction in an offspring’s ability to survive and reproduce within a population. The idea behind this reduction is twofold.
Firstly, if an individual receives the genes from two closely related individuals the chance of receiving a lethal recessive gene becomes larger for the individual. This gene could manifest as disease or disadvantageous physical trait.
Second, over the long-term inbreeding limits the diversity of genes being passed down to offspring because the genes are all coming from two or several related individuals with a similar genetic make-up.
Genetic diversity, termed heterozygosity, is advantageous to wildlife populations because with a diverse amount of genes come the blueprints to adapt to varying amounts of change. This change could be from the environment such as a drought or food shortage or the change could come in terms of habitat destruction or competing with an invading animal species for food, shelter or survival.
Genetic diversity could be looked at as a population’s stock portfolio. Having a diverse portfolio can shield an investor or population in this case, from crashes in one particular market or another. Ultimately, the threat of a species without the genetic depth to adapt is the threat of extinction.
All animal populations are inbred to a degree. The challenge with small populations (less than 100 individuals) is that only a small amount of individuals are contributing to the gene pool each generation putting a ceiling on the overall amount of genetic diversity that can be reached.
Grizzly bears have several social and biological traits that influence the flow of genes around the neighborhood. Grizzly bears are not social creatures and live at relatively low densities compared to other wildlife. How then, is genetic diversity affected by a grizzly bear’s life strategy?
Grizzly bears are polygamous. Finding multiple mates trades genetic material across multiple individuals increasing the combinations of chromosomes possible. No doubt, having an excellent sense of smell to sniff out potential mates at long distances helps grizzlies aide grizzly populations in spreading genetic seed.
Grizzly bears are a long-lived species and can live to 25 years or more. Species with longer generation times and relatively low reproductive rates are slow to experience any effects of inbreeding depression due to the length of time necessary to combine genes enough to begin running low on significantly different combinations.
Naturally, the ability of animals to move throughout an ecosystem can influence genetic diversity. Fragmentation of habitat or barriers to movement can alter how genes flow throughout a population. The grizzly bear population in Yellowstone is an “island” population and has not been connected to any other grizzly population since perhaps the early 1900s. Yellowstone has comparatively lower genetic diversity than other more connected populations such as the Northern Continental Divide Ecosystem (NCDE) or even larger contiguous bear populations further north in Canada.
However, as grizzly bear populations continue to grow in both the Greater Yellowstone and Northern Continental Divide Ecosystems (NCDE) these two populations may someday have genetic exchange. Both ecosystems have observed grizzly bears popping up in areas far outside of what was considered occupied habitat 15 years ago.
For example, several years ago, a grizzly bear turned up outside of Butte, Mont. a location about halfway between both ecosystems. Based on the individual DNA collected from the bear and comparison to genetic diversity in each ecosystems, it was concluded the dispersing bear had traveled from the NCDE.
Highways and human development have proven to be genetic obstacles for grizzly bears as well. The US Geological Survey DNA study in 2004 detected a much greater amount of gene flow across the eastern part of Montana Highway 2 that divides Glacier National Park and the Great Bear Wilderness. The theory is that the more developed, busier western section of Highway 2 may be a barrier to bears moving between Glacier Park and the Wilderness.
However, more recent observations of grizzly bears show expansion of grizzly bears beyond much of what was considered to be occupied habitat as little as 15 years ago. Dispersing bears have wandered into the eastern Montana plains further east than Highway 89 or Interstate 15. Montana Fish, Wildlife and Parks has documented movements of a female.
More recently, the USGS in cooperation with other agencies are evaluating genetic samples from grizzly bears sampled over the past decades. Results indicate that some local inbreeding was present but inbreeding signatures diminished in one bear generation with population expansion and bear movements into the different regions of the ecosystem.
Considering the importance of the variability of genes between individuals in a population the concepts behind habitat connectivity and reproductive strategies become all the more important. Many other factors influence the dynamics of a population beyond the scant few mentioned here. One of the take home messages from genetic variability is that it is good to be the same but your life may depend on being just different enough.
A wealth of information has been learned about grizzly bears in the NCDE over the past 30 years. High survival rates, expanding population, good genetic diversity and stable to increasing population trend indicate that management and recovery efforts to recover the grizzly bear have been successful.
Reader Comments(0)