Grizzly Bear Family Tree

Figure 1. Average locations of 1115 individual grizzly bears in the Northern Continental Divide Ecosystem (NCDE), USA, between 1998 and 2012. Black polygons encompass buffered regions of interest due to low genetic diversity in 2004, clipped by the sampled study area (double lines). Used from Mikle and others, 2016.(Public domain.)

Summary:

Scientists at the USGS used genotypes from 6160 hair samples collected from 1115 grizzly bears between 1998 and 2012 to assess changes in genetic diversity over time in the Northern Continental Divide Ecosystem (NCDE) in northwest Montana.  They found:

• Genetic diversity was lower in 3 southern regions in 2004
• Genetic diversity increased in those regions by 2012
• Initial low diversity resulted from a few animals with very high reproductive success (i.e. a small number of dominant male bears)
• The increase in diversity was due to movement into those areas from throughout the ecosystem

Introduction

Genetic diversity is one index of population health that reflects population size and also a population’s potential to adapt to changes.  Generally speaking, breeding between close relatives, a small population size, and difficulty moving throughout an ecosystem can all lead to lower genetic diversity. On the other hand, increasing genetic diversity can be a sign that bears are able to easily move between regions, bringing new genes to places where they have previously been missing or rare. USGS biologists teamed up with other federal and tribal scientists to collect grizzly bear hair samples from baited hair traps and naturally-occurring rub trees affixed with barbed wire. 6,160 grizzly bear samples were collected, leading to the identification of 1115 unique bears (figure 1).

Genetic diversity in 2004

First, researchers calculated genetic diversity from samples collected in 2004 for each bear and its neighbors and found that three regions that had lower genetic diversity than the rest of the ecosystem (EC, SE, SW in figure 2).

Genetic diversity in 2012

Between 2004 and 2012, the SE and EC regions showed rapid increases in genetic diversity, with levels approaching those in the northern core of the ecosystem near Glacier National Park (figure 2).

Figure 2. Changes in genetic diversity between 2004 and 2012. Large, warm-colored circles indicate higher genetic diversity, while smaller, cool-colored circles indicate less genetic diversity. Notice the large changes in the southern end of the ecosystem. Adapted from Mikle and others, 2016. (Public domain.)

Reproductive success

USGS researchers then built a massive family tree with the 1115 bears and were able to show that each southern region was almost entirely dominated by single extended grizzly bear families in 2004 (figure 3). Individuals within a family often share similar genes, suggesting that this, along with relatively low numbers, was likely to be the cause of the relatively low genetic diversity in 2004.

Figure 3. Distribution of two families descended from two highly successful bears. On the left, a male bear sampled in 2010 was found to have 61 descendants and on the right, an inferred male bear was found to have 101 descendants in the sample. These two examples show how the genetic diversity in an area can be due to few individuals having very high reproductive success. Adapted from Mikle and others, 2016.(Public domain.)

Movement into low genetic diversity regions

Figure 4. Many young bears moved into the southeast region (SE) from long distances away (blue plusses) between 2004 and 2012, showing the high degree of connectivity in the NCDE. New individuals can increase the genetic diversity in an area by bringing new or rare genes.(Public domain.)

Researchers next used the family tree to look at the role of movement, by using the locations of a bear’s parents to determine where that bear came from. In striking out on their own to areas far from where they were born (figure 4), these bears improved the genetic health of the population.  Genetic diversity is usually a very slowly changing measure of the health of the population and the movement we observed was responsible for the rapid increases in diversity. 

Conclusion

This USGS study highlights the importance in maintaining movement corridors for wildlife to ensure that the genetic health of populations is maintained or improved. The large protected areas in the NCDE allowed movement of bears bringing new genes into areas that had historically been dominated by few bear families, increasing the flexibility of the population to adapt to future changes.

Future research

USGS researchers are now working with scientists and managers from the state of Montana and the Canadian provinces of Alberta and British Columbia to include additional bears and analyze international and cross-jurisdictional movements. Analyzing the new data will allow researchers to identify the best way to use family tree data to assess the quality of grizzly bear habitat and barriers to their movement.