Using DNA in Grizzly bear monitoring

Submitted by editor on 13 October 2015.

In our new paper “In the Trap: Detectability of Fixed Hair Trap DNA Methods in Grizzly Bear Trend Monitoring” we explore how local habitat conditions affecting the detection of grizzly bears at fixed hair trap sites.

Large scale grizzly (brown) bear population surveys rely most often on DNA hair trap techniques. Grizzly bears are difficult to monitor for many reasons including large home ranges, low densities, rugged terrain, and cryptic behaviour, all which significantly increase the cost of population surveys. As a result, grizzly bear population surveys in Alberta are infrequent, despite being listed in the province as threatened. One way to reduce costs of DNA hair trap surveys is to adopt a design that relies on a permanent network of hair trap sites rather than more traditional techniques where sites are moved between sampling periods. Strategic placement of fixed hair traps that maximize detection across survey periods is therefore critical for minimizing costs, while still maintaining robust data collection (i.e. biggest bang for your buck). To address this challenge, we first examined what local factors affected variations in grizzly bear occupancy and detection?

Figure 1. A fixed DNA hair trap site situated at the edge of a cutblock.

To answer this question, we conducted a grizzly bear DNA study in west-central Alberta between June and August of 2011 with site attributes measured in the field in August. Grizzly bear habitat quality in the area varies considerably, due to topographic complexity and different land use by humans, which affects how the bears use the landscape. As a result, we investigated local habitat variables relating to landcover type, topographic and forest stand features, food resources, and anthropogenic features as potential factors affecting the detection of bears. We also extended DNA surveys into August – the start of the berry season and one month longer than typical grizzly bear DNA surveys –to test whether the pulsing of available berry crop resources competed with fixed hair trap sites. We used program PRESENCE and the detection history of each hair trap site to investigate the occupancy and detectability of grizzly bears at two spatial scales: local (patch) habitat characteristics and landscape factors.

Figure 2. Grizzly bear hair samples collected at a fixed hair trap site.

When we considered local habitat variables immediately at the site (the patch scale), we found that grizzly bear detection was highest when sites were placed near streams with clover and in intermediate levels of crown closure. When we considered the same variables at a larger (landscape) scale, we found the probability of detection increased near streams and oil and gas wellsites, especially when food resources and wellsite density in the surrounding area was low.  We did not find detectability to vary over time, suggesting that berry crops did not compete with fixed DNA sites during typical survey periods (June to August). Our results highlight that DNA hair trap sampling methods that use a network of fixed sample sites can be used for long-term grizzly bear monitoring programs in west-central Alberta and can be extended longer into the active season. By demonstrating the importance of environmental conditions and resources for grizzly bear detection, this study can help guide optimal placement of fixed hair trap sites. Optimal placement of fixed hair trap sites will increase detection rates and help, in part, to ensure the most cost-effective monitoring results. Future studies should investigate the detection probabilities of other DNA sampling methods, such as scat and hair collected from rub trees alone and in combination with traditional hair snag methods. A better understanding of search effort and optimal study design for long term trend monitoring that uses multiple sources of DNA is needed.