The Impact of Roads on the Demography of Grizzly Bears in Alberta

Introduction

Human access into grizzly bear habitat through roads does not only result in a higher mortality rate but also affect the grizzly bear’s distribution, behaviour and even body conditions. The species’ status has been changed to threatened in Alberta in 2010, however the increase of human activity in bear habitat hinders the recovery of the population.

Background of the Research Project

The focus of this study was to determine the direct effect of roads on grizzly bears. Rather than identifying the amount of human use on the roads the study focused on their overall presence on the landscape and whether survival and reproductive rates are connected to each other in the context of roads. Most of the roads used in the study area are resource-based gravel roads which is important because grizzly bears seem to choose their habitats near such roads.

Variations in survival were estimated based upon factors of human activity. The understanding of the source of population variations is as important as estimating a single point of population trend.

Methods, Materials and Study Area

The study was conducted within the province of Alberta where 42,598 km of road are within grizzly bear habitat. 96.5% of these roads are resource-based gravel roads used by resource extraction industry as well as the public for recreational activities.

For this research project, data collected from 1999-2012 was used. This data originated from a bigger study conducted by the Foothills Research Institute Grizzly Bear Program, which collared grizzly bears and monitored them.

Using survival models from the program MARK, the survival estimates were determined for age and sex groups of bears that were a maximum of 4 years of age. There was no data available for bears during their denning season, resulting in the exclusion of the months of January, February and March from the study. The road density was the main covariate in the study and it was determined by the kilometers of all gravel and paved roads within a 300 m radius of GPS bear location. Next to road density, the distance of each bear location to roads was determined and considered another risk factor for the bears. The estimation of cub survival was more difficult as they were not collared and cub mortalities are very rarely witnessed. Therefore, females with cubs were observed and the times when the cubs was last seen alive and when it was last seen with its mother were noted. If the cub survived until denning season the two numbers were equal, however, if it was lost before then the death occurred between the last time it was seen alive and the last time it was seen with the mother.

For the reproductive analysis litter size was estimated and a later a multi-state model was used to calculate a reproductive rate for female bears.

To determine a threshold of road density in areas where the population was decreasing a demographic model was used.

Results of the Study

Most bears were monitored in the Yellowhead and Grande Cache management unit, each counting 53 monitored bears and leaving the rest of the study area with 36 bears. Out of the 142 monitored bears, 22 died. Seven out of those 22 were illegally poached, three were hunted legally, four were killed due to management actions, two died from road accidents, one died from natural causes and five have an unknown cause of death. A total of 19 deaths were within 500 m of a road, only one of them occurring near a paved primary road. The model showed that female and male bears had different survival rates, however, the survival relative to roads was connected with the age of the bear. Most of the collared bears were within area with road densities of 1.5 km/km2 or less and mortalities were associated with road densities that were larger than 1.0 km/km2. The only exception was with male adult bears, as their mortalities occurred across all road densities.

Regarding cub survival the survival rate was estimated to be 0.55. There were 24 adult females with a total of 53 cubs and 17 mortalities that were documented. Two out of the 17 cases was due to the mother bear dying and therefore the cubs couldn’t survive either.

The estimate of reproduction rate was 2.11 cubs per female per year with 25.8% of all cubs being female resulting in a reproductive rate of 0.272 female cubs per female adult per year. Females with cubs or yearling had a higher mortality risk than females without any cubs or two year olds. However, females in general had a higher survival rate than males.

In the demographic model adult females displayed different survival rates depending on their offspring. A female with cubs or yearlings had a survival rate of 0.90 at a road density of 0.75 km/km2, while a female without any cubs or two year olds had a survival rate of 0.97. If road density measures 1.25 km/km2 and the reproductive state was not considered, adult females had a survival rate of 0.95 in contrast to subadult females with a survival rate of 0.83.

Discussion

Road density does not only affect demography and trend of bear populations but also adds risk to reproduction. In order for the population to not decrease one has to not only consider the survival rate but also the reproductive state. With consideration of the reproductive state the threshold for road density will be reduced.

The mortality risk near watershed areas outside the mountainous zones needs to be managed, since the population has a lower chance to increase in these areas without proper management. Most core areas are within areas of lower road density and about 57% of all secondary areas are within areas with road densities starting at 0.75 km/km2 and going up to 1.25 km/km2 or higher. The thresholds are therefore 0.75 km/km2 if a different survival rate for females with cubs is assumed or 1.25 km/km2 if the survival rate is considered the same for both females with and without cubs. However, 0.75 km/km2 will ensure a more stable grizzly bear population.

When estimating reproduction rates it is important to also include female bears of the age of three as they can already produce cubs before turning four.

The study also found that fragmentation can be a result of high mortality on gravel roads or larger paved highways that bisect parts of the province. It is assumed that also traffic volume has an effect on mortality rate and therefore needs to be monitored and managed on resource roads.

It is also suggested to restrict access to roads in areas where the population trends fall below 1.0 in the demographic model.

The reason why the mortality risk for females with cubs is higher than for females without cubs is unknown. Assumptions suggest that it might be due to the fact that females with cubs occur closer to roads because male bears try to avoid them. Another reason could be forage availability. For that reason, the mortality risk for females with cubs might be higher because they stay closer to roads than other grizzly bears.

Conclusion

Road access into grizzly bear habitat has a large impact on the populations. The study focused on finding a direct effect of roads on grizzly bear through collecting data of collared bears and creating a demographic model. Two threshold estimates for road density were determined, which are 0.75 km/km2 if it is assumed that females with cubs have a higher mortality rate due to roads and 1.25 km/km2 if mortality rates are assumed to be similar for females with or without cubs.

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Source Boulanger J, Stenhouse GB (2014) The Impact of Roads on the Demography of Grizzly Bears in Alberta. PLoS ONE 9(12): e115535. doi:10.1371/journal.pone.0115535