Image Credit: US Fish and Wildlife Service Headquarters, CC BY 2.0, Image Cropped

Are polar bear habitat resource selection functions developed from 1985-1995 data still useful? (2019) Durner et. al, Ecology and Evolution, https://doi.org/10.1002/ece3.5401

The Crux

Ecologists often attempt to predict where species are using the spread of the resources that the species depends upon. This is done because often it’s simply easier to monitor the resources than the species. Resource selection functions (RSFs) are a tool which use the likelihood of a resource being used to predict a species distribution. However, if the landscape the resource is found in changes drastically, a resource selection function may start to be less useful.

In the early 2000s, using data collected in the 80s and 90s, US scientists developed RSFs for polar bears, a species which has regrettably become the poster child for the survival of the Arctic ecosystem. Even back then, the bears’ preferred habitat was receding. Now, with human-driven climate change severely reducing sea ice and markedly altering the bears’ habitat, this week’s authors wanted to know how well those RSFs work nowadays.

What They Did

The initial RSFs were published by Durner et al. in 2009, and utilised environmental variables which included ice concentration, ocean depth, distance to land, and distance to an area with 15% ice concentration. The RSFs worked across the four seasons, although in this model the seasons were considered to be the periods of ice melt, minimum ice coverage, ice growth, and maximum ice coverage. The 1985-1995 data was used as the baseline, with the two subsequent decades compared to this data to see how well the RSFs still applied.

There are 19 recognised subpopulations of polar bears in the Arctic region, and this project used data for 4 of them, ranging from 1985 until 2016. Trackers were placed on adult female polar bears in this time to get an estimate of polar bears’ range.

Did You Know: Problems with the Polar Bear

The polar bear has long been a flagship species for the Arctic. But is tying the success of an entire ecosystem to one species a good idea? There’s some evidence that polar bear populations are capable of adapting, with new prey sources being used. What happens if it turns out that the polar bear does just fine? Does investment in solving the enormous problems in the Arctic ecosystem drop? I’ve written more comprehensively on this before, so I advise you to check out the link below.

Pinning it on the Polar Bear: Problems with Flagship Species

What They Found

Since 1995, the proportion of time polar bears spent in the areas that the RSFs declared to be optimal habitat has decreased. It has most significantly decreased during the periods of ice melt and minimum ice coverage, the two periods when the bears are the most vulnerable. This pattern also followed for the proportion of optimal habitat that made up the polar bears’ home ranges. Especially in the most recent decade, the polar bear habitat shows very little overlap with what the RSF considers to be the optimal habitat.

Problems

As mentioned before, there are 19 different subpopulations of polar bears. The four used here were clustered fairly tightly together (considering how large the population’s total range is). So it’s hard to know how well the data here applies to all 19, and if new RSFs were to be produced, how well they would apply to all 19. Unfortunately, nothing short of a huge international project would solve this particular issue.

So What?

The validity of the original RSFs used have clearly weakened over the past two decades. This isn’t too much of a surprise, as polar bears have recently been forced to expand their range and use habitats that they normally wouldn’t consider. What is interesting is that when the ice was at its maximum, the RSFs were quite accurate. Unfortunately, with climate change not reversing any time soon, using a model which is accurate in the best-case scenario won’t be too useful.