The Executive Summary on the status of the polar bear was just released to the US Fish and Wildlife Service on Sept. 7, and you can read it here:
http://www.usgs.gov/newsroom/special...ve_summary.pdf
As many of us here are aware, the USFWS proposed to list the polar bear as a threatened species under the Endangered Species act in January of this year. At that time they ordered the USGS to assemble the best science available on the viability of the polar bear population to inform recovery efforts. This Executive Summary is that report.
It is not good news. Here are some excerpts:
Quote:
|
Projected changes in future sea ice conditions, if realized, will result in loss of approximately 2/3 of the world's current polar bear population by the mid 21st century. Because the observed trajectory of Arctic sea ice decline appears to be underestimated by currently available models, this assessment of future polar bear status may be conservative.
|
Their predictions are derived from a simple calculation of the environmental carrying capacity for the bear population:
Quote:
1. We divided the range of the polar bear into 4 ecoregions based on major differences in current and projected sea ice conditions. These “ecoregions” were the:
-- Seasonal Ice Ecoregion which includes Hudson Bay, and occurs mainly at the southern extreme of the polar bear range,
-- Archipelagic Ecoregion of the Canadian Arctic,
-- Polar Basin Divergent Ecoregion where ice is formed and then drawn away from near-shore areas, especially during the summer minimum ice season, and
-- Polar Basin Convergent Ecoregion where sea ice formed elsewhere tends to collect against the shore.
Dividing the range of the polar bear into these 4 ecoregions allowed us to make inferences from available knowledge about subpopulations in each ecoregion to the entire ecoregion.
12. Ultimately, we projected a 42% loss of optimal polar bear habitat during summer in the polar basin by mid century.
13. Due to unavailability of telemetry data showing habitats chosen by polar bears in the archipelagic and seasonal sea ice ecoregions, we were unable to project habitat changes in these ecoregions for this analysis.
14. Using a simple deterministic model of future carrying capacity for polar bears, we forecasted that polar bears could be extirpated in the divergent ice ecoregion within 75 years, assuming that sea ice decline follows the mean trajectory predicted by the 10 models we used. If sea ice decline follows the minimum trajectory predicted, extirpation in this ecoregion could occur by year 45.
15. Using the carrying capacity model, we projected populations of polar bears in all other ecoregions to decline at all time steps, with severity of decline dependent upon whether minimum, maximum or mean ice projections were used. The only exception was a slight, temporary, increase in the polar basin convergent ice ecoregion for the 45 year timestep and the maximum ice scenario.
16. Based on a first-generation Bayesian Network model incorporating a range of factors affecting polar bears, we forecasted extirpation of polar bear populations in the seasonal sea ice and the polar basin divergent ecoregions by 45 years from present.
17. We forecasted extirpation of polar bear populations in the polar basin convergent ecoregion by 75 years from present. In the archipelagic ecoregion, polar bears could occur through the end of the century, but in smaller numbers than now.
|
This is a very optimistic analysis, as it ignores recent data that suggest the sea ice is disappearing at a much faster rate than assumed here, and it completely ignores other factors that can cause extinction such as genetic inbreeding and random drift, demographic stochasticity, environmental stochasticity, and hybridization with the grizzly bear. Even if a few polar bears are theoretically capable of subsisting on the dwindling habitat, the probability that they are wiped out by disease or chance climate fluctuation or just bad luck during any given year will be very high.
I think that by mid century, and perhaps well before that time, the population will be mostly genetically subsumed by the grizzly bear, since they seem to be hybridizing more and more frequently as the grizzlies migrate ever more northward with the warming climate.