Forecasting the relative influence of environmental and anthropogenic stressors on polar bears

Abstract

Effective conservation planning requires understanding and ranking threats to wildlife populations. We developed a Bayesian network model to evaluate the relative influence of environmental and anthropogenic stressors, and their mitigation, on the persistence of polar bears (Ursus maritimus). Overall sea ice conditions, affected by rising global temperatures, were the most influential determinant of population outcomes. Accordingly, unabated rise in atmospheric greenhouse gas (GHG) concentrations was the dominant influence leading to worsened population outcomes, with polar bears in three of four ecoregions reaching a dominant probability of decreased or greatly decreased by the latter part of this century. Stabilization of atmospheric GHG concentrations by mid‐century delayed the greatly reduced state by ≈25 yr in two ecoregions. Prompt and aggressive mitigation of emissions reduced the probability of any regional population becoming greatly reduced by up to 25%. Marine prey availability, linked closely to sea ice trend, had slightly less influence on outcome state than sea ice availability itself. Reduced mortality from hunting and defense of life and property interactions resulted in modest declines in the probability of a decreased or greatly decreased population outcome. Minimizing other stressors such as trans‐Arctic shipping, oil and gas exploration, and contaminants had a negligible effect on polar bear outcomes, although the model was not well‐informed with respect to the potential influence of these stressors. Adverse consequences of loss of sea ice habitat became more pronounced as the summer ice‐free period lengthened beyond four months, which could occur in most of the Arctic basin after mid‐century if GHG emissions are not promptly reduced. Long‐term conservation of polar bears would be best supported by holding global mean temperature to ≤ 2°C above preindustrial levels. Until further sea ice loss is stopped, management of other stressors may serve to slow the transition of populations to progressively worsened outcomes, and improve the prospects for their long‐term persistence.

Conservation status of polar bears (Ursus maritimus) in relation to projected sea-ice declines

“…Our projections (table 1) are broadly consistent with expert opinion [12] and Bayesian network model forecasts [5], although methodological differences preclude direct comparison (see electronic supplementary material). Following the Red List guidelines for risk tolerance ([14]; §3.2.3), the high probability of reductions more than 30% in MGPS, and low probability of reductions more than 50%, were consistent with a categorization of vulnerable (i.e.…”

“…All approaches assumed that changes in N were mediated primarily through changes in K or density-independent habitat effects, and that the ratio N / K was stable relative to other factors [17]. These assumptions were established on the basis that polar bears depend fundamentally on sea ice, that sea-ice changes represent the main source of habitat modification for the species [5], and that other potential stressors are either secondary (e.g. contaminants; [5]) or have been managed (e.g.…”

“…These assumptions were established on the basis that polar bears depend fundamentally on sea ice, that sea-ice changes represent the main source of habitat modification for the species [5], and that other potential stressors are either secondary (e.g. contaminants; [5]) or have been managed (e.g. harvest; [6]) for most subpopulations in recent decades.…”

“…Methods to forecast the effects of continued sea-ice loss on polar bears have included structured elicitation of expert opinion [12], Bayesian network models evaluating cumulative stressors [5], demographic projections for individual subpopulations [8] and mechanistic models linking vital rates to environmental factors [13]. To date, there has been no global assessment of polar bear abundance data relative to sea ice.…”

Harvesting wildlife affected by climate change: a modelling and management approach for polar bears

“…) is expected to negatively affect polar bears throughout much of their range, because the species depends fundamentally on sea ice for access to its primary prey (Atwood et al . ). Management and conservation planning will therefore require methods to consider both current population status as well as the anticipated effects of habitat loss.…”