Influence of whitebark pine decline on fall habitat use and movements of grizzly bears in the Greater Yellowstone Ecosystem

Abstract

When abundant, seeds of the high-elevation whitebark pine (WBP; Pinus albicaulis) are an important fall food for grizzly bears (Ursus arctos) in the Greater Yellowstone Ecosystem. Rates of bear mortality and bear/human conflicts have been inversely associated with WBP productivity. Recently, mountain pine beetles (Dendroctonus ponderosae) have killed many cone-producing WBP trees. We used fall (15 August–30 September) Global Positioning System locations from 89 bear years to investigate temporal changes in habitat use and movements during 2000–2011. We calculated Manly–Chesson (MC) indices for selectivity of WBP habitat and secure habitat (≥500 m from roads and human developments), determined dates of WBP use, and documented net daily movement distances and activity radii. To evaluate temporal trends, we used regression, model selection, and candidate model sets consisting of annual WBP production, sex, and year. One-third of sampled grizzly bears had fall ranges with little or no mapped WBP habitat. Most other bears (72%) had a MC index above 0.5, indicating selection for WBP habitats. From 2000 to 2011, mean MC index decreased and median date of WBP use shifted about 1 week later. We detected no trends in movement indices over time. Outside of national parks, there was no correlation between the MC indices for WBP habitat and secure habitat, and most bears (78%) selected for secure habitat. Nonetheless, mean MC index for secure habitat decreased over the study period during years of good WBP productivity. The wide diet breadth and foraging plasticity of grizzly bears likely allowed them to adjust to declining WBP. Bears reduced use of WBP stands without increasing movement rates, suggesting they obtained alternative fall foods within their local surroundings. However, the reduction in mortality risk historically associated with use of secure, high-elevation WBP habitat may be diminishing for bears residing in multiple-use areas.

Density dependence, whitebark pine, and vital rates of grizzly bears

“…If bears were responding to a decline in carrying capacity, however, we would have expected home‐range size and movements to have increased (McLoughlin et al ), bears to have relied on lower‐energy food resources (McLellan ), and body condition to have declined as a consequence (Rode et al , Robbins et al , Zedrosser et al ). To date, there is little support for these conditions in the Yellowstone Ecosystem: female home ranges have decreased in size and are less variable in areas with greater bear densities (Bjornlie et al ), daily movement rates and daily activity radii have not changed for either sex during fall (Costello et al ), bears continue to use high‐quality foods (Fortin et al ), and body mass has not declined (Schwartz et al ). As we discussed previously, percent body fat among adult females has not declined since the early 2000s (IGBST , Schwartz et al ) and, regardless, this effect would be consistent with either interference or exploitation competition and would not explain the changes in vital rates that occurred much earlier than the declines in foods.…”

“…As we discussed previously, percent body fat among adult females has not declined since the early 2000s (IGBST , Schwartz et al ) and, regardless, this effect would be consistent with either interference or exploitation competition and would not explain the changes in vital rates that occurred much earlier than the declines in foods. Current evidence indicates bears showed a functional response to declines in whitebark pine (Costello et al ) and cutthroat trout (Fortin et al ) and compensated for the loss of these particular foods through diet shifts (Schwartz et al ).…”

“…In fact, recent analyses indicate annual cone production is still associated with the annual frequency of grizzly bear mortalities during 2000–2012 (IGBST :24). Even heavily affected stands may contain surviving trees that produce cone crops and grizzly bear use of whitebark pine seeds in such stands has been observed (Orozco and Miles ), although Costello et al () detected a downward trend in fall selection of whitebark pine habitats during 2000–2012. Thus, on an annual basis, good whitebark pine cone production may still have a positive effect on survival of independent bears.…”

Grizzly bear depredation on grazing allotments in the Yellowstone Ecosystem

“…Grazing allotments with whitebark pine, relatively high primary productivity, and open herbaceous forage patches represent high‐quality grizzly bear habitat and therefore have a greater probability of grizzly bear use and interactions with livestock. For example, where whitebark pine is present, grizzly bears will select for whitebark pine habitats from approximately 15 August to 30 September, even in years of poor cone production (Costello et al ). We note, however, that the lack of a relationship between depredation counts and whitebark pine cone production does not support the notion that cattle depredation provides an alternative food source in years of poor cone production.…”

“…We used a spatially explicit index of annual grizzly bear density to represent bear numbers in and around allotments (14 Â 14-km grid cells; Bjornlie et al 2014b). Because many depredations in the GYE have occurred in areas of grizzly bear population expansion, we calculated the absolute distance of cells from the edge of occupied grizzly bear range during 1990-2000and 2000-2014(Bjornlie et al 2014a). Lastly, for each environmental attribute each year of the study period (at both spatial extents), we calculated the average value of cells within grazing allotment boundaries using the zonal statistics spatial analyst tool and used these values as the independent variables in our models.…”