Predicting future grizzly bear habitat use in the Bitterroot Ecosystem under recolonization and reintroduction scenarios: spatial data

Publication Date

2024-07-10

Start Date

2003-05-01

End Date

2023-12-31

Sells, S.N., and Costello, C.M., 2023, Grizzly Bear Space Use in the US Northern Rocky Mountains (ver. 3.0, July 2024): U.S. Geological Survey data release, https://doi.org/10.5066/P91EWUO8.

Understanding habitat selection and potential for connectivity is challenging but key for species of conservation concern, including grizzly bears (Ursus arctos). Over the past century, persecution and habitat loss led to near extirpation of grizzly bears throughout most of their range in the continental United States. Given threats facing the remnant populations, grizzly bears in the continental U.S. were listed as Threatened under the Endangered Species Act in 1975. Six recovery zones, located in different ecosystems, were established in the 1990s and populations slowly increased in four of these areas in the years following listing. Recent numbers included >1,000 individuals each in the Northern Continental Divide Ecosystem (NCDE) and the Greater Yellowstone Ecosystem (GYE), >44 individuals on the U.S. side of the Selkirk Ecosystem (SE), and >50 individuals in the Cabinet-Yaak Ecosystem (CYE). However, no resident grizzly bears are known to currently occupy the Northern Cascades Ecosystem (NCE) or the BE. To assist with recovery planning for the BE, our goal was to predict habitat use within the BE region under scenarios of natural recolonization and reintroduction. Given that naturally recolonizing or reintroduced carnivores, especially habitat generalists like grizzly bears, often select habitats over a broader landscape than designated core areas, a primary aim was to evaluate the relative distribution of selected habitat across recovery area boundaries, ownerships, and jurisdictions. To achieve our study objective, we built on a larger study initiated in Sells et al. (2022). In this first phase, the authors developed integrated step selection functions within currently occupied range in the NCDE to better understand how grizzly bears use habitat. Subsequent application of these models to the NCDE demonstrated high predictive power. In a second phase, Sells et al. (2023a) demonstrated that models developed for the NCDE accurately predicted habitat use in nearby populations and were therefore expected to be transferable and reliable for predicting space use beyond the NCDE. In a third phase, Sells et al. (2023b) demonstrated that multiple simulation methods could help predict connectivity pathways. This present study builds on these first three phases to provide further insight into the potential for advancing conservation and recovery of grizzly bears in the BE. We approached this study through two simulation methods to represent natural recolonization versus a reintroduction of grizzly bears to the BE. All BE simulations predicted that habitat use by grizzly bears would be higher in the northern half of the study area. Under the natural recolonization scenario, use was concentrated in Montana, but became more uniform across the northern BE in Idaho over time. Use was more concentrated in central-east Idaho under the reintroduction scenario. Assuming that natural recolonization continues even if bears are reintroduced, use remained widespread across the northern half of the BE and surrounding areas. Predicted habitat maps for the natural recolonization scenario aligned well with outlier and GPS collar data available for grizzly bears in the study area, with Spearman rank correlations of ≥0.93 and mean class values of ≥9.1 (where class 10 was the highest relative predicted use; each class 1 – 10 represented 10% of the landscape). In total, 52.4% of outlier locations and 79% of GPS collar locations were in class 10 in our predicted habitat maps for natural recolonization. Simulated grizzly bears selected habitats over a much larger landscape than the BE itself under all scenarios, including multiple-use and private lands, similar to existing populations that have expanded beyond recovery zones. This highlights the importance of recognizing and planning for the role of private lands in recovery efforts, including understanding resources needed to prevent and respond to human-grizzly bear conflict and maintain public acceptance of grizzly bears over a large landscape. Study results are presented as rasters of relative predicted habitat use. In these datasets, all results are summarized and quantile binned as classes 1 (lowest relative predicted use) - 10 (highest relative predicted use). Each class is represented by 10% of the study area. The maps can be clipped down as desired to view details of specific areas. (Note: subsetting the maps will mean that the classes no longer each represent 10% of the landscape.) Our predictive maps can facilitate on-the-ground application of this research for prioritizing habitat conservation, human-bear conflict mitigation, and transportation planning.

• Cooperative Fish and Wildlife Research Units

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