Human-mediated shifts in animal habitat use: Sequential changes in pronghorn use of a natural gas field in Greater Yellowstone

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Abstract

To manage America’s 991,479 km2 (245 million acres) of public BLM lands for such mixed uses as natural resource extraction, wildlife, and recreation requires knowledge about effects of habitat alterations. Two of North America’s largest natural gas fields occur in the southern region of the Greater Yellowstone Ecosystem (Wyoming), an area that contains >100,000 wintering ungulates. During a 5-year period (2005–2009), we concentrated on patterns of habitat selection of pronghorn (Antilocapra americana) to understand how winter weather and increasing habitat loss due to gas field development impact habitat selection. Since this population is held below a food ceiling (i.e., carrying capacity) by human harvest, we expected few habitat constraints on animal movements – hence we examined fine-scale habitat use in relationship to progressive energy footprints. We used mixed-effects resource selection function models on 125 GPS-collared female pronghorn, and analyzed a comprehensive set of factors that included habitat (e.g., slope, plant cover type) and variables examining the impact of gas field infrastructure and human activity (e.g., distance to nearest road and well pad, amount of habitat loss due to conversion to a road or well pad) inside gas fields. Our RSF models demonstrate: (1) a fivefold sequential decrease in habitat patches predicted to be of high use and (2) sequential fine-scale abandonment by pronghorn of areas with the greatest habitat loss and greatest industrial footprint. The ability to detect behavioral impacts may be a better sentinel and earlier warning for burgeoning impacts of resource extraction on wildlife populations than studies focused solely on demography. Nevertheless disentangling cause and effect through the use of behavior warrants further investigation.

Highlights

► One polarizing challenge is how best to manage 991,479 km2 of public BLM lands. ► Two large natural gas fields occur in the Upper Green River Basin of Wyoming. ► We focus on habitat selection using mixed-effects RSF models. ► Evidence of a fivefold sequential decrease in patches predicted to be of high use. ► Sequential fine-scale abandonment of crucial winter range by pronghorn.

Introduction

One of America’s most vexing and polarizing challenges is how best to manage the 991,479 km2 (245 million acres) of Bureau of Land Management (BLM) public lands established for multiple uses such as natural resource extraction, wildlife, and recreation. The intersection between energy development and biological conservation affords opportunities both to gather knowledge and to implement findings about how to mitigate impacts to wildlife. As the footprint of human development continues to expand globally into regions that have historically supported abundant wildlife resources, there will be even more pressing needs for long-term data sets, in conjunction with baseline data, to examine changes in life history parameters and behavioral processes.

Western North America contains abundant natural resources, including wildlife populations that still undergo spectacular processes like long-distance migration, a globally-imperiled ecological phenomenon (Berger et al., 2006). Unfortunately, conflicts often arise because the harvest of natural resources is not always compatible with maintaining wildlife populations, thus necessitating choices. These decisions are often contentious because interest groups have vastly different priorities. This puts policy-makers and wildlife managers in the position of needing to make informed decisions about trade-offs, as they attempt to balance the needs of wildlife against people’s desire for energy independence. Ecological theory can guide policy-makers and wildlife managers. For instance, we know from studies based on carrying capacity theory, that a reduction in habitat will ultimately lead to a decline in population size, or when extreme a local extirpation. However, because carrying capacity is not static, and is determined by the complex interplay of many factors (e.g., weather, human-footprints on the landscape), identifying thresholds is difficult. Often the challenge is further complicated by a lack of baseline data on wildlife populations’ behavior or demography against which to assess short-term fluctuations.

Large-scale natural resource extraction has the potential to impact animal movements, habitat use and associated behavior, demography, and population trends (e.g., Bradshaw et al., 1997, Joly et al., 2006, Sawyer et al., 2006, Vistnes and Nellemann, 2008), that includes such North American icons as, caribou (Rangifer tarandus; Bradshaw et al., 1997, Cameron et al., 2005, Noel et al., 2004), greater sage grouse (Centrocercus urophasianus; Copeland et al., 2009, Walker et al., 2007), and mule deer (Odocoileus hemionus; Sawyer et al., 2009). At a broader scale, effects of natural resource extraction span all continents and ecosystems and vary from deserts to tropical forests and polar regions (Contreras-Hermosilla, 1997, Joly et al., 2006, Peres and Lake, 2003).

How wildlife populations respond to increasing human-footprints and habitat loss in the form of energy infrastructure is oftentimes determined primarily by how close the population of concern is to its food ceiling (i.e., carrying capacity; see Stewart et al., 2005). If, for example, a population approaches its habitat’s food ceiling, then impacts of habitat loss and fragmentation may be immediately revealed through either behavior (i.e., habitat or resource selection patterns) or demographic (e.g., changes in survival) responses or both simultaneously. Further, if a population is maintained below its food supply by harvest but its’ habitat is substantially squeezed or fragmented over time, it seems reasonable to expect changes in its spatial ecology even if sufficient food remains available. Mule deer, for example, respond to energy footprints although presumptively held below a given habitat’s food resources (see Sawyer et al., 2009). With respect to the Upper Green River Basin (UGRB) in western Wyoming, we explored the extent to which increasing energy development affected several correlates of pronghorn (Antilocapra americana) biology with a specific emphasis on spatial ecology. Beyond habitat loss and human harvest however, weather exerts strong direct effects on animal movements (Hebblewhite et al., 2005). For species like pronghorn, deep snow may exacerbate risks brought on by habitat loss associated with energy field development.

Two of the largest gas fields in the lower 48 USA (i.e., the Pinedale Anticline Project Area (PAPA) and Jonah Fields, see Fig. 1) occur in the wintering home range of America’s longest terrestrial migrant – pronghorn of the Greater Yellowstone Ecosystem (Berger et al., 2006). This is significant because Wyoming contains an estimated 400,000 of the world’s approximately 700,000 pronghorn and the UGRB herd represents one of the largest in the state (Grogan and Lindzey, 2007, Hoffman et al., 2008).

Our primary study questions are therefore aimed at understanding the interplay of snow and industrial development on habitat selection by pronghorn in this area of extreme energy development. Given that pronghorn are likely kept below their food ceiling (see Stewart et al., 2005) through annual harvest of a mean of 2477 ± 701 pronghorn (e.g., from 2001 to 2009 in Hunt Units 86–91 in the Sublette Herd in the Upper Green River Basin; Wyoming Game and Fish Department, unpub. data), we expected habitat to be a non-limiting factor. If true, then observed resource selection responses of pronghorn to gas field infrastructure may fall below detectable levels (Fig. 2). This annual level of harvest for these six hunt units is over a 4000 km2 area where we conducted monthly distribution flights during the winters of 2005–2010 for which we never counted more than 6500 total animals (unpub. data). As a consequence, we assume that such relatively high human harvest maintained pronghorn below a point at which body condition would be affected by intra-specific competition for food. On the other hand, variation in snow depth in the UGRB and elsewhere (Martinka, 1967) is a key driver of winter movements and food availability. Consequently, we hypothesized that an increasing human-footprint from gas field development over time would sequentially lower the food ceiling through habitat loss independent of effects of snow depth. Our conceptualized interactions among snow, human harvest, and energy-induced habitat loss are depicted in Fig. 2.

There are two general predictions that stem from current BLM and industry proposals to reduce native habitats by 5–14% (BLM, 2006, BLM, 2008): (1) given the harvest-related limitations on population size, the UGRB landscape will retain enough crucial winter range and therefore pronghorn will respond in ways reflecting no biological impacts (i.e., patterns of resource selection will not vary), or (2) pronghorn will show heightened sensitivity to increasingly degraded habitats (i.e., pronghorn will avoid or select against areas in which density of well pads and roads have exceeded a threshold). These dual scenarios enable opportunities to examine fine-scale movements in relation to progressive habitat change while accounting for effects of snow and other variables.

To understand pronghorn use of winter range, we estimated both individual- and population-level resource selection responses to habitat loss, fragmentation, and human activity associated with gas field development and infrastructure using mixed-effects resource selection function (RSF) models to determine which factors influence pronghorn habitat use in gas fields during winter. Further, we examined pronghorn response to gas field development over a 5-year time frame to understand how varying and increasing densities and scale of development and infrastructure impact pronghorn habitat use on their crucial winter range.

Section snippets

Study area

The primary study area within the UGRB was the PAPA and Jonah gas fields (Fig. 1) where elevations range from 2100 to 2800 m. The larger of the two gas fields is the 80,127 hectare (198,000-acre) region designated as the PAPA, while the smaller 12,140 hectare (30,000-acre) Jonah Field is adjacent to the PAPA to the south (Fig. 1). At the end of 2009, 1713 wells had been drilled in the PAPA and 1623 wells had been drilled in the Jonah. However, less than 3% of the physical habitat in the PAPA and

Habitat loss

Disturbance due to development in the gas fields has increased annually. In 2005, habitat loss due to construction of well pads was 9.9 km2 in the PAPA and 11.0 km2 in the Jonah. In 2009, habitat loss due to construction of well pads in the PAPA and Jonah had increased to 12.7 km2 and 14.8 km2, respectively. Over this 5-year span, the total amount of habitat loss due to well pad construction in the PAPA increased by 28.7% and in the Jonah by 34.1%.

Habitat loss in the PAPA from 2005 to 2009 due to

Discussion and conclusions

True impacts of increasing infrastructure, habitat loss, and fragmentation to extract natural resources may be masked or dampened for populations held below a region’s ecological food ceiling by hunting. However, if populations maintained below a food ceiling respond to habitat loss and fragmentation, then we can infer impacts from development for resource extraction can be substantial. When impacts from natural resource extraction on such populations are masked or delayed, then threshold

Acknowledgements

We thank Advanced Telemetry Systems (J. Rosenberg, J. Roth), Jensen Air (T. Jensen), the National Park Service (S. Cain), Quicksilver Air (R. and S. Swisher, P. Johnson, J. Zachowski, D. Rivers, L. Shelton), and, for funding, Shell Exploration and Production Company (A. Davison, J.R. Justus, J. Bickley, D. McMullen, F. Palmer, D. Sinclair), Ultra Petroleum (B. Salinas, C. McKee), and Questar (P. Guernsey). We also thank Sky Aviation (D. Stinson, K. Overfield) and Skytruth (J. Amos). The

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