Ongoing climate change is a threat to biodiversity; although many species will persist. In past climate cycles, many species responded spatially, and shifted their ranges to stay within suitable climate conditions. In warming conditions many species ranges shift higher in elevation or poleward. Tracking upslope movement of suitable conditions can reduce the range sizes for montane species, potentially leading to extirpations of populations. However, not all species will respond the same to changing conditions. Species with different ecological niches will likely exhibit distinct responses to a changing environment, including the extent and direction of spatial responses. Furthermore, differences in the magnitude of niche specialization also indicate which species may be more vulnerable to environmental change. Although climate is fundamental in limiting where a species can persist, climate alone is not the sole determinant of distribution. Therefore, efforts to conserve species in a changing climate benefit from a thorough understanding of the habitat requirements of each species. Patterns of habitat selection can change over both space and time, and at different spatial scales of habitat measurement. Consequently, multiple spatial and temporal scales must be evaluated to accurately assess habitat requirements for each species, and the variation in those requirements.
Climate change has been especially pronounced in high-elevation regions such as the Sierra Nevada, California; accordingly, wildlife in these regions may be particularly vulnerable. Therefore, we characterized the niche space, evaluated variability in habitat use, and projected the range shifts under climate change for multiple small-mammals that are sympatric in much of the Sierra Nevada. Our study species included three ground-dwelling squirrels: yellow-bellied marmot (Marmota flaviventer), Belding’s ground squirrel (Urocitellus beldingi), golden-mantled ground squirrel (Callospermophilus lateralis), and a small lagomorph, American pika (Ochotona princeps). Observations of individual animals were collected from five consecutive years (2008-2012) of visual encounter surveys along 21 10-km transects. The study area transects were distributed over a 320-km latitudinal gradient, covering most of the alpine and subalpine region of the Sierra Nevada.
First, we characterized niche space for the ground-dwelling squirrels, to quantify which ecogeographical variable types (climate, topography, or landcover) were most important in defining the niche of each species. We conducted Ecological Niche Factor Analysis (ENFA) to quantify the niche using these ecogeographical variables to generate indices of ‘marginality’ (magnitude of selection) and ‘specialization’ (narrowness of niche space). All three species demonstrated differential use of niche space, through both marginality and specialization, when compared to the available niche space. Moreover, the relative importance of the variables shaping the niche differed among these species. The niche breadth of these three species was also positively associated with geographic range size. Mammals in high-elevation mountain systems often are perceived as vulnerable to climate shifts, but our results underscore the importance of also including non-climate-based factors in defining the niche of these species. The overall magnitude of niche selection for all three species was driven by a combination of topographic, climatic, and landcover factors; thus, efforts to forecast areas where these species can persist in the future need to evaluate from more than just a climatic perspective.
Next, we evaluated habitat selection at two spatial levels (extents) and two spatial scales (resolutions), across a temporal span of five years (2008-2012) for all four montane mammal species. Habitat availability was based either on visual characterization of habitat at each animal encounter (micro-scale) or from a spatial database (macro-scale), and selection was analyzed at two levels, the transect and the region. Belding’s ground squirrels selected meadow habitat across all levels and scales. Yellow-bellied marmots most frequently selected for meadows; however, other habitat types, such as rocks or shrubs, were sometimes selected for depending on the level and scale. Golden-mantled ground squirrels exhibited variable selection, but they most often selected for meadow or conifer. American pika strongly selected for rocks at the micro-scale, but this pattern was obscured at the macro-scale. Habitat selection for all four species varied by both level and scale, and across years, with stronger and more consistent selection often occurring at the micro-scale. Our results highlight the importance of documenting habitat use through direct observations, as relying solely on spatial databases can obscure the importance of essential habitat types.
Finally, we modeled the current (time of the survey) and future (2070) distribution of suitable areas for all four species using a Random Forest model. Our future predictions showed both gains and losses to suitable area for these species in the Sierra Nevada, with important differences among species. Both golden-mantled ground squirrel and yellow-bellied marmot were predicted to have a net gain of suitable area, while Belding’s ground squirrel and American pika were predicted to experience a net loss. Yellow-bellied marmot, golden-mantled ground squirrel, and American pika all had gains of suitable area, primarily along the high-elevation crest of the Sierra Nevada. Belding’s ground squirrel was predicted to lose suitable area overall, and that loss was primarily along the crest. If climate change continues along the current trajectory, the future of Belding’s ground squirrel in the Sierra Nevada is in peril.
|Advisor:||Van Vuren, Dirk H.|
|Commitee:||Kelt, Douglas A., Hijmans, Robert J.|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-A 82/5(E), Dissertation Abstracts International|
|Subjects:||Wildlife Conservation, Ecology, Climate Change, Regional Studies, Environmental management|
|Keywords:||Callospermophilus lateralis, Distribution, Marmota flaviventer, Ochotona princeps, Urocitellus beldingi, Sierra Nevada, High elevation, Habitat use, Biodiversity, California, Alpine, Subalpine, Mountain ranges|
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