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Spatial and Temporal Variation in Mammalian Diversity of the Colorado Plateau (USA)


Anticipating how species and ecosystems will react to continued climate change is of critical importance to biodiversity conservation and to management of the ecosystem processes on which we rely. Identifying how individual species in a community have responded in the past can be accomplished by evaluating the fossil record on a local and/or regional scale, and by examining spatial patterns of modern abundance and diversity. Here, I explore regional patterns of mammal diversity across the Colorado Plateau (CP), examine local small mammal diversity fluctuations in fossil deposits from northern San Juan County, Utah, through the late Holocene, and assess modern spatial diversity patterns across a range of San Juan County sites.

Over the past century, extraordinary global transformations have taken place, including climate change and land conversion for human use. With these unusually rapid and extreme global changes underway, it is increasingly important to understand the extent to which designated conservation areas have protected biodiversity thus far, and to gauge their potential for continuing to do so in the future. Chapter 1 examines the efficacy of biodiversity preservation in National Park Service (NPS) lands of the CP by using analysis of nested species assemblages to compare surveyed mammal communities to the range map predictions of which species should be present. I find that NPS lands are nested, and, although site diversity is correlated with area, elevational range, budget and visitation, a comparison between species lists compiled from surveys versus range map distributions reveals that the biogeographic patterns prevailing today cannot be distinguished from those prevailing when these NPS sites were established. These broad-stroke patterns define an important context in which to direct future conservation efforts as we attempt to divert and mitigate anthropogenic impacts—past, current, and future.

Modern patterns of diversity are the result of past events and processes that take place on the scale of decades, centuries, and millennia. Detailed paleoecological records from Quaternary deposits are remarkably useful in characterizing these long-term ecological dynamics, but only a handful of Quaternary localities that sample the small mammal community of the CP have been studied to date. In chapter 2, I describe my excavation and analyses of two fossil-bearing alcoves, East Canyon Rims 2 (ECR2) and Rone Bailey Alcove (RBA) (San Juan County, Utah), and quantify diversity and abundance change of the small mammal community as recorded in the fossil samples. Fossil localities with comparable mammal diversity have not been reported from this region previously, so these sites provide novel insight into Holocene mammal diversity in southeastern Utah. Further, these localities contribute to our understanding of natural variation in this system by providing faunal data for a period of recent climate change—cool-wet to warm-dry. AMS radiocarbon dates on 33 bone samples from these sites span ~4.4 ka-present, and shed light on pre-industrial faunal dynamics in the region over the course of environmental change, most notably aridification. I test for an effect of climate on community evenness and relative abundance of 10 small mammal taxa—leporids, perognathines, small sciurids, arvicolines, Cynomys, Neotoma, Dipodomys, Onychomys, Peromyscus, and Thomomys—and find that, in spite of considerable increases in aridity and temperature, neither is significantly correlated with relative abundance or evenness when statistically tested, but there are qualitative patterns consistent with a response to increasing aridity around 1000 years ago.

The CP is home to a diverse complement of species that are experiencing increasing temperature and drought stress today. Understanding how mammal communities might be expected to respond to impending global changes requires a baseline of information on presence, abundance, and spatial variation of species on the landscape today. Chapter 3 describes the results of a preliminary analysis of spatial variation in the small mammals of northern San Juan County at a single point in time, with the objective of learning how species commonly preserved in the fossil record sort geographically in relation to variation in their abiotic environment. I conducted mark-recapture surveys at 8 sites in northern San Juan County, two of which were located in the immediate vicinity of ECR2 and RBA. Over the course of one year, I compiled abundance and presence/absence data on nine species: Neotoma albigula, Onychomys leucogaster, Peromyscus maniculatus, P. truei, Dipodomys ordii, Perognathus flavescens, P. parvus, Ammospermophilus leucurus, and Tamias rufus. In chapter 3, I evaluate spatial differences in species richness using occupancy modeling and metrics of taxonomic difference, and I assess proportional and rank abundance across sites. Although the results are preliminary, some patterns are emerging: sites spanning 50km and 550m elevation range sample the same small-mammal species pool, but abundance of those species varies non-randomly, and sites are less similar in abundance than expected by random distribution of individuals. Species evenness varies among sites, and sites with low evenness are dominated by Peromyscus maniculatus, a “weedy” species with broad habitat requirements. This is also the first report of Perognathus parvus east of the Colorado River, suggesting recent range expansion of the species. Occupancy models indicate that presence/absence of different species is determined by different aspects of their environment, and therefore species will respond idiosyncratically to future environmental changes. However, more survey data is necessary before these patterns can be considered robust or fully explained.

In chapter 4, I compare fossil diversity at ECR2 and RBA to modern diversity at the same sites. I find that evenness reached a peak in ECR2 and RBA between ~1-1.7 ka, then began to decline between ~0.7-1 ka. Evenness of the modern community at ECR2 and RBA is also significantly lower than in the fossil record between ~0.7-4.5 ka. The observed drop in evenness occurred prior to the onset of high-impact, post-European human land uses, like livestock grazing, and is coincident with the time when Ancestral Puebloan populations crashed due to long-term periodic droughts, suggesting a marked environmental change between ~0.7-1 ka. Low modern evenness is consistent spatially—modern evenness at all survey sites is lower than it is for all fossil time bins ~0.7 ka and older, suggesting that this was a landscape-level decline in diversity. These results send a cautionary message: though the basic taxonomic integrity of the small mammal community is still present, abundance and community structure are very different today.

Additional information on past and current diversity of the CP will improve forecasting and establish baselines against which to compare future surveys, allowing us to gauge rates and direction of change, and to prioritize conservation efforts in the future. This study emphasizes the enormous utility of the fossil record in understanding the extent of ecological fluctuations that can be considered “normal” through long periods of time, information which is essential as we struggle to conserve biodiversity in a rapidly changing world.

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