Raptors, rodents, and paleoecology: Recovering ecological baselines from Great Basin caves.

摘要

Forecasting long-term ecosystem responses to climatic and anthropogenic environmental change requires unlocking the wealth of pre-settlement baseline information contained in the recent fossil record. Homestead Cave and Two Ledges Chamber, both long-term owl roosts in the Great Basin of the western United States, contain stratified deposits of regurgitated raptor pellets rich in small-mammal skeletal remains that span the Holocene. Such archives provide unique opportunities for establishing ecological baselines. This, however, requires quantification of the extent to which such "death-assemblages" provide accurate and high resolution pictures of the living communities from which skeletal remains are derived. Using the modern, subfossil, and late Holocene fossil records from these sites, my dissertation combines empirical and theoretical approaches to examine three main factors that could bias the ecological fidelity of raptor-derived small-mammal death-assemblages: (1) temporal changes in the identity of the raptor that concentrated skeletal remains, (2) raptor feeding selectivity across habitats and among prey, and (3) the incorporation of skeletal remains from multiple generations into a single deposit (time-averaging).;First, I develop a discriminant function approach that uses the damage patterns that predators inflict on the skeletal remains of their prey to statistically infer what type of predator produced a death-assemblage of unknown origin. I then apply this method to the stratified record of Homestead Cave to track predator identities through the Holocene, showing that nocturnal owls were the dominant contributors of small-mammal skeletal material throughout the depositional history of the cave. Thus observed changes in taxon dominance through time are not taphonomic artifacts, but reflect true biotic responses of the small-mammal community to environmental change.;Second, I use empirically-parameterized simulations to address how much time a death-assemblage must represent to confidently allow a longer term signal of prey relative abundance to emerge from the shorter-term "noise" of boom-and-bust population dynamics. Results suggest that this requires ∼100 years of summed time. I then use 67 new radiocarbon dates spread across four strata spanning the Holocene at Homestead Cave to empirically explore how a stratum's effective temporal resolution relates to the maximum age range of its skeletal remains. Results indicate that time-averaging of a stratum can range to over 5,000 years. Given the consistently non-uniform distribution of bone ages, however, a stratum's effective resolution is likely an order of magnitude more constrained. Thus raptor-generated small mammal records are time-averaged enough to dampen the effects of short-term population variability, but still have sufficient temporal resolution to detect ecological change at centennial to millennial time-scales.;To evaluate the effects of predator selectivity, seasonal variation, and spatio-temporal averaging on live-dead agreement, I then compare a series of modern snapshot trapping surveys I conducted at Two Ledges Chamber with a series of death-assemblages time-averaged over months to centuries. I find high live-dead agreement across all ecological metrics, indicating that the effects of predator selectivity are minimal, that time-averaging impacts ecological fidelity only at or beyond the century-scale, that the owls sample from all local habitats, and that the small-mammal community at Two Ledges Chamber has remained relatively stable since the late Holocene.;Finally, I evaluate the ability of a live-dead comparative approach to detect the impacts of recent disturbance on desert small-mammal communities. While the Holocene histories of Two Ledges Chamber and Homestead Cave are similar, these sites have diverged dramatically within the last century in the degree to which the surrounding landscape has been impacted by anthropogenic activity. While Two Ledges Chamber has remained relatively undisturbed, Homestead Cave is located in a military training area and is also blanketed by cheatgrass, an invasive species that chokes out native sagebrush steppe. In contrast to patterns seen at Two Ledges Chamber, clear live-dead mismatch is observed at Homestead Cave between modern live and time-averaged surface pellet samples, revealing a system that is species poor and highly uneven relative to its recent past. Local historical trapping records, which show high concordance with the time-averaged surface pellets, provide further evidence that this live-dead mismatch reflects recent ecological change.;The work from this dissertation fills in some major gaps in our understanding of taphonomy in mammalian systems, and has important implications for both neontologists and paleontologists alike. Counter to the concerns of both disciplines, I provide strong evidence that raptor-derived death-assemblages can retain clear ecological information, and that long-term roost sites offer high quality baseline information that can be readily tapped for conservation and remediation efforts.