Research Themes

Paleomiddens

Midden Projects

We examine the ecological and evolutionary response of Neotoma species to temperature change from ~30,000 yr. B.P. to present, at a variety of sites within the mid- to southwestern United States.  This is a topographically complex region, with climatic variability responding to the behavior of the large-scale features of the global climate system.  It is also a region rich in paleoclimatic data; much of it from fossil packrat middens.  These data have been used to reconstruct the vegetation history of the southwest and from that to infer paleoclimate shifts, but had not been applied previously to examine ecological and/or evolutionary change in Neotoma.

 
We measure the radiocarbon-dated fossil fecal pellets contained in large numbers within the paleomiddens, and from them estimate body size, which leads in turn to quantitative predictions about life history, ecological and evolutionary responses to climate change. Since each cave site contains several to dozens of differentially dated middens, a comprehensive investigation can be made of the response of a single woodrat population to climate change occurring over many thousands of years.

 
We find highly significant effects of past temperature fluctuations on Neotoma.  As predicted by Bergmann's Rule, a negative correlation exists between woodrat body size and temperature (Smith et al. 1995; Smith and Betancourt 1998, 2003, 2006).  Body size decreased rapidly at the Pleistocene/Holocene boundary as glacial ice sheets melted and global temperatures increased.  Size did not decrease monotonically, however.  Our data suggest that much of the Holocene was characterized by rapid temperature fluctuations.  This led in some localities to species replacements as climate changed too rapidly for adaptation to occur, or exceeded species thermal tolerances.  Currently, we have data from ~55 caves.  As new ones are added, we are expanding the work to address questions such as:  How similar are the responses of animals in different localities to large-scale climatic change?  We find higher evolutionary rates at the northern portions of the distribution; what does this suggest about local adaptations?  What can we infer about the equability of climate from the midden record?  Thus far, we have demonstrated that body size change precedes changes in plant macrofossils.  Does this represent a more rapid response on the part of woodrats to climate change, or to differences in winter versus summer temperature regimes?  How stable are habitat affinities (say those of N. stephensi for Juniperus spp.) over time?  What are the critical thresholds (temperature or vegetative) for species replacement to occur? What life history consequences result from changes in body size?  Can we use allometric relationships to “scale up” to the likely impacts of body size alterations on the other components of the local community?