Long-term stasis in mammalian community structure: The fossil record as a tool for conservation planning

Background/Question/Methods

Taxon-free metrics of biodiversity health are crucial for present and future conservation efforts in the face of current global change. We investigated the distribution of species in combined diet and body size functional groups over the past 16 million years in the Northern Great Plains to establish a pre-Holocene (before 11,000 ya, when humans arrived in North America) and pre-industrial baseline (11,000-500 ya) of this measure of community structure. To gauge the impact of time-averaging on patterns of community structure change, functional group distributions were compared on two scales: 1) North American Land Mammal Ages (NALMAs), and 2) individual diverse localities.  We statistically compared these distributions using pairwise Fisher’s exact tests with Monte Carlo p-value simulations and Holm p value adjustment, and qualitatively assessed them using Correspondence Analysis. 

Results/Conclusions

When averaged over entire NALMAs, major changes in functional group distribution only take place during the Hemphillian and Holocene.  Individual locality-level patterns also indicate long periods of stasis in the metric (Barstovian-Clarendonian and Hemphillian-Holocene).  One possible explanation for the change during the Hemphillian is a threshold of global climate change, but further study is needed in that regard.  The primary driver of apparent differences between the Holocene and previous time periods is the loss of megaherbivores (>44 kg) during the end-Quaternary extinction event.  Although the extent of time-averaging and other taphonomic biases affect the details of observable patterns per time period, overall, proportional diversity of functional groups is a promising metric for assessing mammalian community health because it is remarkably stable through time and changes only with major external perturbations to ecosystems.