Implications of Immigrant Arrival Times During the Paleocene-Eocene Thermal Maximum for Mammal Biogeographic Response to Modern Climate Change

Abstract

Abilities of taxa to track suitable habitat under climate change is a concern in conservation biology. Projections that assume suitable habitat is limited to currently occupied biomes can produce underestimates of species viability. The geological record is a valuable source of data to test assumptions about habitat tracking because it archives past episodes of climate change. The Paleocene-Eocene Thermal Maximum (PETM) ~56 million years ago was an interval of rapid carbon release (millennial scale) and global warming (~5 C) that caused large geographic range shifts in Earth’s biota. Large, stratigraphically controlled fossil collections spanning the PETM in the Bighorn Basin, Wyoming, document first occurrence data for immigrant mammals and plants, providing evidence of geographic range shifts as well as changes in climate, flora and vegetation. If mammals tracked specific habitats, their intercontinental dispersal would imply continuity of biomes across Holarctica. In that case, intercontinental mammal and plant immigrants should appear concurrently. Instead, mammalian immigrants crossed high latitude belts of warm, temperate forest, then appeared in the Bighorn Basin during the warmest part of the PETM when plant fossils suggest a dry tropical forest. Warm temperate Eurasian plants are rare during the body of the PETM, but become abundant during the recovery, as climate became wetter. Floral change during the PETM recovery is not concurrent with change in mammalian community structure. Distinct patterns of mammalian and plant turnover suggest that mammals did not strictly track plant-defined habitats. Species may be capable of more flexible responses to rapid climate change than current models predict. Efforts that support movement and provide multi-latitudinal networks of protected areas should be prioritized as a viable means to help conserve some species in the face of climate change.