Schrödinger’s mammoth – ecological assembly in the age of humans

Ecologists have long been interested in how species assemble into communities. In particular, they are interested in how species traits, environmental factors, and biotic interactions affect species distributions, and membership and persistence in ecological communities. Determining whether assembly rules exist and what they are is particularly important in the face of ongoing climate change. However, despite decades of study, no clear consensus has emerged. In part because modern studies are limited by the short time scales over which they are able to collect data and by the fact that humans are incredibly successful ecosystem engineers who have affected almost every part of the planet. In contrast, paleontologists have been interested in the interplay between species traits and the environment, and how these relationships change over time in response to global forcing factors such as climate change. Increased knowledge of taphonomic processes has led to an understanding that fossil assemblages preserve reliable information about ecological communities and species interactions. By comparing the structure of these fossil assemblages with modern assemblages, we can begin to identify aspects of community structure that are similar across many taxonomic groups and across long time scales. We can also determine whether and how these patterns have changed with the increasing dominance of humans on the globe. Using a macroecological lens, I examine mammalian community structure over long and short time scales including metrics such as co-occurrence structure, body size distributions, and functional traits. I evaluate how these patterns change over time and with changes in global climate. Finally, I examine how some traits associated with extinction risk have changed over time and the consequences for ecological communities. I find that in paleoecological communities, there are consistent patterns over time in terms of co-occurrence structure, body size distributions, and extinction risk, but that many of these patterns change as human impacts increase including the role that functional traits play in mediating co-occurrence structure. These changes suggest that humans are fundamentally altering ecological communities and resetting ecological assembly rules. Paleontology has a key role to play in identifying the disruptions to assembly rules by humans and what that means for predicting how species are likely to respond to future climate change, habitat fragmentation and the loss of biotic interactions because of extinctions.