Community assembly and disassembly across time

An overarching theme in my research is the way in which species assemble into communities.  I just completed a 6-year NMNH grant with Kay Behrensmeyer and Bill DiMichele and currently have an NSF-RCN grant with Kay Behrensmeyer and Nicholas Gotelli to bring together ecologists and paleobiologists to address the overarching question: what are the patterns and causal processes of animal and plant community assembly and disassembly over geologic time, up to the present-day?  By comparing communities from different time periods, taxonomic groups, environmental and evolutionary histories, we hope to identify general processes that underlie community assembly and disassembly (e.g., Tóth et al., 2014).  As a first step, we analyzed species co-occurrence patterns in plant and mammal communities across time and are assembling datasets on species traits and locality metadata to identify factors that may drive species co-occurrence across time (Blois et al., 2014). Further work has revealed a recent change in species co-occurrence patterns that were relatively constant across the last 300 ma among mammals and plants.  This change began during the Holocene and cannot be attributed to sampling differences between fossil and modern communities or to climate change.  The mostly likely cause is increased fragmentation and dispersal limitation due to human impacts beginning with the spread of agriculture (Lyons et al. 2016a,b,c).


We have initiated a series of follow up studies to try and tease apart the factors that could be responsible for the shift in co-occurrence structure. This work is being led by me and several of the students and postdocs in the group. All of these studies focus on the terminal Pleistocene and the factors affecting the co-occurrence structure of mammals. Our postdoc, Silvia Pineda Munoz has been examining the role of body size on the co-occurrence structure of mammals across this interval. She finds that body size was important in structuring significant pairs of species in the Pleistocene and the Holocene, but not the modern (Pineda-Munoz et al. in prep). Similarly, I find that there is a significant change in the role of trophic interaction across this interval. Certain trophic interactions that used to make it more likely that species would form a significant aggregation or segregation, now make it less likely (Lyons et al. in prep). This seems to be related to a massive homogenization in community composition that occurs coincident with this change in community structure (Fraser et al. in prep). More interestingly, however, there seems to be a fundamental change after the extinction of the megafauna. Prior to the extinction, significant interactions were spread among the community and were weaker and more diffuse than after the extinction (Lyons et al. in prep). Examination of the geographic ranges and climate niche of the victims and survivors of the extinction shows an increase in both immediately following the extinction, but a subsequent decrease in the climate niche of the survivors in the modern (Tóth et al. in prep). When we disentangle the effects of abiotic and biotic factors on species aggregations and segregations, we find that the loss of the megafauna resulted in a loss of biotic interactions that have not been recovered (Tóth et al. in prep).