Humans have an outsized impact on modern ecosystems. Our recent research shows that community assembly patterns changed in the Holocene, coinciding with the expansion of agriculture. In particular, we observe a shift from 300 Ma of aggregated associations to a majority of segregated associations in the present. In order to explore the mechanistic reasons for this observed pattern, we used a database of North American fossil and modern mammal occurrences over the last 40 ky. Our database includes 408 localities for 3 different time periods: late Pleistocene, Holocene and modern. We evaluated whether pairs of species co-occurred randomly, or were statistically aggregated or segregated.
Can species’ traits explain these patterns? To investigate this question, we compiled data on body size for the species in our dataset.
Of the non-random pairs, the proportion of segregated pairs increased from 41% to 59% from the Pleistocene to the Modern. Additionally, 39.7% of the species pairs that changed their association mode across this time interval went from being randomly associated to significantly segregated while just 22% of them changed from random to aggregated.
The average differences in body mass between species in a pair dropped significantly during the Pleistocene-Holocene transition due to the North-American megafaunal extinction. We also found that habitat filtering structured species pairs in the Pleistocene and Holocene but this pattern changes in modern assemblages. Species pairs with smaller body sizes (log10 1-2) tend to segregate more in the present while they used to be more aggregated in the past. Our results suggest that human impacts on modern ecosystems such as habitat fragmentation are affecting the ways in which species interact and share the landscape and that species’ traits mediate these changes.