Species associations of plant and mammal communities from deep time to the recent
A major focus in contemporary ecology is identifying the patterns and causes of species associations. Recent syntheses have suggested that non-random species pairs in modern communities are dominated by species that are significantly segregated; that is species pairs occur together across communities less frequently than would be expected by chance. This dominance of negative species interactions suggests an important role for processes such as competition, habitat selectivity, dispersal limitation and extinction in structuring modern communities. The fossil record provides an excellent opportunity to explore the importance of these processes in structuring communities over longer time scales. Here we analyze the spatial structure of 80 replicated fossil assemblages of plants and animals to reveal the frequencies of aggregated and segregated species pairs and ask whether non-random species associations of plant and mammal assemblages over the last 300 ma are dominated by negative interactions as in the present. We evaluate five possible explanations for our results. These include the spatial or temporal scale of the data, the taxonomic resolution of the included datasets, the mode of collection of the data, climate variability over time, and the uniqueness of the present.
Aggregated species pairs dominate from the Carboniferous (307 ma) to the early Holocene (10,000 bp). However, in a Quaternary fossil pollen sequence, and two Quaternary mammal assemblages, there was a progressive shift towards more segregated pairs in recent times. This trend is also consistent with a meta-analysis of 260 contemporary assemblages, which are dominated by segregated species pairs. The pattern cannot be attributed to effects of spatial or temporal grain and extent of sampling, simple taphonomic artifacts, whether it is an island or mainland assemblage, or to increased variability in climate toward the present. Breakpoint analysis indicates that the trend towards fewer positive associations begins approximately 6000 kya, and may point to the increasing influence of anthropogenic effects over the Holocene. These results suggest that the organization of contemporary and Quaternary plant and animal assemblages may differ from those of deeper time slices.