On the mismatch in the strength of competition among fossil and modern species of planktonic Foraminifera

Aim Many clades display the macroevolutionary pattern of a negative relationship between standing diversity and diversification rates. Competition among species has been proposed as the main mechanism that explains this pattern. However, we currently lack empirical insight into how the effects of individual-level ecological interactions scale up to affect species diversification. Here, we investigate a clade that shows evidence for negative diversity-dependent diversification in the fossil record and test whether the clade's modern communities show a corresponding signal of interspecific competition. Location World's oceans. Time period Holocene. Major taxa studied Planktonic Foraminifera (Rhizaria). Methods We explore spatial and temporal ecological patterns expected under interspecific competition. Firstly, we use a community phylogenetics approach to test for signs of local competitive exclusion among ecologically similar species (defined as closely related or of similar shell sizes) by combining species relative abundances in seafloor sediments. Secondly, we analyse whether population abundances of co-occurring species covary negatively through time using sediment trap time-series spanning 1-12 years. Results The great majority of the assemblages are indistinguishable from randomly assembled communities, showing no significant spatial co-occurrence patterns regarding phylogeny or size similarity. Through time, most species pairs correlated positively, indicating synchronous rather than compensatory population dynamics. Main conclusions We found no detectable evidence for interspecific competition structuring extant planktonic Foraminifera communities. Species co-occurrences and population dynamics are likely regulated by the abiotic environment and/or distantly related species, rather than intra-clade density-dependent processes. This interpretation contradicts the idea that competition drives the clade's macroevolutionary dynamics. One way to better integrate community ecology and macroevolution is to consider that diversification dynamics are influenced by groups that interact ecologically even when distantly related.