CO-OCCURRENCE PATTERNS SUGGEST THAT COMPETITIVE EXCLUSION IS RARE IN FOSSIL AND MODERN MARINE BENTHIC ECOSYSTEMS

A long-running debate in community ecology concerns the degree to which local species pools are shaped by ecological interactions (niche assembly) versus environmental filtering. Competitive interactions are difficult to measure even in modern ecosystems and are exceedingly difficult to establish in the fossil record. However, interactions strong enough to lead to local competitive exclusion are observable in taxonomic co-occurrence patterns. Niche assembly theory predicts that competitive interactions will on average be most intense among closely related and ecologically similar species. An approach recently applied to modern and fossil terrestrial species pools (Lyons et al., 2016) determines whether there are species pairs that co-occur more often than would be expected by chance (aggregated pairs) or less often than expected by chance (segregated pairs, which are potential cases of competitive exclusion). Using a similar approach, we examined modern and fossil soft-substrate marine benthic invertebrate species pools to determine (1) how frequently segregated species pairs occur overall, and (2) whether the frequency of segregated species pairs varies as a function of phylogenetic distance (i.e., whether segregated pairs occur more commonly among congeneric species than among more distantly related species). Lacking phylogenetic hypotheses for most species, we approximate phylogenetic distance using Linnaean ranks. Our results suggest that competitive exclusion is rare in both modern and fossil marine benthic species pools. Among all possible species pairs (random, aggregated, and segregated) in local species pools, the median percentage of segregated pairs is 0%. Although spatiotemporal mixing could obscure species co-occurrence patterns, frequencies of segregated species are comparable in fossil and modern datasets. There is no apparent tendency for segregated pairs to be more common among closely related species. These results are consistent throughout the Phanerozoic and robust to a wide variety of data culls. Our results support previous arguments that competitive interactions may be weak in soft-substrate benthic marine communities, possibly due to the pervasive influences of predation and physical disturbance.