ESTIMATING COMPETITION AND ITS EFFECTS OVER MACROEVOLUTIONARY TIME SCALES

Competition among individuals potentially divert resources important for somatic growth, survival and reproduction. Both intra-specific and inter-specific competition are often studied in the context of density-dependence in ecology, and phylogenetic comparative approaches in macroevolutionary biology. Despite the consequences competition might have on long-term evolution, it is challenging to get direct information on this ecological process from the fossil record. In this presentation, we describe how bryozoans can be used as a model system for studying spatial competition and the consequences thereof in the fossil record. More specifically, we demonstrate that species-level information on competitive outcomes can be gleaned from the fossil record of cheilostome bryozoans over millions of years. Cheilostomes are sessile marine organisms. Most of them need a substrate to encrust in order to initiate their colony and grow. Competition for space is hence unavoidable. Competition outcomes include win-lose, reciprocal overgrowth, and stand-offs, where a high community proportion of the latter two types is indicative of syn-vivo interactions. General findings include how some species are win more often than a null expectation, while others lose more often. Overgrowth outcomes are in part also predictable given species traits.

Given that some species win while others lose, how do the losers maintain a net positive population growth? What are the patterns of species co-existence on longer time scales? We hence ask whether local, short-timescale spatial competition has detectable effects on population growth (and hence extinction risk) over macroevolutionary timescales for different species in the same community. To deal with the incomplete and patchy nature of the fossil record, we develop occupancy modeling, a framework used in statistical ecology to control for incomplete detection and sampling. Using occupancy modeling, we estimate dynamics in the relative abundance of individual species by using data replicated at site and formations, by harnessing information from random effects modeling. We relate these estimates of relative abundance to the competitiveness of different species in species-level data collected from the Pleistocene of the Whanganui Basin of New Zealand, spanning more than 2 million years.