Severity of seabed spatial competition decreases towards the poles.

For more than a century ecologists have considered that competitive interactions between species are more intense at low latitudes [1,2]. This is frequently invoked as either an explanation or a consequence of higher species richness in the tropics, also suggesting that competition shifts from intra- to inter-specific towards the tropics [1]. Another common assumption is that within a community, intraspecific competition needs to be relatively strong, compared to inter-specific competition, in order to enable stable coexistence of species [3]. However, many analyses have found no consistent large scale geographic patterns in the intensity of intra- or interspecific competition [4]. Here, we show a clear latitudinal trend in contest competition for space in nearshore marine environments, for bryozoans (sessile, colonial, suspension feeding animals). Bryozoans form species-rich assemblages with other encrusting fauna and flora (corraline algae), and are highly abundant across the globe [5]. We find that whilst the intensity of competition (percentage of bryozoan colonies involved in direct physical spatial interactions with bryozoan or other encrusters) differed little with latitude, its severity (percentage of bryozoan colonies involved in contests with a win/loss outcome, leading to death of the loser) was three times lower at the poles than in the tropics. The cause of this change in severity was a strong shift in taxonomic relatedness of competitors, from interactions between species of different families dominating at lower latitudes, to mainly intraspecific competition at the poles.