Paper detail

Fractional abundance and the ecology of community structure

The ecological principle of limiting similarity dictates that species similar in resource requirements will compete, with the superior eventually excluding the inferior competitor from the community. The observation that nonetheless apparently similar species comprise a significant proportion of the diversity in any given community has led to suggestions that competition may not in fact be an important regulator of community structure and assembly. Here we apply a recently introduced metric of species interaction, fractional (relative) abundance, to tree species of the tropical wet forest of Barro Colorado Island, Panamá, the particular community that inspired the original model of non-niche or 'neutral' community dynamics9. We show a distribution of fractional abundances between pairs of most closely related congeneric tree species differing from that expected of competitive exclusion, but also inconsistent with expectations of simple similarity, whether such species interchangeability (a fundamental requirement of neutrality) is inferred at the community or the pair level. Similar evidence from a strikingly different dry forest has been linked to the focused, stable competition of a temporal niche dynamic. Taken together with these earlier findings, the results reported here establish a potentially widespread and important role for species interaction in the diversity and maintenance of natural communities that must be considered when inferring process from pattern.