  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,
Panam\'a, 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.
