Although how rare species persist in communities is a major ecological
question, the critical phenotypic dimension of rarity is broadly
overlooked. Recent work has shown that evaluating functional
distinctiveness, the average trait distance of a species to other
species in a community, offers essential insights into biodiversity
dynamics, ecosystem functioning, and biological conservation. However,
the ecological mechanisms underlying the persistence of functionally
distinct species are poorly understood. Here we propose a heterogeneous
fitness landscape framework, whereby functional dimensions encompass
peaks representing trait combinations that yield positive intrinsic
growth rates in a community. We identify four fundamental causes leading
to the persistence of functionally distinct species in a community.
First, environmental heterogeneity or alternative phenotypic designs can
drive positive population growth of functionally distinct species.
Second, sink populations with negative growth can deviate from local
fitness peaks and be functionally distinct. Third, species found at the
margin of the fitness landscape can persist but be functionally
distinct. Fourth, biotic interactions (either positive or negative) can
dynamically alter the fitness landscape. We offer examples of these four
cases and some guidelines to distinguish among them. In addition to
these deterministic processes, we also explore how stochastic dispersal
limitation can yield functional distinctiveness.