Background. Studies of biological invasions focus on negative interactions between
exotic and native biotas, emphasizing niche overlap between species and competitive
exclusion. However, the effects of positive interactions and coexistence are poorly
known. In this study we evaluate the importance of positive, negative, or random
species associations in explaining the coexistence of native and exotic boring polychaetes
inhabiting invertebrate hosts, on the southeastern Pacific coast of Chile. We assess
three hypotheses to explain the observed patterns: positive species interactions, weak
competitive interactions, and competitive intransitivity.
Methodology. To assess the potential effect of competition between native and exotic
polychaetes we analyzed patterns of co-occurrence of species pairs in northern and
southern regions, using the metric of the probabilistic model. Since biotic interactions
can affect not only native species, we also evaluated correlations between native and
exotic polychaete abundance, using reduced major axis regression linear models. To
assess the transitivity of competitive hierarchies we used metrics and analytical methods
based on abundance matrices to estimate species competition and patch transition
Results. On average 50% of the species pairs presented significant weak negative
associations, all associated with the exotic species Polydora rickettsi; the remaining
50% had random associations, and none showed positive associations. However, the
relationship of abundance between native and exotic boring polychates supports a
tendency towards coexistence. At local and regional scales, we observed the presence
of a transitive network competition structure, where the exotic boring polychaete, P.
rickettsi was generally the dominant species.
Conclusions. Our results support that native and exotic boring polychaete species
coexist through weak competitive interactions. Nevertheless, the large number of
random interactions observed indicates that species coexistence can be accountedfor by stochastic processes, as proposed by neutral theory. Coexistence may be a
frequent result of interactions between native and exotic species, although less apparent
than competitive exclusion. Thus, the probabilistic point-of-view used here provides
a statistical tool for evaluating coexistence as a result of exotic and native species’
interactions, an idea which has been proposed in invasion ecology, but largely lacks
empirical support and methodologies for detecting underlying mechanisms. Finally, we
found evidence that P. rickettsi is a successful invader by being competitively dominant,
but not excluding other species.