Making a Bad Situation Worse: An Invasive Species Altering the Balance of Interactions between Local Species

<div><p>Biological invasions pose a significant threat to biodiversity, especially on oceanic islands. One of the primary explanations for the success of plant invaders is direct suppression of competitors. However, indirect interactions can also be important, although they are often overlooked in studies on biological invasion. The shrub <i>Leucaena leucocephala</i> is a widespread island invader with putative allelopathic effects on the germination and growth of other species. We quantified the impact of <i>Leucaena</i> on plant communities richness on an oceanic Brazilian island and, through nursery experiments, investigated the potential for allelopathic effects on the germination of <i>Erythrina velutina</i>, a native species that is often absent from stands of <i>Leucaena</i>. Additionally, in a manipulative field experiment, we examined the direct and indirect effects (mediated by the native species <i>Capparis flexuosa</i>) of the invader on the development of <i>Erythrina</i>. The species richness in invaded sites was lower than in uninvaded sites, and <i>Capparis</i> was the only native species that was frequently present in invaded sites. In the nursery experiments, we found no evidence that <i>Leucaena</i> affects the germination of <i>Erythrina</i>. In the field experiments, the odds of <i>Erythrina</i> germination were lower in the presence of <i>Leucaena</i> litter, but higher in the presence of <i>Leucaena</i> trees. However, the survival and growth of <i>Erythrina</i> were considerably inhibited by the presence of <i>Leucaena</i> trees. The isolated effect of native <i>Capparis</i> on the germination and growth of <i>Erythrina</i> varied from positive to neutral. However, when <i>Capparis</i> and <i>Leucaena</i> were both present, their combined negative effects on <i>Erythrina</i> were worse than the effect of <i>Leucaena</i> alone, which may be attributed to indirect effects. This study provides the first empirical evidence that the balance of the interactions between native species can shift from neutral/positive to negative in the presence of an exotic species.</p>