Resource availability, propagule supply, and effect of nonnative ungulate herbivores on Senecio madagascariensis invasion.
Nonnative invasive herbivores can create complex biotic interactions by differentially feeding on native and nonnative invasive plant species. The herbivores may act as enemies of nonnative plants and prevent them from becoming invasive, or they may facilitate invasion by having a greater negative impact on native plants compared with nonnative plants. It is also possible that within the same ecosystem nonnative herbivores could either facilitate or inhibit invasion under different abiotic or biotic conditions. In this study we experimentally investigated how abiotic (soil nutrients) and biotic (propagule density) conditions influence the effect of invasive generalist herbivores on Senecio madagascariensis, an invasive plant species in Hawaiian dry forest plant communities. We used fenced exclosures to manipulate presence or absence of invasive ungulates (feral goats and sheep), and we used seed addition to manipulate propagule supply of S. madagascariensis. The experiment was replicated in a recently burned and an unburned site to examine how a resource pulse following fire may alter plant-herbivore interactions. There were very few seeds of S. madagascariensis in the seed rain of both sites, and recruitment was four times higher when seeds were experimentally added, suggesting that S. madagascariensis is dispersal limited in this area. Recruitment of S. madagascariensis was five times higher in the burned site compared to the unburned site, suggesting that increased resources promote recruitment. Recruitment was three times higher when herbivores were present compared to when they were excluded, but plants were much smaller when herbivores were present. We conclude that herbivores can alter S. madagascariensis recruitment, even during dry conditions, and that propagule availability influences where S. madagascariensis can become established.