On global change, direct and indirect interactions, and the structure of ecological communities: theoretical and empirical tests

Human induced global change (climate change, CO₂ enrichment, nitrogen deposition, habitat degradation and biological invasions) is the most serious threat to biodiversity. Understanding how ecosystems will respond to different components of global change, and how these responses will affect key ecological processes, has become essential in contemporary ecology. For example, several studies have shown that exotic invasive species have negative impacts on the composition of communities, habitat structure and ecosystem processes. Particularly, exotic species may have negative effects on species interactions due to local extinctions, competition and/or replacement of interactions. Despite the large body of research demonstrating the negative effects of exotic species on native communities, clear responses of the effect of invasive species on seed dispersal mutualisms are scarcely documented or have focused on only a relatively few invasive species. In this dissertation I used exotic species to (1) re-evaluate the inclusion of facilitation in niche theory, (2) study the establishment success and invasion rates of exotic species (i.e. tens rule), and (3) determine how exotic species has influenced community structure by altering the interaction between plants and animals, specifically, plant-seed dispersers mutualisms. In these studies, I established that interactions with exotic facilitators could increase the size of the realized niche by increasing the spatial distribution (reducing dispersal limitations), or by modifying the physical and chemical conditions of the habitats, without altering the size of fundamental niche of native species. I found that the lack of information about failed species introductions and the tendency to report species that have become invasive more than those that have not results in an overestimation of establishment success and invasion rates. Additionally, I found that a suite of exotic vertebrate herbivores and exotic invertebrates lead to a cascade of linked extinctions by disrupting native plant-seed disperser mutualism. Together, my results highlight the negative effect of exotic invasive species and the importance of focusing on interactions among species to understand the causes and consequences of species loss from ecosystems.