Biotic resistance in the tropics: patterns of seed plant invasions within an island

Abstract

Charles Elton proposed that high species diversity and low levels of disturbance provide a measure of biotic resistance against invasions by alien species. While there is some evidence for this hypothesis, there are numerous other factors associated with invasive species richness, and the strength of those relationships is often scale-dependent. Among oceanic island groups, habitat diversity, human population size and economic activity have been identified as among the significant drivers of invasive species richness. However, intra-island patterns are rarely analyzed. We investigate the relationship between the number of invasive seed plant species and human, physical and biotic factors among municipalities of the tropical island of Puerto Rico using Generalized Linear Models. While elevation amplitude and, to a lesser extent, area had significant effects on the diversity of the most abundant invasive species, we found that the best models, according to Akaike Information Criterion, consistently involve a positive relationship between the number of invasive and native species. Moreover, when we examined the relationship between forest reserves and those regions without reserves, proportionately fewer invasive species occur in forest reserves where native species richness is higher, resource competition is presumably greater, and human impacts are no longer as pervasive. Because the invasive species pool consists almost entirely of ruderal species, forest reserves, while not impenetrable, are less susceptible to invasions than the heavily human-impacted landscapes. Consequently, forest reserves may play an important role in slowing the pace of biological invasions on tropical islands.