Invasive Exotic Plants in the Tropical Pacific Islands: Patterns of Diversity

Abstract

Oceanic islands are good model systems with which to explore factors affecting exotic species diversity. Islands vary in size, topography, substrate type, degree of isolation, native species diversity, history, human population characteristics, and economic development. Moreover, islands are highly vulnerable to exotic species establishment. We used AICc analyses of data on 1132 vascular plant species for 15 countries and 114 islands from the Pacific Island Ecosystems at Risk (PIER) project to examine biological, geographical, and socioeconomic correlates of exotic species richness. PIER provides data on the distribution of naturalized non-native plant species thought to pose environmental or economic risk. We hypothesized that the numbers of PIER-listed species would be positively correlated with island size, habitat diversity, and proximity to major source pools for propagules. Further, we expected numbers of PIER-listed exotic species to be similar among islands in the same country and to be greater where human populations were larger and where economic activity was high. Most species (908) were found on ≤ 10 islands. Species number was significantly correlated with island and country areas and with native plant species richness. The strongest model revealed by AICc analyses of island data included log (area) and maximum elevation as well as country membership, substrate type, and presence of an airport with paved runway (an index of economic activity). By country, AICc analyses revealed two equivalent models, both of which included log (area) and per capita gross domestic product as well as a measure of population size (either log (population size) or (population density)). Our analyses provide strong evidence of the roles of biogeographic, environmental, and socioeconomic impacts on the distribution and spread of exotic species.

Naturalization and invasion of alien plants in Puerto Rico and the Virgin Islands

“…Herbs and grasses account for 41 % of all alien species and for 52 % of all species listed as invasive on these islands. This pattern of dominance of herbaceous growth forms has been observed in other alien floras in the Caribbean as well as in other tropical and extratropical islands (Lloret et al 2005;Denslow et al 2009;Arianoutsou et al 2010;González et al 2012). One interesting result of this study is the high proportion of alien trees and shrubs occurring in PRVI.…”

“…Under the current scenario of economic globalization, the dispersal of cultivated plants is promoted and easily achieved even from remote locations. Prevailing economic development in PRVI together with dynamic import markets, high population densities and high urban development, are factors facilitating the influx of alien species, increasing propagule pressure and lastly increasing the likelihood of becoming naturalized or invasive (Denslow et al 2009;Kueffer et al 2010).…”

“…In new habitats where pollen vectors can be limited or absent, facultative selfing could be crucial for the successful invasion of insular ecosystems as they are often characterized by depauperate pollinator communities (Barrett et al 1996;Lloret et al 2005;Denslow et al 2009). In this context, while selfing may ensure reproductive success when pollen vectors are limited, outcrossing may enhance genetic variability and thus increase the probability of adaptation to new habitats (Milbau and Stout 2008).…”

“…Worldwide, insular biomes are considered more vulnerable to invasions than continental ecosystems as they host higher numbers and densities of alien species than equivalent mainland communities (D'Antonio and Dudley 1995;Simberloff 1995;Rejmánek 1996;Simberloff 2000;Denslow 2003;Denslow et al 2009). For example, the number of naturalized alien plants recorded on oceanic islands is almost twice the number of those recorded in similar sized patches on mainland with approximately the same number of native species (Sax et al 2002).…”

Minimizing Risks of Invasive Alien Plant Species in Tropical Production Forest Management

“…In striking contrast to continental forests, tropical oceanic island forests have suffered massively from plant invasions [31,32] (Table 2). Although invasions are promoted by logging (e.g., [33]) and other disturbances, and are often facilitated by invasive alien animals [32,34], invasions of undisturbed sites have also been reported, with the invaders apparently using resources, such as light, that are not being fully utilized by the low-diversity native flora.…”