Why are some species invasive? : determining the importance of species traits across three invasion stages and enemy release of southern African native plants in New Zealand
Abstract
There are many factors that have been proposed to contribute to plant invasiveness in nonnative
ecosystems. Traits of invading species are one of them. It has been proposed that
successful species at a certain invasion stage share particular traits, which could be used to
predict the behaviour of potentially invasive plants at the respective stage. Three main stages
of invasion are distinguished: introduction, naturalization, and invasion. I conducted a stageand
trait-based analysis of available data for the invasion of New Zealand by the flora of
southern Africa. Using 3076 southern African native vascular plant species introduced into
New Zealand, generalised linear mixed model analysis was conducted to assess association of
several species traits with the three invasion stages. The results showed that plant traits were
significantly associated with introduction but fewer traits were associated with naturalization
or invasion, suggesting that introduction can be predicted better using plant traits.
It has been also hypothesized that species may become invasive in non-native ecosystems
because they are removed from the regulatory effects of coevolved natural enemies (Enemy
Release hypothesis). A detailed field study of the succulent plant Cotyledon orbiculata var.
orbiculata L. (Crassulaceae) was conducted in the non-native New Zealand and native
Namibian habitats to compare the extent of damage by herbivores and pathogens. C.
orbiculata is a southern African species that is currently thriving in New Zealand in areas
seemingly beyond the climatic conditions in its native range (occurring in higher rainfall
areas in New Zealand than are represented in its native range). As hypothesised, C.
orbiculata was less damaged by herbivores in New Zealand but, contrary to expectation,
more infected by pathogens. Consequently, the plant was overall not any less damaged by
natural enemies in the non-native habitat than in its native habitat, although the fitness
impacts of the enemy damage in the native and invaded ranges were not assessed. The results
suggest that climatic conditions may counteract enemy release, especially in situations where
pathogens are more prevalent in areas of higher rainfall and humidity.
To forecast plant invasions, it is concluded that species traits offer some potential, particularly
at the early stage of invasion. Predicting which introduced plants will become weeds is more
difficult. Enemy release may explain some invasions, but climatic factors may offset the
predictability of release from natural enemies.... [Show full abstract]
Keywords
plant ecology; invasive species; invasion; invasion stages; introduced plants; naturalization; species traits; herbivory; plant–herbivore interactions; plant pathogens; succulent plants; ecology; southern Africa; southern African flora; Namibia; New Zealand; rainfallDate
2009Type
ThesisCollections
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