Invasion success of exotic plants in natural ecosystems: the role of disturbance, plant attributes and freedom from herbivores

Invasion of natural ecosystems by exotic species is a major threat to biodiversity globally. We assessed two alternative (but not exclusive) hypotheses to explain the success of exotic species in urban bushland on low fertility sandstone-derived soils in Sydney, Australia. These were that success of exotic species is promoted by: (1) plant attributes in particular disturbance types; and (2) freedom from herbivores. We tested these at sites subject to different types of disturbance: nutrient and water enrichment (below stormwater outlets), nutrient enrichment (riparian zones of creeks with an urban catchment) and physical disturbance (tracks), and control sites. At each site we estimated percentage cover of all species and surveyed leaves for damage by herbivores. Species were classified as native, non-invasive exotic or invasive exotic. We found that sites without any disturbance did not support exotic plants. Physically disturbed sites on low fertility soils supported only one exotic species, suggesting that nutrient enrichment is a critical prerequisite for exotic species invasion on low fertility soils. Exotic species cover was highest and native species richness most reduced in areas of highest nutrient enrichment. Both invasive exotic and non-invasive exotic species had significantly lower levels of leaf herbivory than native species, implying that release from pests alone cannot account for the success of invasive species. Specific leaf area of invasive exotic species was consistently higher than specific leaf area of non-invasive exotic and native species, regardless of disturbance type. In physically disturbed sites of higher soil fertility, exotic species were small herbs and grasses of long flowering duration and with small unassisted or wind-dispersed seeds. In sites subject to nutrient-enrichment, exotic species were more likely to be climbers, able to propagate vegetatively, and with seeds dispersed by vertebrates. Thus different plant attributes contribute to exotic species success under different disturbance types. The clearest consistent difference we found between invasive exotic and non-invasive exotic species was in specific leaf area, suggesting that large specific leaf area facilitates invasiveness.