Abstract:

Biological invasions are a pervasive environmental problem and are an important focus of ecological research and environmental management. Research on the function of biodiversity has stimulated an interest in the mechanisms underlying the invasibility of plant communities. Although many theoretical and observational studies suggest that diverse communities are more resistant to invasion by exotic species than less diverse communities, results of experimental studies are not conclusive and this remains a highly debated topic.

In this experiment, a series of manipulated grassland communities with different levels of species diversity and different species functional groups (16 species belong to C3 grasses, C4 plants, forbs and legume respectively) were established to test Elton's diversity-invasibility hypothesis by studying the pattern and process of invasion by Alternanthera philoxeroides. Total biomass of the invasive species, an index of community invasibility, was recorded in each community type. Our results showed that in communities with higher functional group diversity, the biomass of A. philoxeroides was significantly lower due to decreased niche opportunity whereas species diversity alone did not show any significant effects on the biomass of the invasive species. The results showed that community invasibility was negatively correlated with functional group diversity suggesting that the diversity of characteristics of species rather than species diversity itself was important. Niche opportunity for invasive species in communities might be a key determinant influencing its invasibility. The characteristics of functional groups also influenced the success of the invasion. Annual grasses with short life spans and nitrogen-fixing legumes were more susceptible to invasion. Moreover, A． sessilis, which belongs to the same morphological and functional group as A． philoxeroides, caused a significant decrease in establishment of the invader species. This suggests that competition might be more intense within functional groups than across functional groups. Because community invasibility is influenced by many factors and their interactions, the pattern and mechanisms of community invasibility are likely to be more complicated than we have acknowledged so far. More experimental work coupled with theoretical modeling studies are needed to better understand the characteristics of community invasibility.