The relationship between species diversity and communityinvasibility has been discussed intensively in recent years. Traditionaltheory suggests that more diverse communities are less susceptible to invasionthan species-poor communities (Elton, 1958; Lodge, 1993). It is commonlyhypothesized that diverse communities use resources more completely and havemore intense competition than simple communities, and they are thus moreresistant to invasion
Resource availability has been suggested to be a keydeterminant of invasion success (Burke and Grime, 1996; Tilman, 1999). Recentexperiments have shown negative relationships between species or functionalgroup richness and resource availability (Tilman et al., 1997; Hooper andVitousek, 1998; Shea and Chesson, 2002). There are two hypothetical mechanismsfor these relationships: diverse communities have greater variety of means ofcapturing resources than do simple communities, and they use these resourcesmore efficiently, leaving fewer available for invading species and thusreducing invasibility, i.e., resource use complementarity (Knops et al., 1999;Symstad, 2000). Another possible mechanism is that diverse communities have ahigher probability of including species most highly competitive for limitedresources or have a key role in driving the resistance to invasion increases,i.e., sampling effect (Huston, 1997; Wardle, 2001). In this case, the speciescomposition of a community plays an important role in its invasibility becauseof the influence of
At harvest, communities with greater plant species richnesshad significantly lower total biomass of alligator weed. With the increase ofspecies diversity from zero to 16 species per community, biomass of thealligator weed decreased significantly (Figure 3). This effect of speciesdiversity on community invasibility is confounded with the functional groupsdiversity. To avoid such confounding effects, we analyzed the effects ofspecies diversity groups with 4, 8, and 16 species that have the same numberof functional groups. The results indicate that total biomass of alligatorweed was not significantly correlated with the richness of resident species(Figure 4) and ANOVA
The species pool hypothesis suggests that the larger the area of a habitat type and the greater its geological age, the greater the opportunity for speciation and hence the larger the number of available species adapted to that particular habitat, which will in turn lead to higher community diversity.
A species pool, containing all 16 species belonging to thefour functional groups, was established beforehand. All seedlings were grownfrom seeds we collected. Monocultures were established by transplanting theseedlings at densities estimated to allow maximum biomass production for eachspecies (Table 1). In poly-cultures, seedling densities were reduced accordingto the total number of species in the monocultures. For instance, ineight-species communities, the density of each species was reduced toone-eighth its density in monoculture. The resident species were planted on 20March, 2002. At the same time, we cultured lots of alligator weed propagula inHogland nutrient solution in an incubator (KXG-350A, in China) from stems withstipites. One month later, we transplanted twelve alligator weed propagula(with roots 2-2.5 cm) with the same size and performance into the centralsites of ten cement pools and covered them with thin layer of soil.
Biological invasions have happened on a large scale in somany regions of the globe. With this background, there is a need to study theprocess of invasion and the response of communities on a small scale. In thispaper, we report an experimental test on the effects of diversity and the roleof functional groups' traits of the species on invasion. We examined theperformance of the invasive alligator weed () in experimental plant assemblages in which resident speciesrichness and functional groups richness were directly manipulated. Throughintegrating and separating species diversity and functional groups diversityin the established plant communities, we analyze the effects of diversity oncommunity invasibility. Functional groups diversity includes two aspects: thenumber of functional groups (richness) and the types of functional groups(composition). The following questions are explored: (1) Are communities withmore species more resistant to invasion? (2) Are communities with morefunctional groups more resistant to invasion? (3) Is it more difficult forinvasive species to invade communities with species functionally similar tothem? (4) Are there negative relationships between species or functional grouprichness and resource availability?
individual species on resources. On the basis of thediversity-invasibility hypothesis, Lodge (1993) has suggested that communitiesare prone to invasions partly for lacking species which are ecologicallysimilar to the invader. A relevant explanation is the niche limitation ofcommunity assembly theory, i.e., two species' ecological characteristics aretoo similar, and one may competitively exclude the other.
Abstract. The invasion of exotic species intoassemblages of native plants is a pervasive and widespread phenomenon. Manytheoretical and observational studies suggest that diverse communities aremore resistant to invasion by exotic species than less diverse ones. However,experimental results do not always support such a relationship. Therefore, thehypothesis of diversity-community invasibility is still a focus of controversyin the field of invasion ecology. In this study, we established andmanipulated communities with different species diversity and different speciesfunctional groups (16 species belong to C3, C4, forbsand legumes, respectively) to test Elton's hypothesis and other relevanthypotheses by studying the process of invasion. Alligator weed() was chosen as the invader. We found thatthe correlation between the decrement of extractable soil nitrogen and biomassof alligator weed was not significant, and that species diversity, independentof functional groups diversity, did not show a significant correlation withinvasibility. However, the communities with higher functional groups diversitysignificantly reduced the biomass of alligator weed by decreasing its resourceopportunity. Functional traits of species also influenced the success of theinvasion. , in the same morphological andfunctional group as alligator weed, was significantly resistant to alligatorweed invasion. Because community invasibility is influenced by many factorsand interactions among them, the pattern and mechanisms of communityinvasibility are likely to be far subtler than we found in this study. Morecareful manipulated experiments coupled with theoretical modeling studies areessential steps to a more profound understanding of communityinvasibility.
Pärtel M, Zobel M, Zobel K and van der Maarel E (1996) The species pool and its relation to species richness: evidence from Estonian plant communities. Oikos 75: 111–117.
Species richness, defined as the number of species per unit area, is the simplest measure of biodiversity. Small‐scale species richness generally refers to species richness at the scale of a single community, habitat or microhabitat. Understanding the factors that affect and are affected by small‐scale species richness is fundamental to understanding how ecological communities are assembled and function and how biodiversity is maintained. Several factors affect small‐scale species richness, including geographic factors such as the regional species pool, dispersal distance and ease of dispersal, biological factors such as competition, facilitation, and predation as well as environmental factors such as resource availability, environmental heterogeneity and disturbance frequency and intensity. The importance of these factors varies with scale of observation. Further, small‐scale richness can impact aspects of ecosystem function including productivity, stability, and invasibility.
lishment during the vegetative period and flowering seasonof , but after bearing in August, the vegetative bodygradually withered away, and invasion resistance decreased significantly. Thissuggests that resource availability may vary with life history traits and theninfluence its community invasibility. This mechanism in monoculture might alsowork in diverse communities. In these communities, different species may havedifferent history traits and use the same resources at different times. Thus,these communities would provide fewer windows of opportunity for invaders toestablish a population. Furthermore, sampling effects could also operate,i.e., the probability of including the more competitive species or functionalgroups for a given resource in a community may increase as community diversityincreases. In this regard, a plant community may become more susceptible toinvasion whenever the amount of unused resources increases, as reported byDavis et al. (2000)