One of the profound consequences of exotic species invasion is biotic homogenization. Biotic homogenization is caused by some environmental modification that facilitates species invasions or extinctions such that different sites become similar in their biotas. Invasive species, particularly plants, can cause changes in species richness, edaphic factors, hydrological factors, and nutrient cycling in the invaded ranges. Through these changes, they can cause biotic homogenization, in which beta diversity decreases and local plant and animal communities become increasingly similar. Whether this homogenization can erase latitudinal patterns of species diversity and composition has not been well studied. We examined this by comparing soil nematode (genus richness, diversity index, feeding types and life history strategies) and microbial communities (microbial biomass carbon, bacteria: fungi ratio, gram-negative: gram-positive bacteria ratio) in stands of native Phragmites australis and exotic Spartina alterniflora in coastal wetlands ranging from 22.43 °N to 39.19 °N in China.
Results/Conclusions
We found clear latitudinal clines in nematode diversity and functional composition, and in microbial composition, for soils collected from native P. australis. These latitudinal patterns were weak or absent for soils collected from nearby stands of the exotic S. alterniflora. Climatic and edaphic variables varied across latitude in similar ways in both community types. In P. australis there were strong correlations between community structure and environmental variables, whereas in S. alterniflora these correlations were weak. These results suggest that the invasion of S. alterniflora into the Chinese coastal wetlands has caused profound biotic homogenization of soil communities across latitude. We speculate that the variation in P. australis nematode and microbial communities across latitude is primarily driven by geographic variation in plant traits, but that such variation in plant traits is largely lacking for the recently-introduced exotic S. alterniflora. These results indicate that widespread exotic species can homogenize nematode communities at large spatial scales.