On the balance between niche and neutral processes as drivers of community structure along a successional gradient: insights from alpine and sub-alpine meadow communities.

BACKGROUND AND AIMS Neutral theory predicts that the diversity and relative abundance of species in ecological communities do not depend on their specific traits. This prediction remains controversial, as many studies suggest that variations in the niches of species determine the structure of communities. The aim of this study was to test empirically the relative importance of niche and neutral processes as drivers of species abundance within plant communities along a successional gradient. METHODS Information on the abundance (density and frequency) and traits (aboveground individual biomass and seed mass) of > 90 species was collected in alpine and sub-alpine meadows of the Tibet Plateau (China). A successional gradient (1, 3, 15 and 30 years after abandonment) was established in a sub-alpine meadow. The relationships between species traits and their abundance were evaluated using regression models. KEY RESULTS Seed mass was negatively related to both species density (r = -0.6270, P < 0.001) and frequency (r = -0.5335, P = 0.005) in the 1-year meadow. Such relationships disappeared along the successional gradient evaluated (P > 0.07 in the 3-, 15- and 30-year meadows). Data gathered in all sites showed a significant negative relationship between the average individual biomass of a given species and its density within the community (r < -0.30, P < 0.025 in all cases). CONCLUSIONS The results show that seed mass was a key driver of species abundance in early successional communities, and that niche forces may become more important as succession progresses. They also indicate that predictions from neutral theory, in its current form, do not hold for the meadow communities studied.