The relationship between species richness and evenness in plant communities along a successional gradient: a study from sub-alpine meadows of the Eastern Qinghai-Tibetan Plateau. China PLoS One

The relationship between species richness and evenness across communities remains an unsettled issue in ecology from both theoretical and empirical perspectives. As a result, we do not know the mechanisms that could generate a relationship between species richness and evenness, and how this responds to spatial scale. Here we examine the relationship between species richness(S) and evenness (Pielou’s J9 evenness) using a chronosequence of successional sub-alpine meadow communities in the eastern Qinghai-Tibetan Plateau. These meadows range from natural community (never farmed), to those that have been protected from agricultural exploitation for periods ranging from 1 to 10 years. A total of 30 sampling quadrats with size of 0.5 m60.5 m were laid out along two transects at each meadow. Using correlation analyses we found a consistent negative correlation between S and J9 in these communities along the successional gradient at the sampling scale of 0.5 m60.5 m. We also explored the relationship between S and J9 at different sampling scales (from 0.5 m60.5 m to10 m610 m) using properly measured ramet-mapped data of a10 m610 m quadrat in the natural community. We found that S was negatively corrected with J9 at the scales of 0.5 m60.5 m to 2 m62 m, but such a relationships disappeared at relative larger scales ($2 m64 m). When fitting different species abundance models combined with trait-specific methods, we found that niche preemption may be the determining mechanism of species evenness along the succession gradient. Considering all results together, we can conclude that such niche differentiation and spatial scale effects may help t