Relating intra-specific variation in function and performance to relative abundance: An example from Chinese tropical tree seedlings
One of the few 'rules' in ecology is that communities are composed of many rare and a few dominant species. Nowhere is this pattern more evident than in diverse tropical tree communities. However, we still lack an understanding of the factors generating relative species abundance distributions in these systems. Detailed studies of intra-specific variation in function and performance are needed to provide strong tests of leading hypotheses for commonness and rarity. For example, an important hypothesis is that rare species are specialized on habitats that are themselves rare thereby preventing large population sizes. An alternative hypothesis is that rare species represent sink populations ill-suited to all available habitats. These hypotheses can be tested with information about individual performance, but demonstrating how organismal function is linked to performance is required for predictive ecology. Specifically, the first hypothesis predicts that rare species should have relatively high rates of growth and specialized phenotypes with low variance. The alternative hypothesis predicts that rare species should have relatively highly variable rates of growth and there should be a wide range of phenotypes all of which are ill-suited to the available habitats. Here we address these predictions by first quantifying the relationship between variation in growth and species relative abundance. Then we quantify whether intra-specific variation in functional traits is related to species relative abundances and the mean and variation of growth within the species. The study utilizes data we have collected at individual level of more than 1800 individuals of 100 species of seedlings in 218 seedling plots from a tropical rain forest, in China.
Our results show that abundant species exhibit lower variance in relative growth rates and lower variance in functional traits than less abundant species. However, mean relative growth rates were not different for dominant or rare species. These findings provide partial support to the second prediction, where rarer species, in general, exhibit more variable performance linked with low functional specialization, while dominant species exhibit higher and less variable performance and low intra-specific trait variation. Given the results, we propose rare species are transient species that take advantage of temporal fluctuations to bring occasionally individuals to the community. On the other hand, abundant species represent more persistent species exhibiting strong associations with the environmental conditions. The results also suggest that frequently ignored dimensions of biodiversity (intra-specific functional diversity and genetic diversity) contain key insights into community structure.