Conspecific negative density dependence (CNDD), phylogenetic negative density dependence (PNDD) and habitat filtering are often cited as prominent mechanisms maintaining the composition and diversity of communities. Some studies have discussed a potentially confounding influence of habitat filtering when attempting to quantify negative density dependence but this has not previously been well documented.
Prof. CAO Min and his team of Xishuangbanna Tropical Botanical Garden (XTBG) used a population dynamics dataset of 10,316 seedlings for 269 woody plant species for four contiguous one-year census intervals in the 20-ha Xishuangbanna tropical seasonal rainforest dynamics plot in SW China. Using generalized linear mixed models, they explored the relative importance of CNDD, PNDD and habitat filtering for seedling survival.
The researchers built models of seedling survival dependent on the densities of conspecific and heterospecific neighbors and on the phylogenetic dissimilarities between heterospecific neighbors and focal seedlings. Each of those models was built without and with habitat variables to determine the degree to which habitat filtering affected the prevalence of negative density dependence. They wanted to test whether scaling the effects of neighbors by their phylogenetic distances improved model fit. They also wanted to see what the relative importance of CNDD, PNDD and habitat filtering was and how habitat filtering affected the detectability of CNDD and PNDD.
 The results showed that CNDD and habitat filtering simultaneously influenced seedling survival. Replacing heterospecific neighbor densities with phylogenentic diversity indices improved survival models. Taking habitat variables into account elucidated more clearly the negative impacts of conspecific neighbors (seedlings + adults) on seedling survival, and made the species-specific negative effects of conspecific neighbor densities generally stronger. Their study system showed the opposite effect with respect to PNDD. CNDD and habitat filtering both had vital influences on seedling dynamics and the observed effects of conspecific neighbors were the result of an interaction between them. 
  The study entitled “How does habitat filtering affect the detection of conspecific and phylogenetic density dependence?” has been published online in Ecology.

Contact
LIN Luxiang  Ph.D Principal Investigator
Key Laboratory of Tropical Forest Ecology, Xishuangbanna Tropical Botanical Garden, Chinese Academy of Sciences, Mengla, Yunnan 666303, China
Tel: 86-871-65112637
E-mail: linluxa@xtbg.ac.cn

The 20-ha forest plot in Xishuangbanna (image by LIN Luxiang)