The high biodiversity in tropical forests has both fascinated and puzzled ecologists for more than half a century. In the hopes of finding an answer to this puzzle, ecologists have turned their attention to the spatial patterns of such communities and mapped the location of each tree with a stem larger than a pencil in plots covering 25 to 52ha of tropical forest around the world. In a study published in The American Naturalist a German - Sri Lankan research team has now undertaken thousands of spatial pattern analyses to paint an overall picture of the association between tree species in one of these plots in Sri Lanka.
"The problem of studying spatial association between species is that habitat association confounds the effect of plant-plant interactions" says Dr Wiegand, senior scientist at the Helmholtz Centre for Environmental Research in Leipzig, Germany. The breakthrough in their analysis is that it allowed them to disentangle these two effects and to look in a new way at their data. "From previous studies we knew that growth and survival of trees depends quite strongly on their neighbors" say Savitri Gunatilleke and her husband Nimal, both professors at the University of Peradeniya, "we had therefore expected to find strong signatures of positive or negative interactions between species in our data". "However, the fact that not more than 5 percent of the 2070 species pairs we have analyzed showed significant associations is quite remarkable." A conclusion of their study is that neighborhood-dependent processes may equilibrate, thereby producing neutral association patterns in the spatial distribution of trees. "This is certainly not the last word in this debate," says Wiegand "but it is a step towards an understanding of the complexities of the origin and maintenance of species richness in tropical forests."