Tree Diversity in Tropical Forests Is Determined by How the Seeds Disperse

Nowadays, human action has accelerated 1,000 times the process of natural extinction of the species. Stopping the loss of biodiversity can be made only knowing the mechanisms that supports it.

The highest biodiversity is found in the tropical forests, thus, studying the processes that occur in these forests will help understand principles that drive diversity. And, amongst Earth's ecosystems, tropical forest is the most affected by habitat loss, with 0.25 % of it disappearing yearly.

In tropical forests, the number of tree species on a hectare can be by the order of hundreds, while in the most diverse temperate, forest reaches a maximum of few dozens and is usually dominated by a small number of species.

In tropical forests, the trees of the same species often cluster in scales ranging from a few meters to a few hundred meters. It is possible that clustering promotes diversity, by separating species and reducing competition. Limited seed dispersal of tropical pioneer trees (the first trees to grow in a place where trees were removed or fell) limits the spatial distribution of the individuals.

A new study has demonstrated that even the tree species that grow on a mature forest have a limited dispersal power, so there won't be dominant species. This booms species diversity.

Researchers Tristram Seidler and Joshua Plotkin have compared dispersal methods and cluster forming of 561 tree species found on a pristine forest in Malacca Peninsula within a 50-hectare area.

The tree species used ballistic dispersal ("explosive liberation" of seeds when dry mature fruits crack, throwing seeds at a certain distance from the parental tree), gravity, gyration (fruits or seeds endowed with "wings" that allow them to rotate through air, falling at a certain distance from the parental tree), wind, small arboreal animals (small birds that will eat and disperse small fruits), and larger arboreal animals (larger birds and mammals will eat larger fruits.)

The researchers discovered a link between the spatial aggregation and the dispersal method of a tree species. The most aggregated groups were of species with ballistic dispersal, followed by gravity, gyration, wind, and animal dispersal. Trees spread by small animals were more aggregated than those spread by larger animals.

This confirms the hypothesis that larger animals eat bigger fruits, move over larger territories and carry the seeds farther. These correlations could have produced an evolutionary link among species.

The tight association of the wind dispersed trees might be a result of the wind limitation by the canopy, but also by the canopy itself. At smaller scales, the aggregation effect is very strong, but at larger scales - over 200 m - this effect tends to vanish, and does not vary depending on dispersal mechanisms. The same dispersal effect was found on a tropical forest in Panama, where the tree community was formed by totally distinct species.

This leads to the idea that the link between dispersal and abundance is general in the tropical forests. Of course, this study analyzed just one factor responsible to the forest biodiversity.

There are also other factors which have to be taken in account when calculating tree biodiversity in the tropical forests, like how conspecific trees take root in a community.