The Contribution of Seed Dispersers to Tree Species Diversity in Tropical Rainforests

Kakishima, Satoshi, Satoru Morita, Katsuhiko Yoshida, Atsushi Ishida, Saki Hayashi, Takahiro Asami, Hiromu Ito, et al. “The Contribution of Seed Dispersers to Tree Species Diversity in Tropical Rainforests.” Royal Society Open Science 2, no. 10 (October 14, 2015). doi:10.1098/rsos.150330.

An important area of ecological research is the phenomenon of high diversity of tree species in tropical rainforests, particularly how this diversity is created and maintained. While there are two general hypotheses for why this occurs, the niche and neutral theories, neither explains the trend perfectly, and studies continue to explore the finer mechanisms responsible for high species diversity. Kakishima et al. (2015) apply a relatively novel approach of modeling movement of seed dispersers of tree-disperser mutualism and allopatric speciation events to investigate the coexistence of species.

The authors create a “multilayered” 200 x 200 lattice model, consisting of a layer in which tree species occupy individual cells, and n layers for n species of seed dispersers, for a total of n + 1 layers. Each tree species has a corresponding seed disperser that exists in a one-one mutulatism, although this assumption is relaxed later in the study. In the plant layer, each cell is either occupied by tree species i or empty. Animal diapers move throughout their own layer, although are limited in cells they are allowed to visit, which is meant to represent a “niche”. However, these cells are randomly distributed throughout the lattice, and do not resemble a gradient, as would be expected if a niche were to be partly driven by the environment. Reproduction and mortality are highly simplified in this model, with the only population dynamics experienced in either plants or animals being a constant mortality rate that is specific to tree species. Allopatric speciation is incorporated via three fragmentation processes, by which the landscape is fragmented in half, and each animal species population is allowed to “evolve” by adjusting the cells they are and are not allowed to visit.

There are several parameter spaces that allow for coexistence between tree species. High niche separation of the animal dispersers (modeled by a large number of sites which each individual species cannot visit) allowed for the highest number of species, whereas no restriction on movement allowed only the tree species with the lowest mortality to persist. If the species specificity of seed dispersal is relaxed (dispersers can move more than one species of tree), coexistence decreases, although this relationship can be mediated by increasing the number of sites an individual disperser cannot move into. When speciation is allowed to occur, it is only intermediate niche shifts that allow for coexistence. Small and large niche shifts resulted in competitive exclusion and random distribution of species, respectively.

Modeling the individual movements of seed dispersers, and the resulting tree dynamics, illustrates the important role of niche heterogeneity in dispersers in promoting species coexistence. This was also highlighted by the step of the model, which differentiated dispersers’ niches further, and allowed for coexistence of all species. An especially interesting part of this paper is that the tree dynamics were not modeled at all, except for the tree mortality, and yet coexistence could still be attained, relying solely on the movement of seed dispersers.

A cartoon of the modeling framework described above. a & b show dispersal steps while c demonstrates the allopatric speciation.