Herbivores and the Number of Tree Species in Tropical Forests

TLDR

Low‑land tropical forests are characterized by a high number of tree species, low density of adults per species, and long distances between conspecific adults. The author proposes that these patterns largely result from predator effects on seeds and seedlings. The model examines how predator efficiency, dispersal, seed‑crop size, and environmental severity affect adult density, conspecific spacing, and species richness, predicting that lower predator efficiency in drier or temperate zones permits dominant species to monopolize the habitat. Events that increase predator efficiency reduce adult density or increase spacing, creating more habitat space for other species and thus higher total tree species richness, provided seed sources persist over time.

Abstract

A high number of tree species, low density of adults of each species, and long distances between conspecific adults are characteristic of many low-land tropical forest habitats. I propose that these three traits, in large part, are the result of the action of predators on seeds and seedlings. A model is presented that allows detailed examination of the effect of different predators, dispersal agents, seed-crop sizes, etc. on these three traits. In short, any event that increases the efficiency of the predators at eating seeds and seedlings of a given tree species may lead to a reduction in population density of the adults of that species and/or to increased distance between new adults and their parents. Either event will lead to more space in the habitat for other species of trees, and therefore higher total number of tree species, provided seed sources are available over evolutionary time. As one moves from the wet lowland tropics to the dry tropics or temperate zones, the seed and seedling predators in a habitat are hypothesized to be progressively less efficient at keeping one or a few tree species from monopolizing the habitat through competitive superiority. This lowered efficiency of the predators is brought about by the increased severity and unpredictability of the physical environment, which in turn leads to regular or erratic escape of large seed or seedling cohorts from the predators.