Chapter 7

LargeTropicalForest

DynamicsPlots:Testing

Explanationsforthe

MaintenanceofSpecies

Diversity

Jess K. Zimmerman, Jill Thompson, and Nicholas Brokaw

OVERVIEW

Large tropical forest dynamics plots (FDPs) play a significant role in developing theory and testing hypotheses that may
explain the species diversity of tropical forests. Here we summarize the contribution of FDPs belonging to the network
coordinated by the Center forTropical Forest Science (CTFS). In CTFS FDPs all trees and shrubs with diameter at breast
height of 1 cm or more are identified and measured for diameter, and their locations are mapped in plots that range in
size from 2 to 52 ha (most are 16–50 ha). By virtue of their large size, the FDPs present a comprehensive picture of
relative species abundances and species distribution in tropical forests, including the contribution of rare species. The
plots have demonstrated that the shape of the species abundance curves is similar in a wide geographical and structural
range of tropical forests. Mapped locations of trees and repeated censuses (usually at 5-year intervals) provide data
on spatial and temporal dynamics critical to testing theoretical explanations for high species diversity in tropical
forests. In this chapter we concentrate on three potential explanations for the high level of species diversity observed
in tropical forests that have received particular attention using data from the FDPs: (1) neutral theory, which is a null
model for community dynamics; (2) negative density dependence (NDD); and (3) gap specialization and dynamics.The
differences among these explanations reflect, among other things, rare species advantage and the relative importance
of the life-history characteristics of each species, and how they may determine the community dynamics.The value of
neutral theory as a general explanation for the high species diversity of tropical forests is weakened by the abundant
phenomenological evidence from FDPs that demonstrates that NDD operates in most tropical forest. Although a small
number of “pioneer” tree species and non-tree species are specialized for growing in canopy gaps, information from
large FDPs does not support a strong role for gap specialization as an explanation for the high diversity of tropical
forest trees.

INTRODUCTION

How to explain the great species diversity of most
tropical forests is a fundamental issue in tropical
ecology (Connell 1978, Leighet al. 2004), and
one that also demonstrates the great difficulty of
testing ecological theory in the tropics. In most
tropical forest communities, only a few species

are common, and many species are rare. In such

diverse communities it is difficult to obtain a suf-

ficient sample of trees to provide an estimate of

community diversity and structure, or determine

the demographic parameters of individual species,

in order to test ecological theory. One solution is

to census large areas (i.e., tens of hectares) of for-

est, thereby assuring that the sample comprises a