The New Neotropical Companion

soil fertility and undergrowth density, both a measure

of plant productivity. Large mammalian species tend to

range widely and maintain relatively constant densities

over large areas, but small species vary dramatically in

numbers and diversity from one study site to another.

Does Disturbance Help Explain Why

There Is Such High Overall Diversity?

As I discussed in chapter 7, disturbances are common

in the tropics. Climate and weather disturbances, in

a manner roughly analogous to predators, may act to

reduce the competitive edge of any one species against

another (plate 9- 31). The game of competition is in

essence restarted after the disturbance.

Disturbance intensity and frequency are critical

variables in considering disturbance per se as a force for

maintaining high species richness (fig. 9- 1). Consider

what would happen if an area were harshly and

frequently disturbed. The severe physical conditions

would act to preclude high species richness. Not many

species could adapt. This may be, in part, why there

are so few species in the high latitudes relative to the

tropics. But consider also what would happen if an

area never experienced disturbance. Species richness

will theoretically decline due to inevitable interspecific

competition among species. As with Goldilocks’s

porridge, which had to be “just right,” disturbance

frequency and intensity must be neither too severe

nor too limited. In theory, moderate, intermediate

disturbance should result in maximal species richness.

This model, termed the intermediate disturbance

hypothesis (IDH), was first argued by Joseph Connell

to account for high species richness in both rain

forests and coral reefs. It postulates that intermediate-

level disturbance is locally patchy but regionally

continuous, and that the overall disturbance regime is

sufficiently gentle as to maintain high species richness

but sufficiently frequent to prevent extinction through

interspecific competition. The model envisions the

tropics as a mosaic of different- aged disturbance

patches (recall the discussion in chapter 7). Bottom line:

essentially every place is in some state of recovery from

disturbance— there is no real equilibrium in tropical

forests. This means that there is no definitive assemblage

of species that will form the “end point” of development

in any forest community. Instead, species assemblages

will change as affected by disturbance events. There

have been studies that test the intermediate disturbance

model and most add support to it.

There is no shortage of possible topics to study if

you are game for looking more deeply into the factors

responsible for the generation and maintenance of

high species richness and biodiversity in the tropics.

Plate 9- 31. A severe hurricane cleared much of this mangrove

forest on an offshore cay in Belize. Disturbance, of various

degrees of magnitude, occurs in all forests and may be a

major factor affecting interspecific competition and keeping

forests in states of high- diversity nonequilibrium. Photo by

John Kricher.

High

Low

Diver

sity

Disturbances frequent Infrequent

Soon after a disturbance Long after

Disturbance large Small

b

A. Colonizing B. Mixed

A

B

C

C. Climax

b c c

c c

Canopy

Understory

Figure 9– 1. This figure depicts the possible effect of

disturbances ranging from frequent (left) to infrequent (right)

on the diversity of tropical ecosystems. Adapted from Connell,

J. H. 1978. Diversity in tropical rain forests and coral reefs.

Science 1999: 1302–1310. Reprinted with permission from

AAAS.

chapter 9 why are there so many species? 153