Tropical Forest Community Ecology

18 Jérôme Chave

provide an evaluation of the relative importance
of dispersal and of environment in the shapin gof
beta-diversity patterns.

UNDERSTANDING THE CAUSES OF

SPATIAL VARIATION IN SPECIES

DIVERSITY

The goal of community ecology is to explain
patterns, not just to document them. Theoreti-
cians have now developed models that can be
used to derive predictions for beta-diversity pat-
terns. Hubbell (2001) provided a good overview
of these theoretical attempts and classified them
into two groups, referred to as niche-assembly the-
ories and dispersal-assembly theories (Figure 2.3).
Before delvin ginto the neutral theory’s predic-
tions of beta-diversity patterns, I shall first discuss
the more traditional theory, that of habitat spe-
cialization, and its predictions for patterns of
beta-diversity.

Niche-assembly theories

Niche-assembly theories posit that species are
distributed not randomly but as a result of

environmental constraints and competitive dis-

placement. Two types of niche-related mecha-

nisms may act upon a species and determine its

presence at a given site, namely aphysiological

filterand abiotic filter, a distinction that is far from

new even in the tropical ecology literature (see

questions (a) and (c) in Poore 1968, p. 144).

By physiological filter (or “stress,”sensuGrime

1977), I mean that certain environmental fea-

tures prevent the establishment of plants that do

not present specific adaptations to the chemical

composition of soils, water availability, or light

availability. Soils may contain metals toxic for

certain plants, either naturally in acidic soils or

as a result of human contamination (aluminum,

lead, cf. Baker 1987). Water availability, includ-

in glen gth of the dry season, is another crucial

environmental feature for plant species vulnera-

ble to embolism and cavitation in the xylem water

column (Tyree and Sperry 1989, Engelbrecht

and Kursar 2003). Other species may be unable

to establish in the anoxic soils prevailin gin

periodically flooded forests or peat swamps, where

other plants thrive (Ashton 1964, Webb 1969,

Newberyet al. 1986, Tuomisto and Ruokolainen

1994). Finally, plants may develop protections

against herbivores that are specific to the habitats

Niche-assembly models Dispersal-assembly models

Figure 2.3 Theoretical distributions of plant species. Four species are represented and they occupy six patches.
Accordin gto the niche-assembly theory (left) the distribution of the species is mostly due to the species’ preference for
environmental conditions (the patternin gof the patch matches the pattern of the locally most adapted species).
Accordin gto the dispersal-assembly theory (ri ght) the environment is homo geneous across patches, but species may
have dispersed from different regions, which is reflected in their current local distribution.