Tropical Forest Community Ecology

Colonization-related Trade-offs in Tropical Forests 183

this mechanism of coexistence, which shows that
these trade-offs can potentially make a strong
contribution to species diversity maintenance
in homogeneous environments (Skellam 1951,
Levins and Culver 1971, Horn and MacArthur
1972, Armstrong 1976, Hastings 1980, Tilman
1994). However, the conditions for competition–
colonization trade-offs to maintain diversity
arestringentandlikelytobeuncommoninnature,
and in the absence of these conditions these
trade-offs need not be a powerful or even signif-
icant force enhancing coexistence (Geritzet al.
1999, Levine and Rees 2002, Kisdi and Geritz
2003a).
Other colonization-related trade-offs can
contribute to habitat partitioning among species,
and thereby to coexistence in heterogeneous envi-
ronments. A trade-off between fecundity and the
ability to tolerate low resource conditions or harsh
habitats can mediate coexistence when there is
spatial variation in resource availability or the
harshness of conditions among local regeneration
sites (Levine and Rees 2002). A trade-off between
fecundityanddispersaldistancecanmediatecoex-
istence in the presence of spatial variation in the
density of sites suitable for regeneration (Yu and
Wilson 2001).
While competition–colonization trade-offs have
been the subject of extensive theoretical inves-
tigation and of empirical research in a num-
ber of plant communities (Turnbullet al. 1999,
Coomes and Grubb 2003), other colonization-
related trade-offs have rarely been studied. In
tropical forests, there has been little research even
on competition–colonization trade-offs. The lim-
ited consideration of such trade-offs in tropical
forests has focused mainly on whether they might
contribute to the coexistence of shade-tolerant
and gap-dependent species (Connell 1978, Leigh
et al. 2004). Nonetheless, there are a number
of relevant empirical studies in tropical forests
whose results shed light on the potential for
colonization-related trade-offs to contribute to the
maintenance of species richness in these diverse
plant communities.
In this chapter, I review the theory and evidence
regarding the contribution of colonization-related
trade-offs to diversity maintenance in tropical
forests.Throughout, I devote the most space to the

competition–colonization trade-off, because of its

premier position in the literature on colonization-

related trade-offs and thus the abundance of

relevant theory and empirical studies. I begin by

briefly reviewing the relevant theory, identifying

the key assumptions and predictions of models

in which these trade-offs contribute to diversity

maintenance. I then consider the methods that

can be used to document these trade-offs and their

roles in real communities, and evaluate the rel-

evant empirical evidence from tropical forests in

particular. I end with recommendations for future

research and a summary of what we can conclude

thus far.

THEOR YON

COLONIZATION-RELATED

TRADE-OFFS AND DIVERSITY

MAINTENANCE

Equalizing versus stabilizing influences

Colonization-related trade-offs have the potential

to exert equalizing and/or stabilizing effects on

diversity maintenance,sensuChesson (2000).

Equalizing influences minimize fitness differences

among species that would otherwise lead to

competitive exclusion (Chesson 2000), making

dynamics less exclusionary and more neutral

(sensuHubbell 2001). If there is merely par-

tial equalization so that species remain less than

perfectly equal, dynamics are near-neutral, and

the weaker species are deterministically excluded

(Zhang and Lin 1997, Yuet al. 1998), albeit

at a slower rate than they would be without

the trade-off (Figure 11.1a–d). If there is per-

fect or complete equalization, species become

equal in competitive ability, and thus are subject

to neutral drift (Hubbell 2001) (Figure 11.1e,f).

In contrast, stabilizing influences actively con-

tribute to diversity maintenance by increasing

negative intraspecific interactions relative to neg-

ative interspecific interactions (Chesson 2000).

This ensures that each species is relatively advan-

taged when rare and disadvantaged when com-

mon, which tends to kee ps pecies from extinction

or monodominance (Figure 11.1g,h). Stabilizing

influences make dynamics less neutral, but in a