372 Chris J. Peterson and Walter P. Carson
patterns of succession were highly repeatable
and very similar within broad regions (see also
MacMahon 1981). Tilman’s model assumed that
light and soil resources (particularly nitrogen)
were inversely correlated durin gsuccession. Thus
dominant species were superior competitors at
equilibrium for the ratio of light and nutrients
that occurs at a given period during succession.
As the ratio of these two resources changes,
different species would gain a competitive advan-
tage and become dominant. In this model, space
is implicit, while adequate propagule availability
is assumed amon gall species (Table 22.1). The
most important life-history traits are the require-
ments of a species for light and soil resources and
how this defines their competitive ability. Tilman
(1985) did not consider effects of dispersers or
herbivores.
Shade tolerance
Huston and Smith (1987) developed an indi-
vidualistic model mainly for secondary forest
succession that relied on inversely correlated
life-history traits. In particular, they focused on
longevity, sapling establishment rate, and max-
imum size, age, and growth rates (Table 22.1).
They argued that population-level models are too
simplistic to explain successional dynamics even
whenpopulation-levelmodelsproducepredictions
that are confirmed in nature. They emphasized
variation in conditions, particularly light lev-
els, at the scale of individuals, and showed that
when competition for light is strong, the trade-
off between shade tolerance and other life-history
traits may be the primary mechanism that under-
lies species replacement (Table 22.1). Huston and
Smith demonstrated through the use of com-
puter simulations that hypothetical species that
differed in growth rate or rate of sapling estab-
lishment, and shade tolerance or maximum size,
would reproduce actual successional dynamics.
As with several of the previous models, this model
assumed adequate propagule availability and did
not consider any animal impacts. Although they
mentioned variation in dispersal ability among
species, their model was not spatial. Similar to
the resource ratio model, this model focused on
competitive interactions (primarily for light) as
major drivers of successional change.Hierarchical causesPickettet al. (1987) developed a hierarchical
successional framework that explicitly consid-
ered nearly all conceivable causes of turnover
durin gsuccession. Unlike the other successional
models, this approach focused far greater atten-
tion on site characteristics, rather than only
species characteristics. Pickettet al. established
that there are three fundamental causes of com-
positional change and its variation during suc-
cession: (1) site availability, (2) differential species
availability (e.g., identity and number of propag-
ules of potential colonists), and (3) differential
species performance (e.g., differential competi-
tive ability of species that actually colonized the
site). This approach emphasized neglected aspects
of succession such as how initial site size and
landscape configuration could alter successional
processes. The advantage of this framework was
that one could choose amon gan exhaustive
list of processes that could influence succession
(e.g., colonization limitation) at any given site and
pick ones that applied to a give locale. The draw-
back of this approach was that all mechanisms
were implicitly given equal importance within
each position of the hierarchy and succession was
viewed as relatively site specific, leavin glittle room
for generality. Regardless, this exhaustive hierar-
chical framework undoubtedly applies to tropical
systems. We suggest that this paper should be read
by anyone studyin gsuccession because it is com-
prehensiveandconsidersnearlyallpossiblecauses
for species turnover.DO STUDIES OF
POST-AGRICULTURAL
SUCCESSION INCORPORATE OR
TEST TEMPERATE MODELS?
Overall we found that few studies of tropical
post-agricultural succession were designed to test
the above models or were motivated by these
models. Indeed, amon g61 publications reviewed
none identified that their objectives included