some similar assumptions. For instance, our model
contains the influence of body mass on individual
production and mortality rates (equations 12.1),
two components of allometric theory. Second, com-
munity evolution models account explicitly for the
coevolutionary dynamical process that is supposed
to underlie the patterns revealed by allometric
theory.
Consider one of the main results of allometric
theory, i.e. the distribution of species abundances
as a function of body size. Damuth (1981) showed
with empirical data that the densityDof a given
species is related to its mean body mass, notedxby
the relationshipD¼kx0.75. Since the mean meta-
bolic rateMof an individual is linked to its body
size by the relationshipM¼k^0 x0.75(Kleiber 1961),
the total amount of resourcesEconsumed by a
given species in the system should beE¼MD¼
kk^0 x^0 , i.e. the energy consumed by a species is inde-
pendent of its body mass. This prediction is called
the energetic equivalence rule (Damuth 1981; Nee
et al. 1991). Although the mechanism that is sup-
posed to lead to this equal partitioning of resources
among species is somewhat vague, coevolution of
species that share a same set of resources has been
invoked (Damuth 1981; Maiorana and Van Valen
1990). This influential rule has been tested using
empirical data with both successes (Damuth 1981,
1991, 1993; Marquetet al. 1990; Neeet al. 1991; Long
and Morin 2005) and failures (Brown and Maurer
1986; Greenwoodet al. 1996; Cyr 2000; Cohenet al.
2003; Russoet al. 2003) Although it was initially
derived for species within a single trophic level, it
was later extended by others to systems that con-
tain multiple trophic levels. Allometric theory then
predicts that the exponent that links density and
body size is1, so thatD¼kx^1 (Brown and Gil-
looly 2003).
Interestingly, the model presented in section
12.2.1.2 contains some components that are simi-
lar to the ingredients used in Damuth’s energetic
equivalence rule. The allometric relationships
used for production and mortality rates are in-
ferred from individual metabolism, and the
model simulates species coevolution on shared
resources, the mechanism that was proposed for
the emergence of the perfect sharing of resources
between community members. Therefore, it is
possible to test this mechanism (keeping in
mind, of course, the limits of the model’s as-
sumptions) and to see for which parameters, if
any, the predicted links between population den-
sity or energy use and body size are observed.
The results show that population density is a
decreasing function of body mass, but the expo-
nent of the relationship depends on the strength
of competitive interactions and on the niche
width of consumers, so that coevolution does
not lead to an equal partitioning of energy
among species (Loeuille and Loreau 2006). This
example illustrates how community evolution
models may give additional insights to the allo-
metric theory of ecology. Such models can in-
clude allometric components when they
consider body size as an evolving trait. Because
they consider dynamical components of popula-
tions instead of focusing on the equilibrium
communities, they may also be used to test me-
chanisms assumed to explain allometric patterns.12.4 Conclusions, and possible extensions of community evolution models
Community evolution models make three major
contributions to community and ecosystem ecolo-
gy. First, they extend classical pairwise coevolu-
tionary models to large, complex ecosystems, with
new results. Take the example of how evolution, or
coevolution, affects population dynamics. In small
communities, some studies show that evolution or
coevolution may have stabilizing effects (Pimentel
1961; Saloniemi 1993; van Baalen and Sabelis 1993)
while others suggest the contrary (Abrams and
Matsuda 1997; Yoshidaet al. 2003). As we have
pointed out in section 12.3.1, the results seem to be
less ambiguous in more complex community evo-
lution models, in which evolution tends to produce
large assemblages of species that are stable on a
demographic timescale.
Second, they provide, for the first time, insights
into the evolutionary emergence of entire food
webs or ecosystems. Classical evolutionary models
have mostly considered evolution or coevolution of
pre-existing species. In community evolution mod-
els, species themselves emerge spontaneously from
the evolutionary dynamics of the system.176 FUTURE DIRECTIONS