Geoscience Reference
In-Depth Information
It is important to consider how the variability at the individual level alters
basic properties in the species-level food web (
Gilljam et al., 2011
). In the
context of competitive communities, it has been shown that individual varia-
tion increases coexistence if variation among individuals in many dimensions
is species-specific (
Clark, 2010
). In our food web data, the relationship
between intraspecific variability in connectivity, sampling effort and
species-level connectance is straightforward. If—as shown in empirical
data—a few individuals contain most of the prey items regardless of the
location and time of the sampling, then a small increase in sampling thresh-
old collapses species-level connectance and the food web becomes discon-
nected. An alternative explanation for the greater number of abundant fish in
some environmental situations can be based in the high variance in number
of prey among individuals. As shown by the empirical data, this variance
decreases species-level connectance, and thus following May's corollary, this
increases the probability of higher stability and species coexistence (
Cohen
et al., 1990; May, 1973
). The lower variance from the neutral expectation in
individual number of prey requires—at least for the Guadalquivir estuary
food web—twice the sampling effort to entirely disconnect the network.
Most food web theory uses mean-field equations based in the law of mass
action to describe the dynamics of populations of interacting individuals.
While ignoring both the scale of individual interactions and their spatial
distribution (
Pascual, 2005
), population-mean approaches are useful as neu-
tral expectations to study alternative mechanisms driving the observed data.
Our model working at individual level uses the same basic assumptions, that
there is a well-mixed system at individual level, which is the root of the
Lotka-Volterra equations and their many descendants (
Lotka, 1956
). The
much larger variance observed in the individual number of prey compared to
the model expectation and the small fraction of variance accounted for by
variations in the size of individual predators raise a number of important
questions. Is the variance of individual number of prey caused by the spatial
structure of the resources? Is it imposed by the differences in digestion rate or
metabolic constraints and foraging behaviour of individuals? How does such
variance alter the speed between the ecological and the evolutionary dynam-
ics? Our final aim has been to reconcile these questions by setting a baseline
throughout scales—from individual-level processes to food web dynamics.
B. Perspective
Important future advances linking population genetics, genetic differentiation
and speciation in the context of food web dynamics and spatial landscapes will
be to develop analytical relationships among the important parameters, par-
ticularly mutation-speciation and trophic rate by using explicit individual
