Species Competition and Predation - Ecological Systems - page 225

Fig. 13.1 Representation of a subset of a food web with arrows showing directions of effects from
one species to another. Differences in the thicknesses of arrows are meant to indicate differences in
the strengths of effects. The different symbols distinguish trophic levels, and their size differences
are meant to imply differences between species in population size within a trophic level. The
letters R and P indicate that they are respectively resources and predators of the species, N , in the
middle trophic level, with subscripts labeling species within a trophic level. These subscripted
letters serve simultaneously as species labels and as the population densities of the species. The
recursive arrows for resources mean they experience direct intraspecific competition of some form
giving density feedback to themselves, but not direct interspecific feedback within the resource
trophic level. An assumption like this is common in the Lotka-Volterra models discussed in this
essay, and may also be applied to the predators too
How species in a guild interact with each other, and ultimately coexist or instead
exclude each other, leads to an understanding of how communities are structured,
that is, an understanding of the relationships between the traits of different species
that allow them to come together to form a long-lasting community, or in other
words are assembled [ 20 , 21 ]. Competition can be understood by considering the
linkages between focal-guild species and their resources in the trophic level below.
Linkages with the tropic level above lead to an understanding of apparent competi-
tion [ 22 ].
The arrows in the diagram show directions of effects. A species benefits from
arrows pointing to it from a lower trophic level and suffers from arrows pointing to
it from a higher trophic level. Chains of arrows define pathways of effects. Compe-
tition and apparent competition can be understood by pathways leading from the
middle trophic level back to that level. For example, the species N 2 has pathways
going from itself to each resource species, R , back to itself. These pathways
contribute to intraspecific competition for N 2 . The idea is that an increase in the
density of N 2 leads to greater consumption of each resource species, R , reducing
their densities and thereby reducing the availability of these resources for N 2 .
Pathways from these resources back to other species contribute interspecific com-
petition. Thus, the pathways from N 2 through R 3 and R 4 back to N 3 lead to
interspecific competition for N 2 on N 3 . Through these various pathways, increasing
the density of N 2 feeds back negatively to itself and to other species in the same