Environmental Engineering Reference
In-Depth Information
those resources in similar ways, and susceptible to the
same or similar natural enemies. The standard of similar-
ity in this definition is not precise, and varies depending on
the purpose of the investigator.
Natural enemy
An organism that benefits its own reproduction or survival
by harming the individuals of a given species, commonly
by feeding on them.
Niche overlap
For any pair of species, the degree to which density
dependence through feedback loops is concentrated
between species compared to within species. It is
measured by the quantity
r
which varies between zero
for no overlap (no interspecific density dependence) and 1
for complete overlap (interspecific density dependence is
on average equal to intraspecific density dependence).
Predator
A species that gains food by killing and consuming
individuals of the species in the ecological guild in
question.
Species average
fitness
For a given species in a guild, it is a numerical measure of
how well that species is adapted to the environment with
the property that it predicts which species would dominate
if the niche overlaps,
r
, were all equal to 1. It is normally
related to the long-term average per capita growth rates of
the species measured at fixed levels of competition and
apparent competition. In this essay, the fitnesses
k
are
obtained from per capita growth rates at zero levels of
competition and apparent competition, which are achieved
by setting all members of a guild at zero density. These
growth rates are then divided by scaling factors that correct
for differences between species in their levels of sensitivity
to competition and apparent competition.
Stable coexistence
The tendency of the members of a guild to recover when
individually perturbed to low density, allowing their long-
term persistence in the presence of interactions with other
guild members.
Definition of the Subject
Competition and predation are key interactions between species, and are major foci
of thought and study in community ecology. They are believed to be major forces
structuring natural communities, having critical roles in the determination of species
diversity and species composition, and are regarded as important drivers of evolution-
ary processes. The relationships between the niches of different species determine
how they interact through competition and predation, which then have key roles in
