- Several mechanisms have been proposed (Holt 1977, Holt and Kotler 1987,
Holt 1987) by which a shared predator can mediate the reduction or exclusion
of one prey species by another (apparent competition). A novel twist on this
idea is that one prey species can act as a spatio-temporal corridor for the
predator to reach the other prey leading to a reduction in that prey's numbers
("quantitative apparent competition") or even to its elimination
("qualitative apparent competition"). The key elements in this mechanism are
that interactions are local, and dispersal is short-range.
- In this work, interest focuses on a target prey species, with high birth
rate and a non-target species, which provides an additional resource for a
predator which is introduced to control the pest. It is shown that the
non-target prey species can facilitate the control of the pest, and that this
role is enhanced when interactions are localized. Thus, models that do not
account for the localization of interactions may underestimate the potential
for control, or may overestimate the impact of the predator on the non-target
prey species. These results have important implications for the theory and
practice of biological control.
- Figures 2-8 follow. See
Apparent Competition and Biocontrol for the accompanying text.
FIGURE 2: Transfer diagram for the three species model showing
probabilities of moving from one state to another. Parameters are defined
in the text.
FIGURE 3: Time series for a local dispersal model demonstrating
qualitative apparent competition (Here b1=.008, b2=.006,
d=.004, d1=.08, d2=.04, beta=.001, beta1=.2,
beta2=.1, delta1=.005, and delta0=.0001).
When the pest and non-target prey are introduced concurrently the pest
displaces the non-target species (a). When the pest and predator are present
the two species persist, going through several initial large oscillations and
later smaller oscillations (b). When all three species are introduced together
(c), the pest is driven extinct and the non-target species persists with the
predator.
FIGURE 4: Time series for global dispersal model demonstrating for the
same parameters given in Figure 3. As in the global dispersal situation, when
the pest and non-target prey are introduced concurrently the pest displaces
the non-target species (a). Similarly, when the pest and predator are present
the two species persist, however the oscillations damp through time (b). For
this dispersal regime, there are two alternative outcomes when all three
species are introduced together. When initial densities are low, the
non-target species is driven extinct and the pest persists with the predator (c).
When initial densities are high, the pest is driven extinct and the non-target
species persists with the predator, as in the local dispersal case (d).
FIGURE 5: Regions in which pests and/or non-target prey persist for the
global dispersal parameters in Figure 4. Three initial prey densities were used
(.05, .1 and .15 for each species), paired with a range of initial predator
densities (0 through .5 at intervals of .02). The winning species for each
combination of initial densities was determined by running the model 4 times
for 10,000 generations for each parameter set and noting which prey species
was present at the end. For each trial only one species remained at the end.
In the upper region that species was the non-target prey. For the middle region
the winner was sometimes the pest and sometimes the non-target species. At the
lowest densities the pest always persisted.
FIGURE 6: Densities of the predator, pest and non-target prey over
time demonstrating quantitative apparent competition. When dispersal is global
(a), the density of the non-target prey is approximately the same as it is for
the mean field model (.13). However, when dispersal is local (b), the density of
non-target prey similar to the mean field density when pests are absent (.17).
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