Priyanga Amarasekare. 2000. Coexistence of competing parasitoids on a patchily distributed host: local vs. spatial mechanisms. Ecology 81:1286-1296.
Appendices
Appendix A: Potential mechanisms for parasitoid coexistence.
Ecological Archives E081-014-A1.
Both parasitoid species are specialists on eggs of harlequin bugs. No other insects inhabiting the coastal sage scrub ecosystem produce eggs sufficiently large to accommodate these two parasitoids. Hence, it is unlikely that coexistence occurs via specialization on distinct resources. Coexistence via partitioning the same limiting resource is a more likely possibility. I discuss four potential mechanisms and why they are unlikely to apply to the host-multiparasitoid system being studied. The two plausible hypotheses are discussed in the text.
Competition and predation
Coexistence may occur in the face of asymmetric inter-specific competition if
the superior competitor is differentially susceptible to a predator (Sih et
al. 1985, Tilman and Pacala 1993) or hyperparasitoid (May and Hassell 1981).
This hypothesis seems unlikely since neither parasitoid is known to be attacked
by hyperparasitoids (Walker and Anderson 1933, Maple 1937, Huffaker 1941, Sjaarda
1989) and the adults are too minute and short-lived to be prey to any insect
predator.
Priority effects and dispersal
When the outcome of competition depends on initial conditions, coexistence can
occur if local populations are linked by small amounts of dispersal (Levin 1974).
This hypothesis is unlikely for two reasons. First, priority effects arise when
both species exhibit a stronger effect on the other species than on conspecifics.
Data show a strong asymmetric effect of Ooencyrtus on Trissolcus
but not vice versa (Sjaarda 1989). Second, one should expect a mosaic
of patches where each species achieves numerical dominance in a subset of patches
and maintains a small population in the other patches (Levin 1974). Such a pattern
is inconsistent with the observed spatial pattern of Trissolcus-only
and two-parasitoid patches.
Temporal niche partitioning
Each parasitoid should be more effective at exploiting the host during periods
when the other species is not (Hutchinson 1961). A trade-off between competitive
ability and tolerance for a physical factor such as temperature or humidity
(Tilman and Pacala 1993) could lead to temporally segregated parasitism patterns
(Utida 1957, Huffaker and Kennett 1966, Takagi and Hirose 1994, Roland and Embree
1995). Trissolcus is more cold tolerant than Ooencyrtus (Sjaarda
1989), which allows it to emerge earlier in the year (February-March as opposed
to April-May for Ooencyrtus). However, parasitism patterns of the two
species overlap for the better part of the year (Fig. 2). Moreover, asynchrony
in emergence patterns in itself cannot explain why some patches are colonized
only by Trissolcus.
Spatial niche partitioning via
aggregation of parasitoid attacks
If the two parasitoid species have aggregated spatial distributions, or attacks,
that are independent of one another, they will have higher encounter rates with
conspecifics (Hassell et al. 1991) than with competitors of other species (May
and Hassell 1981). This will result in stronger intra-specific than inter-specific
competition, thus facilitating coexistence via niche segregation (May and Hassell
1981, Hogarth and Diamond 1984). One would hence expect a high degree of superparasitism
but little or no multiparasitism (Kakehashi et al. 1984). In this system, however,
attacks by the two parasitoid species overlap down to a single egg clutch. Multiparasitism
is frequent, while superparasitism is absent in Trissolcus. These observations
suggest that aggregation of attacks is unlikely to be a major force in parasitoid
coexistence. Moreover, spatial niche partitioning per se cannot explain
the spatial pattern of one-parasitoid and Trissolcus-only patches.
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