Appendix A. Coexistence mechanisms previously studied.
Local density-dependent processes
I have previously excluded three local density-dependent processes: alternative hosts, priority effects, and selective predation (Amarasekare 2000a). A fourth mechanism involves species' differences in larval developmental periods. Ooencyrtus' developmental period is 1.5 to 2 times longer than that of Trissolcus. Analysis of a stage structured model with developmental delays (P. Amarasekare, unpublished results) suggests that this difference in itself is unlikely to enhance trade-off mediated coexistence.
(i) Species-specific responses to variation in temperature and humidity — If the two parasitoids respond differentially to variation in temperature and humidity, each species will be most effective at exploiting hosts at different times of the year. Such temporal segregation can allow coexistence if it increases the strength of intraspecific competition relative to interspecific competition (Chesson 2000). The parasitoids experience seasonal variation in temperature and humidity (e.g., low temperature and high humidity in winter and spring, and high temperature and low humidity in summer and fall). Temporal niche partitioning could occur if Trissolcus is more efficient at host exploitation in the spring and Ooencyrtus is more efficient in the summer. However, it is unlikely that Trissolcus decline in the summer is a response to low humidity per se because it can achieve very high summer parasitism rates in the absence of Ooencyrtus (Amarasekare 2000a).
(ii) nonlinear competitive
responses — Coexistence is possible if the two parasitoids P.
Amarasekare Intraguild Predation and Temporal Variation 2 have
nonlinear resource consumption rates such that the superior resource
competitor experiences greater variation in resource abundance. When
such variation arises from nonlinear functional responses (Armstrong
and McGehee 1980; Gurney and Nisbet 1998; Abrams 2006), coexistence
requires that fluctuations in resource abundance be large enough to
allow the inferior competitor to invade when rare (Armstrong and
McGehee 1980; Gurney and Nisbet 1998). The host-parasitoid system shows
population-level responses that are inconsistent with this mechanism:
host populations with greater fluctuations cause exclusion
of the inferior resource competitor (Ooencyrtus), while host populations with smaller fluctuations allow coexistence (Amarasekare 2003a). Moreover, parasitoid coexistence is possible even in the absence of host fluctuations (Amarasekare, unpublished data). Theory shows that resource fluctuations in combination with nonlinear functional responses aremore likely to allow coexistence if multiple limiting resources are involved (Abrams 2006).
With a single limiting resource, as in the host-parasitoid system, conditions for coexistence are quite restrictive (Armstrong and McGehee 1980; Abrams 2006).
A second mechanism that leads to nonlinear consumption rates is temporal variation in attack rates (Holt et al. 2003). Coexistence requires the superior resource competitor to exhibit greater fluctuations in the attack rate. This appears unlikely in the host-parasitoid system. Because the inferior resource competitorâ€™s (Ooencyrtus) parasitism rate is more sensitive to low temperatures, it is likely to experience greater fluctuations in the attack rate (Sjaarda 1989; P. Amarasekare, unpublished data). With pure resource competition, greater fluctuations in the inferior competitor's attack rate cannot allow coexistence (Holt et al. 2003). However, such variation may enhance trade-off mediated coexistence by reducing the strength of IGP. This possibility remains to be investigated.
(i) Spatial niche partitioning at the local community scale — This can occur if the parasitoids have aggregated distributions, or attacks, that lead to stronger intraspecific than interspecific competition (May and Hassell 1981; Hogarth and Diamond 1984; Taylor 1993). This mechanism is unlikely for the following reasons. The absence of superparasitism in Trissolcus and the high incidence of multiparasitism lead to strong spatial overlap at scales as small as a single egg clutch. In fact, parasitism data show no evidence of intraspecific aggregation and interspecific segregation. The frequency of both species parasitizing eggs within the same clutch are significantly greater than that expected by chance (expected frequency: 0.08, observed frequency (Mean ± SE): 0.47 ± 0.09; G-test, P < 0.01, n = 26 clutches). This is the opposite of expected outcome if substantial interspecific segregation were occurring.
(ii) Spatial niche partitioning at the metacommunity scale — Dispersal experiments showed that parasitoid coexistence was possible even in the absence of a competition-colonization trade-off (Amarasekare 2000b). Theory developed to investigate alternative spatial mechanisms (Amarasekare and Nisbet 2001) predicted coexistence via source-sink dynamics if there is spatial variation in competitive rankings and the inferior competitor's dispersal rate is below a critical threshold. This mechanism is unlikely because the two parasitoids lack spatial variation in competitive rankings. In 12 years of field studies of over 50 local communities, the direction of competition is always the same with Trissolcus declining in the presence of Ooencyrtus (Amarasekare 2000a, b, 2003a).
Abrams, P. A. 2006. The prerequisites for and likelihood of generalist-specialist coexistence. American Naturalist. 167:329–341.
Amarasekare, P. 1998. Coexistence in a spatial context: empirical and theoretical perspectives. Ph.D. thesis, University of California, Irvine.
Amarasekare, P. 2000a. Coexistence of competing parasitoids on a patchily distributed host: local vs. spatial mechanisms. Ecology, 81:1286–1296.
Amarasekare, P. 2000b. Spatial dynamics in a host-multiparasitoid community. J. Anim. Ecol., 69:201–213.
Armstrong, R. A., and R. McGehee. 1980. Competitive exclusion. American Naturalist 115:151–170.
Chesson, P. 2000. Mechanisms of maintenance of species diversity. Annual Review of Ecology and Systematics, 31:343–366.
Gurney, W. S. C., and R. M. Nisbet. 1998. Ecological dynamics. Oxford University Press, New York, New York, USA.
Holt, R. D., M. Barfield, and A. Gonzalez. 2003. Impacts of environmental variability in open populations and communities: "inflation" in sink environments. Theoretical Population Biology 64:315–330.
Sjaarda, N. 1989. Interactions between the harlequin bug, Murgantia histrionica and the egg parasitoids, Trissolcus murgantiae and Ooencyrtus johnsonii: factors affecting patterns of parasitism in a southern California coastal sage habitat. Ph.D thesis, University of California, Davis, California, USA.