Species coexistence in a host-multiparasitoid community: The interplay between biotic and abiotic environmental variation

Background/Question/Methods

We investigate how biotic and abiotic environmental variation influence species coexistence in a host – multiparasitoid community inhabiting the coastal sage scrub in Santa Barbara Co., CA, USA. The host species, the bordered plant bug [Largus californicus] is attacked by three parasitoids (an egg parasitoid [Gryon largi]) as well as a parasitoid wasp [unidentified sp.] and a tachinid fly [Trichopoda pennipes] that attack nymphal and adult life-stages). This research is motivated by a striking pattern observed in the host’s population dynamics.  In many Hemipteran insects, adults exhibit more stable dynamics than eggs and nymphs because of invulnerability to attack by natural enemies and because fluctuations due to seasonal temperature variation are attenuated due to stage-structure. The bordered plant bug provides an exception to this pattern: adults are vulnerable to attack by natural enemies and exhibit high variability in abundance. We hypothesize that it is the interplay between the bug’s sensitivity to temperature-variation and attack by enemies at multiple life-stages that drives its population dynamics. We test these mechanisms using a combination of mathematical models, field censuses, and laboratory experiments in temperature-controlled incubators and using an automated video system.

Results/Conclusions

A mathematical model of host population dynamics parameterized with data from laboratory experiments predicts that in a constant environment with no seasonal temperature fluctuations, bug populations should exhibit oscillations in abundance associated with a stable limit cycle. These oscillations arise from positive density-dependence generated by a developmental time-delay due to a short adult life-span relative to nymphal developmental period and a high rate of reproduction. This positive feedback is counteracted by negative density-dependence that arises via a decline in fecundity with increasing adult abundance, likely due to intra-specific competition. We investigate bug population dynamics under ambient seasonal variation by quantifying the temperature-responses of life history traits in laboratory experiments and incorporating these responses into the dynamical model. Lastly, we use an automated video system to investigate host-parasitoid interactions, focusing on key behavioral traits such as searching efficiency and handling time that are crucial to understanding host-parasitoid dynamics. Our results have implications for host-multiparasitoid interactions in variable environments and practical applications for biological control of agricultural and invasive pest species.