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Title: Implications for coevolutionary dynamics of a tri-trophic interaction between the orange-tip butterfly, its host plants and primary parasitoid in a heterogeneous landscape
Author: Davies, William James
ISNI:       0000 0004 6059 2466
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2016
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Coevolutionary interactions change across landscapes, leading to the formation of geographic selection mosaics. Analyses of coevolutionary dynamics have so far focused on interactions between two trophic levels. The tritrophic interactions between the pierid butterfly Anthocharis cardamines (the orange-tip), its brassicaceous host-plants Cardamine pratensis (lady's smock) and Alliaria petiolata (garlic mustard), and primary parasitoids Phryxe vulgaris (Tachinidae) and Cotesia saltator (Braconidae), are known to vary across mainland Britain and Continental Europe. In Britain, northern A. cardamines populations tend to utilize C. pratensis and southern ones A. petiolata; the primary parsitoid in this country is P. vulgaris. In Sweden, the butterfly does not show a strong preference for any brassicaceous hostplant, and the primary parasitoid is C. saltator. In this thesis, I investigate likely selection pressures operating on these interactions in a single study site on the Wirral peninsula in northern England. Host use affects the emergence timing and dispersal of A. cardamines males; specimens utilizing C. pratensis emerge earlier than those utilizing A. petiolata, and are also smaller. Since small size is linked with depressed dispersal, utilization of C. pratensis results in an "emerge early and wait" mate seeking strategy; conversely, utilization of A. petiolata is associated with an "emerge late and rove" strategy. These alternative strategies are likely adapted to the varying density of locally emerging females across the landscape, with high density populations tending to be associated with C. pratensis and low density ones with A. petiolata. In dense populations, late emerging males will be at a disadvantage since females mate only once; in the study population, their predicted fitness always declines to < 1 in late season. This is coupled with a 'stay-or-go' response, in which a proportion of late emerging males immediately emigrate to a low density continuum outside the study area, where early emergence is less critical for fitness. Such a response could help maintain sink populations by averting Allee effects (decreased population growth due to low mate encounter rates). Late instar A. cardamines larvae are heavily parasitized by P. vulgaris. This could select for early vacation of the host-plant at small larval size, whereas the size-fecundity relationship in females should select for prolonged growth to larger size. Mathematical models indicate that high rates of parasitization are sufficient to overturn the fecundity benefit of large size, but fall short of maintaining strong stabilizing selection for an optimal winglength. The tendency of some larvae to move off their host-plant before the final instar resting phase is probably a direct evolutionary response to parasitism risk. A. cardamines larvae are pre-dispersal seed-predators; an early flowering ecotype of C. pratensis has likely been selected to avoid egg-laying A. cardamines females. In turn, the butterfly appears to be invading this host-plant's phenological space, with selection favouring small, early emerging females which oviposit on it. This cautions against interpreting the recent advance in A. cardamines' phenology solely in terms of a response to climate change. Spatio-temporal variation in the intensity of these effects likely contributes to the ongoing coevolutionary dynamics within this tritrophic system.
Supervisor: Saccheri, I. J. ; Hurst, G. D. D. Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral