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UBC Theses and Dissertations

Spatial heterogeneity in natural selection and local adaptation to ungulate herbivory in Plectritis congesta Skaien, Cora


Understanding how organisms respond to environmental change via genetic and plastic responses can help predict species occurrence and persistence in changing landscapes. I quantified phenotypic and genotypic variation in the annual plant Plectritis congesta to test for the effects of spatial variation in ungulate herbivory on plant traits. I first surveyed 285 island and mainland populations in British Columbia, Canada, to estimate how ungulates, climate, and population isolation affect fruit phenotype and plant height. I then tested for local adaption in common gardens open to and protected from deer, and for adaptive plasticity in common gardens protected from herbivores, using populations exposed and naïve to ungulate herbivory. I then estimated genetic variance, heritability, evolvability and plasticity for plant height, shape and branch number, and assessed how these traits changed in response to intraspecific competition. Ungulate occurrence explained substantial population-level variation in phenotype, but climate and isolation had minor effects. In island populations, plants naïve to ungulate browsers were 2.6 times taller and 3.4 times more likely to produce winged fruits than plants from historically exposed populations. I observed local adaptation in common gardens open to and protected from herbivores, wherein plants from naïve populations were three times more abundant than plants from exposed populations after five years of protection from browsing. In contrast, plants from exposed populations survived three times better and were twice as fecund as plants from naïve populations when browsed, due to later bolting and flowering. Trade-offs in plant height and fecundity occurred in response to intraspecific competition: height increased 1.5 to three times as density increased but led to a ~20-30% decrease in fecundity. Moderate additive genetic variance and evolvability in traits under selection suggest a capacity for rapid evolution in 2-18 generations, similar to that shown in island populations of other taxonomic groups. My results suggest that spatial heterogeneity in browsing by ungulates can drive local adaptation in P. congesta populations, resulting in context-dependent trade-offs that influence fitness and elicit adaptive plasticity. Existing variation between island populations of P. congesta has the potential to provide long-term stability when faced with rapid environmental change.

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