PS 71-135
Simulated herbivory reveals both induced and constitutive strategies to resist herbivory in Plectritis congesta dependent on plant origin and associated natural herbivory rates
Theory suggests that natural selection can promote local adaptation and differentiation of populations in morphological, phenological or other traits influential of individual fitness and population persistence, especially in heterogeneous selective landscapes. Most recently, spatial variation in herbivory, pollination and individual fitness was shown experimentally to account for spatial variation in stipe-length in the perennial herb Primula farinosa. We tested if spatial heterogeneity in natural selection via herbivory also drives morphological differentiation in Plectritis congesta, a winter annual that varies dramatically in size and shape on islands with and without resident deer in southwestern British Columbia, Canada. We simulated herbivory on plants from 12 populations known to experience high or low herbivory rates in nature. Chamber-grown control plants (n = 197) and experimental plants artificially browsed to 2.5 cm at 51 days post-germination (n = 71) were assessed to estimate the effect of browsing on plant size, shape, fruit production and fruit distribution as a consequence of population origin and the fraction of tissue removed. We predicted that plants from populations with resident deer would be taller, allocate more fitness to a single inflorescence, and suffer more browsing damage and fitness reduction than plants from populations with resident deer.
Results/Conclusions
Both selective background and the proportion of tissue removed influenced elements of plant fitness and morphology. Control plants from herbivore-free islands were twice as tall, had half as many branches and had double the height-width ratio of plants from islands with deer, confirming a genetically differentiated influence on morphology. Morphological differences between selective environments resulted in plants from herbivore-free islands losing 48.3% of tissue on average given simulated herbivory compared to 14.3% on average in plants from islands with resident deer. Seed number decreased and branch number increased with the proportion of tissue removed regardless of plant origin. In contrast, the percentage of seeds present in side versus terminal inflorescences of clipped plants differed by origin; increasing in those from herbivore-free populations but decreasing in plants from populations that co-occur with deer. Our results indicate that plants from high herbivory environments express constitutive defenses against herbivory via growth form and strategic allocation of reproductive effort to multiple side branches. Conversely, plants from low herbivory environments prioritized height and the production of seeds in a large terminal inflorescence, but maintained the ability to respond to simulated herbivory via the opportunistic production and allocation of inflorescences to additional lateral branches.