The underappreciated role of harsh rocky outcrops as herbivore refugia for CA jewelflowers

Sarah R. Ashlock, Evolution and Ecology, University of California, Davis, Davis, CA and Sharon Y. Strauss, Department of Evolution and Ecology, University of California, Davis, Davis, CA

Background/Question/Methods
The niche of a species reflects dynamic combinations of abiotic tolerances and biotic interactions. Identifying and understanding the factors that shape species distributions remains a central topic in ecology. While abiotic factors are typically thought of as niche determinants, biotic interactions like herbivory and competition can profoundly limit plant species distributions to a subset of the available habitat, or realized niche.
California native mustards in the genus Streptanthus are found in landscapes with edaphic and topographic heterogeneity. Northern species are typically restricted to harsh rocky outcrops on hot dry south facing slopes, despite being surrounded by richer and moister soils. We investigated the relative contributions of habitat suitability, herbivory and competition in determining the realized niche of three species of Streptanthus. We conducted transplant experiments with seedlings on and off of rocky outcrops and manipulated the presence of herbivores and competitors with cages and weeding. We recorded levels of herbivory, vegetation cover, and size of experimental transplants as well as naturally occurring plants on and off outcrops. Lifetime fitness will be assessed in late spring. To explore herbivore habitat use, we surveyed scat of deer, rabbits and gopher mounds along transects on and off outcrop habitats.
Results/Conclusions
Herbivore pressure restricts natural populations of Streptanthus to a subset of edaphically and topographically challenging habitat. Herbivory rates were higher off the outcrop than on the outcrop in natural populations, and herbivory tended to increase with distance from the outcrop in experimental transplants. Probability of mortality was higher off the outcrop, often due to increased herbivory. Complementary to these results, scat and track surveys indicate herbivore traffic is much higher in zones off the outcrop than on the outcrop. We predict as precipitation decreases in the spring, areas surrounding outcrops likely offer more suitable growing habitat, but herbivory may restrict populations to harsher outcrops. The role of rocky outcrops as herbivore refugia has been underappreciated. Previous work and ongoing experiments suggest competition also plays a role in restricting Streptanthus to suboptimal habitat. Vegetation cover was significantly higher off the outcrop than on the outcrop. Seedling mortality was lower in competitor removal treatments than control transplants off the outcrop. Our results suggest that both herbivory and competition are important biotic interactions limiting Streptanthus to habitats that may not support optimal growth, thereby enforcing habitat specialization.