Microsite availability, not floral herbivory, limits recruitment in peripheral native thistle populations

Abstract

Variation in insect herbivory can drive variation in plant fitness and population dynamics. However, our ability to predict the ecological contexts in which insect herbivores will reduce plant fitness or population growth is limited. In theory, populations at the periphery of a plant species' biogeographic range are expected to experience reduced herbivory. Further, in montane landscapes, elevation is expected to drive variation in abiotic conditions and variation in plant–insect interactions. Specifically, less insect herbivory may occur at cooler, higher elevations. To examine these predictions, we quantified effects of inflorescence- and seed-feeding insect herbivores in populations of the short-lived, monocarpic, perennial forb Cirsium canescens (Platte thistle) in montane grasslands in Colorado, USA. We asked: (1) Does insect flower head herbivory and pre-dispersal seed predation limit Platte thistle lifetime seed production? (2) Does this insect herbivory limit seedling recruitment? (3) Does ecological context, including spatial—especially elevational—and temporal variation, affect the outcome of these interactions? We conducted insect exclusion experiments in three years at five sites over 52% of Platte thistle's elevation range in our region. We compared both lifetime viable seed production and seedlings recruited between plants with ambient versus insecticide-reduced levels of flower head herbivory. Insect herbivory on flower heads significantly reduced Platte thistle lifetime viable seed production at all sites, independent of elevation. Unexpectedly, however, increasing seed by reducing herbivory did not lead to a proportional increase in seedling recruitment. The relationship between viable seed production and seedling recruitment per plant was non-linear, decelerating across the range of seed production achieved by both plants exposed to and protected from flower head herbivory. While elevation altered Platte thistle flowering phenology, it did not influence insect damage, viable seed production, or seedling recruitment. These results show that flower head- and seed-feeding insect herbivores strongly reduced Platte thistle lifetime viable seed production, a key component of maternal fitness, in these peripheral populations. Yet, the herbivory did not determine population recruitment, suggesting post-dispersal processes limit recruitment here. Further, elevation did not drive context-dependent variation in the insect herbivore outcomes.