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

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 predispersal 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 vs.
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.