Variation in demography and effect of insect herbivory on Platte thistle (Cirsium canescens) between biogeographic range center and range edge

Abstract

Insect herbivory can reduce plant fitness, but the strength of these effects varies greatly in space and time. The Abundant Center Hypothesis (ACH) states that species will encounter more stressful abiotic conditions at the edge of their biogeographic range and that the species’ abundance and performance will decline from range center to edge. Lower plant densities and more stressful conditions at the range edge may affect specialist herbivore attack rates and plant compensatory ability. Platte thistle (Cirsium canescens) is a monocarpic species whose range is centered in the Nebraska Sand Hills and reaches its western limit in central Colorado. This study addressed 1) How do key Platte thistle demographic rates differ between range edge and center? and 2) How do insect herbivore effects on Platte thistle fecundity and recruitment differ between range edge and center? I addressed these questions by monitoring of juvenile thistles and using insect exclusion experiments on adult Platte thistles. Survival of large juvenile thistles was higher in the range center, and probability of transitioning to reproduction in small juvenile thistles was significantly higher at the range edge, seeming to support the predictions of the ACH. The reduction in seed production and seedling recruitment caused by insect herbivory was not significantly different between range center and edge, but seed production and seedling recruitment overall were significantly higher at the range edge, defying predictions of the ACH. Better reproductive performance at the range edge may be result from different levels of pollination success, whereas differences in juvenile thistle demography may reflect greater abiotic stress at the range edge. My findings seem to support a growing sentiment in ecology that the ACH is too simple to be able to accurately predict the patterns of plant reproductive success and demography at species biogeographic range edges.