When can herbivores slow or reverse the spread of an invading plant? A test case from Mount St. Helens

Author Posting. © University of Chicago, 2005. This article is posted here by permission of University of Chicago for personal use, not for redistribution. The definitive version was published in American Naturalist 166 (2005): 669-685, doi:10.1086/497621Here we study the spatial dynamics of a coinvading consumer-resource pair. We present a theoretical treatment with extensive empirical data from a long-studied field system in which native herbivorous insects attack a population of lupine plants recolonizing a primary successional landscape created by the 1980 volcanic eruption of Mount St. Helens. Using detailed data on the life history and interaction strengths of the lupine and one of its herbivores, we develop a system of integrodifference equations to study plant-herbivore invasion dynamics. Our analyses yield several new insights into the spatial dynamics of coinvasions. In particular, we demonstrate that aspects of plant population growth and the intensity of herbivory under low-density conditions can determine whether the plant population spreads across a landscape or is prevented from doing so by the herbivore. In addition, we characterize the existence of threshold levels of spatial extent and/or temporal advantage for the plant that together define critical values of "invasion momentum," beyond which herbivores are unable to reverse a plant invasion. We conclude by discussing the implications of our findings for successional dynamics and the use of biological control agents to limit the spread of pest species.This effort would not have been possible without the support of the National Science Foundation through its Mathematical Biology and Ecological Studies programs (awards NSF OCE-9973212, DEB-9973518, DEB-0235692, and DEB-0089843). The University of Maryland, Washington State University–Vancouver, and the Banff International Research Station for Mathematical Innovation and Discovery provided additional support. M.L. acknowledges support from Mathematics of Information Technology and Complex Systems Canada, a Natural Sciences and Engineering Research Council operating grant, a Contract Research Organisation grant, and a Canada Research Chair