Invasion Dynamics of Cytisus scoparius: A Matrix Model Approach

Abstract

It is at the level of population dynamics that an invasion either fails or succeeds. By elucidating patterns of variation in population growth rates or demographic rates, it is possible to forge a connection between quantitative field data and theoretical ideas about invasiveness, invasibility, and rates of spread. Demographic models also provide a tool to guide control strategies for invasive pests. Here I report the results of a demographic study of Cytisus scoparius, an exotic shrub on the west coast of North America. I used matrix population models to describe demographic patterns in six populations (three in prairies and three in urban fields) and across advancing stages of invasion. At the edge of the invading front, all populations showed finite rates of increase (λ) >1; however, prairie populations were increasing much more rapidly than urban ones. While many individual vital rates differed between prairie and urban populations, Life Table Response Analysis revealed that seedling establishment made by far the largest contribution to the difference in growth rate between the habitats. Establishment is much higher in the prairies, which are also less anthropogenically disturbed and show higher plant species diversity. From the edge of the invading population to the center, λ generally decreased, and the elasticity pattern changed from one evenly distributed across life history stages to one dominated by the survivorship of large adults. Comparing the matrix model predictions to direct estimates of invasion (change over time of various measures of density and biomass), λ was most closely correlated with the increase of total biomass. From a control perspective, elasticities did not suggest one particularly sensitive life history stage (“Achilles heel”) for this pest plant. A simulation was used to evaluate the potential efficacy of biological control agents that attack seeds. Based on model predictions, under current conditions a control agent would have to destroy over 99.9% of seeds in prairies, and 70% of seeds in urban populations, to suppress the invasion of C. scoparius populations.