Abstract

1 To assess the effect of annual environmental variability on the long-term population dynamics of the invasive woody perennial Prosopis glandulosa var. torreyana (honey mesquite), we employed elasticity analyses of annual, mean and periodic matrix models. Growth, survival and reproduction were recorded for 1306 individuals in a 1-ha plot over a 4-year period. The volume of each individual was estimated, and transition matrices with nine size classes were constructed. Standard matrix analysis was performed, and the relative importance of individual life-cycle components to changes in the finite rate of population increase (lambda) was determined. 2 Periodic matrix analysis projected a 29% annual increase in population size (lambda= 1.29), while annual projection lambda-values varied between 0.99 and 1.44. For both methods, elasticity of seedling recruitment was always very low in all 4 years, and the highest elasticities were generally associated with permanence in the same size class. 3 Periodic and annual projections predicted similar elasticity patterns, with the relative contribution to a change in lambda of different demographic processes changing between 'good' and 'bad' years. However, annual matrices identified two bad years, with fecundity and growth elasticity decreasing relative to survival, whereas periodic analysis identified only one such year, and elasticity changes were seen only in fecundity and survival.