Background/Question/Methods:
Analysis of matrix population models yields a prescription on how to intervene in organism life cycles to change population growth. One application is controlling a plant invader by targeted disruption of its life cycle, which involves prospective analysis identifying transitions in the life cycle that potentially influence population growth, and retrospective analysis to identify which of these is most amendable to manipulation. Here we ask how robust is this prescription to changes in (1) initial conditions (represented here by time of disturbance), (2) variation in community structure (represented here by a range in community configurations from a single-species plant population to multi-species system consisting of two herbivore species on a plant population that is also interacting with a background vegetation)? We conducted a Life Table Response Experiment that examined the independent and interacting effects on tansy ragwort (Senecio jacobaea) populations of three levels of plant competition (unaltered, clipped, and removed), two levels of Cinnabar moth (Tyria jacobaeae), two levels of ragwort flea beetle (Longitarsus jacobaeae), and two timing of initial disturbance (fall and spring). We analyzed a density-independent matrix model with time invariant demographic parameters quantified for transition of ragwort plants between three stages: 1st year juveniles, 2nd year juveniles, and adults.
Results/Conclusions:
We found that plant competition and insect herbivory each reduced the finite growth rate of ragwort populations; the effect of the flea beetle was stronger and faster than that of the moth. Elasticity structures were similar across most treatments, with highest values associated with ‘biennial transitions’ in the ragwort life cycle; exceptions involve the ‘cinnabar moth’ herbivore treatment, with highest values associated with ‘perennial transitions.’ Models, experiments, and observations agree on causes and consequences of phenotypic plasticity in ragwort’s life cycle: cinnabar moth ‘induces’ the perennial pathway in the ragwort life cycle; the ragwort flea beetle ‘blocks’ the perennial pathway. We conclude that (1) sensitivity and elasticity structure can be used to improve the diagnosis of invader vulnerabilities, (2) the diagnosis is fairly robust to variation in community structure, and (3) combining the manipulation of disturbance, colonization, local interactions (competition, mutualism, and herbivores) can achieve biological control using fewer control-organism individuals and species, while helping to identify why we have invasions in the first place.