A seasonal, density-dependent, stage-structured harvest model for the management of an invasive weed

Background/Question/Methods

Management by harvest (the often selective removal of individuals from a population) is a common control tactic for invasive species. Harvest strategies, however, may backfire if density-dependent factors increase the survival of the individuals that escape harvest (e.g. if removing part of an overcrowded population affords the remainder more resources). These undesirable compensatory effects make it difficult to reduce, and may even amplify, long-term population densities, rendering attempts at management counterproductive. Although key population processes, including compensatory effects, are determined by seasonality and the timing of harvest, modeling the associated nonlinear density-dependent dynamics and seasonal effects raises challenging mathematical issues. To address these issues, we have developed a periodic, nonlinear, stage-structured matrix population model that is seasonally-explicit and incorporates selective harvest with respect to stage and timing.

Results/Conclusions

We have applied our model to the management of garlic mustard (Alliaria petiolata), a biennial European weed that is aggressively displacing North American woodland species. Garlic mustard life history was partitioned into relevant stages and seasons based on major life cycle transitions, and several popular garlic mustard management strategies (pulling flowering adults, clipping flowering or fruiting adults, applying herbicide in the spring or fall, and using weevils for biocontrol) were assessed for their projected effects on long-term population dynamics. We find that reducing seed germination or allowing adults to resprout after clipping has minimal effects on population size in all seasons, while reducing rosette, adult, or seed bank survival is more effective in decreasing average annual densities. We additionally find that population dynamics, including potential compensatory effects, depend on the season in which harvest is applied, and that each harvest strategy differentially increases or decreases densities in each season. These results affirm the importance of stage and seasonal considerations in directing population management efforts.