Abstract
Invasive plants often occupy large ranges in the introduced region and consequently, local population dynamics vary in ways that affect the potential for biological control. We used matrix models to describe how density and population growth rate of Centaurea solstitialis varies in time and space. Matrix models were parameterized with data collected over 4 years from invasions at the coast, interior valleys and Sierra Nevada Mountains in California (USA). Asymptotic population growth rates (λ) varied dramatically across all populations and years (0.24–6.45), density varied by an order of magnitude and had a measurable effect on survival and λ in all populations. We used simulations to estimate the degree to which a biocontrol agent would need to reduce plant survival to control the weed. Because seedling survival was dependent on density, an agent that reduced seedling density had the effect of increasing the probability that the remaining plants survived to flowering. Interestingly, this meant that in the highest density populations the plant had the largest compensatory response to agent attack and experienced decline (λ ≤ 1.0) only after heavy losses (≥90%) to the agent. Conversely, in populations where density was so low that it had only a weak effect on survival, the agent was able to control the plant (λ ≤ 1.0) at much lower levels of attack (≤50%). In other words, the impact of a biocontrol agent is predicted to be lower where the plant reaches its highest densities because the surviving plants, now experiencing less intraspecific competition, are more likely to survive to flowering and produce more seeds. This may also be true for other invasive species in which strong density dependent processes are operating. For this reason, prospective agents ought to target density-independence vital rates.
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Acknowledgements
We thank Dan Doak, Carla D’Antonio, Laurel Fox, Kris Hulvey, Alden Griffith and two anonymous reviewers for helpful comments that improved this manuscript. Marci Beitch, Maggie Barr, Aaron Johnson and Joe Braasch ably assisted with the collection of data in the field and the lab. In particular, Joe Braasch deserves thanks for his excellent work counting and categorizing thousands of seeds. SMS is grateful for the support of the Jean Langenheim Fellowship in Plant Ecology and Evolution and an NSF Doctoral Dissertation Improvement Grant DEB-0808337 during the first 2 years of the data collection portion of this study.
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Swope, S.M., Satterthwaite, W.H. & Parker, I.M. Spatiotemporal variation in the strength of density dependence: implications for biocontrol of Centaurea solstitialis . Biol Invasions 19, 2675–2691 (2017). https://doi.org/10.1007/s10530-017-1476-3
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DOI: https://doi.org/10.1007/s10530-017-1476-3