Submitted to: Oecologia
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/12/2010
Publication Date: 7/1/2010
Citation: Swope, S.M., Parker, I.M. 2010. Widespread seed limitation affects plant density but not population trajectory in the invasive plant Centaurea solstitialis. Oecologia. 164:117-128. Interpretive Summary: A central goal of invasive plant management is to understand the factors that regulate the recruitment of new individuals into a population because such information improves our ability to manage invasions. This is especially true for plants with an annual lifecycle such as yellow starthistle (Centaurea solstitialis) because each year “re-invasion” of the site must take place entirely from seed. In some plant populations, there is a strong relationship between the number of seeds entering a patch of ground and the number of seedlings (and ultimately flowering plants) that establish. In such populations, recruitment is limited by the availability of seed and any increase or decrease in seed production is expected to have a concomitant effect on recruitment. But the link between seed production and seedling abundance is rarely straightforward. Three (3) processes have the potential to decouple recruitment from seed production: a scarcity of germination microsites, granivory and density dependent mortality. All seeds require a suitable microsite for germination. Although some species are more stringent in their microsite requirements than others, there is a finite number of such sites in any given patch of ground. When plans produce more seed than there are germination microsites available, increasing seed input will not lead to an increase in seedling recruitment. Seed predators may strongly mediate recruitment but only when they remove seeds that are otherwise likely to have germinated, i.e., in populations in which recruitment is seed limited but not microsite limited. Finally, strong density-dependent mortality at any life stage prior to flowering can reduce or even eliminate gains in recruitment that are the result of higher seed input. We used a combination of experimental and observational studies to determine how seed input is related to seedling and flowering plant establishment in yellow starthistle in California. In our seed addition experiment we added seeds to uninvaded areas at the following levels: 0 seeds added (this served to determine if a seedbank was present or if there was any meaningful immigration into out plots from the adjacent invasion), 50, 500 and 1000 seeds added to plots that measured 0.5m×0.5m. We excluded granivores from half the plots in each seed addition level. We conducted this experiment at three study sites across yellow starthistle’s range in California (Coast, Interior Grasslands, and the leading edge of the invasion in the Sierra Nevada Mountains) because these sites differed in abiotic characteristics important to germination and survival and so might produce very different results. We coupled the experimental approach with observational studies within established yellow starthistle invasions at each site to estimate seed rain, seedling recruitment and mortality at natural densities. Seed limitation occurred in both experimental and observational plots in all populations meaning that as we added more seed, more seedlings established. Germination microsites saturated at 500 seeds at the Coast and Interior sites but not at the Sierra site. Granivores were actively feeding at the sites but had no effect on starthistle recruitment. Density increased mortality only in the site with the highest seedling density (20-500 seedlings per plot); as a result, the general patterns we saw in seedling abundance persisted through to the flowering stage. Seed-limited populations such as these ought to be highly sensitive to losses to seed predators and many biological control agents, including those established for starthistle, are seed predators. However, there was a strong effect of density on fecundity; seed production was nearly constant in plots across all seed addition levels (50, 500, 1000 seeds added). Thus seed predators (including biocontrol agents) reduce flowering plant density but not the per
Technical Abstract: In some plant populations, the availability of seeds strongly regulates recruitment. However, a scarcity of germination microsites, granivory or density dependent mortality can reduce the number of plants that germinate or survive to flowering. The relative strength of these controls is unknown for most plant populations and for exotic invaders in particular. We conducted a seed addition experiment (0, 50, 500 and 1000 seeds added to 0.25m2 plots) with a granivore exclusion treatment in a field setting to explore how these factors interact to regulate populations of the widespread invader Centaurea solstitialis (yellow starthistle) in western North America. We conducted our experiments at three study sites across the longitudinal range of C. solstitialis in California, and sites differed in abiotic characteristics important to germination and survival. We coupled the experimental approach with observational studies within established C. solstitialis populations to estimate seed rain, seedling recruitment and mortality at natural C. solstitialis densities. Seed limitation occurred in both experimental and observational plots in all populations. Granivores were actively feeding at the sites but had no effect on C. solstitialis recruitment. Density increased mortality only in the site with the highest seedling density (20-500 seedlings per 0.25m2). The seed limitation that was evident at the seedling stage persisted to flowering. Seed-limited populations such as these ought to be highly sensitive to losses to seed predators and many biological control agents, including those established for C. solstitialis, are seed predators. However, flowering plant density was decoupled from seed production by a strong compensatory response in the surviving plants; seed production was nearly constant in plots across all seed addition levels (50, 500, 1000 seeds added). Thus flowering plant density is reduced by the established biocontrol agents, but seed production compensates to replace the population every generation, and no long-term decline is predicted.