Population‐level compensation impedes biological control of an invasive forb and indirect release of a native grass

Abstract

The intentional introduction of specialist insect herbivores for biological control of exotic weeds provides ideal but understudied systems for evaluating important ecological concepts related to top-down control, plant compensatory responses, indirect effects, and the influence of environmental context on these processes. Centaurea stoebe (spotted knapweed) is a notorious rangeland weed that exhibited regional declines in the early 2000s, attributed to drought by some and to successful biocontrol by others. We initiated an experiment to quantify the effects of the biocontrol agent, Cyphocleonus achates, on Ce. stoebe and its interaction with a dominant native grass competitor, Pseudoroegneria spicata, under contrasting precipitation conditions. Plots containing monocultures of each plant species or equal mixtures of the two received factorial combinations of Cy. achates herbivory (exclusion or addition) and precipitation (May-June drought or "normal," defined by the 50-year average) for three years. Cy. achates herbivory reduced survival of adult Ce. stoebe plants by 9% overall, but this effect was stronger under normal precipitation compared to drought conditions, and stronger in mixed-species plots compared to monocultures. Herbivory had no effect on Ce. stoebe per capita seed production or on recruitment of seedlings or juveniles. In normal-precipitation plots of mixed composition, greater adult mortality due to Cy. achates herbivory resulted in increased recruitment of new adult Ce. stoebe. Due to this compensatory response to adult mortality, final Ce. stoebe densities did not differ between herbivory treatments regardless of context. Experimental drought reduced adult Ce. stoebe survival in mixed-species plots but did not impede recruitment of new adults or reduce final Ce. stoebe densities, perhaps due to the limited duration of the treatment. Ce. stoebe strongly depressed P. spicata reproduction and recruitment, but these impacts were not substantively alleviated by herbivory on Ce. stoebe. Population-level compensation by dominant plants may be an important factor inhibiting top-down effects in herbivore-driven and predator-driven cascades.

Biological control and precipitation effects on spotted knapweed (Centaurea stoebe): empirical and modeling results

“…The data are inconsistent about the effects of precipitation on other survival and growth stages. Other results suggest that drought relief increased rosette mortality or has minimal effects on juvenile densities (Corn et al 2007, Ortega et al 2012, so no precipitation effects were included for growth or rosette survival. There is an interaction between precipitation and the effect of plant competition on spotted knapweed (Maron and Marler 2008); however, data do not indicate a correlation between precipitation and stem density or number of flowers (Ortega et al 2012; T. R. Seastedt, unpublished data).…”

“…In the 10-30 years following the release of biological controls, spotted knapweed populations have declined while biological control populations have increased or persisted , 2008, Seastedt et al 2007, Gayton and Miller 2012. However, an experiment by Ortega et al (2012) demonstrated that spotted knapweed populations exhibited a compensatory response adult mortality: replacement of dead adults with increased numbers of rosettes becoming adults. The result was that there was no net change in adult density despite an increase in adult mortality due to biological controls (Ortega et al 2012).…”

“…However, an experiment by Ortega et al (2012) demonstrated that spotted knapweed populations exhibited a compensatory response adult mortality: replacement of dead adults with increased numbers of rosettes becoming adults. The result was that there was no net change in adult density despite an increase in adult mortality due to biological controls (Ortega et al 2012). This finding suggests that seed reduction and increased mortality of adult plants might not be sufficient to overcome growth from juvenile plants.…”

“…This finding suggests that seed reduction and increased mortality of adult plants might not be sufficient to overcome growth from juvenile plants. A further complication when interpreting results is that drought has been suggested as a contributing factor to the observed declines in spotted knapweed populations (Corn et al 2007, Story et al 2008 or as an alternative explanation instead of reductions due to biological controls (Ortega et al 2012). Experimental manipulations have been conducted on many stages of the life cycle; however, integration of these findings to compare how individual effects impact long-term population dynamics has not been undertaken.…”

Native Plant Establishment Success Influenced by Spotted Knapweed (Centaurea stoebe) Control Method

“…Following invasion, knapweed forms peripherally enlarging monocultures that seed profusely, have allelopathic effects on susceptible species, and negatively impact ecosystem richness and diversity (Watson and Renney 1974, Schirman 1981, Tyser 1992, Sheley et al 1998, Kedzie-Webb et al 2001, Thorpe et al 2009). While it is known that knapweed can be controlled using a variety of methods including herbicide application (Rice • 283 et al 1997), biological controls (Story et al 2006, but see Ortega et al 2012), and controlled burning (MacDonald et al 2007), resurgence of knapweed is very likely without the formation of a competitive, native community (Sheley et al 1996), especially on sites in which seeds from previous infestations are present in the seed bank (Sheley et al 1996, Carpinelli et al 2004. As diverse native plant communities are more capable of resisting knapweed invasion (Kennedy et al 2002, Bakker and Wilson 2004, Maron and Marler 2007, it is important during any restoration project to ensure that such diverse native systems are reestablished.…”