Biocontrol attack exacerbates resource- and pollen- limitation in its target weed (Centaurea solstitialis) across a soil moisture gradient

Background/Question/Methods

Centaurea solstitialis is a highly invasive thistle in California. It has an annual lifecycle and is dependent on insect pollinators for reproduction. Seed set may be limited by pollen receipt or by resources available to mature pollinated ovules. Whether seed set is pollen- or resource- limited ought to be influenced by local conditions. In high stress sites (low soil moisture), plants ought to experience resource limitation, while in more benign microhabitats (higher soil moisture), they may experience pollen limitation. Attack by the foliar pathogen Puccinia jaceae f.s. solstitialis– a biocontrol agent – may exacerbate resource limitation because infected plants have smaller taproots which limit their access to soil moisture during flowering. Infection may also exacerbate pollen limitation because infected plants are smaller and may be less conspicuous or rewarding to pollinators. The goal of this study was to determine whether attack by a biocontrol agent exacerbates either resource- or pollen- limitation in its target weed along a soil moisture gradient. I conducted a field experiment across a naturally occurring gradient of soil moisture (south-facing slope, north-facing slope and ridge); plants were experimentally infected with the pathogen and received supplemental pollen in a factorial design.

Results/Conclusions

Plants growing in the more stressful microhabitats (south-facing slopes and ridges) had lower seed set than plants growing on the more benign north-facing slopes suggesting greater resource limitation in seed set in the stressful microhabitats. Infected plants also had lower seed set than uninfected plants in south-facing slopes. Pollen addition led to higher seed set in both infected and uninfected plants on the north-facing slopes. Importantly, infected plants had a larger response to pollen addition than did uninfected plants indicating that they experienced greater pollen limitation. Pollen addition had no effect on seed set for plants on the south-facing slope regardless of infection status. This indicates that the biocontrol pathogen exacerbated the degree to which seed set was pollen limited in the benign microhabitats, and exacerbated the degree to which seed set was resource limited in the most stressful microhabitats. These results demonstrate that pollinators play a significant role in the success of this annual invader and that a biocontrol agent that exploits this dependence may have a greater than expected impact on its target weed.