Print PagePlants interact with numerous enemies and mutualists over the course of their lifetimes, potentially giving rise to indirect interactions and non-additive outcomes. The topic of multispecies interactions has received much attention in recent years. But the influence of the abiotic environment on these interactions and their outcomes remains under-studied.
How the abiotic environment alters indirect interactions is also central to the use of biocontrol agents to manage invasive plants because the current approach is to release multiple agents for each weed species, which tend to span numerous environmental gradients in the invaded range.
I conducted a field experiment to explore how a foliar pathogen (a new biocontrol agent) affects its host, the invasive plant Centaurea solstitialis, and its interactions with two insect seed predators (well-established biocontrol agents). I conducted this experiment on adjacent patches of sandstone-derived soils and serpentine soils. Serpentine soils are of particular conservation value and they also have very low Ca/Mg ratios and Ca is essential to a plant’s ability to respond (via systemic acquired resistance, SAR) to pathogen infection. I hypothesized therefore that serpentine plants may be more susceptible to the pathogen’s direct effects than sandstone plants and this may also change any potential indirect interactions with the seed predators.
Results/Conclusions
The pathogen had a direct, negative effect on plant biomass and inflorescence production on both soil types although the pathogen’s direct impact was not greater on serpentine plants. The pathogen also had an indirect, positive impact by reducing seed predation. Both species of seed predators consistently ate fewer seeds when the plant was infected than when it was healthy, even when infected plants produced seeds with higher nitrogen content. This supports SAR as the mechanism underlying reduced seed consumption.
The net impact, i.e., the relative strength of the numerous direct and indirect interactions was contingent upon abiotic conditions (soil type). On the sandstone soil, the pathogen biocontrol acted as much as a mutualist as an enemy because its direct, negative impact was entirely canceled out by its indirect, positive effect. On the serpentine soil, pathogen infection reduced lifetime seed output by half.
Soil type fundamentally changed the outcome of the biocontrol agents’ impact on the plant. This combination of agents may reduce C. solstitialis abundance on serpentine soil (to the degree that recruitment is seed-limited) but not on adjacent non-serpentine soils. More broadly, biocontrol impacts may well vary substantially across an invader’s range.
