Modelling the effect of two biocontrol agents on the invasive alien tree Acacia cyclops—Flowering, seed production and agent survival

Invasive alien plants play an important role in our changing world and have a considerable negative impact on ecosystems in many parts of the world. One effective control strategy involves the use of biocontrol agents which either damage the invasive alien plant itself or reduce the production of seeds. In this paper, we focus on seed-attacking biocontrol agents. To be able to predict the impact of biocontrol agents on the seed production pattern provides a way of comparing the impact of different biocontrol agents before they have actually been released. This becomes complex in systems where more than one biocontrol agent is present and where both depend on the same resource. In this paper, we examine a system consisting of two seed-attacking biocontrol agents (a midge and a weevil) and one invasive alien plant (Acacia cyclops). We used a non-spatial but temporal model to predict the impact of these biocontrol agents on flowering and seed set. We used a global sensitivity analysis (i.e. analysing the impact of several parameter simultaneously) to evaluate the impact of selected parameters which were considered uncertain but important. We could show that the system, even without biocontrol agents, fluctuates for most parameter sets between years, and that the fluctuations increased after the release of the biocontrol agents, although the number of seeds produced was, on average, substantially smaller. We found that the two biocontrol agents could usually not coexist, highlighting that special care needs to taken in selecting each additional biocontrol agent to avoid driving an already established biocontrol agent to extinction. These observations make a strong case for long-term monitoring after the release of biocontrol agent.