A review of the causes and consequences of geographical variability in weed biological control successes
© 2020 The impacts of different biotic and abiotic limiting factors on introduced biological control agents and their weed targets may result in geographically variable control, with implications for ongoing management and plans to improve efficacy. Geographic variability in control successes, however, may be underreported. Using the comprehensive catalogue, “Biological control of weeds: A world catalogue of agents and their target weeds”, we assessed whether geographic variability in successful control is common, whether variability in programs could be attributed to limiting factors, and which factors were most likely responsible for variable success. In 38% of all reviewed programs, success was deemed spatially variable but nearly half of variable programs had unknown or unreported limiting factors. We discuss the factors that can contribute to geographic variability in agent abundance and associated success of weed biological control, provide case studies and current biological control practices, and finally discuss the potential ecological and evolutionary consequences of this variability. Published data to assess whether geographic patterns of variability were predictable, or to quantify variability along environmental gradients are lacking. Research that addresses the strength of agent-host interactions across environmental gradients, such as temperature or precipitation, or examines spatial variability in a metapopulation or landscape context may provide the best understanding of control failures and contribute to a biogeographic framework within which to evaluate ongoing and future biological control projects.