Environmental Variability and Ontogenetic Niche Shifts in Exotic Plants May Govern Reinvasion Pressure in Restorations of Invaded Ecosystems

When restoring ecosystems dominated by exotic plants, reinvasion pressure, or the rate of new exotic recruitment following mature exotic removal, can vary broadly between similarly invaded habitats. Reinvasion pressure strongly influences restoration costs and outcomes but is difficult to predict. Ontogenetic niche shifts (ONSs, changes in niche breadth or position during development) in exotic species paired with interannual variation in abiotic conditions may decouple pre-removal mature exotic density and average reinvasion pressure. Identifying such decouplings could improve restoration efficiency by informing site selection and management strategies, but requires estimates of average reinvasion pressure that mandate greater understanding of its principle drivers. We hypothesize that reinvasion pressure is predominantly driven by exotic propagule abundance and spatiotemporal availability of realized recruitment windows, which are periods of variable duration that permit exotic establishment from propagules. Realized recruitment windows are based on the “safe sites” concept but account for ONSs and are determined by abiotic conditions and interspecific interactions with recipient communities. Biotic resistance or facilitation may increase or decrease times required for establishment by influencing exotic growth rates or altering niche availability and may permit or preclude establishment in marginal abiotic conditions. We discuss factors influencing reinvasion pressure, basic approaches to estimate reinvasion pressure, and potential ways to increase management efficiency under different reinvasion pressure scenarios. Accurate estimates of reinvasion pressure could improve restoration efficacy, efficiency, and predictability in ecosystems dominated by exotic plants. We argue that greater theoretical and practical considerations of reinvasion pressure and ONSs are merited.