Socio-ecological mechanisms for persistence of native Australian grasses under pressure from nitrogen runoff and invasive species

Abstract

Nitrogen runoff in certain southeastern Australian grasslands promotes the invasion of exotic grassland species at the expense of native species. Mitigation programs can reduce runoff and thus support native species, but their success may require the awareness and support of local populations. This situation represents a coupled socio-ecological system, since nitrogen runoff caused by local populations can enable the invasion of exotic grassland species, which can in turn stimulate a social response to restore the native species. Our objective is to use a mathematical model to identify potential socio-ecological mechanisms for the persistence of native grassland species, and the parameter regimes for which these mechanisms operate. We couple a model of southeastern Australian grassland dynamics with a model of human social dynamics concerning runoff mitigation. Nitrogen runoff can enter the ecosystem either through local sources under control of a human population, or through global sources not under their control. Humans learn mitigating behaviour socially, and respond to the prevalence of native and exotic grassland species. We find that socio-ecological dynamics introduce broad parameter regimes that are not present in the ecological system in isolation from the human system. We identify two mechanisms for native grassland persistence: one is associated with significant reductions in runoff rates and/or cost of runoff mitigation programs, resulting in a stable state where the native grassland species exists or dominates, with or without the support of socio-ecological feedback. A second mechanism associated with higher rates of nitrogen input supports persistence of the native species through oscillations in species abundance and mitigation behaviour in the human population. However, this state is less favourable to the native species because the oscillations may become extreme in amplitude. Finally, we find that increasing the cost of mitigation programs not only reduces mitigating behaviour in the population but also (more surprisingly) increases the tendency for the system to destabilize into a regime of oscillations in native species biomass. We conclude that multiple socio-ecological mechanisms could potentially support native species in grassland ecosystems under stress from nitrogen runoff and invasive species. Further research can refine such models to inform policy in the face of nonlinear socio-ecological responses.