Exotic Mammals and Invasive Plants Alter Fire-Related Thresholds in Southern Temperate Forested Landscapes

Human activity has resulted in the emergence of fire-induced forest-shrubland mosaics in many southern temperate forest landscapes. In New Zealand (NZ), this transformation was caused by the introduction of fire to previously very low fire-frequency ecosystems. High levels of seed and seedling predation by exotic mammals and the reduction of avian pollination and seed dispersal further influence vegetation change and can cause successions to become completely arrested. Analogous dynamics occur in many other Southern Hemisphere forest landscapes where changes in fire regimes and the introduction of mammalian herbivores and predators have resulted in forest-shrubland mosaics. We developed a spatially explicit state-and-transition model to disentangle the effects of altered fire regimes; invasive pyrophyllic shrub species; seed and seedling predation by exotic mammals; and lowered rates of long-distance seed movement on long-term landscape dynamics and state transitions. Our modelling highlights a threshold in fire frequency where forest systems move to a fire-induced early-successional shrubland state. Because early-successional vegetation in NZ and many other southern temperate forests is much more flammable than mature late-successional vegetation, this shrubland state will persist until fire is suppressed. Our modelling suggests also that in the absence of fire there are critical thresholds in seed predation beyond which succession may become completely arrested in a mid-successional state. Even moderate levels of predation slow regeneration and move the tipping point at which forests shift to early-successional shrublands in the direction of less frequent fire. The presence of invasive pyrophyllic shrubs in the landscape also influences the location of thresholds in fire frequency at which state shifts occur. Restoring these landscapes to their formerly forested condition will require the removal of fire as well as mammalian seed/ling predators and the reinstatement of pollination and dispersal services. Removal of seed predators may act as an effective component of fire control by allowing secondary successions to proceed more quickly, thereby reducing landscape-level flammability.