Factors preventing the recovery of New Zealand forests following control of invasive deer

There is currently great interest in restoring ecosystems affected by invasive organisms. In New Zealand, deer were introduced during the nineteenth century, causing dramatic changes to the understory composition and structure of some forests. Deer prefer to browse on short tree species, mostly associated with early successional stands and moist-fertile sites, but many tall tree species are browsed only when other food sources are scarce. The government has invested heavily in control programs aimed at reducing deer numbers and thereby restoring forests to something approaching their preinvasion composition and structure, but have met with only limited success. Based on a literature review, we give several examples of situations in which deer impacts may not be reversible, including the following: (1) palatable species remaining highly browsed even at low deer densities as a result of diet switching; (2) occupation of vacated niches by plant species not eaten by deer; (3) local extinction of seed sources; (4) fundamental alterations to successional pathways; (5) shifts in ecosystem processes; (6) other exotic animals becoming naturalized and weakening the effectiveness of single-species control; and (7) exotic plants weakening the effectiveness of single-species control. We consider the contributions that scientific research can make to effective forest restoration, including empirically based forest-dynamics models that place regeneration in the context of other processes, such as disturbance, soil fertility, and multiple invasive organisms.