Restoring temperate ecosystems after long-term ungulate impact: A global perspective
It has been known for over 100 years – since the modern founding of ecology – that high abundances of wild ungulates threaten the conservation and restoration of natural ecosystems. One family of ungulates in particular, deer (Cervidae), have become widespread throughout temperate regions during this period. But much remains unknown about the outcomes of landscape-level management actions to control deer populations, in part because vegetation responds slowly and can change along different trajectories. The responses of ecosystems to herbivore removal may also have important implications beyond biodiversity, such as for increasing terrestrial carbon sinks.
Here, I identify generalities in how long-term landscape-level control of deer populations has led to vegetation recovery across permanent forest plots in New Zealand, Canada, and the UK. Simulation models provide an important tool to complement plot-based findings, and I showcase a spatially-explicit model developed to predict the spread of upland birchwoods in the Scottish Highlands. Forest simulation models can easily be extended to generate predictions about the responses of other conservation priorities following deer management, e.g. carbon stocks.
Vegetation changes slowly following deer population reduction because of the: (i) depletion of seed sources of deer-preferred plants; (ii) preferential browsing of forage even under low deer densities; (iii) intervening changes in abiotic conditions associated with processes other than deer browsing, e.g. abiotic disturbance; (iv) formation of alternate vegetation types under high deer browsing; (v) variation in browse damage of deer-preferred species with plant community composition (e.g. associational resistance or susceptibility); and (vi) slow growth rate of plants relative to the amount of biomass consumed by herbivores. Simulation modelling confirms that deer browsing strongly controls transitions of saplings to taller (>3m) height tiers, and further identifies suitable ground cover for seedling establishment as a prerequisite for vegetation recovery and future carbon sequestration. Into the future, implementing science-based management recommendations, e.g. culling, and their societal acceptance will likely be the largest obstacle to achieving vegetation recovery rather than an understanding of ecosystem and restoration ecology.