2009 ) Landscape-level vegetation recovery from herbivory: progress after four decades of invasive red deer control . Journal of Applied Ecology , 46 , 1064 – 1072, , , , & (
Given the ubiquity with which ungulates are becoming increasingly common in temperate terrestrial systems, partly because of anthropogenic influences (i.e. removal of predators, milder winters and supplementary feeding), understanding the responses of herbivore-degraded ecosystems is a highly desirable objective. Current knowledge of how vegetation communities respond to ungulate browsing is largely drawn from comparisons between fenced (unbrowsed) and unfenced (browsed) areas. Replicated exclosure treatments, if imposed for long enough and distributed across a suitable environmental gradient, can provide novel insights into the complex interplay among woody plants, herbivore and the environment (e.g. Olofsson et al. 2009). Yet for practical reasons such experimental approaches are often limited in size, replication and/or location. An alternative approach is to assess the impact on vegetation communities of natural reductions or human culling of ungulate populations. Interpreting the response of vegetation to natural declines in ungulate population is complex since the same factor (e.g. climate change) responsible for ungulate declines may also impact directly on vegetation (Vors & Boyce 2009). While culling can avoid such pitfalls it is rarely undertaken over a large enough spatial and temporal scale to provide a detailed picture of ecosystem response. For this reason, the research by Tanentzap et al. (2009) is distinctive and represents one of the few studies to explicitly examine long-term (>30 years) patterns of change across multiple vegetation communities following herbivore reductions. Such an analysis bears important consequences for the management of wild herbivores and for linking ecological understanding to restoration actions (sensuOrmerod 2003).
Between 1861 and 1919, more than 250 red deer (Cervus elaphus scoticus) were released in New Zealand for recreational hunting and further releases continued until 1926. By the 1960s, red deer ranged over most of the country measurably impacting forest composition and structure by reducing populations of the most palatable plant species (Elton 1958). In the Murchison mountains of southern New Zealand, limited red deer control began in 1948, but population reductions were considered minimal until systematic culling began in 1962. To assess the response of vegetation to reduced deer densities, permanent plots were established in the area as early as 1969.
As a result of systematic culling, deer numbers were reduced by 92% and it was generally believed that continuous deer control since 1962 would reduce deer densities to levels at which regeneration of palatable species may occur. Tanentzap et al. (2009) report the results arising from recent resurveys of these long-established plots. Although some metrics did recover over time in response to reductions in browsing (i.e. recruitment of palatable tree seedlings and shrubland and grassland structure), the overall rate was slow (c. 30 years for relatively small but significant increases). While herbivore control is essential in restoring herbivore-disturbed ecosystems, the results suggest that solely manipulating herbivore population numbers may not always be a useful management objective in allowing for vegetation recovery. Vegetation recovery in response to long-term and significant herbivore reductions may be limited by several factors, including the slow growth rates of New Zealand species, density-dependent diet switching by deer, altered successional trajectories and below-ground processes. As a result, after 39 years, even low densities of herbivores may restrict ecosystem recovery, and restoration may require a long-term perspective on the order of decades. Management strategies can accelerate recovery by protecting existing palatable plants within deer exclosures, and planting or seeding palatable species within these refugia. However, in addition to increasing seed sources, restoration may only become apparent following large-scale disturbance events and canopy turnover.