Linking Restoration and Ecological Succession (Springer ... - Inecol

Chapter 3 Aboveground–Belowground Linkages, Ecosystem Development, and Ecosystem Restoration 51
term, herbivores can stimulate decomposers by promoting compensatory plant
growth (and hence NPP), returning organic matter as labile fecal material, enhancing
foliar nutrient concentrations and impairing succession, thus preventing
domination by later successional species that produce more recalcitrant litter
(McNaughton 1985, Augustine and McNaughton 1998). Herbivores can also
depress decomposers through reducing NPP of plant species by tissue removal,
inducing production of defense compounds, and promoting succession (thus
encouraging domination of plant species that produce more recalcitrant litter)
(Pastor et al. 1993). At the landscape scale the most important belowground
effect of herbivores is usually through alteration of vegetation successional
pathways. The effects of herbivores on vegetation succession (Bardgett and
Wardle 2003, Sankaran and Augustine 2004; Fig. 3.2) in turn alter the densities
of decomposer organisms and rates of processes that they regulate.
Human-induced changes in densities of browsing mammals have important
consequences for both the aboveground and belowground subsystems worldwide;
these include the introduction of mammals to new regions (where they
function as invaders), the promotion of conditions that allow native mammals
to reach unnaturally high densities, and the reduction of natural mammal populations,
sometimes to extinction (reviewed by Wardle and Bardgett 2004).
For each of these mechanisms, several examples exist of how human-induced
shifts in densities of browsing mammals have affected vegetation succession and
consequently the decomposer biota. This is indicative of important herbivoreinduced
effects on feedbacks between the aboveground and belowground subsystems,
and collectively the impacts of these herbivores may represent an
important, though often unrecognized, component of global change (Zimov
et al. 1995, Wardle and Bardgett 2004, Burney and Flannery 2005). Restoration
activities would be aimed at returning mammal densities to the levels at
which they would occur in the absence of human activity (either by increasing
or reducing their densities), and thus encouraging the ecosystem to more closely
resemble its prehuman condition. It is apparent that alteration of mammal densities
and therefore intensity of herbivory will in turn alter the successional
trajectory of the vegetation.
New Zealand rainforests provide excellent opportunities to study the consequences
of human-induced alterations of herbivore densities on aboveground–
belowground feedbacks. Several species of large browsing mammals were introduced
to New Zealand between the 1770s and 1920s, the most pervasive
of which are the European red deer and feral goats. Before human settlement,
browsing mammals did not exist in New Zealand, making this system ideal for
investigating the impacts of introducing a whole functional group of animals
to an environment where they were previously absent. Further, in the 1950s
to 1980s the former New Zealand Forest Service established several hundred
fenced exclosure plots (typically 20m × 20m) throughout New Zealand’s forests
to assess browsing mammal impacts on vegetation; a subset of these are still
effective and they provide real opportunities for studying ecological impacts
of browsing mammals over the order of decades. Through exclosure studies, it
has been demonstrated that deer have important and consistent effects on vegetation
composition, through consuming and removing fast growing, palatable,
broad-leaved species, and encouraging their replacement by unpalatable monocotyledonous
species, ferns, and small-leaved species (Conway 1949, Wallis
and James 1972). This is consistent with the model of herbivores promoting