Environmental Engineering Reference
In-Depth Information
height and above-ground biomass of all three species. Some studies in the
UK report a high number of seedlings within fenced enclosures compared
with unfenced areas (Sykes and Horrill, 1979; Marrs and Welch, 1991;
Staines, 1995). Historical records have also been used that suggest that
deer may prevent natural regeneration of Scots pine (
Pinus silvestris)
in
the United Kingdom. With respect to stocking densities, studies inves-
tigating the impact of ungulates foraging on upland heaths in Scotland
suggest that red deer (
Cervus elaphus
) at stocking densities of .1 deer/20
ha can prevent tree regeneration (Staines et al., 1995).
Persistent high levels of disturbance are also thought to af ect ecosystem
function, particularly where these eliminate important functional groups
that af ect ecosystem processes. Groups of grass species may be signii cant
in maintaining the productivity of savanna ecosystems (Walker et al.,
1981). Walker et al. found that grasslands with persistent intensive grazing
by settled peasant farmers had lower levels of productivity than moderate
opportunistic grazing practices employed by nomadic pastoralists. In the
former case, productive functional groups declined because herbivores
showed a preference for the most palatable species, whilst in the latter
case these preferred species were able to persist in the sward and adapt to
change and instabilities caused by grazing and drought, thereby maintain-
ing structural resilience.
Overgrazing may exacerbate the high inter-annual variation in produc-
tivity on many rangelands. Walker (1988) has observed a much higher
phenological diversity in semi-arid systems not subject to heavy grazing
compared with those that are intensively grazed. On lightly grazed areas
he noted an even mix of early, mid- and late season grasses that were
able to respond to rainfall wherever it occurred in the season. Heavy
grazing leads to an absence of highly palatable early season species that
are replaced by later growing species (Silva, 1987). The implication being
that forage production was lower and more unstable on heavily grazed
areas compared with lightly grazed land because the sward was not able
to respond to early season rains. In the Serengeti, McNaughton (1985)
has also shown that forage production was more stable where the number
of species contributing to biomass was high compared with swards where
relatively few species contributed to forage production.
Many complex ecosystems that aim to maximize heterogeneity (such as
non-equilibrium systems) may be threatened by intensive grazing regimes
that attempt to restrict livestock movements. This may have a negative
impact on the stability and sustainability of the system. In areas where the
fodder resource is widely dispersed seasonally and spatially, restrictions
on stock movements by using paddocks can lead to land and vegetation
degradation (Hof man and Cowling, 1990). Increasing the connectivity
