Environmental Engineering Reference
In-Depth Information
the amount of clover relative to grass. In this manner combined cattle
and sheep grazing systems may be more productive than single species
systems.
Examples of rangeland management systems that attempt to encourage
diversity in herbivore populations to enhance resilience include replac-
ing monocultures of domestic livestock with multispecies game systems
and combined cattle/game ranches such as the CAMPFIRE (Communal
Areas Management Programme for Indigenous Resources) programme in
Zimbabwe (Cumming, 1993). Scholes and Walker (1993) have suggested
that events such as i re and herbivory play an important role in maintain-
ing the diversity and resilience of such systems. Here, the reduction of such
perturbations is thought to reduce landscape diversity and the ability of
the system to survive similar shocks in the future.
Moreover, in savannas (Walker et al.,
1981) and
Agrostis- Festuca
grass-
land in Britain (Hulme et al., 1999) groups of grass species are important
in maintaining the system's productivity. In other habitats, such as boreal
and deciduous forests in North America and Europe, where insectivorous
bird species are considered to be instrumental in controlling outbreaks of
forest insect pests, overall species diversity is important for maintaining
stability (Morris et al.,
1958;
Tinbergen, 1960; Campbell and Sloan, 1976;
Holmes et al., 1979; Takekawa et al., 1982; Holling, 1988; Maquis and
Whelan, 1994). There may also be indirect ef ects of diversity with some
species inl uencing the survival of other species, such as key plant species
determining the course of successional processes through the provision of
so-called 'nurse ef ects'. Several studies have observed a greater number of
seedlings beneath mature trees compared with more open areas (Grii n,
1971; Borchert et al., 1989; Espelta et al., 1995). Similarly, shrub species
may inl uence seedling establishment, acorn consumption and the extent of
browsing by herbivores (Morgan, 1991; Callaway, 1992; Herrera, 1995).
Despite the importance of biodiversity in contributing towards eco-
logical services many ecosystems are undergoing profound change due
to economic development. Heavily disturbed agricultural ecosystems in
many parts of the world are threatened by human intervention, which
has resulted in a loss of biodiversity, productivity and resilience. Land
managers frequently need to know how biodiversity is af ected by the
level and intensity of management and what will be the result of that
change. The signii cance of human-induced disturbance and environmen-
tal perturbation is the subject of the next section.
Ecosystem disturbance and biodiversity
Considerable insight into the understanding of conservation biology
has been gained through knowledge of the ef ects of human-induced
