Biodiversity conservation in managed landscapes - Environmental Management - page 85

deliberately maintain as many as a dozen breeds of camel in the Sudan
because they are able to exploit the vegetation of extreme environments,
including deserts and other uncultivated land. The loss of these hardy
animal breeds therefore means a reduction in the area of human habitat
(Köhler- Rollefson, 1993).
In Mediterranean dehesa systems good conservation practices that
promote biodiversity have arisen because local farmers recognize that a
diverse system helps reduce variation in productivity from year to year.
Local farmers do not necessarily have biodiversity conservation goals
in mind as a management aim. Nevertheless, biodiversity coincides with
certain production goals such as improved stability of production under
unpredictable environmental conditions. Large dif erences in climatic,
geological and topographical gradients contribute to a considerable
degree of variation in productivity across the regional landscape. Climatic
factors are instrumental in dictating plant and animal dynamics and prod-
uctivity. Consequently, a long history of anthropogenic inl uence has led
to the development of a high level of management and functional com-
plexity as a means of ensuring stability. Attempts to reach an understand-
ing of the relationship between management practices and environmental
variability have prompted research ef ort into the interactions between the
individual components that comprise this complex ecosystem including
tree, herbaceous and shrub, and livestock components.
For example, holm oak is preferred by dehesa farmers because it
favours the growth of highly productive perennial herbaceous species
through improved retention of soil moisture, modii cation of micro-
climate, improved nutrient availability and improved soil properties
(Marañon, 1986; Jof re and Rambal, 1993). Marañon (1986) has observed
a much higher phenological diversity in dehesa systems that include a tree
component (Figure 4.2).
Groups of perennial herbaceous species may be signii cant for maintain-
ing productivity because they are able to utilize nutrients and moisture
more ef ectively. These include Agrostis castellana, Dactylis glomerata,
Lolium perenne and Phalaris aquatica , which were all found more fre-
quently beneath tree canopies than in the open i eld (Jof re et al., 1988).
Jof re et al. (1988) hypothesized that dif erences in nitrogen utilization
occurred between annual and perennial species and that the ei ciency of
nitrogen utilization by herbaceous species was af ected by the tree canopy.
They report higher nitrogen mineralization in grasslands with perennials
compared with annuals and greater nitrogen mineralization below the tree
canopy.
Farmers have evolved complex farming systems specii cally to be able
to exploit resources in species-rich environments. Diverse multispecies