Biodiversity conservation in managed landscapes - Environmental Management - page 91

of an under-connected system may also cause the system to change to a
new stable state. An example of such a change has occurred in semi-arid
shrublands, where the erection of fences has restricted animal move-
ments, leading to localized feeding and degradation of vegetation and soil
resources (Hof man, 1988).
There is evidence that overgrazing may trigger the transition from one
ecosystem to another, for example from forest to grassland or grassland to
a shrubby semi-desert (Holling, 1973; Westoby et al., 1989). This evidence
suggests that state and transition models (STM) are appropriate in many
rangeland situations where vegetation is best described by a set of discrete
'states' and a set of discrete 'transitions' between states. During a transi-
tion the system jumps to another state if a threshold is exceeded. This can
be triggered by i re, rainfall or grazing and the system never rests halfway
through a transition. They use an STM in eastern Australia to show how
once ecological thresholds are exceeded the system shifts from a woodland
with a grass understorey to a less productive shrubby state. The concept of
ecological thresholds suggests that there are limits to the ability of ecosys-
tems to withstand environmental perturbation. If such limits are exceeded,
ecosystems may shift to a less productive phase.
In some circumstances human management and spatial landscape
change may undermine ecosystem processes. For example, some empirical
studies have demonstrated that habitat fragmentation may reduce para-
sitism rates on herbivorous insects at dif erent spatial scales (Kruess and
Tscharntke, 1994; Roland and Taylor, 1997). Similarly, silvicultural prac-
tices may reduce parasitism rates of spruce bark beetle ( I. typographus ) in
central Sweden (Weslien and Schroeder, 1999).
Clearly persistent intensive use of resources can degrade ecosystems
and impact negatively on key functional groups. However, the level and
nature of disturbance appears to be an important factor. Some examples
of managed ecosystems indicate that an element of disturbance caused
by human intervention may actually enhance biodiversity. In the next
section we explore the impact of low and moderate levels of disturbance
on biodiversity, productivity and resilience.
Moderate ecosystem disturbance and biodiversity
MacArthur and Wilson (1967) suggest that some disturbance can promote
diversity because dif erent species respond to disturbance in dif erent ways.
They i rst characterized species as either r - or K - strategists, which have
evolved mechanisms to optimize resources in quite dif erent environments.
The former ( r -strategist) refers to species that attempt to maximize growth
in an unconstrained environment, reproduce quickly, disperse widely and
are of smaller size and shorter lifespan. On the other hand K - strategists