Evidence for multiple states in nature is extremely sparse. Forest insects may be
held at low density by warblers but can erupt to high density where warblers do not
regulate them (Ludwig et al. 1978; Crawford and Jennings 1989). There are a few
examples where mammals act as the predator. White-tailed deer maintain different
plant communities by feeding on young trees. Two tree densities can be found depend-
ing on whether young trees can escape this herbivory or not (Augustine et al. 1998).
Similarly, elephants in Serengeti can maintain two different densities of Acaciatrees.
When fire prevents regeneration and mature trees die of old age a grassland is pro-
duced that elephants can maintain by feeding on and regulating juvenile trees. When
overhunting by humans removed elephants (in both the 1880s and 1980s), trees escaped
herbivory and formed a mature savanna. After both periods of removal elephant
numbers increased; they fed on the mature trees but did not return the woodland to
grassland (Sinclair and Krebs 2002). Examples of multiple states where mammals are
prey are also rare (see Section 10.7.1). Outbreaks of house mice and European
rabbits in Australia may be interpreted as changes from a predator-regulated to a
food-regulated state (Pech et al. 1992). The collapse of the “forty-mile” caribou herd
of Yukon may be evidence of multiple states. Similarly, managers culled wildebeest
in the Kruger National Park, South Africa to reduce their numbers. When culling
ceased wildebeest numbers continued to decline through lion predation (Smuts
1978). In general, these examples illustrate that more than one state can occur under
a given set of climatic conditions.
In some cases the change in state has undesirable consequences for management
and conservation. For example, in the semi-arid regions of the Negev–Sinai in Israel
and Egypt, and in the Sahel of Africa, a shrub and herb layer acts as a blanket on
the soil, retaining moisture and heat overnight. During the day thermal upcurrents
carry moisture from both the soil and transpiring plants to upper levels where it
condenses as rain. This supplies the plants and soil, completing a positive feedback
self-sustaining system when in an undisturbed state. In contrast, overgrazing by live-
stock leaves a denuded soil surface, higher surface albedo, and a cooling at night.
There are fewer thermals, and these carry less moisture. Thus, overgrazed areas have
much lower precipitation, and this is also a positive feedback (Otterman 1974; Sinclair
and Fryxell 1985). The vegetated state switches to the denuded one through the dis-
turbance of overgrazing, that is, there is a threshold level of disturbance (grazing)
where one state switches to another. A similar positive feedback switch in vegeta-
tion state occurs in Niger where overgrazing has altered vegetation structure leading
to reduced water retention, increased soil loss, and further vegetation loss. The
system is now locked into this reduced state (Wu et al. 2000).
However, good examples of multiple states are rare, a few known from lakes, rivers,
coral reefs, grasslands, and forests (Knowlton 1992; Augustine et al. 1998; Dent
et al. 2002). The relevance to management is that multiple states are an emerging
property of ecosystems that will rarely be predicted from the study of single species.
Some states arise from excessive disturbance (see below). Thus conservation needs
to plan for more than one natural state whilst avoiding unnatural states due to exces-
sive human disturbance.
Figure 21.1 illustrates the pathways for energy and nutrients. However, it does not
indicate where the regulation occurs in the system. It can come via the food supply
(bottom-up), from predation (top-down), or both. In the absence of predators (or
ECOSYSTEM MANAGEMENT AND CONSERVATION 371
21.8 Regulation of top-down and bottom-up processes
