fruiting plants are fed upon by birds (Levey 1990). Temperate conifer forests experi-
ence fire at frequencies of 50 –200 years, maintaining a mosaic of stands of different
age and species composition, and a diversity of habitats for birds and mammals
(Bunnell 1995).
In tropical savannas, frequent fire dominates the system. It impedes plant succes-
sion, maintaining an open tree canopy (<30% cover) and a grass understory. This
fire regime provides the optimum habitat for the high diversity of ungulates in East
Africa (Frost 1985). Fire is also required to maintain specific habitats: in North America
the endangered Kirtland’s warbler (Dendroica kirtlandii) requires fire to create its jack
pine (Pinus banksiana) habitat (Probst 1986).
Other forms of disturbance are hurricanes and floods. Hurricanes are important at
the 10–20° latitudes along coastlines. Their periodicity is usually measured in dec-
ades and they have physical restructuring effects by opening up forests, and altering
shores, estuaries and riverbanks, and sedimentation rates. Flooding of rivers and
estuaries is more frequent and universal. Some river flooding is necessary to main-
tain nesting habitat on sandbars for least terns (Sterna albifrons) and piping plovers
(Charadrius melodius) in the USA, and on braided rivers of New Zealand for endan-
gered black stilts (Himantopus novaezealandiae) (Boyce and Payne 1997).
One of the important consequences of disturbance is that it creates heterogeneity,
or patchiness, in habitats, particularly because it reverses succession. A mosaic of
patches of different age from the time of the disturbance leads to different com-
binations of habitats and species, an aspect that is important for ecosystem manage-
ment and conservation. In particular, heterogeneity creates the mosaic of sources and
sinks (Pulliam 1988; see Section 7.7.4). Sources are good habitats where species are
self-supporting and surplus animals emigrate from these to sinks. The latter are poorer
habitats that are not self-supporting. However, sinks provide a vital role in allowing
non-breeding individuals to survive while they wait for opportunities to obtain
territories in source habitats. Sinks provide the compensation in a population for unpre-
dictable disturbances that reduce breeding populations. Protected areas, therefore, should
contain both source and sink habitats.Disturbances that are too frequent or too extreme can radically alter an ecosystem,
changing it to a different state. Persistent overgrazing can result in denudation, as
mentioned earlier for semi-arid areas (Wu et al. 2000), or in a change from grass-
land to woodland, as in savanna areas (Walker et al. 1981). Other forms of human
overdisturbance are often the underlying cause for invasions of exotic species that
can take over and maintain new states (Vitousek et al. 1996).
More rarely, single extreme natural disturbances can change ecosystem states: the
‘Wahine’ storm of 1968 destroyed the beds of aquatic macrophytes in Lake Ellesmere
in the South Island of New Zealand, and the physical change in the lake sediments
has prevented their return. The resident population of black swans (Cygnus atratus)
numbered 40,000–80,000 in the 1950s and 1960s. They used the weed beds for food
and to raise young. Mortality from the storm itself, starvation, and reduced breed-
ing because of a lack of suitable habitat rapidly reduced the population to less than
10,000 in subsequent years, and they have never returned to their original numbers
(Williams 1979; A. Byrom, pers. comm.).
Earthquakes and their resulting tsunamis are another form of disturbance that
can cause sudden effects in an ecosystem with long-term consequences. Botswana inECOSYSTEM MANAGEMENT AND CONSERVATION 375
21.10.2Disturbance
and ecosystem
management