Environmental Engineering Reference
In-Depth Information
include species that optimize growth in a climax successional phase or
a crowded environment, are highly adapted to stable equilibrium condi-
tions, are less l exible, more vulnerable to change, are generally longer
lived and do not disperse as well. High levels of disturbance may lead to
species-poor habitats since they favour the persistence of competitive,
opportunistic
r
species better adapted to cope with disturbance (Miller,
1982). Conversely, undisturbed environments that do not undergo change
may support less diversity because they favour the persistence of domi-
nant
K
-strategists. Linder et al.
(1997) examined the ef ects of i re history
on stand structure and plant diversity in Swedish forest reserves. They
concluded that the reintroduction of i re represents an important means
of disturbance that was necessary to promote diversity of l ora and fauna
in the area. Continued i re suppression has changed successional patterns
and altered stand structure. Late successional species such as spruce domi-
nate due to lack of i re, and pioneer species such as pine, silver birch and
aspen are decreasing in number because they require i re disturbance to
regenerate. This appears to accord with MacArthur and Wilson's (1967)
theory where undisturbed environments may therefore support less diver-
sity because climax species are favoured. Linder et al.
(1997) recommend
prescribed burning to ensure a relatively wide range of successional stages
to promote biodiversity over the longer term.
Higher habitat diversity due to moderate disturbance can also be
explained by niche relations and the manner in which species divide up
limited resources for their survival
(Schmida and Wilson, 1985). They may
divide up the available space (for example, by selecting dif erent habitats)
or energy resources (for example, by adopting dif erent diets). Some studies
serve to demonstrate that moderate levels of human activity may enhance
biodiversity by opening up new niches, providing new food or protection
from predators and by diversifying micro-habitats. For example, struc-
tural heterogeneity is thought to be important for bird species diversity
and vegetation indexes have been developed to quantify structural diver-
sity particularly in relation to bird species (MacArthur and MacArthur,
1961; Willson, 1974; Erdelen, 1984). Several studies indicate that a decline
in structural diversity (James and Wamer, 1982; Terborgh, 1985; Ratclif e,
1993; Telleria and Carrascal, 1994) and l oristic diversity (Lynch and
Whigham, 1984) leads to less bird species diversity. This is coni rmed by
Casey and Hein (1983) and Dambach (1944) who all reported that wood-
land heavily browsed by deer supported fewer bird species than woodland
that was not grazed (although see DeGraaf et al.,
1991).
Schemske and Brokaw (1981) provide empirical evidence to show
that moderate disturbance in tropical forests caused by natural tree falls
resulted in the greatest diversity of bird species. Clout and Gaze (1984) in
