Environmental Engineering Reference
In-Depth Information
Diversity Begets Ecosystem Stability
Ecologists have been long been concerned with understanding how ecosys-
tems withstand being buffeted by natural and manmade disturbances. One
of the earliest formal explanations invoking a critical role for diversity in
stabilizing long-term ecosystem function was developed by Robert
MacArthur (1955). It is now known as the Diversity-Stability Hypothesis.
MacArthur derived his explanation after wondering why it was that in some
ecosystems the abundances of most species changed little in the face of ab-
normal changes in the abundance of one species in that system, whereas in
other ecosystems, species abundances fluctuated widely.The former would
be called a stable system and the latter an unstable one (Box 8.1). MacArthur
(1955) argued that the stability difference was tied directly to the pattern of
interconnectedness or food web linkages among the species in the system.
To understand what MacArthur was driving at, imagine two systems
with identical numbers of plant species (P), herbivore species (H), and car-
nivore species (C) organized into food webs (figure 8.2). Food webs I, II,
and III differ only in the way the species are linked together.Yet, these sim-
ple differences can have profound implications for the ability of these sys-
tems to buffer the effects of a disturbance. For example, any disturbance that
lowers the abundance of plant species 1 (P
1
) will be felt more strongly in
food web I than in food web II or III.The reason is that in food web I,
herbivore species 1's ( H
1
) livelihood is directly tied to the abundance of plant
species 1 and the carnivore species depend wholly (C
1
) or partly (C
2
) on
the abundance herbivore species 1.Thus, fluctuations in the abundance of
plant species 1 will reverberate right up the food chain and cause significant
fluctuations in the abundance of herbivore species 1 and the two carnivore
species. In food web II, the effects of fluctuations in plant species 1 will be
buffered because herbivore species 1 now has an alternative resource that
can help meet shortfalls in its resource supply. In other words, food web II
has a greater diversity of species interconnections than food web I.This in-
terconnectedness increases the diversity of resources that higher trophic lev-
els can draw upon to mitigate shortfalls arising from fluctuations in the
abundance of any single resource species. By extrapolation, the more in-
terconnected species are in a food web (cf. food web I versus III) the more
stable are the systems (MacArthur 1955).This is because the greater
diver-
sity
in the lines of dependency (feeding linkages) among species in highly
interconnected food webs allows species higher up in the food chain to
