untitled

to occur because of dominance and keystone effects. Thus, if we lose species from a

system, initially we are unlikely to see much reduction in function for two reasons.

One is that the species lost could be minor players in the system, and a drop in func-

tion is not observed until a keystone or dominant is lost. Thus, a drop in function

occurs anywhere in the sequence (even at the beginning) depending on when the

important species are lost. A second reason, however, is that in a diverse com-

munity the first few species to be lost can be replaced by other species that take over

their function. This involves the idea that there are redundant species in the system

(Walker 1992, 1995). An indication of this effect is seen in the progressive loss of

fish species due to human harvesting of the oceans. Initially, productivity of the fisheries

was maintained through the replacement of lost species by others that increased.

However, after a delay of several decades, so many species were lost that the remain-

ing ones were unable to replace them and fish harvests collapsed ( Jackson et al. 2001).

The second way of looking at ecosystem function is to consider how it changes

with the degree of disturbance (Fig. 21.4b). With minor disturbances some species

can compensate for lost ones and maintain the function. However, at some point the

disturbance is so large that there are insufficient species to replace the lost ones. So

far most of the evidence for these concepts has come from plants and the stability

of productivity and nutrient cycles (Vitousek and Hooper 1993; Naeem et al. 1995;

Naeem and Li 1997; Naeem 1998, 2002).

The distortion of ecosystem processes can lead to unwanted ecosystem effects,

conservation threats to individual species, and expensive ecosystem management.

For example, Australia before European settlement was largely covered in eucalypt

woodland. In the past century agriculture has removed nearly all of this woodland,

especially in Western Australia. Originally eucalypts kept groundwater levels down

through transpiration processes. Once the trees were removed, groundwater levels

rose, water evaporated at the soil surface, and saline deposits made the soil unsuit-

able not only for crops but also for native biota. Now large areas of Australia have

a major problem with salinization of soil and groundwater upwelling, with a result-

ant decline in agricultural productivity. In response to this ecological (and economic)

problem, Australia has had to adopt the expensive policy of revegetation (McFarlane

et al. 1993; Nulsen 1993).

382 Chapter 21

Number of species Disturbance intensity

Ecosystems function Ecosystems function

Replacement

No replacement

Replacement

No replacement

(a) (b)

Fig. 21.4(a) Hypothetical relationship between ecosystem function and species number. If species can replace the function of
lost species then system function follows the solid line. If there is no replacement then function follows the broken line.
(b) Ecosystem function relative to the degree of disturbance that can deplete species diversity. Low disturbance (left on x-axis)
is tolerated because species can replace each other. Higher disturbance can cause a rapid decline in function when redundancy
of species is used up.