Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 261

density of two species of piscivorous fi sh - walleye ( Stizostedion vitreum ) and north-
ern pike ( Esox lucius ). In total, more than 2 million fi ngerlings of the two species
were stocked beginning in 1987 (Figure 9.14a) and total piscivore biomass responded
rapidly, stabilizing at 4-6 kg ha −1 . The biomass of zooplanktivorous fi sh declined, as
a result of increased predation by the piscivores, from 300-600 kg ha −1 prior to
biomanipulation to 20-40 kg ha −1 in subsequent years. The consequent reduction in
predation pressure on zooplankton (Figure 9.14b) led, in turn, to a switch from
small zooplankton grazers ( Daphnia galeata mendotae ) to the larger and more effi -
cient Daphnia pulicaria . The increased grazing pressure had the desired effect of
reducing phytoplankton density and increasing water clarity (Figure 9.14c).

9.8 Ecosystem
services and
ecosystem health

Having considered a range of impacts of human activities on ecosystem functioning,
attention can now be turned to the question of how to 'cost' these impacts or, more
particularly, how to put a value on the ecosystem service(s) that have been lost
(Section 9.8.1). Beyond this, managers have another requirement to help them
identify and prioritize ecosystems in trouble - easily measured indicators of
the 'ecosystem health' of terrestrial, freshwater and marine ecosystems (Sections
9.8.2 - 9.8.5).

9.8.1 The value of
ecosystem services

Some human effects are more signifi cant than others for the welfare of nature and
of humans, but until recently it was not possible to set benefi ts and costs against
each other so that managers can identify priorities for action. The private economic
benefi ts of exploiting a forest for timber or a coral reef for fi sh (or for coral) are
usually clear. But the public benefi ts associated with the pristine ecosystem (or of
less intensive exploitation) are harder to quantify. Considerable ingenuity is required
to assign value to benefi ts that accrue to people from natural ecosystems - ecosystem
provisioning services such as wild food and forest products and ecosystem regulating
services such as maintenance of the chemical quality of natural waters, buffering of
human communities against fl oods and droughts, protection and maintenance of
soils, regulation of climate, and breakdown of organic and inorganic wastes.
The most straightforward approach is to compare the value of retaining a particu-
lar habitat in a relatively undisturbed condition as opposed to intensively exploiting
it. Going beyond the mere calculation of private benefi t to incorporate the dollar
values of diverse public benefi ts of ecosystem services, Balmford et al. (2002) ana-
lyzed fi ve case studies. Two of these concern tropical forest (Figure 9.15a,b). High-
impact (and unsustainable) logging in Malaysia produces the highest private benefi ts
(to t he loggers), but after taking ecosystem services into account (nontimber forest
products, fl ood protection, carbon accumulation in vegetation reducing atmospheric
carbon dioxide and counteracting global warming - Chapter 11), the economic value
of high-impact forestry is actually 14% lower than for sustainable, low-impact
logging. In the case of Cameroon, conversion to plantation (oil palm and rubber)
was compared with conversion to small-scale agriculture or retention of low-impact
forestry. The value of all ecosystem services combined is highest under sustainable
forestry while, at the opposite extreme, plantation conversion actually makes a net
loss when both private benefi t and ecosystem services were included in the
analysis.
Analysis of a mangrove ecosystem in Thailand showed that the private benefi t
from shrimp farming shrinks almost to nothing when the economics take into