Applications from food web and ecosystem theory - Ecological Applications: Toward a Sustainable World - page 269

Human disease risk
Lyme disease (and other tick-transmitted diseases) is part of a complex food web
involving ticks, small mammals, birds, reptiles, large mammals and oak trees. The
transmission of malaria (and other diseases carried by mosquitoes) is similarly
affected by a variety of species with which the mosquitoes interact. Based on
an understanding of these interactions, managers can sometimes reduce
disease risk.

Harvest management
Harvesting one species invariably affects others in the food web and it can be risky
to treat a marine fi shery as though the target species exists in isolation. In reality,
any fi shery has direct and indirect consequences, generally reducing the abundance
of favored large fi sh and, through a reduction in top predators, allowing a prolifera-
tion of smaller species further down the food chain. By overfi shing the largest and
most desirable species, fi shers increasingly rely on species from much lower down
the food chain - known as 'fi shing down marine food webs'.

Managing agriculture to minimize fertilizer input and nutrient loss
Intensive agriculture leads to pollution, by phosphorus or nitrogen, of the waterways
that drain the land when there is a mismatch between the application of fertilizer
and its use by the intended recipients (crop plants, plantation trees). Approaches to
minimize fertilizer loss from the land include maintenance of ground cover of veg-
etation year-round, mixed cropping and careful return of organic matter to the soil.
In addition, restoration of the vegetation zones between land and water (wetlands
and riparian vegetation) can reduce inputs to the aquatic ecosystem because the
plants and microorganisms remove some of the dissolved nutrient as it fi lters
through the soil.

Ecosystem services and ecosystem health
Benefi ts accrue to people from natural ecosystems - ecosystem provisioning services
such as wild food and forest products and regulating services such as high water
quality and maintenance of soils. It is important for managers to be able to put a
value on ecosystem services that are put at risk by human economic activities.
Activities that are economic from the point of view of private benefi t often turn out
to be uneconomic when public benefi ts are also taken into account.
Many ecosystems have been degraded by human activities. Managers describe
ecosystems as 'unhealthy' if their community structure or ecosystem functioning
has been fundamentally upset. To help managers identify and prioritize ecosystems
in trouble, easily measured indicators of 'ecosystem health' have been developed.

The fi nal word

As a retired banker Aretha understands classical economics pretty well, but she was
surprised to fi nd that such a wide range of ecosystem services are now having dollar
values placed on them. She was struck by the thought that often one ecosystem
service has to be traded off against another. ' Harvesting abalone (a provisioning
service) and conserving the sea otter (arguably a cultural service) are both great objec-
tives, but they cannot be achieved in the same place at the same time. A balancing act is
required. In the same vein, the negative effects on ecosystem functioning of exotic trout
need to be balanced against the improved recreational opportunities they provide. ' Then