Environmental Engineering Reference
In-Depth Information
nutrient concentrations rose steeply from the early 1970s, more or less eradicating
ecosystem services such as the provision of seafood and of clean water for swim-
ming. Xu et al. (2004) have proposed a suite of ecosystem health indicators that
encompass physical state (e.g. t urbid it y), chemical state (e.g. d i s solved oxygen near
the seabed), biological state (e.g. mangrove biomass, phytoplankton biomass, fecal
coliform numbers) and the state of ecosystem services (e.g. seafood productivity and
quality, feasibility of marine farming, safety of contact recreation). The management
response has been the implementation of the Tolo Harbor Action Plan, which aimed
to signifi cantly reduce pollutants and excess nutrients entering the rivers that fl ow
into Tolo Harbor. It is gratifying to note that the majority of state indicators have
improved a little during the 1990s but there is still a way to go before regaining the
full suite of ecosystem services that existed in 1970.
Summary
Food webs
The infl uence of a predator population on its prey, or of one competitor on another,
is easy to envisage. But the infl uence of a species often spreads via a series of indirect
effects throughout the food web. The food web is often the starting point for predict-
ing the consequences of the arrival of an invader, for planning the removal of preda-
tors of an endangered species or for devising a pest control strategy. Managers need
to be aware that some species (keystone) are much more infl uential in the food web
than others.
Top-down and bottom-up control of food webs
A carnivore or parasite, through its infl uence on a herbivore, may reduce consump-
tion of plants whose biomass increases as a result. Thus, plant biomass may some-
times be controlled top down . In other cases plant biomass is controlled bottom up
by the supply of resources needed for photosynthesis - light, carbon dioxide, water
and nutrients, such as phosphorus and nitrogen. Nuisance blooms of algae in lakes
may be controlled by increasing predation on herbivores (top down) or by reducing
the delivery of excess nutrients (bottom up) via improved sewage treatment or
reduction of agricultural runoff.
Energy and nutrient dynamics in ecosystems
Plant communities are most productive where sunlight, water and nutrients are in
good supply. Some of the energy locked in plant biomass passes along food chains
to herbivores and their consumers - the grazer system. A proportion of plant
biomass dies and moves as dead organic matter into the decomposer system, with
its bacteria and fungi, detritivorous animals and their consumers. Living organisms
extract chemicals from their environment, hold on to them for a while, then lose
them again. Ecosystem ecologists are interested in how the biota moves chemical
elements between the various living and nonliving compartments of the ecosystem.
Certain species play key roles in determining patterns of energy and nutrient fl ux,
and biosecurity managers need to pay particular attention to invaders that have a
novel method of resource acquisition or that change the links between ecosystem
compartments. Ecological stoichiometry is concerned with the relative availability
of different elements - particularly the ratio of carbon to nitrogen. Stoichiometric
knowledge can sometimes be used to manipulate soil carbon content to facilitate
the restoration of a native plant community.
Search WWH ::




Custom Search