Environmental Engineering Reference
In-Depth Information
Figure 9.1). The starfi sh benefi ts a suite of inferior competitors (algae growing on the rocks together
with a range of invertebrates) by preferentially consuming the dominant competitors (barnacles
and, in particular, mussels). When the starfi sh are removed, the inferior competitors disappear
from the scene. Managers need to keep in mind not only that a species of interest will have both
direct and indirect interactions in the food web, but also that some are far more infl uential than
others.
Trophic cascades and top-down and bottom-up control of food webs
Many examples have come to light where a carnivore (or parasite), through its infl uence on a her-
bivore, is responsible for relaxing the impact of the herbivore on its plant food, whose biomass
increases as a result. The idea can be extended to a food chain with four links - in this case the
top predator may reduce an intermediate predator, relaxing predation on a herbivore, leading to
enhanced grazing and a reduction in plant biomass. These patterns are known as trophic cascades
and the species at the top of the food chains can reasonably be called keystone species. One way
to reduce nuisance algal blooms in lakes is to manipulate a cascading food chain to benefi t the
herbivores.
In trophic cascades, the dominant control on biomass in the food web is exerted from the top
down . In particular, whether plant biomass is high or low is a direct consequence of the impact of
a top predator. But this is not always the case. It is by no means unusual for plant biomass to be
controlled bottom up by the supply of the resources needed for photosynthesis - light, carbon
dioxide, water and nutrients such as phosphorus and nitrogen. Everyone will be aware that the
application of fertilizer usually has the effect of increasing plant productivity because previously
limiting nutrients become more readily available. This is bottom-up control of plant biomass. An
alternative way to reduce nuisance algal blooms in lakes is thus to identify and eliminate the delivery
of excess nutrients by improved sewage treatment or reduction of agricultural runoff.
Ecological energetics
Net primary productivity (NPP) is the rate of production of new biomass by plants. Plant communi-
ties can be expected to be most productive where sunlight is in good supply, and where suffi cient
water and plant nutrients are available to fuel plant growth. Some of the energy locked in plant
biomass passes along food chains involving herbivores and their consumers - collectively known
as the grazer system . But because herbivores are never 100% effi cient at consuming NPP, some
plant biomass dies and fi nds its way as dead organic matter (DOM) into the base of another com-
ponent of the food web - the decomposer system , with its bacteria and fungi, detritivorous animals
and their consumers. Note that each trophic grouping (herbivores, carnivores, detritivores, etc.)
is less than 100% effi cient at assimilating what is consumed (some is lost as feces, which also
enter the DOM compartment) and less than 100% effi c i e n t a t t u r n i n g what is assimilated into new
biomass (some energy is lost as respiratory heat). The rate of production of new biomass by
organisms other than plants is known as secondary productivity .
Taking all these points into account, Figure 9.2 illustrates some gross differences in the fl ux of
energy through contrasting ecosystems. First note that the decomposer system is responsible for
the majority of secondary productivity, and of respiratory heat loss, in almost all the world's eco-
systems. The grazer system holds little sway in terrestrial communities (Figure 9.2a,b) because of
low herbivore consumption and assimilation effi ciencies, and it hardly exists at all in many small
streams and ponds because NPP is so low (Figure 9.2d). These small water bodies depend for their
energy on dead organic matter that enters from the surrounding terrestrial environment. The grazer
system plays its strongest role in plankton communities because a large proportion of NPP is
consumed alive and it is assimilated at quite high effi ciency (Figure 9.2c). Certain species may play
key roles in determining the overall productivity and pattern of energy fl ux in an ecosystem. Thus,
management sometimes relies on understanding the ecosystem role of endangered keystone native
species, or of potential invaders that could fundamentally change the fl ux of energy and nutrients
(below).
Nutrient dynamics
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 accumulates, transforms and
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