Principles of terrestrial ecosystem ecology.pdf

11
Trophic Dynamics
Introduction
Although terrestrial animals consume a relatively
small proportion of net primary production
(NPP), they strongly affect energy flow
and nutrient cycling. In earlier chapters we
emphasized the interactions between plants
and soil microbes, because these two groups
directly account for about 90% of the energy
transfers in most terrestrial ecosystems. Plants
use solar energy to reduce CO 2 to organic
matter, most of which senesces, dies, and
directly enters the soil, where it is broken down
by bacteria and fungi. Similarly, most nutrient
transfers in ecosystems involve uptake by
plants and return to the soil as dead organic
matter, where nutrients are released and made
available by microbial mineralization. In most
terrestrial ecosystems the uncertainties in our
estimates of primary production and decomposition
exceed the total energy transfers from
plants to animals. It is perhaps for this reason
that terrestrial ecosystem ecologists have frequently
ignored animals in classical studies of
production and biogeochemical cycles. Aquatic
ecologists, however, have been unable to ignore
animals because most of the energy and nutrients
are transferred from plants to animals
rather than directly from plants to dead organic
matter (see Chapter 10). Perhaps for this
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Trophic dynamics govern the movement of carbon, nutrients, and energy among
organisms in an ecosystem. This chapter describes the controls over trophic dynamics
of ecosystems.
reason aquatic ecosystem ecologists have generally
led the theoretical developments in this
aspect of ecosystem ecology.
Understanding the factors governing energy
and nutrient transfer to animals has societal
implications. Most human populations depend
heavily on high-protein foods derived from
animals. The exponentially increasing human
population requires more food in a world
where many people already face an inadequate
food supply. An ecologically viable strategy for
efficiently providing food to feed the growing
human population requires a good understanding
of the ecological principles regulating
the efficiency with which plants and animals
support their growth and maintenance.
Overview
Energy transfers define the trophic structure of
ecosystems. The simplest way to visualize the
energetic interactions among organisms in an
ecosystem is to trace the fate of a packet of
energy from the time it enters the ecosystem
until it leaves—without worrying about the
identity of the organisms involved (Lindeman
1942). Trophic transfers involve the feeding of
one organism on another or on dead organic
matter. Plants are called primary producers or