Principles of terrestrial ecosystem ecology.pdf

autotrophs because they convert CO2, water,
and solar energy into biomass (see Chapter 5).
Heterotrophs are organisms that derive their
energy by eating live or dead organic matter.
Heterotrophs function as part of two major
trophic pathways, one that is based on live
plants (the plant-based trophic system) and
another that is based on dead organic matter
(the detritus-based trophic system). The second
trophic system is less immediately obvious to
aboveground animals like us and is often overlooked,
even though it usually accounts for
most of the energy transfers through animals.
Consumers (also termed secondary producers)
are organisms that eat other live organisms.
These include herbivores, which eat plants,
microbivores, which eat bacteria and fungi,
and carnivores, which eat animals. A group of
organisms that are linked together by the linear
transfer of energy and nutrients from one
organism to another are referred to as a food
chain. Grass, grasshoppers, and birds, for
Roots
Detritus
Phytophagous
nematodes
Saprophytic
fungi
Bacteria
Figure 11.1. Pattern of energy flow through belowground
portions of a grassland food web. Food webs
consist of many interconnecting food chains, includ-
Collembolans
Cryptostigmatid
mites
Noncryptostigmatid
mites
Fungivorous
nematodes
Earthworms
Enchytraeids
Bacteriophagous
nematodes
Flagellates
Bacteriophagous
mites
Overview 245
example, form a food chain. Those organisms
that obtain their energy with the same number
of transfers from plants or detritus belong to
the same trophic level. Thus plants constitute
the first trophic level; herbivores, the second;
primary carnivores, the third; secondary carnivores
that eat mainly primary carnivores, the
fourth, etc., in a plant-based trophic system
(Lindeman 1942, Odum 1959). Similarly, in the
detritus-based trophic system, bacteria and
fungi directly break down dead soil organic
matter and absorb the breakdown products
for their own growth and maintenance. These
primary detritivores are the first trophic level in
the detritus-based food chain and are fed on by
animals in a series of trophic levels analogous
to those in the plant-based trophic system (Fig.
11.1).
Although food chains are an easy way to
visualize the trophic dynamics of an ecosystem,
they are a gross oversimplification for organisms
that eat more than one kind of food.
Predaceous
nematodes
Amoebae
Nematode -
feeding mites
Predaceous
mites
Predaceous
collembolans
ing plant-based (heavy solid lines) and detritusbased
(dashed lines). (Modified with permission
from Biology and Fertility of Soils; Hunt et al. 1987.)