Primary productivity brings solar energy into
ecosystems, some of which is converted to potential
energy incorporated into organic compounds. As
energy passes through food webs, it does so in material
form as high- energy, structurally complex compounds
that make up organisms, organic waste, and eventually
detritus. Thus energy and chemical elements are tightly
coupled as they move through food webs. Energy
is ultimately lost as heat, never recycled. In stark
contrast, elements, the atoms and molecules of life, are
recycled. The material basis for nature is the usage of
key elements (in various proportions), each essential
to life’s biochemistry; those elements are “shared” by
life- forms in the process of biogeochemical cycling.
Because Earth has no significant input of matter from
space (a year’s worth of meteorites adds up to very
little), atoms present in waste products and dead tissue
must be reacquired, recycled back to living tissue.
Decomposition and subsequent recycling is the process
by which life’s chemicals move between the living and
nonliving components of an ecosystem. Recycling is a
by- product of decomposition, and decomposition is
the means by which decomposer organisms acquire
energy and nutrients.
In a rain forest, energy in the form of biomass (from
net primary productivity— think of a leaf, for example)
will ultimately move in one of two paths. It may be
consumed as part of living tissue, as when a caterpillar
chews a leaf, in which case it will begin moving through
the food web, possibly to pass through several animals.
The path is called the grazing food chain (plant captures
sun’s energy, caterpillar eats part of plant, antbird
consumes caterpillar, forest-falcon eats antbird). Or
it may remain within the leaf structure until the leaf
eventually drops from the tree, at which time the energy
becomes available to the multiple organisms composing
the soil community. This latter direction moves energy
directly into what is termed the decomposer food web.
The decomposer food web is a rich and diverse array
of organisms that are heterotrophic (meaning that they
require organic carbon sources), ranging from vultures
to bacteria and fungi that rely on dead material and
waste products as their energy source. A glance at a
lush, green rain forest, plus a dash of pure logic, is
enough to show that the vast majority of the energy
captured during photosynthesis is destined to directly
enter the decomposer food web. If it were otherwise,
trees, shrubs, and other green plants would show
far more leaf damage than they typically do (plates
6- 1– 2). Most net primary productivity remains as
potential energy in the structural tissue of leaf, bark,
stem, and root. This potential energy will eventually be
released by a host of soil community organisms as they
unpretentiously make their livings below your muddy
boots among the forest litter and soil.
Numerous species of microbes, in particular fungi
and bacteria, are the principal organisms in this
ongoing and essential process of decomposition, one
of nature’s most fundamental processes. Using a series
of reduction- oxidation reactions, microbes ultimately
convert complex high- energy organic tissue back into
simple low- energy inorganic compounds, making
them available for uptake by the root systems of plants.
Many other organisms also significantly contribute
to decomposition: slime molds, actinomycetes,
protozoans, and hordes of animals ranging from vultures
to arthropods, earthworms, and other invertebrates.
Termites (discussed later in this chapter), are uniquely
important decomposers in tropical ecosystems. This
host of life- forms collectively influences the complex
process of converting a senesced fig leaf, a dead sloth,
or a tapir’s feces back into basic chemical elements
(plate 6- 3).
The soil community forms such a diverse, complex
food web that it may rival the biodiversity found in the
leafy canopy. Few studies have made detailed estimates
of such parameters as fungal biomass or pathways of
energy movement among the constituent flora andChapter 6. Essential Dirt: Soils and Cycling
Plate 6- 3. A fallen leaf in a tropical humid forest undergoes
a complex process of decomposition involving multiple
organisms, as the minerals contained in the leaf are eventually
released back into the abiotic element pool, and again
available to plants. Photo by John Kricher.81