fauna of the decomposer community. Those that exist
demonstrate the high complexity of the microbial
components of the decomposer community.
Fungi are abundant in the tropics, as well as
elsewhere on Earth. An individual fungal strand is
called a hypha, and a network of hyphae is called a
mycelium. In some tropical forests the mycelium, a
mesh- like interconnected array of fungal strands, is
sufficiently dense as to be visible on the forest floor.
Fungi, like bacteria, are essential decomposers. They
ultimately liberate atoms back to the soil. In addition,
many fungal species, collectively termed mycorrhizae
(discussed below) are essential to trees and other plants
in aiding the uptake of atoms from the soil.
Microbial organisms facilitate a process called
humification, in which complex soil organic matter
(humus) is maintained at the interface between the
tree roots and soil. Humus helps aerate the soil. Humus
particles are negatively charged and by electrostatic
attraction act to help retain mineral nutrients in the soil,
such as potassium, magnesium, and calcium, that carry
positive charges. Without such electrostatic attraction
rain could leach these essential minerals from the soil.
Soil represents a temporary repository for mineral
nutrients such as phosphorus, nitrogen, calcium,
sodium, magnesium, and potassium. Each of these
minerals, as well as others, is necessary for biochemical
reactions in organisms (fig. 6- 1). A shortage of any
one of them can significantly limit productivity. For
example, phosphorus and nitrogen are important in
the structure of nucleic acids (DNA and RNA) as well
as proteins and other necessary molecules. Magnesium
is an essential part of the chlorophyll molecule, without
which photosynthesis could not occur. Sodium is
essential for the functioning of nervous and muscular
systems in animals.
Consider how an atom is cycled. A leaf drops to
the ground. Inside the leaf are billions of atoms, but
let’s select a single atom of phosphorus (a propitious
choice, as you will see later). This phosphorus
atom may initially pass through a termite or other
invertebrate that consumes the dead leaf tissue, only
to be returned to the litter through elimination of
waste or the death and subsequent decomposition
of the creature itself. Indeed, the atom may move
through numerous organisms before becoming part
of the humus. Or the atom may be directly taken up
by a fungus. This same atom eventually may pass
through several dozen fungal and bacterial species,
each of which gains a modicum of energy by ingesting,
digesting, and thus decomposing the deceased leaf
(or termite). Within days the phosphorus atom, likely
bound through chemistry to other atoms, becomes
part of the inorganic components of the soil. At that
point a tree root grabs the phosphorus, aided in
doing so by the mutualistic fungi called mycorrhizae
(discussed below), which penetrate or grow atop tree
roots and take up the element and pass it along to the
tree. The phosphorus atom will henceforth serve the
biochemistry of the tree. The cycle is now complete.
This nutrient cycling is also called biogeochemical
cycling, a term that describes the fundamental process
of chemicals moving continuously between the bios
(living) and the geos (nonliving) parts of an ecosystem.
The movement of minerals in an ecosystem is strongly
influenced by both temperature and rainfall; thus,
abetted by high temperatures and ample rainfall,
biogeochemical cycling is efficient in the tropics.
Heat stimulates evaporation. As plants warm they
evaporate water; this heat- driven pumping process,
called transpiration, cools the plants and returns water
to the atmosphere. Transpiration is an essential process
in plant physiology. It brings water and minerals
up from the soil, helps cool the plant, and supplies
essential water needed for metabolism. Water from
rainfall is taken up by plants and transpired, returning
to the atmosphere, under the stress of tropical heat.
Nowhere is this continuous process of transpirationFigure 6– 1. Simplified compartment model showing the
recycling system of an ecosystem such as a tropical forest.
Compartments are not to scale. Note that decomposers
ultimately make mineral nutrients such as phosphorus
available again to primary producers, which take them
up during the process of photosynthesis. Reprinted with
permission from Kricher, John. Tropical Ecology. Princeton, NJ:
Princeton University Press, 2011.82 chapter 6 essential dirt: soils and cycling