Science of Water : Concepts and Applications

Water Ecology 159
FIGURE 6.2 Notonecta (left) and Corixa (right). (Adapted from Odum, E.P., Basic Ecology, Saunders
College Publishing, Philadelphia, 1983, p. 402.)
organism is a predator of or prey for some other organisms. Odum refers to an organism’s niche
as its “profession” (Odum, 1975). In other words, each organism has a job or role to fulfi ll in its
environment. Although two different species might occupy the same habitat, “niche separation
based on food habits” differentiates two species (Odum, 1983). Comparing the niches of the water
backswimmer and the water boatman reveals such niche separation. The backswimmer is an active
predator, while the water boatman feeds largely on decaying vegetation (McCafferty, 1981).
ECOSYSTEM
Ecosystem denotes an area that includes all organisms therein and their physical environment.
The ecosystem is the major ecological unit in nature. Living organisms and their nonliving
environment are inseparably interrelated and interact with each other to create a self-regulating
and self- maintaining system. To create a self-regulating and self-maintaining system, ecosystems
are homeostatic, i.e., they resist any change through natural controls. These natural controls are
important in ecology. This is especially the case because natural controls tend to be disrupted by
people through their complex activities.
As stated earlier, the ecosystem encompasses both the living and nonliving factors in a particular
environment. The living or biotic part of the ecosystem is formed by two components: autotrophic
and heterotrophic. The autotrophic (self-nourishing) component does not require food from its
environment but can manufacture food from inorganic substances. For example, some autotrophic
components (plants) manufacture needed energy through photosynthesis, whereas heterotrophic
components depend upon autotrophic components for food.
The nonliving or abiotic part of the ecosystem is formed by three components: inorganic
substances, organic compounds (link biotic and abiotic parts), and climate regime. Figure 6.3 is a
simplifi ed diagram showing a few of the living and nonliving components of an ecosystem found
in a freshwater pond.
An ecosystem is a cyclic mechanism in which biotic and abiotic materials are constantly exchanged
through biogeochemical cycles. Bio refers to living organisms and geo to water, air, rocks, or solids.
Chemical is concerned with the chemical composition of the Earth. Biogeochemical cycles are driven
by energy, directly or indirectly from the sun.
Figure 6.3 depicts an ecosystem where biotic and abiotic materials are constantly exchanged.
Producers construct organic substances through photosynthesis and chemosynthesis. Consumers
and decomposers use organic matter as their food and convert it into abiotic components;
that is, they dissipate energy fi xed by producers through food chains. The abiotic part of the
pond in Figure 6.3 is formed of inorganic and organic compounds dissolved and in sediments
such as carbon, oxygen, nitrogen, sulfur, calcium, hydrogen, and humic acids. Producers such as
rooted plants and phytoplanktons represent the biotic part. Fish, crustaceans, and insect larvae
make up the consumers. Mayfl y nymphs represent detrivores, which feed on organic detritus.