The dominant species in a climax community interact with
and depend on nonliving (abiotic) factors in that environment.
The most important abiotic factors in an environment, and on the
SAT II Biology, are the chemical cycles, the availability of sunlight
and oxygen, the character of the soil, and the regulation of these
various phenomena. These abiotic elements, along with living matter,
make up an ecosystem.
Inorganic elements such as carbon, nitrogen, and water
pass through the environment in various forms. These elements are
vital to life: they are consumed, excreted, respired, and otherwise
utilized by living things. The passages of these elements between
organisms and the abiotic environment are called the chemical cycles.
The Carbon Cycle
The carbon cycle begins when plants use CO2 from
the air to produce glucose, which both animals and plants use in
respiration and other life processes. Animals consume some of these
plants as a source of food. Animals use what they can of the carbon
matter and excrete the rest as waste that decays into CO2.
Plant and animal respiration releases gaseous CO2.
The carbon that plants and animals do use remains in their bodies
until death. After death, decay sends the organic compounds back
into the Earth and CO2 back
into the atmosphere.
The Nitrogen Cycle
Nitrogen is a vital component of amino acids and nucleic
acids, which are the fundamental units of proteins and DNA. The
nitrogen cycle begins with inert atmospheric nitrogen (N2),
which is generally unusable by living organisms. Nitrogen-fixing
bacteria in the soil or on the roots of legumes transform the inert
nitrogen into nitrates (NO3–)
and ammonium (NH4–).
Plants take up these compounds, synthesize the 20 amino
acids found in nature, and transform them into plant proteins; animals,
typically only able to synthesize eight of the 20 amino
acids, eat the plants and produce protein using the plant’s materials. Plants
and animals give off nitrogen waste and death products in the form
of ammonia (NH3). One
of two things can happen to the ammonia: (1) nitrifying
bacteria transform the ammonia into nitrites (NO2)
and then to nitrates (NO3–),
which reenter the cycle when they are taken up by plants; (2)
denitrifying bacteria break down the ammonia to produce inert nitrogen
The cycling of water and phosphorus are also important,
as these substances are limited and vital to the life processes
of most organisms.
The Water Cycle
The majority of the Earth’s water resides in the oceans
and lakes, which act as water storage depots. This water escapes
into the atmosphere through evaporation and condenses into clouds.
Precipitation in the form of rain, snow, hail, etc., returns water
to the ocean and lakes and also brings water to dry land. Water
on land may either return to the oceans and lakes as runoff or penetrate
into the soil and seep out as groundwater.
Oxygen, Sunlight, and Competition
Oxygen and sunlight are both vital to most forms of life.
The relative abundance or lack of oxygen in a particular geographic
or physical locale will create competition among organisms and drive
evolution. Oxygen is abundant in the atmosphere and is therefore
readily available to terrestrial species. But in order to penetrate
aquatic environments, oxygen must be dissolved in water, where it
exists in smaller concentration.
Like oxygen, sunlight is necessary to life for most organisms.
In terrestrial species, competition for sunlight has pushed evolution
of plants, with some plants growing broader leaves and branching
to capture more rays. Sunlight cannot travel through water as easily
as it can travel through air, so at great ocean depths, light is
scarce. At these sorts of depths, autotrophic organisms have to
find some way to produce energy that does not use light, such as
The nature of soil determines which populations can be
sustained in a given ecosystem. High acidity inhibits most plant
growth but may be ideal for some plants that are better adapted
to acidic soil. The texture of the soil and amount of clay it contains
affect its ability to retain water, while the presence of minerals
and decaying organic matter influences the types of plant life that
can be supported.