Chapter 3
Ecology is the scientific study of the interactions of organisms
and their environment. All these organisms live and interact in the
biosphere. The biosphere is the part of Earth where life exists. To
understand relationships within the biosphere, ecologists study
events and organisms that range in complexity from a single
individual to the entire biosphere. Ecologists study many levels
of organization.

• individual organisms
• species—a group of similar organisms that breed and
produce fertile offspring
• population—a group of individuals of the same species that
live in the same area
• community—a collection of different populations that live
together in an area
• ecosystem—all of the organisms living in a specific place,
together with their physical environment
• biome—a group of ecosystems with the same climate and
similar dominant communities
• biosphere—the part of the planet (land, water, and air)
where all life exists

Scientists conduct ecological research using three basic
approaches: observing, experimenting, and modeling. All of
these approaches rely on the application of scientific methods
to guide ecological inquiry. Observing is often the first step in
asking ecological questions. Observations can also be used when
designing experiments and making models. Experiments can be
used to test hypotheses. They may be done in a laboratory or in
the field. Modeling helps scientists understand complex processes.

3–2 Energy Flow
Organisms use energy from the environment for life processes.
Living things get energy in different ways. Sunlight is the main
energy source for life on Earth.
Organisms that use the energy in sunlight or chemicals to make
food are called autotrophs. Autotrophs, also called producers,
make food in two ways.
• Some autotrophs use light energy to make food. This process
is called photosynthesis. In photosynthesis, carbon dioxide
and water are changed to carbohydrates and oxygen. Plants,
some algae, and certain bacteria carry out photosynthesis.

• Some types of organisms rely on the energy stored in
organic chemical compounds. The process in which
autotrophs use chemical energy to make carbohydrates is
called chemosynthesis. Only certain types of bacteria carry
out chemosynthesis.

Many organisms rely on other organisms for energy and food.
These organisms are called heterotrophs. Heterotrophs also are
called consumers. There are many types of heterotrophs.
• Herbivores, such as cows, get energy by eating only plants.
• Carnivores, such as snakes, get energy by eating only animals.
• Omnivores, such as humans, get energy by eating both
plants and animals.
• Detritivores, such as earthworms, feed on the remains
(dead matter) or wastes of other organisms.

• Decomposers, such as fungi, break down organic matter.
Energy flows through an ecosystem in one direction. It flows
from the sun (or inorganic compounds) to autotrophs and then
to heterotrophs. A food chain shows how living things transfer
energy by eating and being eaten. For example, a food chain might
consist of grass (producer), an antelope (herbivore), and a coyote
(carnivore). A food web links together all of the food chains in an
ecosystem. For example, rabbits may also feed on the grass in the
food chain above. These rabbits may be eaten by the coyotes. The
feeding relationships of the grass, rabbits, antelopes, and coyotes
make up a food web.

Each step in a food chain or food web is called a trophic level.
Producers are at the first trophic level. Consumers make up higher
trophic levels. Each consumer depends on the trophic level below
it for energy.

Ecological pyramids are diagrams that show the relative
amounts of energy or matter at each trophic level. Only about 10
percent of the energy available at one trophic level is passed on
to organisms at the next trophic level. Three types of ecological
pyramids are
• Energy pyramids. Energy pyramids show how much energy
is available at each trophic level.
• Biomass pyramids. Biomass pyramids show the biomass,
or total amount of living tissue, at each trophic level.
• Pyramid of numbers. Apyramid of numbers shows the
relative number of individual organisms at each trophic level.

3–3 Cycles of Matter
Energy and matter move through the biosphere very differently.
Unlike the one-way flow of energy, matter is recycled within
and between ecosystems. Matter, including water and nutrients,
moves through organisms and parts of the biosphere through
biogeochemical cycles.

The Water Cycle. All living things need water to survive.
Water cycles between the ocean, atmosphere, land, and
living things. Many processes are part of the water cycle.
For example, during evaporation liquid water changes to
a gas. Transpiration is the evaporation of water from the
leaves of plants. Water changes from a gas to a liquid through
the process of condensation. Water vapor in the atmosphere
condenses into tiny droplets that form clouds. When the
droplets get large enough, they fall to Earth’s surface
as precipitation.

Nutrients are chemical substances that organisms need to
survive. Every living organism needs nutrients to build tissues
and carry out essential life functions. Like water, nutrients are
passed between organisms and the environment through
biogeochemical cycles.

The Carbon Cycle. Carbon is a key part of living tissue.
Photosynthesis and cellular respiration are parts of the carbon
cycle. Human activities such as burning fossil fuels are also
parts of the carbon cycle.

The Nitrogen Cycle. Organisms need nitrogen to build proteins.
Different forms of nitrogen cycle through the biosphere.
• Nitrogen gas is the most abundant form of nitrogen on
Earth. However, only certain kinds of bacteria can use this
form directly.
• These bacteria change nitrogen gas into ammonia in a
process called nitrogen fixation. Other bacteria in the soil
convert ammonia into nitrates and nitrites.
• When organisms die, decomposers return nitrogen to
the soil. Other bacteria change nitrogen compounds
called nitrates back into nitrogen gas. This process is
called denitrification.

The Phosphorus Cycle. Most phosphorus is stored in rocks and
ocean sediments. This phosphorus is slowly released into water
and soil and then used by organisms. Phosphorus is a key part
of DNA and RNA.

Primary productivity is the rate at which producers form
organic matter in an ecosystem. Nutrient availability affects
primary productivity. A nutrient that is scarce or cycles slowly
through an ecosystem is a limiting nutrient. A limiting nutrient
can affect ecosystem health.
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