Key Terms
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ecology
population
biosphere
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biotic factor
community
habitat
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abiotic factor
ecosystem
Millions of species live in Earth's diverse environments, which range from scorching deserts to bubbling deep-sea vents,
from lush rain forests to your school's grounds. This section introduces the study of how living things interact with these
varied environments.
The Study of Ecology
The scientific study of the interactions among organisms and between organisms and their environments is
called ecology. Ecologists, scientists who conduct research in ecology, test their hypotheses outdoors in the field as well
as in the laboratory. Ecologists also use computer models to understand these complex interactions. Understanding
ecology helps people make decisions about environmental issues, such as how best to manage forests, rivers, and other
natural resources.
An organism's environment consists of other organisms as well as nonliving factors. The prokaryotes, protists, animals,
fungi, and plants in the environment are called biotic factors. The nonliving physical and chemical conditions are
called abiotic factors. Ecologists study the relationships among biotic and abiotic factors at five increasingly broad levels:
organisms, populations, communities, ecosystems, and the biosphere.
Individual Organisms The smallest unit of ecological study is the individual organism. A blue sweetlip fish is one example
of an organism in a coral reef environment. Other examples of organisms in a coral environment include other fish,
sponges, and the small coral animals that make up the reef. Ecologists ask questions about the adaptations that enable
organisms to meet the challenges posed by their environments. For example, how are the reef organisms affected by
the changing sunlight levels to which they are exposed?
Blue Sweetlips
Populations A population is a group of individual organisms of the same species living in a particular area. A group of
sweetlip fish in the reef environment is an example of a population. In contrast, all the fish living around a reef do not
form a single population because they represent more than one species. Ecologists often ask questions about factors
that affect the size and growth of a population. For instance, what factors limit the number of sweetlip fish living around
a reef?
Communities The coral reef is home to a collection of living things including fish, coral animals, microscopic algae, and
all other organisms living in and around the reef. All of the organisms inhabiting a particular area make up a community.
Ecologists investigate interactions among the organisms in a community. For example, how do different species of algaeeating fish compete for food? How might a disease that strikes coral animals affect the other species in the community?
Ecosystems An ecosystem includes the abiotic factors and the biotic factors in an area. A coral reef ecosystem includes
the reef's many species and its nonliving conditions, such as the water temperature and amount of sunlight. Questions
at the ecosystem level may relate to the flow of energy and chemicals. For example, how does nitrogen move within the
reef ecosystem?
Biosphere The broadest level of ecological study is the biosphere. The biosphere is the sum of all Earth's ecosystems.
You can picture the biosphere as the "envelope" of air, land, and water that supports and includes all life on Earth: the
atmosphere to an altitude of several kilometers; the land to a depth of about 2 kilometers; lakes and streams; and the
oceans to a depth of several kilometers. Questions at the biosphere level involve global issues, such as investigating the
effects of climate change on living things.
Aside from energy, which enters the biosphere as sunlight and exits as heat, the biosphere is essentially a closed system.
This means that the chemicals in the biosphere's living and nonliving things mostly come from within the system—they
are not supplied from the outside. (One exception is the occasional meteorite that adds material to Earth.)
Patchiness of the Biosphere
The biosphere is not spread out uniformly around the planet. Looking at Earth from a space shuttle, you would see that
the biosphere is "patchy"—like a quilt of different environments, including land and oceans, lakes and ice. Zoom in
closer to observe just one continent, and you would see an uneven distribution of ecosystems such as deserts,
grasslands, forests, and rivers. A still smaller area, such as a wilderness, may contain patches of woods, fresh water, and
marshes. All these environmental variations are due mainly to differences in abiotic factors such as temperature, soil
type, and the availability of water and light.
This patchiness creates a number of different habitats, or specific environments in which organisms live. Each habitat
has characteristic abiotic and biotic factors. For example, a wilderness may include several habitats: patches of forest,
marshy areas, and a river. The forest patches are home to trees and certain species of birds and animals. The marshy
land areas provide a habitat for many species of prokaryotes, fungi, and insects that live in and on top of the wet soil.
And within the river habitat is a surface "microhabitat" that supports floating plankton and swimming animals, while the
deeper water is a separate microhabitat, with different light and temperature conditions and a different community of
organisms.
Key Abiotic Factors
The organisms that inhabit forests are adapted to the characteristic physical conditions of forest habitats. These abiotic
factors include sunlight, water, temperature, soil, and wind.
Sunlight
The sun provides light and warmth and is the energy source for almost all ecosystems on Earth. Sunlight
powers photosynthesis by plants, the main producers in most terrestrial (land) ecosystems. Within a forest, much less
sunlight reaches the forest floor than reaches the tops of the tall trees. This varying amount of sunlight creates different
microhabitats. In aquatic (water) environments, sunlight provides energy for photosynthetic producers such as algae.
The fact that sunlight only penetrates a short distance into a body of water affects where algae live within a lake or
ocean.
Water Water is essential to all life on Earth. All organisms contain water—in fact, you consist of nearly 70 percent
water! Among its many important properties, water can dissolve gases such as oxygen and solutes such as salt, as you
read in Chapter 4. Terrestrial organisms have adaptations that keep them from losing too much water and drying out.
For example, some plants such as pine trees have needle-shaped leaves with a waxy coating. These leaves minimize the
amount of water that evaporates into the air. Aquatic organisms also must balance their water uptake and water loss.
Otherwise, their cells may burst or shrivel due to osmosis.
Temperature Most life exists within a fairly narrow range of temperatures, from about 0°C to about 50°C. Few
organisms can maintain an active metabolism below 0°C for long, and most organisms' enzymes are denatured (they
lose their shape and stop working) above 50°C. However, extraordinary adaptations enable certain species to live at
extreme temperatures. Some species of prokaryotes, for example, can thrive in hot springs as hot as 80°C and around
deep-sea vents, which are even hotter.
Soil
Soil is the product of abiotic forces (such as ice, rain, and wind) and the actions of living things (such as
microorganisms, plants, and earthworms) on the rocks and minerals of Earth's crust. The structure and chemical makeup
of soil and rock in an area affect the types of plants that grow there. For example, areas with a certain type of dry,
nutrient-poor soil are often dominated by little bluestem grasses. These grasses have extensive roots that obtain scarce
moisture in the soil. Their long, narrow leaves roll up in hot, dry weather, reducing water loss. In aquatic environments
as well, the characteristics of the underlying sand or rock affect the types of plants and algae that can grow. This in turn
influences the other organisms found there.
Wind Wind can affect the distribution and the activities of organisms in several ways. Wind moves clouds and rain
over Earth's surface. Wind also stirs up water in ponds, lakes, and streams, creating currents that in turn bring up
nutrients from the bottom. Many land plants depend on wind to help disperse their pollen and seeds.
Severe Disturbances
Major natural disturbances that affect ecosystems include fires, hurricanes, tornadoes, drought,
floods, and volcanic eruptions. Some disturbances, such as volcanic eruptions, are so infrequent that organisms have not
acquired evolutionary adaptations to them. Other disturbances, such as fires, occur frequently in some communities.
Many organisms in such communities have adapted to these disturbances. For example, in dry scrublands where fires
are common, many plants can regrow from their roots after a brush fire destroys their aboveground parts.
Concept Check 34.1 (on separate paper, or typed & sent to nyoshuff@gmail.com)
1. Draw a diagram showing the relationship among the five levels of ecological study.
2. Describe what is meant by the "patchiness" of the environment.
3. Explain the importance of sunlight as an abiotic factor in terrestrial ecosystems.
4. Choose a particular ecosystem. List 3 key biotic and abiotic factors and give an example of each
5. Explain why it is more accurate to define the biosphere as the global ecosystem, than as the global community.
6. Write a definition for each of the underlined key terms found in Concept 34.1.