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16..1 What Is an Ecosystem?
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An individual organism is made up of many smaller systems that work together. Each
system has a structure (its parts) and a function (what it does). This is true for cells, tissues,
organs, organ systems, and the entire organism. In this section, you will learn how individual
organisms are grouped into larger systems.
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How ecosystems are organized
Ecosystems
levels of
organization
An ecosystem is a group of organisms interacting with one another and
their physical environment. Ecosystems can be as big as the ocean, or as
small as a jar of water containing microscopic organisms. A Texas tall-grass
prairie is an example of an ecosystem. A prairie ecosystem is made up
of the plants, animals, and other organisms that live there, plus nonliving
things such as soil, air, water, sunlight, and nutrients. The living and
nonliving parts function together to remain in balance-similar
to the way a
single organism's parts work together to keep the organism alive.
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The diagram below shows the levels of organization of ecosystems.
Individual organisms are grouped into populations. Populations of different
species of organisms are grouped into communities. An ecosystem consists
of a community of organisms and their physical environment.
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Ecosystem
A community and its physical surround!ngs
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Oornmunity
A group of different populations
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ecosystem a group
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Individual orqanlsms and species
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~pecies of the The first level of organization of an ecosystem is an individual organism
Texas tallgra~s of a particular species. A species is a group of organisms that are capable
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of reproducing in order to produce fertile offspring. Fertile means "able
to reproduce." A typical ecosystem contains individuals of many different
species of animals, plants, and other types of organisms. For. instance, Texas
tall-grass prairie ecosystems ~re dominated by plant life such as grasses and
wildflowers. These plants provide food and shelter for many' animal species
including snakes, small mammals, deer, birds, and a large variety of insects.
The illustration below shows just a few of the many different species of
organisms ~hat live in the prairie.
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Reading 16.1 What Isan Ecosystem?
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Populations
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What is a A population is a group of individuals of the same species living in a given
population? area. For example, a population of clover plants in a tray consists of the total
number of plants in that tray. A population of clover plants in a field would
be much larger (Figure 16.1).A typical ecosystem contains populations of
different species including animals, plants, fungi, and bacteria.
Growth rate
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p'op'Lliation~ a grou~~ofind:10r~ua"
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Populations change as old members die and new members are born. In
nature, populations often stay about the same size from year to year.
Sometimes, though, populations grow or decline very fast. The change
in size of a population over time is called its growth rate. Growth
rates can be positive, negative, or neutral. The graph below shows data
for a population of deer. What can you tell about the growth rate of this!
population? Identify places on the graph where the growth rate is positive,
negative, and neutral.
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Limits to In order to grow, a population needs energy, nutrients, and space. Energyj.
population
and nutrients both come from food. Therefore, population growth is limited
growth by the amount of food available. Population growth is also limited by space.
Different organisms require different amounts of space in which to live. If any
of these variables are too limited, a population will not continue to grow.
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Figure 16.1: Each populdiion ofcfover
plants consists of the number of plants in I :
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the group.
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A community is defined as all of the populations of different species in
an ecosystem. Different species in a community interact with one another
to obtain food. living space. and other resources in the ecosystem. To be
considered a community, the different populations in an ecosystem must live
close enough to interact with one another.
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comm~nrty ~ all of tfle!Ropulations
irl arlecosystem
of dWfeteritspecies
biotic factors - the' Ilvirg parts of
an ecosystem incluclirg Individuals,
populations, and communities
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An examplr.of In prairie ecosystems, grazing (plant-eating) animals such as deer. elk, and
a commufjuty bison are. part of a community that depends on tall grasses for food and
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shelter. Other animals in tl:iecommunity such as insects, rabbits, and mice
also depend on the grasses for food and shelter. Animals such as coyote,
fox, hawks,
owls, and othef I birds eat some of the animals
that feed off
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the grasses. Microscopic bacteria that live in the soil are also important
members of the community because they take nutrients from the air and
make them available to the: grasses to help them grow (Figure 16.2).
abiotic factors - the nonliving
parts of an ecosystem inCluding
temperature, sunlight, soil, water,
and air
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Biotic and Ecosystems are made up of communities and include living and nonliving
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parts. The living parts of an ecosystem are called biotic factors. These
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include individuals, populations of different species, communities, and
their interactions. The nonliving parts of an ecosystem are called abiotic
factors. These include temperature, sunlight, soil, water, and air.
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Changes in one or more abiotic and/or biotic factors
result in changes to an ecosystem.
organiJms
and t~eir
environments
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Changes in any of these factors (amount of rainfall, decline in a population,
overgrazing, e~ result in changes to other parts of the ecosystem. For
example, a long period of scarce rainfall, or drought, may impact all parts of
an ecosystem. Populations of plants that have low tolerance to drought may
decline. Populations of animals that depend on those plants for food may also
decline. Populations of drought-hardy plant species may increase as a result
of the decrease in other plant populations. Grazing animals in a community
may begin to depend on the drought-hardy species of plants for food.
Figure 16.2: Bacteria that live in soil
are important
members of a prairie
ecosystem community.
Reading 16.1 What Is an Ecosystem?
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