Science High School Reviewer | Grade 4 Science | Life Science | Exploring Ecosystems | Lesson 11: Changes in the Environment

Environments naturally change over time. These changes can occur very
slowly or very quickly. Changes to an environment can affect ecosystems and
the species that inhabit them.

The Process of Change

This process of gradual change from one community of organisms
to another is called succession. Succession occurs as the environment
changes. Changes in an environment affect the communities within it.
As communities change, conditions might change also. New conditions
allow different communities to grow.

One Step at a Time
In most cases, succession occurs in stages. If conditions are
right, bare land might become grassland. Grassland will give
way to shrubs. Shrub land will become a forest. Communities
grow and replace one another until there is a stable community
with few changes.
Changes in climate may also affect ecosystems. Climate is
the average temperature, winds, and rainfall for an area over
many years. Climates change very slowly over a long period
of time. More than 15,000 years ago, snow and ice covered
parts of North America. Trees, grasses, and many flowering
plants could not grow. As the climate became warmer, plants
and animals moved in. Eventually the plants and animals
formed the forest communities we see today.

Changing Species
In the 1800s and early 1900s, settlers watched huge
flocks of passenger pigeons fly over the Great Smoky
Mountains. By 1915, not a single passenger pigeon
was alive. The passenger pigeon had become extinct.
The entire species had died out and was gone forever.
Species sometimes become extinct because they
cannot adjust to changes in Earth’s environment.
Climate changes, volcanoes, and even meteors
have caused extinction throughout Earth’s history.
In modern times, human activities such as habitat
destruction and hunting are the main reasons species
become extinct. Once a population drops below a
certain number, the species may not be able to recover.
Populations of some species have been reduced so
much that they are in danger of becoming extinct.
These species are called endangered species.
Species that may soon become endangered are
threatened species. Both endangered and threatened
species sometimes leave an area to search for a more
suitable environment.

Species Then and Now
Fossils show us that life on Earth has not always
been the same as it is now. Over long periods of time,
changes in the environment have caused species to
change or adapt as well. Scientists can compare fossils
of organisms that lived long ago with organisms that
are alive today.
Woolly mammoths lived long ago. They have since
become extinct, but many are preserved as fossils.
Some mammoths have been frozen solid. Scientists
compare these frozen mammoths with modern
elephants. They both have large tusks and long noses.
Their skeletons are also very much alike. Both the
woolly mammoth and modern elephants are classified
in the same family.
Fossils can also tell us about the
environment long ago. Sometimes
fossils of marine creatures
are found in dry climates.
This tells scientists that
long ago shallow seas
must have covered the area
where the fossils were found.

Rapid Changes
A hurricane’s strong winds rip up trees and flatten plants.
Heavy rains and huge waves flood a coastal community.
Lightning strikes a tree, starting a forest fire that burns almost
everything in its path.
Hurricanes, floods, and fires, along with volcanic eruptions
and earthquakes, are natural events that can quickly change
the landscape. These rapid changes may force species to
leave the area because the resources they need are no
longer available.

Although some rapid events are destructive, they also
play an important part in keeping an ecosystem balanced.
Fires help clear away dead and dying plant matter,
making more room for new plants to grow. Some trees,
such as the Table Mountain pine, have sealed cones that
open when they come in contact with the heat of a fire.
The ash from a volcanic eruption enriches the soil.

Natural Disasters
In the spring and summer of 1993, huge amounts of
rain caused the Mississippi and Missouri Rivers to overflow.
Some areas were flooded for almost 200 days. The floods left
thousands of acres of land covered with sand and mud.
The floods affected many plants and animals. Grasses and
trees died because of too much water. Birds had fewer offspring
because many nesting places were destroyed. But the populations
of some fish increased. They used the flooded areas to feed
and reproduce.