Phenomenon

On May 18, 1980, in the state of Washington, the volcano Mt. St. Helens erupted, dramatically changing the landscape for hundreds of miles in every direction. NASA has captured images showing changes in the landscape since that blast. Below shows the landscape surrounding the volcano in 1984 and 2013. Lighter, gray regions are where vegetation was stripped away by lava and ash; darker, green regions show plant growth.

Image by NASA Earth Observatory; https://earthobservatory.nasa.gov/images/146735/mount-st-helens; public domain

Observations and Wonderings

  1. What are you observing about this phenomenon?
  1. What are you wondering about this phenomenon?

Focus Questions: Give your best response prior to learning about this topic. Your initial answers may change as you explore this unit.

  1. What do you suppose the impact was on plant and animal communities following the volcano eruption?

  1. Would the impact have been the same if the ecosystem disturbance was a flood, fire, or overhunting?

  1. List 4-5 species you think may have colonized the Mount St.Helens region first. Why did you make your choices?

Bio.1.4 Stable Ecosystems

Organisms Depend on the Environment and Each Other


What other species do you need to survive?

Species cannot live alone. All life needs other life to survive. The image shows a bluestreak cleaner wrasse eating algae and other small organisms off a giant moray eel. This is an example of a symbiotic relationship.

Interdependence of Living Things

All living things depend on their environment to supply them with what they need, including food, water, and shelter. Their

environment consists of physical factors—such as soil, air, and temperature—and also of other organisms. An organism is an individual living thing. Many living things interact with other organisms in their environment. In fact, they may need other organisms in order to survive. This is known as interdependence. For example, living things that cannot make their own food must eat other organisms for food. Other interactions between living things include symbiotic relationships and competition for resources.

Since living things depend on their environment and other organisms, changes to the environment or one population can affect other populations. The numbers

and types of organisms in an ecosystem are not usually static because ecosystems adjust to gradual changes. As conditions change, the numbers and types of species in the ecosystem gradually change over time. If conditions remain stable, what will happen to the number and types of organisms in the ecosystem?

Moderate Changes

Minor to moderate changes happen from season to season. Seasonal floods, hunting, climate change, and other factors can create changes to an ecosystem in a short period of time. The changes are typically not big enough to completely disrupt the ecosystem, but do affect the populations that inhabit the ecosystem. Ecosystems typically recover from this type of disturbance quickly.

Extreme Changes

A catastrophic event, such as a volcano, can destroy an existing ecosystem and also set the stage for a new ecosystem to be created. Primary succession occurs when an area has never been colonized; in other words, there are no living organisms or even soil in the area. Bacteria and lichens that can live on bare rock, along with wind and water, help weather the rock and form soil. Once soil

begins to form, plants can move in. At first, the plants include grasses and other species that can grow in thin, poor soil. As more plants grow and die, organic matter is added to the soil. This improves the soil and helps it hold water. The improved soil allows shrubs and trees to move into the area.

Primary Succession. New land from a volcanic eruption is slowly being colonized by a pioneer species.   

Secondary Succession occurs in a  formerly  inhabited  area that

was disturbed. The disturbance could be a fire, flood, or human action such as farming. This type of succession is faster because the soil is already in place. In this case, plants such as grasses, birch trees, and fireweed are the first plants to grow. Organic matter from the first plants improves the soil. This lets other plants move into the area.

Secondary Succession. Two months after a forest fire, new plants are already sprouting among

the charred logs.

Putting It Together

Phenomenon

Image by NASA Earth Observatory; https://earthobservatory.nasa.gov/images/146735/mount-st-helens; public domain

Focus Questions

  1. After reading this chapter, how would you modify your ideas about the impact of a volcanic eruption on the forest ecosystem surrounding Mt. St. Helens? Discuss the difference in impact of an extreme disturbance, such as a volcano, and a moderate disturbance, such as a flood or fire.

  1. Revisit the list of species you made at the beginning of the chapter. After reading about succession, what changes would you make to your list and why?

Final Project

A disturbance to ecosystems in many areas of the Western United States is wildfire. Imagine you are the supervisor of a large National Forest in Utah, which is at high risk of wildfire, based on recent drought and other changes in climate. Make a plan to minimize fire’s impact on the forest ecosystem as well as neighboring communities. Consider the challenge of more residences being built in forestlands.

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