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Earth Science Lesson 5
Ecology (Grade 6)


Instruction 5-5

Energy Transformation in the Ecosystem | The Food Web and the Physical Environment | Organism Populations | Biomes | Resources in the Ecosystem | Summary

Resources in the Ecosystem
CCSTD Science Grade 5 5.e.

Throughout this Lesson, we've been telling you about the relationships among organisms and resources in various ecosystems and biomes.

As you recall, a population in an ecosystem is considered "stable" when the resources available in the ecosystem can support the number of organisms in it. This is also called the "carrying capacity" of the ecosystem.

Primary resources include, but are not limited to:

  • Food and Water: A given supply of food and water might be enough for a low-density population, but if the population grows, the supplies may run out. In that case, competition between the individuals in the population usually develops.

  • Light: Plants, especially, are extremely sensitive to light. If plant-density increases, plants that don't get enough light will die.

  • Space: This is an obvious limiting factor, especially when you think of fish in a lake, bacteria in a petri dish, or plants in a forest.

The areas occupied by most populations do have limited resources (food, water, light, space and supplies to build and repair dwellings, from birds' nests and beaver dams to peoples' houses). These factors limit population growth and often bring about death rates that are exactly equal to the birth rates. This is one way a population reaches stability.

We also told you about abiotic (non-living) limiting growth factors such as weather, storms, fires, earthquakes or floods. Any of these factors can affect the supply of food and water. They can also destroy building materials used by the biotic (living) inhabitants.

To explore simple mathematical models for the carrying capacity of various ecosystems, click HERE.

Human Beings in the Ecosystem

In our own ecosystems, our lives are sustained by a tremendous flow of goods and services from the environment.

These include renewable natural resources like water, food and forest products. They also include nonrenewable resources like certain kinds of fuel, which we discussed in Sixth Grade Earth Science Instruction 3-2.

Natural-resource industries play a vital economic function in many national and local economies. They range from providing livelihoods for the poor to massive trade by multinational corporations. Food, forest products, minerals and energy are key commodities in the global economy.

Words to Remember

Although the world is filled with natural resources, there is not an inexhaustible supply. Ecosystems can be threatened, and here are some of the colorful words people use to describe various stages in that situation:

Drawdown is the process by which the dominant species uses up the surrounding resources faster than they can be replaced. This means that resources must be borrowed from other places and times (like the future).

Overshoot is the result of continued drawdown. This happens when the use of resources in an ecosystem exceeds its carrying capacity and there is no way to borrow, replace or recover what is lost. Easter Island is a good example. When it was first settled about 1,000 years ago, it was covered with trees and had a rich, sophisticated culture. But as the population grew to 4,000, all the trees were cut down for building material and firewood. The fertile soil was exhausted, too. The shortage of food and firewood led to warfare and chaos. So by the time Captain Cook arrived in 1775 there were only 630 Easter Islanders left, living a marginal existence.

Crash is what happens after overshoot and it is a sharp decline in population. Once a population has exceeded the carrying capacity of its environment, there is nothing that can be done until the population falls back down to where the available resources can sustain it. This is what happened in the Irish Potato Famine of 1845. For well over a century, the Irish people had depended on one crop: the potato. Their population grew from 2 to 8 million. Then a parasitic fungus came along and destroyed the potato crops. Within a generation, the country was devastated and more than half of the population had died or emigrated (many to the United States).

Die-off, unfortunately, has happened throughout history in the worlds of botany and zoology. It occurs when the resources of a species are so depleted that the whole species dies off (or goes extinct). Fortunately now that we understand how this happens, we may be able to prevent it from ever happening again.

Video Instruction
*Availability of You Tube video links may vary. eTAP has no control of these materials.

Reading List:
from the California Department of Education

Title, Author

Far North (An Exploration of Natural Resources Through the Story of a Plane Crash and its Survivors) (Hobbs, Will)

Garbage & Recycling: Environmental Facts and Experiments (Morgan, Sally; Harlow, Rosie)

for Students, Parents and Teachers

Now let's do Practice Exercise 5-5 (top).


You have now completed this Lesson and are ready to do the Problem and Test sections. You may wish to review any or all of the topics before answering the questions that follow. Good luck.

Next Page: Problems (top)