Species are generally defined as specific kinds of plants and animals that reproduce their own kind and that do not interbreed with other species in natural environments. Plant and animal species do not exist in a vacuum. They interact with other species and the environment. The basic unit for measuring a species is its population (How many are there?). In a stable ecosystem, the population of each species is in balance. Example: birds have enough seeds to eat and there are enough seeds left over to sustain the plant species that made the seeds.

Tropical rain forests contain many species. Typically, the populations of these species are in balance, even including native Indians. The extent of population balance determines whether a species thrives or faces extinction. Modern humans, however, may upset that balance by aggressive agricultural practices.

This lesson explores some basic ideas about species, populations, and the factors that determine whether or not they are in balance.

After completing this lesson, each student should be able to:

• Explain what is meant by species, population, and population balance.
• Know what "ecological diversity" means and why it is important
• Recognize elements of the environment that might upset population balance
• Predict what might happen after a population becomes out of balance

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