ECOSYSTEMS:  In earlier grades, students learned about what plants and animals need to survive.  In the Ecosystems unit, students learn how different species affect and are affected by each other and their environment. An ecosystem  has been defined as, "An ecological community together with its environment, functioning as a unit."  (The American Heritage^® Dictionary of the English Language: Fourth Edition.  2000).  For a more detailed encyclopedic discussion of this topic, see "ecology" article in Columbia Encyclopedia, http://www.bartleby.com/65/ec/ecology.html.

KEY CONCEPTS:

• Energy Cycle:  Students learn how energy passes from the sun to plants ("producers"), which transform the sun's energy thhrough photosynthesis to produce food.  Some of the plant's stored energy passes to animals which eat the plants, or to animals which eat those animals ("consumers").  When plants or animals die, energy still stored in them is used by other organisms ("decomposers"), which break down dead matter into nutrients which can then be used by plants in their growth, starting the cycle all over again.
• Food Chains and Webs:  A typical food chain would include a producer being eaten by a first-order consumer, which might be eaten by a second-order consumer, whose remains are eventually consumed by decomposers.  For example, grain (producer) gets eaten by a mouse (first-order consumer), which in turn gets eaten by an owl (second-order consumer), which eventually dies and whose remains get eaten by various organisms such as bacteria (decomposers).  Since consumers and decomposers often eat more than one kind of organism (mice eat different kinds of plants, and coyotes eat just about anything), different food chains overlap and intersect to form food webs.
• Water Cycle:  Although the earth is mostly covered by water, only a small percentage of the earth's water is liquid fresh water needed for land-based plants' and animals' survival.  The water cycle involves the process of evaporation of water from oceans (only the water evaporates, not the dissolved salts etc.), ponds, puddles, etc.  The evaporated water (water vapor) condenses in clouds and falls as precipitation back to earth.  Some of the precipitation evaporates, some drains into the soil, and some is taken up by plants or animals to support their life. 
• Nitrogen Cycle:
• Oxygen/Carbon Dioxide Cycle:  Plants use carbon dioxide in photosynthesis and release oxygen into the surrounding air or water.  Animals use oxygen in their breathing and release carbon dioxide into the surrounding air or water. 
• Other Environmental Factors:  Everything in an environment affects the wellbeing of all of the living organisms in an ecosystem.  Some organisms are adapted to be able to withstand a wider range of temperatures or of salinity or of pollution than others.  We will focus on some of these issues to help students understand the effects of pollution and of other environmental disturbances.  Students should be better equipped to understand the debate on global warming.