Ecosystems and Biodiversity Lesson 3 : Ecosystems

The hydrologic cycle, or water cycle, is the continuous movement of water above, below, and on the surface of the Earth. The hydrologic cycle is powered by the sun’s energy and moves water from one location on Earth to another. Along the way, water may change state from liquid, to gas (water vapor), to solid (snow and ice). For example, water evaporates from the ocean into the atmosphere, forming clouds that may be blown to the Great Lakes, where they will rain or snow onto Michigan’s land. While some of this water soaks into the ground, where it may be stored in aquifers, much of the rain and melted snow may run off into wetlands or inland lakes, which then empty into rivers that flow to the Great Lakes. The water in the Great Lakes may again evaporate into clouds, continuing the water cycle. As water moves through the water cycle, it passes through terrestrial and aquatic ecosystems; passes through plants, animals, and humans; and is used in some way, in all ecological processes.

The role of plants in the water cycle is often not understood or is underestimated by students. It is estimated that about 10% of the moisture in the atmosphere is released by plants through transpiration. In one day, a single large tree in full leaf can “lift” over a ton of water from the soil, distributing it to every leaf on the tree. Transpiration refers to the loss of water vapor by plants, through their stomata (tiny holes in leaves). During the growing season, one large tree may transpire several hundred gallons of water into the air every day. An added bonus is that this transpiration has a cooling effect, cooling as much air as would six windowunit air conditioners (for the same large tree)! While a single tree may not have a large effect, an entire forest can transpire enough water so that it affects climate and rainfall in surrounding areas. Step 3 of Lesson 3A includes an investigation related to transpiration.

While the world will never run out of water, having an adequate supply of clean affordable fresh water in the places where it is needed to meet human demands is not guaranteed. Already, shortages of clean water are causing disease, death, economic hardships, and political conflicts for many people in the world, especially in Africa, Asia, the Middle East, and in the western United States. In Michigan, we are fortunate to be surrounded by the Great Lakes, which account for 20% of the world’s surface freshwater supply and for 90% of all of the freshwater in the United States. However, the quality and quantity of the Great Lakes water supply are threatened by various forms of pollution, global warming, and demands for water diversion by areas with insufficient supplies of useable water. At the end of this lesson, students are encouraged to consider how water supplies are threatened and what personal actions they can take to help guarantee access to clean water supplies for future generations.