Picture the following chain of events: hundreds of oak caterpillars munching holes in the leaves of an old oak tree while squirrels and jays collect acorns and gobble caterpillars too.
The squirrel stops to scratch a flea. A pair of chickadees hop through the branches collecting caterpillars for their chicks that are waiting eagerly in their hole nest in the trunk. Animal droppings, egg shells, leaves and dead insects fall to the ground beneath the tree. Soil organisms such as earthworms, mushrooms and bacteria consume this organic matter, breaking it down into nutrients that can be taken up by the tree roots.
Squirrels live in oak trees and gather the acorns to eat during the winter. However, squirrels don’t remember where they bury every acorn so the acorns that are forgotten may eventually become the new oaks. Sunlight helps them to grow and the rain waters the seeds.
In addition to the squirrels there are also owls that swoop down on unsuspecting squirrels, chipmunks, mice and other small mammals that live in the forest. They keep the population from getting too large. This relationship between the hunted and the hunter keeps the community in balance.
In northern Canada, for example, lynx hunt snowshoe hares as their favorite prey. As the hare population shrinks, the lynx population follows into decline. With less lynx hunting them, the hare population rebounds and the cycle continues.
Communities can be small or large but many communities share common physical characteristics such as the forest in which they all live or a pond that they all use. These shared features bring communities together into a larger unit called an ecosystem.
In an ecosystem, each plant will be consumed by different kinds of animals, and in turn, each animal may eat many different types of food. Ecologists (scientists who study ecosystems) call plants “Producers,” because of their ability to trap the sun’s energy and use it to produce sugars. The following chart summarizes the categories that have been established to describe the different ways that plants and animals get food to survive:
As plants are eaten by herbivores, and herbivores are eaten by carnivores, nutrients are transferred from one organism to the next. Scientists use the word “biomass” to stand for the matter or substances that make up organisms, both living and dead. This biomass includes the chemical energy stored in the tissues of the organism.
When one organism is eaten by another, much of the original biomass is unavailable to the second organism. In the case of plants for example, the roots, dead leaves, and stem of the plant may be left behind. Some of the biomass of the animal that was eaten may be left unused as bones or feathers. The animal doing the eating is losing heat energy to its surroundings. It is also eliminating waste products that contain chemical energy.
You lose energy as well. You eat in order to maintain your body temperature at about 35°C (98°F), but you also produce waste products and you “leak” energy or heat at a rate equivalent to a 100 Watt light bulb every second.
This transfer and loss of energy and biomass can be illustrated with a pyramid. The following drawings show the loss of energy and biomass that might occur each time one kind of organism is eaten by another. An ecosystem usually needs many producers to support fewer herbivores, and many herbivores to support fewer carnivores.
Ecosystems can have very complex food webs linking plants with thousands of large and microscopic organisms that pass energy along through the system and into neighboring ecosystems. From the microscopic to the global perspective, life is revealed as a continuum of relationships linked to the past and to all parts of Earth’s physical environment. As the science of ecology has matured, so has our understanding of how humans are part of the food web and how they are affecting the biosphere.
Fifty years of research into the effects of the pesticide DDT is revealing the impact of this human-made chemical on the global food web. When it is eaten, DDT becomes linked to the fatty tissues of the body, since it is fat soluble. As larger fish or birds eat thousands of small organisms such as insects, the small amount of DDT in their bodies gets stored in the fatty tissue of the larger animal. The larger animal passes its “dose” of DDT onto a still larger predator, which may consume many “doses” in its lifetime.
On every continent scientists have found harmful amounts of DDT in organisms such as fish, eagles, and humans that are near the top of the food pyramid. DDT was banned in the United States in 1973. American companies continued to make DDT for other countries, so the pesticide is still used outside the U.S. and continues to invade the food web of the world.
Lead, mercury, and PCP are other examples of common and highly toxic substances that can accumulate in the tissue of organisms and be passed through the food web. As communities around the world have become educated to the harmful affects of chemical pollution, grass roots organizations have sprung up to resist pollution and toxic waste dumps in neighborhoods.
Question 2.5. Find out what toxic chemicals are a problem in your home, school, and community. What are the sources of the pollution? What groups are working to solve the problem?
Imagine a giant see saw with two people standing at the central balance point. If the people are the same mass, they can carefully walk at the same rate to the opposite ends of the see saw and back again without causing one end to touch the ground.
The balancing mechanisms within living systems, such as the ocean or a whale, are never this simple. Some parts of the system produce substances that are used and broken down by other parts of the system. Living systems usually have a way of reusing substances that build up in one area of the system.
Think of a relay game for the giant see saw that uses at least three people of different sizes and involves the exchange of at least two items. If one end of the board touches the ground, this represents a break-down of the game and all players lose. If you manage to have your players moving around, exchanging things and never touching the ground you will have achieved a dynamic equilibrium.
The health of an ecosystem like a pond depends on the parts of the system being in dynamic equilibrium with each other. Daphnia, small pond organisms, feed on green algae floating in the water.
Let’s suppose the environment and food supply for the daphnia are excellent, enabling the animals to rapidly increase in number. As the daphnia reproduce and compete for food, the population of algae is reduced. A shortage of food can cause the adult daphnia to produce fewer offspring. Some daphnia may starve. Provided some algae survive, as the population of daphnia declines, the population of algae can rebound. As food becomes plentiful again, the daphnia can increase in number until they again exceed the ability of the pond plants to maintain their populations.
The daphnia’s response to the declining food supply is a simplified example of a negative feedback response. Negative feedback responses bring a system back into balance, thereby acting to reduce overall change in the system.
In most ponds there are populations of fish and other predators that eat the daphnia. These predator populations also provide negative feedback mechanisms by keeping the daphnia from drastically reducing the algae populations. Negative feedback responses between organisms in a food web tend to keep the populations in dynamic equilibrium so that no organism wipes out its food supply.
Question 2.6. Study the drawing of the pond ecosystem (page 17). Describe at least two negative feedback responses that could maintain a balance between parts of the pond system.
Sometimes the dynamic equilibrium in an ecosystem becomes disrupted. Substances coming from outside the system may destroy the balance between parts of the system. For example, if sewage enters the pond, the oxygen level in the water drops because bacteria in the water use up the oxygen as they break down the waste. Organisms like daphnia and fish that require lots of oxygen may be killed. As they decompose the oxygen level drops further causing more organisms to die.
This is an example of a positive feedback response causing the system to change in a new direction. Without plentiful oxygen in the water, many kinds of plants and animals will die and be replaced by ones that tolerate low levels of oxygen. Positive feedback responses increase disturbance to a system.
Hole nesting birds such as chickadees and many kinds of woodpeckers and owls require mature forests to reproduce and successfully raise young. The following is a hypothetical graph of a positive feedback response resulting from reduction of forest habitat.
Ecosystems are affected by what happens in neighboring ecosystems because organisms, water, air, nutrients, and energy move from region to region. Whales, waterfowl, and caribou are a few examples of migratory animals that require international research and political cooperation to protect dwindling populations. Oil spills in the Canadian tundra, drainage of wetlands in Minnesota, and use of DDT for mosquito control in the marshes of Mexico can separately and together have devastating effects on animals such as ducks, geese, and cranes, which cross international borders during their life cycle.
We live in an exciting and sobering time of scientific collaboration and debate coupled with community awareness and action. A growing body of research reveals that humans are responsible for positive feedback responses that affect entire regions and even global systems. To better understand the gigantic systems and microscopic systems that affect all life, scientists around the world are sharing data and planning joint research projects.
Informed citizens are working together in families, schools, community groups, and national organizations to prevent positive feedback responses from reducing the diversity of Earth’s life systems. These national and international efforts can be viewed as negative feedback responses resulting from the growth of knowledge and education about our global system.
The box below shows examples of human activities that have influenced ecosystems. Which activities do you think might cause positive feedback in the system, and which might promote negative feedback to the system?
Can you think of more examples of human impact on living systems? Add them to the list.
See also the NASA Earth Observatory page, World of Change http://earthobservatory.nasa.gov/Features/WorldOfChange/ ,