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Many plants depend on animals for pollination and seed dispersal, and animals depend on plants for food and shelter

This standard addresses the much larger concept of interdependence. All organisms depend on other organisms for their existence. As we noted in Standard 2a, all animals ultimately depend on plants for matter and energy. This standard adds that plants also provide shelter for animals.

The interdependence of plants and animals with respect to pollination and seed dispersal illustrates another major concept in biology, co-evolution. Plants and animals have evolved together. This principle is beautifully illustrated in the co-evolution of flowering plants and insects. Plants develop flowers with colors, shapes, odors, and food to attract particular species of insects. The insects, in turn, develop adaptations that make them better suited to recognize and utilize particular plants. These insect adaptations then cause further specializations in plants in an unending tale of mutual adaptations.

In any particular environment, some kinds of plants and animals survive well, some survive less well, and some cannot survive at all

On the simplest level, this standard is almost too easy to teach. Can a whale survive in a desert? What would happen if you tried to plant a rose bush in the ocean? Each species has adaptations that enable it to successfully live and reproduce in the environments it normally inhabits. Any given environment can support only a small range of organisms compared with the millions of different species that exist.

On a deeper level, this standard can guide us as we analyze the results of changes in an environment. Examples include a major oil spill, a forest fire, a volcano, or the introduction of an invasive species. After each of these events, plants and animals that used to survive quite well in a particular environment may not survive at all or may have greatly reduced numbers. Other organisms that previously could not survive in the environment can now flourish there. Teachers and students can use the web and local agencies to investigate distant and local ecosystems that have been perturbed, and how these changes affect who can live well in the ecosystem.

Plants are the primary source of matter and energy entering most food chains

All ecosystems with their food chains and food webs ultimately rely on a class of organisms called producers who capture the energy that sustains the entire ecosystem. In essentially all the familiar ecosystems, organisms in the plant kingdom play this role. Through the process of photosynthesis, plants ranging from redwood trees to grasses to microscopic plankton in the ocean capture the energy of sunlight and package this energy in chemical form in sugars.

There are some ecosystems that rely neither on plants nor on photosynthesis. Generally the producers in these ecosystems, such as hot springs or hydrothermal vents in the ocean floor, capture chemical energy from non-carbon elements and package that chemical energy in the form of carbon-containing molecules that other organisms can use as sources of energy and matter. These ecosystems have the same kind of organization as those based on photosynthesis with producers, consumers, and decomposers.

Photosynthesis not only captures energy, but it also takes carbon dioxide gas from the air and makes organic molecules from this inorganic form of carbon. Initially these organic molecules are sugars, but these sugars are then built into the proteins, DNA, carbohydrates, and fats that make up living organisms. When you pick up a dry log, almost 90% of the weight of that log came from carbon dioxide gas that the tree made into sugar through photosynthesis!

Since photosynthesis both captures energy and converts carbon dioxide gas into sugars, this process done by plants is the primary source of both matter and energy for food chains. Many texts focus on the energy pyramid in ecosystems, how energy flows from producers to herbivores to carnivores. It is important to realize that the matter in those organisms is also flowing from plants to animals that eat plants to animals that eat these herbivores.

Most people understand that organisms need matter to grow. However, they often do not realize how much matter fully-grown organisms need just to keep the bodies that they have. Our bodies are being continually broken down and rebuilt at the molecular level. We constantly exchange matter with the environment as we excrete the wastes from this internal recycling and take in new building blocks to reconstruct our molecules. This recycling is so extensive that more than 98% of the atoms in your body were not in your body two years ago! Therefore, it is important that plants, via photosynthesis, provide both the matter and the energy that enters food chains.