Energy flows into the biological world form the sun. Life exists on Earth because photosynthesis makes it possible to capture some of the light energy from the sun and transform it into the chemical energy of organic molecules. These organic compounds compose what we call food. The amount of organic material that the photosynthetic organisms of an ecosystem produce is called primary productivity.

Primary productivity determines the energy budget of an ecosystem. All of the organisms in an ecosystem are chemical machines driven by the energy captured in photosynthesis. The organisms that first capture energy, the producers, include plants, some kinds of bacteria, and algae. Producers make energy-storing molecules. All other organisms in an ecosystem are consumers, which obtain energy to build their molecules by consuming plants or other organisms.

Ecologists assign every organism in an ecosystem to a trophic level, which is determined by the organism’s source of energy. Energy moves from one trophic level to another. The lowest trophic level of any ecosystem is occupied by the producers: plants in most terrestrial ecosystems and algae and bacteria in aquatic ones. Producers not only use the energy of the sun to build energy-rich sugar molecules, but also absorb nitrogen and other key substances from the environment and incorporate them into biological molecules. It is important to realize that plants respire as well as produce. The roots of a plant, for example, do not carry out photosynthesis, because there is no light underground. Roots obtain energy the same way as everybody does—by using energy-storing molecules produced elsewhere (in this case, the leaves of the plant).

At the second trophic level are herbivores, animals that eat plants. They are the primary consumers in ecosystems. Cows and horses are herbivores, as are caterpillars and ducks. A herbivore must be able to break down the plant’s molecules. Simple sugars and starches present no problem, but the digestion of cellulose, a molecule made of sugar units linked together, is a chemical feat that only a few organisms have evolved the ability to perform. Most herbivores rely on helpers. A cow, for instance, has a thriving colony of bacteria in its gut that digests cellulose. Humans cannot digest cellulose because we lack these bacteria. This is why a cow can live on a diet of grass but you cannot.

At the third trophic level are secondary consumers, animals that eat herbivores. Such flesh-eating animals are called carnivores. Tigers, wolves, and snakes are carnivores. Some animals, such as bears, are both herbivores and carnivores, eating both plants and animals. They use the simple sugars and starches stored in plants as food, but they cannot digest cellulose. Such animals are called omnivores. Humans are omnivores. Many ecosystems contain a fourth trophic level made of carnivores that consume other carnivores. They are called tertiary consumers, or top carnivores. A hawk that eats a snake is a tertiary consumer. Very rarely do ecosystems contain more than four trophic levels.

In every ecosystem there is a special class of consumers called detritivores, which include fungal and bacterial decomposers, vultures, and worms. Detritivores obtain their energy from the organic wastes and dead bodies that are produce at all trophic levels. Bacteria and fungi are known as decomposers because they cause decay. Decomposition of bodies and wasters releases nutrients back into the environment to be used again by other organisms.

3. The food web is unavailable.

4. Choose any primary consumer and explain what would happen to the food web if this organism becomes extinct. Be specific and discus the effects by naming specific organisms and what changes would take place in there numbers, habits, and behaviors.

Pocket mouse- If this primary consumer were removed, the producer it mainly feeds on, the Indian paintbrush, would have a slight growth in its population, but the other animals who feed on the Indian paintbrush would most likely compensate.

But if the pocket mouse were to be removed, populations of gopher snakes would take a drastic downward turn because the pocket mouse is its main source of food.

Meanwhile, with less gopher snakes, their secondary food source, the kangaroo rat population, would explode. Kangaroo rats are a primary consumer of Indian paintbrush, so the rat’s population explosion would cause a decrease in the Indian paintbrush population.

The hawk would also be affected by the loss of the pocket mouse, because one of its main sources of food is the gopher snake. With a decreased hawk population, because of the loss of pocket mice, and the subsequent loss of gopher snakes, the animals that are usually the hawk’s prey would turn upward. However, an example like the Gila monster, who is a prey for hawk, would stay about the same, because Gila monster’s mainly feed on pocket mice, which in this whole example, have been removed.

So, if you were to remove a primary consumer, in this case the pocket mouse, the affects on the food web could be devastating.

5. Explain if there would be a different consequence if a top consumer were removed. A producer?

If you were to remove a top consumer, say a king snake, from the food web, it would throw the whole food web out of whack. One of the king snake’s main food sources is the sidewinder snake. With the king snake gone, the sidewinder population would take off. With an increased sidewinder population, the ground squirrel population would decrease, because the ground squirrel is the sidewinder’s main food source. With a lowered ground squirrel population, the prickly pear cactus population also would take off, because the ground squirrel feeds on the prickly pear. Since little else feeds on the prickly pear, there would be a larger population of the cactus.

If you were to remove a producer, say the Indian paintbrush, this could be the most devastating of all. If you remove the Indian paintbrush, there would be a decrease in all of the secondary consumers which feed on it. Let’s take one food chain, for example - the pocket mouse. Pocket mice feed on Indian paintbrush, and with no Indian paintbrush, the pocket mouse population would decrease. Then, everything that mainly feeds on the pocket mouse would decrease, such as the gopher snake. With a decreased gopher snake population, there would be a decrease in the hawk population, because the hawk feeds on the gopher snake. The removal of any producer could lead to a total food web breakdown.