Links in the chain
Plants use sugars for growth, maintenance, and reproduction. Measures the growth of a plant in your house or garden. Over time its stem becomes taller and wider and the roots longer, and new leaves sprout. Plants also store sugars in seeds, fruits, and tubers. Seeds need a supply of energy to allow the rapid growth of a new plant. Fruits contain sugars to attract animals. Bulbs are fattened extensions of the stem that are held underground, serving as the planet's food reserves in winter or in times when nutrients are in short supply from photosynthesis.
Sit down in the garden on a warm summer day and you'll see that many animal populations take advantage of the food stored in plants. Caterpillars munch on leaves, while many beetles prefer tree bark. Birds feast on fruit, often gobbling seeds and all. The seeds are later deposited in new places complete with a built-in source of fertilizer. Squirrels must crack open the hard shells of the nuts they gather to reach the nutritious meat inside. Gophers tunnel beneath the soil in search of tubers and bulbs. This passage of energy from plants to animals is called a food chain. But the food chain doesn't stop here. Toads snap caterpillars off the undersides of leaves at night. Woodpeckers hammer holes in the bark to find beetle larvae. Local cats hunt gophers. A raccoon eats fruit, but it's just as likely to make a meal of birds’ eggs.
Ecologists describe food chain to follow the flow of energy from one organism to the next. In reality, the feeding relationships between populations in a community are more like webs, because most organisms eat more than one type of food.
Food-Web diagrams, showing the connections between organisms, are like road maps. Each feeding strategy is called a trophic level. As the source of energy for the system, producers are placed at the base of the diagram. Primary consumers eat producers. A grasshopper is a primary consumer because it eats grass leaves. Primary consumers may also be called herbivores. The garter snake that eats the grasshopper is a secondary consumer, or carnivore. The term "carnivore" can be confusing, however, because it also applies to the hawk that catches the snake. It's more accurate to say that the hawk is a third-level, or tertiary, consumer.
Like the raccoon in the garden, some organisms fill more than one trophic level. These are the omnivores. American robins are a good example. Robins are first-level consumers when eating berries but become second-level consumers when they catch grasshoppers. Black bears are also omnivorous; their diet includes animal prey and scavenged meat. Scavengers eat dead organisms that they have not killed. For some animals, like black bears, scavenged food makes up only part of their diet. Bald eagles get a lot of their nutrition from scavenged meat, but they also capture fish and ducks. You will often see crows pecking at roadkill, but the eggs and nestlings of smaller birds are another part of their diet. At the other extreme, vul-tures show no interest in hunting. Carcasses provide their entire food source.
Invertebrates such as beetles, Wood lice, millipedes, earthworms, and slugs belong to a group of scavengers called detritivores. Depending on the species, they consume dead animals, animal waste, or dead plants. As detritivores eat, organic matter is shredded into smaller pieces. De composers can then move in. These organisms break organic material down to its most basic chemical parts. Bacteria decompose animal remains, while fungi work on plants. De composers do not eat food and digest it internally. Instead, they produce enzymes that cause matter to break apart. They absorb the elements needed for nutrition and energy; the remainder is left in the environment for later reuse.
It's rare to find food chain that have many more than five levels (aside from scavenging and decomposition). There's a good reason for this. An organisms takes in a lot of food in its lifetime. But much of this food is used up. Life is hard work! The transformation of food to energy and energy to life processes creates heat. Excess heat is lost to the environment; the organism cannot use it again. In physics this situation is called entropy and is described by the second law of thermodynamics: when energy is converted from one from to another, the amount of useful energy decreases. On average, only about 10 percent of the energy from one trophic level gets passed to the next.
In other words, when a grasshopper eats grass, it receives one tenth the amount of energy that the grass made during photosynthesis. The snake gets one-tenth of the grasshopper's stored energy-just one-hundredth of what was in the grass. This situation can be depicted as a pyramid. The wide base represents producers, with the most energy. At successive trophic levels the amount of energy decreases, so the pyramid narrows. The tip of the pyramid symbolizes the highest-level consumer. Very little of the original energy made available by producers remains at this level.
Think back to the garden. Did you ever notice that there are far more plants than insects? In turn, there are more insects than birds. This is true in any food chain or ecosystem. The amount of biomass, or biological weight, decreases at each trophic level in approximate proportion to the loss of energy.
Food webs support every living thing on Earth. But they are easily disrupted. This sometimes happens as a result of overexploitation, as in the oceans and forests. Australia's cane toad exemplifies what can happen when people call introduce species into a food web. A small group of cane toads was transported from Hawaii to Australia in 1935. Sugarcane farmers hoped that the toads would eat beetles that had been killing their plants. The toads proved useless in this capacity. But they ate many other types of insects, including the bees that pollinate native Australian plants. Cane toads even ate birds'eggs, frogs, and small mammals. Because of a toxin in the toad's body that protects it from predators, the cane toad has thrived. Today the species is so widespread that it is difficult to control.
Ecologist Aldo Leopold described an opposite effect in his book A Sand County Almanac. He and his friends came upon a pack of wolves while hunting. Like most people in the mid-twentieth century, they considered wolves to be pests-a threat to human safety, reckless killers of livestock, and competitors for deer. The hunters killed the wolves. Leopold later regretted this decision;he became an advocate for predators and for ethical hunting. He explained what happened after the wolves were killed: