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In this lesson, you'll learn about food chains, food webs, and the different roles that organisms play in an ecosystem. You'll also learn about how energy flows through an ecosystem.
The Sonoran Desert in the Southwestern United States may seem like a desolate place, but looks can be deceiving, because a wide array of wildlife can be found in this very dry environment. You might wonder how animals can make a living in the desert - after all, what is there for them to eat? Just like in every other ecosystem, all animals must eat other organisms, or, at the very least, secretions of other organisms, to acquire energy. We can follow a sequence of organisms that feed on each other to create a food chain, or a sequence of organisms that feed on each other.
Since a food chain follows the sequence of organisms that feed on each other, it always starts with an organism that gets its energy from an abiotic source, which is usually light from the sun. An organism that gets its energy from an abiotic source is called a producer. Since plants get their energy from sunlight, they are producers, and one of the common producers in the Sonoran Desert is the prickly pear cactus. Many different animals eat the fruit of the prickly pear cactus, including Harris's antelope squirrel. The squirrel is a consumer because it gets its energy from other organisms. In this case, the squirrel gets its energy from the fruit of the prickly pear cactus.
In turn, the squirrel can be eaten by another consumer, the diamondback rattlesnake, and the rattlesnake can then be eaten by a roadrunner? and no, that is not a mistake. In the Sonoran Desert, roadrunners are deadly predators that will commonly pick up rattlesnakes by the tail and smash their head repeatedly onto the ground in a whip-like fashion. Once the rattlesnake is dead, the roadrunner will swallow it whole. In cases where the rattlesnake is too large to swallow all at once, the roadrunner will partially swallow the snake, with its tail still hanging out of its beak. As the first parts of the snake are digested, the roadrunner will then continue to swallow the rest of the snake.
The roadrunner itself can also be eaten by yet another predator: the Red-tailed hawk. You thought I was going to say a coyote, didn't you? Well, it turns out that, much like in the cartoon, adult roadrunners are just too fast for a coyote to catch, although coyotes will eat roadrunner eggs and chicks. In any case, the Red-tailed hawk is one of the few animals able to catch an adult roadrunner and make a meal of it, which puts the hawk at the top of our food chain.
So let's take a closer look at our food chain, which can be divided up into different trophic levels, or the levels of a food chain where organisms obtain their energy. The food chain starts with a producer, the prickly pear cactus, which obtains its energy from sunlight. The prickly pear is eaten by Harris's antelope squirrel, which, because it is the first consumer in the food chain, is called the primary consumer. The squirrel is eaten by the diamondback rattlesnake, which is called the secondary consumer. The rattlesnake is eaten by the roadrunner, which is the third consumer in the chain and therefore called the tertiary consumer. And finally, the fourth consumer in the chain, the Red-tailed hawk, is the quaternary consumer.
Now although this food chain is a possible sequence of events that could happen in the Sonoran Desert, each of the animals in the food chain is usually capable of eating other food items as well. Each of them could also be eaten by other animals in the desert, which means that there are a seemingly limitless number of other possible food chains. So, to more accurately describe energy flow through an ecosystem, ecologists can construct a food web, which is a combination of food chains that are interconnected to create a network of feeding relationships. Just like a food chain, energy enters the food web at the trophic level of the producers, which can also be called autotrophs. In the desert, autotrophs are often some type of cactus, grass, or shrub.
Plant-eating animals, or herbivores, such as grasshoppers, butterflies, jackrabbits, and desert tortoises, feed at the trophic level of the primary consumer. Carnivores, or animals that only eat other animals, like scorpions, snakes, spiders, hawks, owls, and mountain lions, can feed at the trophic levels of the secondary, tertiary, or quaternary consumer, or even higher if a particular food chain is long enough. And then there are some animals that eat both plants and animals, called omnivores. Omnivores, such as antelope squirrels, roadrunners, coyotes, and ringtails, can potentially feed at any of the consumer trophic levels.
There's just one more trophic level to talk about, and that's the level of the detritivore, which is an organism that feeds on waste products or dead organic material. Examples of detritivores include vultures, fungi, and bacteria. Since detritivores can take dead organic material and bring the stored energy back into the food web, they can be thought of as the recyclers of the ecosystem. After all, if there's energy just lying around, some organism will fill the niche and make use of this energy, and that's exactly what detritivores do.
Now, let's look at how much energy actually flows through the food web. We know that plants and animals use energy in their daily activities, and we also know that plants and animals store energy within their tissues. So, how much energy is used by an organism, and how much is stored? The answer actually varies based on many factors, such as the type of organism, but there are a few key rules of thumb that we can use to estimate how much energy is used and how much is available to the next trophic level of the food web.
First, let's start with the autotrophs. Organisms that use photosynthesis to harness energy are only able to convert about one percent of the total light energy that hits them into usable chemical energy. Of that energy that is harnessed by producers through photosynthesis, about 30% is used during metabolic processes and dissipated as heat, and about 70% becomes stored energy.
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What about consumers? Consumers do not consume all of the energy stored in producers; they only consume about one-seventh of all of the energy stored, which turns out to be about ten percent of the total energy that is harnessed by producers through photosynthesis. They also use energy very differently than producers, and that is largely due to the fact that consumers are usually mobile. Movement uses a lot of energy, and in addition, some consumers, like birds and mammals, also use a lot of energy specifically to create their own heat and maintain body temperature. However, secondary, tertiary, and quaternary consumers consume more of the stored energy available to them. So, when all is said and done, the consumers at each trophic level, on average, pass along about ten percent of the total energy they receive to the next trophic level that feeds upon them. Most of the other 90% of the energy is used in various functions, such as metabolism and movement, and is ultimately dissipated as heat, and whatever isn't used or eaten by predators is consumed by detritivores.
So, if we take the total photosynthetic energy that is produced by autotrophs and ask how much of it is passed along to the primary consumers and then stored and made available to secondary consumers, we'd take ten percent to represent the energy passed along to primary consumers and multiply it by 0.1 to represent the ten percent passed along to secondary consumers, and our answer would be one percent. If we go up another level in the food web, we multiply this number by 0.1 again, and the amount available to tertiary consumers would be one tenth of a percent. By extension, the amount of energy available to quaternary consumers would be a mere one-hundredth of a percent of the total energy produced by autotrophs through photosynthesis. Whatever energy isn't used at a particular level and isn't passed on to the next higher trophic level is eventually consumed by detritivores, which will also use a little less than 90% of the energy they consume and contribute about ten percent back to the food web.
Let's review. A food chain is a sequence of organisms that feed on each other. Since a food chain follows the sequence of organisms that feed on each other, it always starts with an organism called a producer, which gets its energy from an abiotic source, usually in the form of light from the sun. An organism that eats the producer, or a part of the producer, forms the next link in the food chain and is called the primary consumer. A consumer is an organism that gets its energy from other organisms. The next link in the food chain that eats the primary consumer is called the secondary consumer. A tertiary consumer eats the secondary consumer, and sometimes a quaternary consumer will eat the tertiary consumer.
Because organisms usually have more than one food source and more than one other organism that feeds upon them, food chains can be combined to form a food web, which is a combination of food chains that are interconnected to create a network of feeding relationships. Food webs are much more complicated than food chains, but more accurately depict the feeding relationships within an ecosystem.
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