T
trofia
Vieras
Tästä selviää. Aluksi kysymys: sinut laitetaan autiolle saarelle, mukanasi kukko ja kana, vettä ja muutama säkki viljaa, mutta saarella ei voi kasvattaa mitään, koska siellä ei ole multaa jossa voisi kasvattaa viljaa. Mikä on paras strategia selvitä pisimpään hengissä. Suurin osa vastaa tähän kysymykseen väärin.
I have an old exam question that has tripped up many students. It goes like this: Imagine that you have been stranded on a small desert island. You have plenty of water, but your only food is a crate of chickens (of both sexes) and several sacks of grain. There is no soil on this island, so you can’t grow anything. What is your best strategy to make your food supply last the longest?
Ecosystems run on energy, plain and simple. To better understand and compare different ecosystems, biologists have developed a number of techniques for quantifying energy production and flow in ecosystems. As I hope to show you, this is not just an academic exercise but one that has a direct impact on human society. I’ll cover three main points:
When comparing ecosystems, it is useful to start with how much and how fast energy is captured. The term for this concept is primary productivity, the measurement of the rate of biomass production in an ecosystem.
Not all ecosystems are equal in size or primary productivity. For instance, the open oceans cover much of the planet but they are not very productive. Deserts are another example of a widespread ecosystem with low productivity. In contrast, reef systems and tropical rainforests are both very productive, but they don’t occupy much of the planet. Several environmental factors such as climate, seasonality, and nutrient availability interact to determine the productivity of an ecosystem.
When I first went to college, I had a limited amount of money in the bank and only a small amount coming in from a part-time job. Each month, I had to make a budget that would see me through until the next paycheck. My budgets were extremely tight. You may be in a similar circumstance, and so are ecosystems.
The amount of primary productivity determines the energy spending limits of an ecosystem. A reef community or tropical rainforest with very high productivity supports a rich and diverse community of organisms. In contrast, the open ocean or a desert with low productivity supports a less varied community of organisms.
Now that we’ve discussed how energy enters ecosystems, let’s consider what happens once the energy is there. Here is the overriding principle: Energy flows through an ecosystem. We describe this flow in terms of food chains, from producers (such as plants) to primary consumers (such as animals that eat plants) to secondary consumers (such as animals that eat animals) and so on. At each step of the way, there is less and less energy available. Stated another way, the available biomass is constantly decreasing. Why is that so?
Think about any type of consumer you are familiar with—a squirrel, a caterpillar, your dog, or even you. Most of the food energy consumed by any of these animals is used to power the activities of life. The food you eat provides the energy to power your metabolism, but eventually most of the energy you consume is lost as heat.
As an example, consider a monarch caterpillar, a primary consumer. Monarch caterpillars eat only milkweed, a producer. If you weigh all the milkweed that a caterpillar eats, and then weigh how much mass the caterpillar gained, you’d discover something very interesting: Only about 15% of the milkweed biomass eaten by the caterpillar is actually converted into caterpillar biomass. In other words, if a caterpillar eats seven grams of milkweed, only about one gram of it will turn up in the body of the caterpillar. The other six grams are used to power caterpillar metabolism or are simply waste that the caterpillar could not process. This loss of 85% between steps in a food chain is fairly typical. Such losses between steps in a food chain create what is known as an energy pyramid.
For any ecosystem, the most energy is available at the producer level. Moving up, each step of a food chain involves about a 90% loss in energy. Or let me put it this way: If you have 100 pounds of corn, it can support only about 10 pounds of cow, which in turn can only support only about one pound of you. This is an energy pyramid. Which is why, in any ecosystem, there are many more herbivores (such as deer) than there are top predators (such as wolves). It takes about 10 pounds of deer to support each pound of wolf. In any ecosystem, as you scale the energy pyramid, you’ll find about a ten-fold reduction in the mass of organisms at each level.
Now that we have discussed how energy enters and flows through an ecosystem, think back to the test question I posed. Once you understand the realities of food pyramids, this is an easy question, but you might be surprised how many students miss it. Many people think the best strategy is to feed the grain to the chickens so that the chickens could produce eggs and offspring. Then they would eat the extra chickens and eggs. You should now know that this won’t work. Ninety percent of the energy in the grain will be lost as the chickens go about the business of being chickens. This is a big waste of energy that you can’t afford on a desert island. To maximize your time for survival on the island, you should first eat the chickens, and when they are gone, eat the grain. You’ll get the maximum possible amount of energy available in your desert island ecosystem.
Of course, this question is somewhat analogous to the human population on the earth. There is only so much biomass available each year on the planet. A diet from high on the food chain requires a larger share of the global productivity than a diet that features foods low on the food chain. And that’s some food for thought.
I have an old exam question that has tripped up many students. It goes like this: Imagine that you have been stranded on a small desert island. You have plenty of water, but your only food is a crate of chickens (of both sexes) and several sacks of grain. There is no soil on this island, so you can’t grow anything. What is your best strategy to make your food supply last the longest?
Ecosystems run on energy, plain and simple. To better understand and compare different ecosystems, biologists have developed a number of techniques for quantifying energy production and flow in ecosystems. As I hope to show you, this is not just an academic exercise but one that has a direct impact on human society. I’ll cover three main points:
- First, I’ll talk about how energy is captured by ecosystems.
- Next, we’ll consider how energy flows through ecosystems.
- Finally, we’ll review that test question I posed a minute ago.
When comparing ecosystems, it is useful to start with how much and how fast energy is captured. The term for this concept is primary productivity, the measurement of the rate of biomass production in an ecosystem.
Not all ecosystems are equal in size or primary productivity. For instance, the open oceans cover much of the planet but they are not very productive. Deserts are another example of a widespread ecosystem with low productivity. In contrast, reef systems and tropical rainforests are both very productive, but they don’t occupy much of the planet. Several environmental factors such as climate, seasonality, and nutrient availability interact to determine the productivity of an ecosystem.
When I first went to college, I had a limited amount of money in the bank and only a small amount coming in from a part-time job. Each month, I had to make a budget that would see me through until the next paycheck. My budgets were extremely tight. You may be in a similar circumstance, and so are ecosystems.
The amount of primary productivity determines the energy spending limits of an ecosystem. A reef community or tropical rainforest with very high productivity supports a rich and diverse community of organisms. In contrast, the open ocean or a desert with low productivity supports a less varied community of organisms.
Now that we’ve discussed how energy enters ecosystems, let’s consider what happens once the energy is there. Here is the overriding principle: Energy flows through an ecosystem. We describe this flow in terms of food chains, from producers (such as plants) to primary consumers (such as animals that eat plants) to secondary consumers (such as animals that eat animals) and so on. At each step of the way, there is less and less energy available. Stated another way, the available biomass is constantly decreasing. Why is that so?
Think about any type of consumer you are familiar with—a squirrel, a caterpillar, your dog, or even you. Most of the food energy consumed by any of these animals is used to power the activities of life. The food you eat provides the energy to power your metabolism, but eventually most of the energy you consume is lost as heat.
As an example, consider a monarch caterpillar, a primary consumer. Monarch caterpillars eat only milkweed, a producer. If you weigh all the milkweed that a caterpillar eats, and then weigh how much mass the caterpillar gained, you’d discover something very interesting: Only about 15% of the milkweed biomass eaten by the caterpillar is actually converted into caterpillar biomass. In other words, if a caterpillar eats seven grams of milkweed, only about one gram of it will turn up in the body of the caterpillar. The other six grams are used to power caterpillar metabolism or are simply waste that the caterpillar could not process. This loss of 85% between steps in a food chain is fairly typical. Such losses between steps in a food chain create what is known as an energy pyramid.
For any ecosystem, the most energy is available at the producer level. Moving up, each step of a food chain involves about a 90% loss in energy. Or let me put it this way: If you have 100 pounds of corn, it can support only about 10 pounds of cow, which in turn can only support only about one pound of you. This is an energy pyramid. Which is why, in any ecosystem, there are many more herbivores (such as deer) than there are top predators (such as wolves). It takes about 10 pounds of deer to support each pound of wolf. In any ecosystem, as you scale the energy pyramid, you’ll find about a ten-fold reduction in the mass of organisms at each level.
Now that we have discussed how energy enters and flows through an ecosystem, think back to the test question I posed. Once you understand the realities of food pyramids, this is an easy question, but you might be surprised how many students miss it. Many people think the best strategy is to feed the grain to the chickens so that the chickens could produce eggs and offspring. Then they would eat the extra chickens and eggs. You should now know that this won’t work. Ninety percent of the energy in the grain will be lost as the chickens go about the business of being chickens. This is a big waste of energy that you can’t afford on a desert island. To maximize your time for survival on the island, you should first eat the chickens, and when they are gone, eat the grain. You’ll get the maximum possible amount of energy available in your desert island ecosystem.
Of course, this question is somewhat analogous to the human population on the earth. There is only so much biomass available each year on the planet. A diet from high on the food chain requires a larger share of the global productivity than a diet that features foods low on the food chain. And that’s some food for thought.
