Chapter+2

Food Chains, Food Webs and Energy Flow


Except for a few exceptions, all of the **energy** for all life and human technology comes from the **SUN**. Animals and humans can't eat sunshine. Plants are the first level in the **food chain**. They convert sunlight to food for animals (though the plants may not look at it that way).

The flow of **energy** through life is not an endless cycle. The energy doesn't go round and round getting used over and over again and never wearing out. Its passage through the **food chain** can better be described as in and out. As energy moves up the food chain there is less and less of it to go around. That's the main reason there aren't very many big fierce predators compared to the herbivores. Not enough energy for them! We'll talk about this in detail in another section, but a brief explanation goes something like the following: Most of the **solar energy** that falls on the earth is not used by plants. It bounces back to space or heats the air, oceans, and ground, and makes weather, among other things.

The **plants** only get a little bit of the solar energy that hits the earth. The **herbivores** only get a little bit of the energy that hits the plants. The **carnivores** and **decomposers** only get a little bit of the energy that was eaten by the herbivores.

// (Most of the plant energy that is consumed by a herbivore is used by that herbivore to keep itself eating, breathing, walking, and staying warm. Only a little bit is left over for the carnivore or decomposer that eats the herbivore.) //

At the end of the chain there isn't much of that original solar energy left.

**Therefore....**

We need fresh sunshine everday and new plants have to keep growing. Otherwise the whole amazing system would quickly run out of energy and everything alive would come to a **//"dead"//** end.



**Energy above flows (or jumps in bites?) from sun to primary producers (plant) to primary consumers (mouse) to secondary consumers (coyote) to decomposers (bacteria, etc.)**



This flow of **energy** is transported through the animals by a system biologists call the **food chain**. A food chain in a given ecosystem is usually very complicated but it is useful to think of the food chain in the four simple steps or levels described below.

The drawing below only shows three steps. Which one is missing?
 * // Scroll down to read about the 4 food chain steps... //**

** Primary Producers ** -- **Green plants** and certain types of **bacteria** and **algae** are the **primary producers** because they are the ones that produce usable energy for the rest of the living organisms on earth. They use energy from the sun to make **sucrose**, **glucose**, and other compounds that other life forms can eat and "burn" for energy. In each one of those sugar molecules a little bit of the sun's energy is stored in a form that we can call chemical energy. But it might better be called "potential energy" since it is a sort of "doing-nothing-for-now-waiting-to-happen" kind of energy (for more on this visit [|Energy Changes]).

** Herbivores ** - Herbivores are the plant eaters. They have the ability to digest the plants they eat and release the energy stored in the plant cells for their own use. Some examples of animals in this group are deer, cows, elephants, rabbits, elks, zebras, most insects, and birds that eat fruit and seeds. Sometimes scientists call this level of the food chain the **Primary Consumers**.

** Carnivores ** - These guys are the meat eaters. **Predators** and **scavengers** are in this group. Sometimes this level in the food chain is refered to as the ** Secondary Consumers **. They eat the guys that eat the plants and sometimes they eat each other. Most of these animals can't eat plants at all. They would starve to death if it weren't for the Herbivores digesting the plants first. They've got the glamour job but they're really pretty helpless without all the boring plants and herbivores. Cats and dogs, killer whales, sharks, spiders, snakes, wolves, vultures, hawks, eagles, crocodiles, and many other fierce predators that for some reason we are especially fascinated with, are in this group.

They're like carnivores and herbivores, because they also have to get their energy from the cells of animals or plants. The difference is they prefer their food dead - very dead. What do you think? Are maggots decomposers or carnivores (or just yucky little things we'd rather not think about)?
 * Decomposers ** - These are not the guys that sit around unwriting songs[[image:http://www.ftexploring.com/ftimages4/erthwrm1.gif width="120" height="135" align="right" caption="Earthworms are considered one of the decomposers."]] and symphonies (get it? The opposite of composers?). They are the guys that eat up dead bodies - both plant and animal. And aren't we glad they do. This group of useful critters are // mostly bacteria and fungus //, but also, according to our sources, includes maggots, dung beetles, earth worms, sow bugs (shown below), and many other eaters of dead organic matter. Without them there would be a lot of dead bodies lying around.

Energy Flow and Pyramid of Numbers


 * Do you know why there are** **more herbivores than carnivores**?

In a food chain, energy is passed from one link to another. When a herbivore eats, only a fraction of the energy (that it gets from the plant food) becomes new body mass; the rest of the energy is lost as waste or used up by the herbivore to carry out its life processes (e.g., movement, digestion, reproduction). Therefore, when the herbivore is eaten by a carnivore, it passes only a small amount of total energy (that it has received) to the carnivore. Of the energy transferred from the herbivore to the carnivore, some energy will be "wasted" or "used up" by the carnivore. The carnivore then has to eat many herbivores to get enough energy to grow. Because of the large amount of energy that is lost at each link, the amount of energy that is transferred gets lesser and lesser ...


 * The further along the food chain you go, the less food (and hence energy) remains available.**

The above [|energy pyramid] shows many trees & shrubs providing food and energy to giraffes. Note that as we go up, there are fewer giraffes than trees & shrubs and even fewer lions than giraffes ... as we go further along a food chain, there are fewer and fewer consumers. In other words, a large mass of living things at the base is required to support a few at the top ... many herbivores are needed to support a few carnivores

Most food chains have no more than four or five links. There cannot be too many links in a single food chain because the animals at the end of the chain would not get enough food (and hence energy) to stay alive. Most animals are part of more than one food chain and eat more than one kind of food in order to meet their food and energy requirements. These [|interconnected food chains] form a **food web**.

 **A change in the size of one population in a food chain will affect other populations.**

This interdependence of the populations within a food chain helps to maintain the balance of plant and animal populations within a community. For example, when there are too many giraffes; there will be insufficient trees and shrubs for all of them to eat. Many giraffes will starve and die. Fewer giraffes means more time for the trees and shrubs to grow to maturity and multiply. Fewer giraffes also means less food is available for the lions to eat and some lions will starve to death. When there are fewer lions, the giraffe population will increase.

**Symbiosis** is a broad term used to describe any close, long-term relationships between species. Parasitism, mutualism, and commensalism are all specific types of symbiosis.

**Parasitism** denotes a relationship in which one species benefits at the cost of the other. Parasitism is among the most successful life history strategies in the world. It has been argued, quite convincingly, that pressure from parasites could have been one of the most powerful forces driving evolution on the planet. Almost every organism has some form of parasite specialized to exploit it.

Fleas are a classic example of parasitism, living on cats, dogs, humans, and other mammals, sucking their blood for subsistence.

There are also "parasitoids" – organisms which are parasites in their larval form. Ichneumon wasps, for example, lay their eggs in other insects.

There’s also "brood parasitism", which is practiced by brown-headed cowbirds and some other birds. These birds lay their eggs in the nests of other birds, tricking the nest owner into caring for parasite’s young.

Many of our worst diseases are caused by parasites: malaria, for instance, is a blood parasite. Ringworm is another, less deadly, parasite on humans – a fungus that lives in the skin of its host.


 * Mutualism** denotes a relationship in which both species benefit. These relationships have also been very powerful forces in evolution; consider the highly specialized shapes of certain flowers and the butterflies with correspondingly specialized mouthparts to access the nectar.

==== Bees, butterflies, some bats, hummingbirds (and many more) have evolved mutualistic relationships with flowering plants. The plants often develop flowers that can only be accessed by specific species, and that species is able exploit the flower’s nectar, while transferring pollen to other flowers. This biological team up where two separate species are evolving together and influencing each other extensively is called "coevolution". ====

Another great mutualistic relationship is that between cleaner wrasse and larger fish: these little guys are granted amnesty by fish that would normally eat them because they will swim into larger fish’s mouths and clean up leftovers. This helps keep the larger fish’s mouth clean and provides a meal for the wrasse.

==== Perhaps the most familiar example of mutualism is that between humankind and our pets and livestock. In all instances of domestication, we offer these animals food and shelter and they offer us companionship, transportation, food, etc. ====

**Commensalism** denotes a relationship in which one benefits but the other isn’t harmed or helped in any significant way.

Think of hermit crabs: they depend on the empty shells of snails for their homes, but they don’t actually harm the snail to obtain it. They still need the snails, but they don’t have any significant impact on them.

Another good example is spiders and plants. Web-weaving spiders depend on plants to provide structures on which to weave their webs. But, again, the spiders have no appreciable impact of the lives on the plants they weave their webs on.

** Succession **

Words to Know
**Climax community:** A relatively stable ecosystem characterized by large, old trees that marks the last stage of ecological succession.

**Ecosystem:** An ecological community, including plants, animals, and microorganisms, considered together with their environment.

**Opportunist species:** Plant species with short life-spans that devote most of their energy to producing seeds.

**Pioneer plants/communities:** Plants or communities that are the first to be established in an area previously empty of life.

**Primary succession:** Succession that takes place on an area that was originally completely empty of life.

**Secondary succession:** Succession that occurs in an area where life once existed but has then been destroyed.

In [|ecology], **succession** is the replacement of one biological [|community] by another. Succession can be //primary// or //secondary//. **Primary succession** occurs on bare rock or soil that has never been colonised by organisms before. Examples would be [|sand dunes] and [|lava flows]. **Secondary succession** occurs on land which has been colonised before, but has been disturbed back to some earlier state. Examples would include a drained [|reservoir], cleared [|forest], or [|ploughed] field. Succession is a process of ecological change in which a series of natural communities are established and then replaced over time. Ecologists (scientists who study the relationships of organisms with their living and nonliving environment) generally recognize two kinds of succession, primary succession and secondary succession. Primary succession takes place on an area that is originally completely empty of life. As an example, an area that has been covered by a flow of lava has, for a time, no life at all on it. Over a period of time, however, various kinds of organisms begin to grow in the area. Over time, the variety of life-forms changes as succession continues. Secondary succession is far more common. It occurs in an area where life once existed but has then been destroyed. For example, imagine a forest that has been destroyed by a wildfire. Again, for a period of time, no living organisms may exist in the area. Before long, however, certain types of plants begin to reappear. And, as with primary succession, the nature of the plant communities gradually change over time.

The stages in ecological succession
The changes that take place during any form of succession depend on a variety of environmental factors, such as the amount of moisture, temperature, and wind. One possible scenario for primary succession might begin with the appearance of simple plants, such as lichens and mosses. Such plants are able to spring up in tiny cracks in the rocks in which water and dissolved minerals collect. When these pioneer plants die, they decompose and begin to form soil in which other, more complex plants can begin to grow. The second stage of plants might consists of grasses, herbs, and small shrubs. A characteristic of these plants is that they devote a great deal of energy producing huge numbers of seeds. They may live only one year, and spend the greatest part of their energy to ensuring that offspring will arise the following year. Species of this kind are known as opportunist species. Grasses are a common example of opportunist species. Plants that make up the early stages of succession also die, decompose, and contribute to the growing layer of soil. This process takes place over hundreds or thousands of years, however. Eventually, the soil is able to support more complex plants, such as larger shrubs and small trees including aspen, black spruce, and jack pine. These plants gradually take over from earlier communities since they are taller, have more leaves, and can capture more sunlight that was originally captured by simpler plants. In the final stages of succession, taller trees begin to grow. They, in turn, block out the sunlight needed by smaller trees and replace them. The final stage of ecological succession is known as a climax community. A climax community in the scenario outlined here might consist of birch, white spruce, and balsam fir.

Secondary succession
The general trends that take place during secondary succession are similar to those for primary succession. Imagine that a forest has been cleared for agriculture and then abandoned at a later date. In this case, a pioneer community consisting of lichens and mosses is not needed. Soil, rich or not, is already available. In such a case, the first plants to reappear might be annual (living one year) weeds, such as crabgrass. At a somewhat later date, the weedy community might be replaced by perennial (those that live year after year) weeds, and then by shrubs, a pine forest, and finally a mature forest consisting of oaks, maples, elms, and other large, long-living trees.

As succession goes forward, the nature of plant communities changes significantly. Instead of sending out many seeds each year, as in a pioneer community, trees in more mature communities devote their energies to sending out roots, branches, leaves, and other structures. Indeed, as they grow larger and create more shade, they actually prevent the germination (first life stages) and growth of their own seeds and seedlings.

Climax community
Ecologists refer to the final, highest stage of ecological development in an area as the area's climax community. That terms refers to a relatively stable community that is environmentally balanced. Climax communities are more a theoretical than a real concept. Certainly it is possible to recognize in old-growth communities areas that change relatively slowly compared to the earlier, more dynamic stages of succession.

Illustration of (1) a climax forest (2) destroyed by wildfire and (3 and 4) its eventual recovery. Secondary succession occurs in an area where life once existed but has then been destroyed. However, change in ecological communities is a universal phenomenon. Thus, even the climax state cannot be regarded as static. For example, even in old-growth communities succession on a small scale is always occurring. That succession may involve the death of individual trees and the growth of new ones. As environmental conditions change, even climax communities themselves continue to evolve.



Click here for PowerPoint on Food Chains:

Click here for PowerPoint on Sucession:

Click here for chapter review:

Click here for reading guide 3 ,4 ,5 & 6:

Click here for an excellent link to better understand succession:http: []