I. Organisms and Their Environment

I. Organisms and Their Environment

Name ______Assignment # ______

Date ______Period ______


I. Organisms and Their Environment

A. Ecology is the study of how organisms interact with the living and nonliving things that surround them.

1. The most important concept in ecology is that all living things are dependenton each other and interact with one another and with the physical environment.

2. Environment– every living and nonliving thing that surrounds an organism.

B. Parts of an Ecosystem

1. Ecosystem – all the living and nonliving things that interact in a specific area.

a. For example an ecosystem can be a pond, field of corn, fish tank, woodland, ocean or a decaying log.

2. Biotic Factors – the living parts of the environment.

a. For example bacteria, plants and animals.

3. Abiotic Factors – the nonliving parts of the environment.

a. For example soil, water, physical space and energy.

4. Habitat – the ‘home’ of a particular species.

a. For example fields, forests, oceans, streams and deserts serve as homes for various species.

5. Population – the individuals of one species that live in the same area.

a. For example, all the squirrels in the town of Lancaster.

6. Community – different populations in the same area.

a. For example all the dogs, cats, people, squirrels, birds, (etc.) in the town of Lancaster.

7. Biosphere – all of the earth’s ecosystems combined.

a. The biosphere is every single place on Earth that contains living organisms.

b. Throughout the biosphere, organisms interact and compete for resources, such as food, space, and shelter.

C. Environmental Limits on Population Size

1. The survival of organisms depends on the physical conditions of the environment and on the resources available to the organism.

2. If species had unlimited resources they could produce populations of infinite (unlimited) size.

3. However, within any ecosystem resources such as oxygen, carbon dioxide, water, nutrients, space, and sunlight are finite (limited).

4. Because the resources are limited, organisms must compete with one another to survive.

a. Competition – the struggle for resources among organisms.

b. Within any one species, competition keeps the size of that species’ populationbalanced.

c. Populations tend to increase or decrease depending on the resources that are available at the time. However over long periods of time the size of the population remains stable.

5. Limiting Factors – anything that limits the size of a population.

a. Limiting factors can be abiotic (nonliving) such as:

i. The amount of dissolved oxygen in a pond may limit the kinds and numbers of fish that can live there.

ii. The amount of sunlight filtering through a forest may limit the number of green plants living on the forest floor.

iii. The intensity of light, the temperature range, minerals that are available in the water or soil, or the type of rock or soil in the ecosystem.

b. Limiting factors can also be biotic (living) such as:

i. The relationship between predators and their prey.

  • Predator – kills and eats other organisms.
  • Prey – killed for food.

ii. As predators kill and eat their prey, they limit the growth of the prey population.

iii. If too many prey animals are killed, predators begin to starve, and the predator population is reduced.

iv. With fewer predators, the size of the prey population begins to increase.

6. Carrying Capacity – the number of organisms of any single species that an ecosystem can support.

a. Carrying Capacity of a species is determined by the amount of energy, water, oxygen, minerals, and the interactionsof its organisms.

b. For example, a field’s carrying capacity for a population of foxes is affected not only by the climate, but also by the number and kinds of other populations present:

i. If there are many mice for the fox population to eat, the fox population may increase.

ii. If there are many viruses affecting the foxes, their population may decrease.

iii. A population of organisms in a given area will increase until the carrying capacity of that area is reached.

II. How Organisms Interact with Each Other

A. Ecological Niche – the role that each species plays in the ecosystem. A species’ niche includes everything about that organism including when it sleeps, where it lives, what it eats, when it eats, etc.

1. Only one species at a time can occupy a specific niche.

a. If two species attempt to fill the same niche (role) in an ecosystem, competition results.

b. Usually, one species will be more suited to the niche, which forces the other species to move on or face elimination.

c. Eventually, only one species will occupy each niche.

2. Although not true, sometimes it appears as if different populations occupy the same niche:

a. For example, deer and moose often live in the same area, and seem to eat the same plants at the same time of the day. A closer examination reveals that the deer and moose prefer to eat different plants and only compete when food is very limited.

b. Similarly, several bird species may seem to nest and feed in the same tree. In reality, it is more probable that the birds are nesting in different parts of the tree and eating different insects.

3. Competition for a particular niche often occurs when a foreign species enters an area. The new species may be more successful than the native species, partly because the newcomer may not have any natural enemies to control its population.

a. Humans frequently bring foreign species into an area either on purpose or accidentally.

i. One accidental example is the zebra mussels that were brought to the Great Lakes on cargo ships. The zebra mussel has become a major problem in New York waterways.

B. Relationships in an Ecosystem

1. In every ecosystem, populations of different species are linked together in a complex web of interactions.

a. Sometimes these relationships are competitive (organisms act against each other) and occasionally they are cooperative (organisms help each other).

i. Cooperative Relationships:

  • Mutualism – a relationship between two organisms in which both are helped. Example: Termites have single-celled organisms living in their intestines. These unicellular organisms help the termites to digest their food. The single-celled organisms are helped because they gain a place to live and plenty of food, and the termites can make use of a food supply that they would not be able to digest without this cooperative relationship. Another example: Lichens are made of algae and fungus. The algae cells make food for themselves and the fungus through photosynthesis. The fungus holds water and minerals that they both use. They live better together than either could live alone. Lichens can be found on rocks or tree bark.
  • Commensalism – a relationship between two organisms in which one is benefited and the other is unaffected. For example: A remora is a small fish that attaches to other larger fish, such as sharks. The shark carries the remora for a free ride while the remora eats leftovers that the shark does not eat. It does not hurt the shark.

ii. Competitive Relationships:

  • Parasitism – a relationship between two organisms in which one is harmed and the other is helped. The organism that is harmed is called the host. For example: A mosquito feeds on an animal’s blood. The animal would be the host organism.

Another example: Bacteria live in the lungs of humans and cause pneumonia. Humans would be the host organism.

2. Food Chains – shows what eats what but only in a straight line relationship.

a. Organisms are described partly based on how they get their food:

i. Autotrophs or Producers – organisms that produce their own food; they are the source of energy for all other living things on Earth.

ii. Heterotrophs or Consumers – must acquire food (energy) by eating other organisms.

  • Herbivores – heterotrophs that survive on plant tissues.
  • Carnivores – heterotrophs that eat other animals.
  • Omnivores – heterotrophs that eat both plantsand animals.

iii. Decomposers – an organism, generally bacteria or fungus, that eats dead organisms and organic wastes.

  • Decomposers recycle materials that are then reused by producers.

iv. Scavengers – animals (such as vultures) that eat dead organisms.

  • They are natures “clean up crew.”
  • Scavengers are NOT decomposers. Dead organisms and wastes still have to be broken down by decomposers after scavengers eat them.

v. Parasites – organisms that attack other live organisms (called host organisms), but rarelykill them.

  • Parasites usually live on or in the body of their host. For example, ticks may live on a dog and also feed on its blood.

3. Food Webs are web-like diagrams that show the more complex feeding relationships among producers, consumers, and decomposers.

a. Normally, each organism feeds on more than one kind of organism. Because organisms normally have more than one food source, food chain diagrams are too simple.

b. The food web shows the many interconnected food chains that exist in the ecosystem. How many food chains do you see in the food web below? Answer: 3 food chains

c. Organisms have several food choices, so ecosystems can sometimes remain stable even when one population shows a major decline in numbers (because the consumers of these decreasing organisms may be able to pick another food choice).

i. The consumers that feed on the declining population simply rely more heavily on one of their other food choices until the declining population recovers.

III. Energy Flow Through An Ecosystem

A. Introduction

1. Almost all organisms use energy, either directly or indirectly, from the sun to power their life processes. (Reminder: Life processes include nutrition, excretion, transport, synthesis, growth, respiration, regulation, and reproduction.)

a. Producer organisms capture energy from the sun and store it in the chemical bonds of the food molecules they make.

2. The energy producers get from the sun is eventually used up or lost from the organism in (2) two ways:

a. The organism uses up some energy while performing its own life processes.

b. As the energy in food is converted to the useable form of energy known as ATP, most of the energy is converted to heat and is lost to the environment.

3. If the organism is a producer, it must make food (through photosynthesis) continually. If the organism is a consumer, it must eat foodcontinually. Why?... because its energy is:

a. Used during life processes.


b. Lost to the environment as heat.

4. Only the energystored in the body tissues of each organism is passed on to the next consumer in the food chain.

5. Because of this loss of energy, most of the original stored energy is lost in just a few steps of the food chain.

6. For this reason, food chains are usually quite short.

7. An energy pyramid is a diagram that shows the transfer of energy through a food chain or food web.

a. Each block of the energy pyramid represents the amount of energy that is available to the organisms in the next higher block.

b. Producer organisms contain the most energy because they capture energy directly from the sun and store it in the chemical bonds of the food molecules they make.

c. Each level is smaller due to the loss of heat as the organisms carry on their life activities.

d. In spite of this constant loss of energy to the environment, life continues because the suncontinues to provide energy.

Use the diagrams below to answer the questions that follow.


1. List the letters of the above diagrams in order from producers to secondary consumers in a food chain. __c__, __a__, __b__.

2. Write the letter of the diagram from above that best matches each of these phrases:

a. __c__ needs sun’s energy to make food.

b. Consumers are __a__ and __b__.

c. A producer is __c__.

d. A primary consumer is __a__.

e. A secondary consumer is __b__.

f. __b__ gets the least energy available.

3. Count the numbers of living things in each of the pictures above and record the numbers here: Rabbits __4__, Fox __1__, Carrots __7__.

4. On the blanks at the left of the pyramid, write the names of the living organisms from above. On the arrows at the right of the pyramid, write each of these terms: producer, primary consumer, and secondary consumer.

fox secondary consumer

rabbits primary consumer


5. Explain why the pyramid has fewer things living at the top instead of at the bottom. There is less energy available to them for survival______.

6. Where does the energy go? It is used during life processes and lost as heat to the environment______.

7. Explain why the pyramid does not have an arrow showing energy going back down to the bottom. Energy only moves in a one way direction__.

B. Recycling and Reusing Materials

1. Decomposers (bacteria and fungi) extract the last bit of energy contained in dead organisms (as well as the energy in their waste products) and use it to carry out their own life processes.

a. In the process of doing this, decomposers return the raw materials (contained in the once-living organism) to the soil.

b. Decomposition – the process of breaking down dead organisms, and their waste products, into raw materials and returning those raw materials to the ecosystem.

c. Examples of decomposers are bacteria and fungi.

2. Because of the actions of decomposers, the atoms and molecules in living things cycle through both the biotic (living) and abiotic (nonliving) parts of the biosphere.

a. Chemical elements such as carbon, hydrogen, oxygen, and nitrogen, (the four most abundant elements that make up the bodies of living things: H,O,N,C) pass through food webs and are combined and recombined in different ways in different living organisms.

b. For example, plants trap carbon dioxide and water molecules in energy rich compounds (such as glucose) during the process of photosynthesis. When plants need energy for their life activities or are eaten by a consumer, these molecules may be broken down and used by the organism. During cell respiration, molecules (such as glucose) are broken down by the cells and returned to the environment in the form of carbon dioxide and water (and energy is released).

c. Photosynthesis:

energy and carbon dioxide and water produce glucose and oxygen

energy + CO2 + H2O  C6H12O6 + O2

d. Cell Respiration:

glucose and oxygen produce energy and carbon dioxide and water C6H12O6 + O2  energy + CO2 + H2O

e. The glucose and oxygen that are produced by photosynthesis is needed for cell respiration. The carbon dioxide and water that is produced by cell respiration is needed for photosynthesis.

f. Energy is NOT recycled but the molecules are.

IV. Differences Among a Species are a Good Thing!!

A. Biodiversity – describes how many different species are present in one ecosystem. If there are a lot of different kinds of species in one ecosystem, then there is a lot of biodiversity in that ecosystem.

1. The greater the biodiversity of an ecosystem, the more stability the ecosystem has. For example:

a. A natural forest contains many different species of trees. If disease or insects attack one population of trees, nearby trees of another species are likely to survive. In this example, there will still be trees left in the forest for organisms to use, which preserves the ecosystem.

b. In addition, the mix of species in the ecosystem makes it difficult for disease organisms to move quickly through this environment. Here biodiversity slows the spreading of disease.

c. In contrast, on a tree farm where all of the trees planted are of a single species (low biodiversity), the entire population could be seriously damaged by a single

disease or insect attack, leaving no trees left for the organisms to use. This situation seriously harms the stability of the ecosystem.

2. The interactions between organisms may allow an ecosystem to remain stable for hundreds or thousands of years.

3. In established, stable ecosystems, populations tend to increase and decrease in size in a predictable pattern. Over time, however, the size of the population remains relatively stable. For example:

a. When the prey population increases, a large food supply causes the size of the predator population to increase. Because each predator requires many prey to meet its energy needs, the prey population rapidly decreases.

b. With the decline in a prey population, some of the predators begin to starve. When only a few predators remain alive, the prey population reproduces and greater numbers of prey survive; this causes the prey population to increase in number. Then the cyclerepeats itself.

4. The loss of biodiversity in an ecosystem upsets the stability of the ecosystem.

a. Removing species from an environment often causes instability due to the loss of organisms that were filling critical ecological niches.

5. Many species may be lost when natural disasters or human activities cause habitats to be destroyed.

a. Clearing large areas of tropical rain forest has disrupted many ecosystems; some may never recover.

i. Although some species may be able to return to a damaged ecosystem, others with critical roles may be totally lost.

ii. Different populations of organisms may have depended on each other so much in the original ecosystem that the ecosystem may never be restored to its original state if biodiversity is lost.