Ecosystems and How They Work - AP Environmental Science Review

You should already have a good background vocabulary in some basic ecological terms. This will serve as a review and will also introduce some new terms.

Review text: Chapter 4, pages 81-107

Chapter 5, pages 108 - 131

Chapter 8, pages 186 – 205

Chapter 9, pages 206 - 233

ECOLOGY: Is the study of the relationships between organisms and their environments.

- it describes and explains how organisms interact with other organisms and with their non-living environments.

- NO organism lives completely on it's own, IT depends on other organisms and other non-living factors that affect it.

e.g. FISH - depends on plants for most of the oxygen it needs

- during respiration the fish produces carbon dioxide that the plant needs for P/S

- the fish feeds on small plants and animals

- waste products from fish provide nutrients that both other plants and animals need

- the plants provide hiding places for fish and places where the fish can lay their eggs

- the fish may serve as food sources for other animals ,e.g., mammals, turtles, etc.








NO organism can live completely on it's own.
















POPULATION: A group of individuals of the same species. The same area may have several populations.

COMMUNITY: Is all the living things in an area. For example, a pond community may contain populations of Canada Geese, ducks, frogs, water lilies, etc.

BIOME: Is a large geographic area with a characteristic climate. The world has 13 main biomes. Canada has seven biomes.

Tundra, Boreal Coniferous Forest, Temperate Deciduous Forest, Grassland, Mountain, Great Lakes Forest Ecotone, Chapparal.

Each biome has a characteristic set of plants and animals and is made up of several communities.












BIOSPHERE: Is the region on earth in which all life exists, from the lower atmosphere to the bottoms of oceans. The biosphere is made up of biomes.


ECOSYSTEM: Is an interacting system that consists of groups of organisms (biotic) and their non-living (abiotic) environment.

The abiotic part includes water, soil, temperature, light, wind, rain,etc.

CLIMATE is the overriding factor that determines the general nature of an ecosystem. If the climate is always hot and dry the soil will likely be sandy. This will in turn require plants and animals that can live in desert conditions.




Thus, if plants in a lake change, the animals, the soil on the bottom, the amount of oxygen in the water, and all other factors will be affected. The CHAIN OF EVENTS when one factor in an ecosystem is changed is long and complex.

An ecosystem can be any size. Freshwater ecosystems range in size from a fishbowl aquarium to ponds and lakes.

HUMANS DO NOT OFTEN THINK ABOUT THE ECOLOGICAL CONSEQUENCES OF THEIR ACTIONS. For example, building a weir (small dam) in a river to produce trout habitat so that humans could fish for trout can have devastating consequences.

- it worked only downstream from the weir

- the water gained oxygen as it rushed over the weir

- the water became cooler due to increased evaporation

- the faster, more turbulent water scoured the bottom, washing away fine sediments and leaving cobble and gravel

All these conditions were desirable for trout, but upstream from the weir, the water was trapped and became slow moving and warmer. These conditions are not good for trout. What, then, was accomplished by building the weir?



HABITAT: The habitat of an organism is the place in which it lives. An ecosystem such as a lake has many habitats .e.g., trout live in deep, cool water while the habitat of a sunfish is the warm water shallows at the edge of the lake. HABITATS may overlap, for example, a trout may visit warm water to feed on smaller fish or to escape a predator.

NICHE: The niche of the organism is its total role in the community. For example, the niche of frogs in a pond is to feed on insects and to become food for snakes and other animals.

Think of HABITAT as the address of the organism and the NICHE as the organism's occupation or job.

THE NICHE OF AN ORGANISM DECIDES IT'S MAIN HABITAT.e.g. Sludgeworm feeds on dead decaying organic matter, that is common only on the bottoms of small lakes and streams, ponds, etc. It is however, uncommon on the bottoms of fast lakes and streams.



COMPETITION: If two organisms have the same habitat and similar niches, they will compete with one another. Often competition causes one organism to leave the habitat, but remain in the same general area. For example, red-winged blackbirds occupy the shallow area near the shore of a marsh while yellow-headed blackbirds occupy deeper areas. By splitting up the marsh in this way, the birds avoid competing in the same habitat.

Interspecific Competition: competition vs. other species. E.g.?

Intraspecific Competition: competition vs. own species. E.g.?



Ecologists call feeding levels trophic levels.









Can a first-order consumer also be a second-order consumer?

OMNIVORE: Animals which are herbivores and carnivores.

Carnivores that live on animals are called PREDATORS and the animals they are eating are called the PREY.

SCAVENGERS: Are animals that feed on dead organisms. In some cases, such as a bald eagle, you can be a predator one day and a scavenger the next day. Snails and crayfish in ponds eat both dead plants and animals are called scavengers.

SAPROPHYTES: Fungi and bacteria that feed on dead organisms.

DECOMPOSERS: These organisms are mainly bacteria and fungi such as yeast and moulds. They breakdown and feed-on dead organic matter and return valuable nutrients to the water (or soil). If decomposers were not present, aquatic ecosystems would soon clog up with wastes and dead organisms. Once nutrients are returned to the water, they can be used again to help producers grow.








Food Chain - feeding sequence in which each kind of organism

eats the one below it in the chain.










In most ecosystems the interconnections between living things are more complex than those found in a simple food chain. Most plants are eaten by many different herbivores,

and most herbivores are eaten by more than one type of carnivore, etc.




Food Web - Interconnecting of food chains, since organisms are often in more than one food chain.















Many unusual relationships exist between organisms. One of these is called symbiosis. Symbiosis means "living together" and it is defined as a close association between two organisms of different species in which at least one of the two benefits. There are three kinds of symbiosis: (a) Parasitism, (b) Mutualism and (c)Commensalism.


(a) Parasitism: Is a symbiotic relationships between two organisms in which one organism benefits and the other suffers harm. The organism that benefits is called the "parasite" while the one that is harmed is called the host. e.g., frogs and turtles often have leeches attached to them.

(b) Mutualism: Is a symbiotic relationship between two organisms in which both organisms benefit.e.g., one species of hydra is green on the inside because it contains green algae in its interior. The hydra provides the algae with a secure place to live while the algae supplies the hydra with food made by photosynthesis.

(c) Commensalism: Is a symbiotic relationship in which one organism benefits and the other organism neither benefits or suffers harm.e.g., freshwater turtles can be seen with green hair (algae) on their backs. It needs keratin (present in turtle shells) and does not harm nor benefit the turtle.




Food Pyramid of Numbers - A pyramid based on the number

of organisms at each trophic level. It is ideal for

studying how many organisms a given ecosystem can support.

It is used in population studies.

NOT ALL pyramids have a regular shape. Think about the hundreds of mould organisms that feed on a dead fish! Although this is a simple pyramid, it is not used much by ecologists because it only treat all organisms in terms of numbers while size can play an important role.


Food Pyramid of Biomass - Based on the total mass of

organisms at each trophic level. It makes more sense to talk about the masses of organisms at each trophic level than about numbers. For example, you don't care about the total number of potatoes you ate for dinner; rather you care about the total amount or mass of potatoes.

Problem - This system implies that equal masses of organisms have equal energy contents, which is not so. Different types of tissues have different energy contents. The most noticeable difference occurs between plant and animal tissue. An organism can usually obtain about 20% more energy eating 1 gram of animal than by eating 1 gram of plant.

Food Pyramid of Energy - Shows how much food energy is

available at each tropic level in a particular ecosystem.

Since organisms consume other organisms for energy, this

pyramid best represents the relationships between trophic


Since energy is lost along a food chain, ALL pyramids of energy look like the following example:











This shows that the energy tapers off to almost nothing.




ONLY a small percentage of energy is transferred to the next trophic level, since each organism uses most of the energy it receives. Once used, ENERGY cannot be recycled. LESS THAN 10 % OF THE ENERGY TAKEN IN IS AVAILABLE AT THE NEXT TROPHIC LEVEL.











How does energy get into an ecosystem?

How does it reach every organism in the ecosystem?

Where does the energy go?


ALL LIVING THINGS NEED MATTER AND ENERGY. The matter is needed to make new cells and repair worn-out parts. The energy is needed to power life processes such as movement and growth.


ECOSYSTEMS AND ENERGY PATHWAYS:All living things need ENERGY to carry out the daily activities of life. We get our ENERGY from the food we

eat. All food webs are based on PRODUCERS which collect SOLAR ENERGY from the sun and convert it to CHEMICAL ENERGY VIA A PROCESS KNOWN AS PHOTOSYNTHESIS!










THIS CHEMICAL ENERGY is stored in plant structures such as roots and seeds, where it is usually called FOOD ENERGY. This FOOD ENERGY is first used by the plants themselves for their own life functions (cell respiration). When herbivores eat these plants, this stored plant energy is transferred to them. Likewise, when a carnivore eats a herbivore, the stored energy in herbivores is transferred to the carnivores.



Energy Flow is One-Way: The passing of energy along a food chain is not very efficient. A great deal of energy is lost at each trophic level. e.g.,

Thus, energy is one-way along a food chain. For an ecosystem to keep operating, energy must always enter it from the sun.


An ecosystem needs not only energy in order to function, it also needs MATTER. Matter is used for life processes such as growth. This matter is called NUTRIENTS. Some examples of nutrients you require include: oxygen, nitrogen, magnesium, carbon, hydrogen, etc.


Nutrients flowing through a food chain is not lost, as in the case of energy, it is RECYCLES back to the producers. Producers get their nutrients from the soil, water and air. Herbivores get these nutrients when they eat producers. Carnivores get the same nutrients when they eat herbivores. The decomposers break down animal wastes and dead organisms and release these nutrients back into the soil, water and air so that they can be reused by producers. The path each nutrient follows is called a NUTRIENT CYCLE.

The Water Cycle:









Water vapour enters the atmosphere through transpiration from vegetation. It also enters the atmosphere by evaporation from bodies of water and the soil. In the cool upper atmosphere, this water vapour condenses, forming CLOUDS. In time, enough water vapour collects in the clouds to cause precipitation. When this occurs, some of the water runs along the surface of the ground to a stream, pond or other body of water. This water is called surface runoff. The water that soaks into the ground does so by the process of percolation. Some of this water gradually percolates to the bedrock. It then becomes ground water and gradually runs back into lakes and other bodies of water. Some of the water in soil moves up to the roots of plants by capillarity. Upon decomposition of plants and animals, water present in their tissues is released into the atmosphere.


The Carbon Cycle:

Carbon is another important nutrient that all organisms need. Carbon is present in the atmosphere as carbon dioxide. Producers use CO2 via P/S to make food. Now the carbon is in the producers. Herbivores eat the plants and they in turn are eaten by carnivores. Now the carbon is in the animals. Both plants and animals respire. During respiration, CO2 is returned back into the atmosphere. Decomposers also return CO2 to the atmosphere by decomposing wastes and dead organisms.

LATELY the carbon cycle, due to the increased burning of fossil fuels and the deforestation of rain forests, is not in balance. CO2 is being added to the atmosphere faster than producers can remove it. Ocean water holds over 50 times as much CO2 as does the air.





The Nitrogen Cycle:

Nitrogen is required by all living organisms in order to make proteins. Almost 78% of the atmosphere is nitrogen. However, in order for nitrogen to be absorbed in must be in the form of nitrate (NO3). In this form, plants can absorb nitrogen. The changing of nitrogen to nitrates is called NITROGEN FIXATION and is accomplished by certain types of bacteria and blue-green algae, as well as lightening. Plants use these nitrates to make proteins, which an animal will receive when it eats the plant. When plants and animals die, bacteria change their nitrogen to ammonia (NH3), which has a very distinct and pungent odor. This ammonia is in turn converted to nitrites (NO2) and then to nitrates (NO3) by bacteria. As well, some bacteria convert nitrates and nitrites to nitrogen to complete the cycle.










The Phosphorus Cycle:

Phosphorous is another important nutrient which is important to all living things. For example, adenosine triphosphate, ATP, is a phosphorous bearing compound that plays a key role in energy storage and supply in a cell. Phosphorous normally occurs in water and soil as inorganic compounds. Phosphates (PO4) are a common form. These compounds are absorbed by plants and used to make organic compounds such as ATP. When animals eat plants, phosphorus is passed on to them. When dead organisms or waste decays via bacteria, organic forms of phosphorous are released into the water or soil. Bacteria decompose these organic forms into inorganic forms.















Succession is the gradual replacement of one community of living things by another. All ecosystems, whether on land or in the water will undergo a gradual change with time. For example, a freshly dug pond will have a bare bottom. However, as time passes, plants gradually colonize the bottom. Thus, changes in the animal community and in the abiotic environment accompany the changes in the plant community.

Suppose you sat in a boat at the center of a freshly dug pond for hundreds of years. The pond would undergo a complete succession and the following is what would likely happen:


Since the pond is quite deep, sufficient light would not be able to reach the bottom, thus, the bottom would consist of nothing but parent earth material.



As the years pass, soil runoff from the land and the decay of dead organisms would add soil to the bottom. The pond would gradually become more shallow so that green plants can grow on the bottom. These submergent or underwater plants would soon cover the bottom of the pond. Among these plants you would have Chara (stonewart), Elodea (Canada waterweed), Ceratophyllum (coontail), and Cabomba (fanwort).

The submergent plants on a pond or lake are of great importance to ecosystems. They provide:

- a habitat for a wide range of organisms

- food for many organisms

- oxygen for the water


Over the years, the decay of the submergent plants adds further humus to the bottom of the pond. In time, the pond is shallow enough that floating-leafed plants such as water lilies can grow. These plants in turn make the pond even more shallow. Now it can support the growth of emergent plants such as cattails, rushes and sedges. Such plants are called emergent because part of them sticks out or emerges form the water. The pond is now called a REEDSWAMP.

All emergent plants are flowering plants. Two species of emergent plants that you might be familiar with include Vallesneria (wild celery) and Potamogeton (pondweed).

Generally this group of plants is most important to waterfowl such as ducks, geese, swans, woodcocks and other birds. They feed off the leaves and seeds of some of these plant species.

Some potamogetons have broad leaves which float on the surface. These normally occur among other floating-leafed plants such as white water lilies (Nymphaea), yellow water lilies (Nuphar) and water shield (Brasenia). These plants provide a unique habitat for a wide variety of animal species.

In calm stagnant areas of ponds and lakes, an interesting group of plants called the duckweeds can be found. These plants float on the surface of the water and produce tiny flowers while their roots hang below the surface. Duckweeds often form a mat on the pond which is so thick that birds can walk on it.


Three other common types of emergent plants include the bulrush, the spike rush and the sedge. These can often be found near the shores of ponds and lakes. From a distance all of these look just like tall grasses. All three plants provide food for a wide variety of wildlife including moose, beaver, muskrat, elk, deer, grouse and waterfowl.


Four other common types of emergent plants also include the pickerel weed, Calla, Alisma and the cattail which can live in water just a few centimeters deep. All of these plants form a habitat for a wide variety of amphibians and reptiles as well as providing food for many species of wildlife.


The decay of emergent vegetation still fills the pond even further. Now no open expanse of water remains. The reedswamp has changed into a marsh. As the islands of land become drier, shrubs may begin to grow on them. The marsh has changed into a carr. As the land becomes still drier, trees begin to grow. The carr has become a swamp.


You can usually see this succession on observing any pond. Look carefully at the first three stages of succession as you look at the pond along a line that runs from the deepest spot to the edge.















Changes in other organisms accompany changes in plants during succession. In the pioneer stage most of the organisms are PLANKTON (the word means small, drifting organisms). Most plankton are microscopic and some are producers while others are consumers. The plankton that are producers are called PHYTOPLANKTON. Most of these are algal protists such as diatoms and flagellates. The plankton that are consumers are called ZOOPLANKTON. Some of these are protozoan protists while many others are animals such as rotifers and tiny crustaceans. The zooplankton are mainly herbivores feeding on phytoplankton. Then these in turn become food for carnivores such as insects and fish.

The submergent vegetation stage is dominated by plants that grow entirely underwater. Most animals living in this habitat are adapted for life in a submerged stage. Most are gill breathing insect larva such as mayflies, damselflies and dragonflies. In most ponds the submergent stage is an important source of oxygen and food for the pond ecosystem.

Plants that grow partly in and out of the water dominate the emergent vegetation stage. It is interesting to note that many of the animals that live in this vegetation are also adapted to a life that is partly aquatic and terrestrial. Amphibians such as frogs and toads are one example while reptiles such as snakes and turtles are another such example.



Five things gradually happen as succession proceeds:

- species diversity increases

- population numbers increase

- the number of niches increases

- the total biomass of the ecosystem increases

- the amount of organic matter (living and non-living) increases

All of these things add to the complexity of the ecosystem. This complexity makes the ecosystem more stable. Many plants and animals will have numerous niches which means more food chains and food webs will be formed. Thus there is less chance of the entire community collapsing if one or two species disappear.

Terrestrial Succession: Primary Succession












Secondary Succession: