Wednesday, December 13, 2006

Topic 4.1 - Communities and Ecosystems

4.1.1 Define ecology, ecosystem, population, community, species and habitat.

  • Ecology - the study of relationships between living organisms and their environment.
  • Ecosystem - a community interacting with the abiotic elements in the environment
  • Population - a group of organisms of the same species occupying a specific location at a specific time
  • Community - a group of populations living and interacting with each other in a specific location
  • Species – Organisms of similar structure and function that can interbreed and produce fertile offspring.
    • Not species: Horses and Donkeys can breed to form mules, but mules are not fertile.
  • Habitat - the environment in which a a species normally lives or the location of a living organism
  • Biotic: Living organism in an ecosystem
  • Abiotic: Non living factors in an ecosystem
  • Biosphere: Air, water, soil on earth that supports life.

4.1.2 Explain how the biosphere consists of interdependent and interrelated ecosystems.

Define Biosphere: Air, water, soil on earth that supports life. Consists of a thin layer of independent and inter related ecosystems that cover the earth.


· Food chains or a food web. The food chain is a linear and simple feeding relation, where one organism has one type of food and is eaten by one type of organism. However, a food web is a more complex and it includes more variety of organisms, each of which can feed on a variety of other organisms and is fed upon by a variety of organisms.

· carbon dioxide produced by one ecosystem can be carried in the winds to another and there be used in photosyntheis. (humans and plants)

· Organic cycles such as the water cycle

· transpiratory return of water to the atmosphere

4.1.3 Define autotroph (producer), heterotroph (consumer), detritovore and saprotrophs (decomposer).

  • Autotrophs: Organisms that use an external energy source to produce organic matter from inorganic moledcules.
    • They also are called primary producers as they make their own food.

- Plants are Autotrophs as they convert solar energy from the sun into chemical energy stored in their biomass.

§ For communities based on consumption of pants – solar energy is the initial energy source

- Photosynthetic bacteria can also be considered Autotrophs

  • Heterotrophs; Organsims that use the energy in organic matter, obtained from other organisms.
    • Three types:

- Consumers: feed on other living organisms. They’re capable of taking a live organism, killing it and eating it.

§ Locusts, sheep, lions

- Decomposers

§ Detritivores feed on dead organic matter by ingesting it.

§ Dung beetles, earthworms, the bottom feeders

§ Saprotrophs feed on dead organic matter by secreting digestive enzymes into it and absorbing the products of digestion. Given time, they can completely break down any organic matter (including cellulose and lignin) to inorganic matter such as carbon dioxide, water and mineral ions.

§ Bread mould, mushrooms, fungi

4.1.4 Describe what is meant by a food chain giving three examples, each with at least three linkages (four organisms). 4.1.5 Describe what is meant by a food web. 4.1.6 Define trophic level.

Populations do not live in isolation – they live together with other populations in communities.

Community: A group of populations living together and interacting with eachother in an area.

Food Chain’s

  • linear and simple feeding relations, where each member in a sequence feeds on another. Each stage in a food chain is called a trophic level

Trophic levels

  • The position of an organism in a food chain. Every time you move up a level in the food chain you move to a different trophic level.
    • In a good chain, an organism can only occupy one trophic level. However, in a food WEB, an organism can occupy several.
    • Different trophic levels:
      • Producer (autotroph, this is where the food chain begins)
      • Primary consumer (eat the producers, herbivores)
      • Secondary consumer (eat the primary consumers, carnivores)
      • Decomposers not included???

food web

  • a diagram that shows all the feeding relationships in a community. Arrows are drawn to indicate the direction of energy flow.
    • They more complex than a food chain and include a larger variety of organisms. Each of these feed on a variety of other organisms and they are in turn fed on by more organisms.
      • (Therefore, if one species becomes extinct the ecosystem will still be able to exist.)

Food chains always start with photosynthetic producers (plants, algae, plankton and photosynthetic bacteria) because, uniquely, producers are able to extract both energy and matter from the abiotic environment

· energy from the sun

· 98% of their matter from carbon dioxide in the air

· 2% from water and minerals in soil

All other living organisms get both their energy and matter by eating other organisms.

Although this represents a “typical” food chain, with producers being eaten by animal consumers, different organisms use a large range of feeding strategies (other than consuming), leading to a range of different types of food chain.

Food chains can also only show some of the trophic relationships in a community.

Top carnivore

A consumer at the top of a food chain with no predators.


A consumer that eats plants or animals.


A human that chooses not to eat animals (humans are omnivores)


Microscopic marine organisms.


“Plant plankton” i.e. microscopic marine producers.


“Animal plankton” i.e. microscopic marine consumers.


An animal that hunts and kills animals for food.


An animal that is hunted and killed for food.


An animal that eats dead animals, but doesn't kill them


Dead and waste matter that is not eaten by consumers


Alternative word for detritus


An organism that consumes detritus (= detrivores + saprophytes)


An animal that eats detritus.


A microbe (bacterium or fungus) that lives on detritus.




Two organisms living together for mutual benefit.


Relationship in which only one organism benefits

4.1.7 Deduce the trophic level of organisms in a food chain and a food web.

4.1.8 Construct a food web containing up to 10 organisms, given appropriate information.

4.1.9 State that light is the initial energy source for almost all communities.

  • Light is the initial energy source for almost all communities.

4.1.10 Explain energy flow in a food chain. 4.1.11 State that when energy transformations take place, including those in living organisms, the process is never 100% efficient, commonly between 10-20%. 4.1.12 Explain what is meant by a pyramid of energy and the reasons for its shape.

Energy absorbed by living organisms is only available to the next trophic level if it remains as chemical energy in the growth of the organism.

Three things can happen to the energy taken in by the organisms in a trophic level:

  • It can be passed on to the biomass of the next trophic level in the food chain when the organism is eaten.
  • It can become stored in detritus. This energy is passed on to decomposers when the detritus decays. This happens when
    • Organisms die before being eaten
    • Some parts aren’t eaten such as hari and bones
    • Some parts are indigestible and pass out as feces
  • It can be converted to heat energy by
    • inefficient chemical reactions
      • cell respiration
    • radiated by warm bodies
    • through friction due to movement.
      • The heat energy is lost to the surroundings, and cannot be regained by living organisms.

These three fates are shown in this energy flow diagram:

Eventually all the energy that enters the ecosystem will be converted to heat, which is lost to space.

Any exchange of energy is never 100% efficient, and usually only 10-20% of energy is passed up along trophic levels.

  • Gross Primary Production = net production + energy lost through respiration
  • Net primary production = (energy through ingestion – energy lost through respiration and feces)
  • The energy lost via respiration and faeces = (energy gained through ingestion – NP)
  • the photosynthetic efficiency = (gross primary production / incident solar energy) x 100
  • The percentage of energy lost between ingestion by grasshoppers and ingestion by spiders = [(energy ingested by grasshoppers – energy ingested by spiders) / energy ingested by grasshoppers] x 100

To sum it up:

Explain how energy enters a community, flows through it and is eventually lost.

  • energy enters as light / sunlight;
  • trapped by plants / producers / autotrophs;
  • converted to chemical energy in photosynthesis;
  • passed to first consumers when they eat plants;
  • passed from consumer to consumer / passed along the food chain by feeding;
  • lost from the community as heat;
  • lost as a result of cell respiration/metabolism/movement;
  • approximately 90 % lost / 10 % passed on between trophic levels;
  • number of trophic levels limited by amount of energy entering into the ecosystem;
  • energy is lost between trophic levels as defecation/loss of feces/excretion;
  • passed to decomposers after death of organisms / parts of organisms;
  • energy is lost along trophic levels due to uneated parts


In general as you go up a food chain the size of the individuals increases and the number of individuals decreases. These sorts of observations can be displayed in ecological pyramids, which are used to quantify food chains. There are three kinds:

1. Pyramids of Numbers.

· show the numbers of organisms at each trophic level in a food chain.

· The width of the bars represent the numbers using a linear or logarithmic scale, or the bars may be purely qualitative.

· The numbers should be normalised for a given area for a terrestrial habitat (usually m²), or volume for a marine habitat (m³).

2. Pyramids of Biomass

· convey more information, since they consider the total mass of living organisms (i.e. the biomass) at each trophic level.

· The biomass should be dry mass (since water stores no energy) and is measured in kg m-2.

· The biomass may be found by drying and weighing the organisms at each trophic level, or by counting them and multiplying by an average individual mass.

· Pyramids of biomass are always pyramid shaped, since if a trophic level gains all its mass from the level below, then it cannot have more mass than that level

o (you cannot weigh more than you eat).

· The "missing" mass, which is not eaten by consumers, becomes detritus and is decomposed.

3. Pyramids of Energy

· Diagrams that show how much energy flows through each trophic level in a community

· kJ/m2/year

· Food chains represent flows of matter and energy, so two different pyramids are needed to quantify each flow. Pyramids of energy show how much energy flows into each trophic level in a given time, so the units are usually something like kJ m-2 y-1. Pyramids of energy are always pyramidal (energy cannot be created), and always very shallow, since the transfer of energy from one trophic level to the next is very inefficient The “missing” energy, which is not passed on to the next level, is lost eventually as heat.

Draw a clearly labeled pyramid of energy for a set of named organisms and explain its shape.

  • energy pyramids show amount of energy per trophic level in the community;
  • organisms without chlorophyll / consumers / heterotrophs eat producers / organisms
  • with chlorophyll / autotrophs to obtain energy;
  • each link of a food chain loses energy of movement and heat from the chain;
  • only the energy retained by the molecules of the organisms at the time it is
  • consumed can contribute to the next level of the pyramid / roughly 10 % of energy
  • available at each trophic level is converted into new biomass in the trophic level
  • above it;
  • pyramids of energy always have a normal pyramidal shape (unlike pyramids of
  • numbers or biomass);
  • this pyramid assumes that the ecosystem is in balance / that no food is being
  • transported into the system;

.4.1.13 Explain that energy can enter and leave an ecosystem, but that nutrients must be recycled.

A community and its abiotic environment function together as a system called an ecosystem.

· Recycling of nutrients is one example of the interaction between living organisms and the abiotic environment in an ecosystem.

o (Energy is not recycled. It is supplied to ecosystems in the form of light, flows through food chains and is lost as heat.)

· Nutrients are not usually re-supplied to ecosystems – they must be used against and again.

Nutrient Recycling

· Matter cycles between the biotic environment and in the abiotic environment.

  • Simple inorganic molecules (such as CO2, N2 and H2O) are assimilated (or fixed) from the abiotic environment by producers and microbes, and built into complex organic molecules (such as carbohydrates, proteins and lipids).
  • These organic molecules are passed through food chains and eventually returned to the abiotic environment again as simple inorganic molecules by decomposers. Without either producers or decomposers there would be no nutrient cycling and no life.

· The simple inorganic molecules are often referred to as nutrients.

· Nutrients can be grouped as:

o major nutrients (molecules containing the elements C, H and O, comprising >99% of biomass);

o macronutrients (molecules containing elements such as N, S, P, K, Ca and Mg, comprising 0.5% of biomass); and

o micronutrients or trace elements (0.1% of biomass). Macronutrients and micronutrients are collectively called minerals. While the major nutrients are obviously needed in the largest amounts, the growth of producers is usually limited by the availability of minerals such as nitrate and phosphate.

There are two groups of decomposers:

  • Detrivores are animals that eat detritus (such as earthworms and woodlice). They digest much of the material, but like all animals are unable to digest the cellulose and lignin in plant cell walls. They break such plant tissue into much smaller pieces with a larger surface area making it more accessible to the saprophytes. They also assist saprophytes by excreting useful minerals such as urea, and by aerating the soil.
  • Saprophytes (or decomposers) are microbes (fungi and bacteria) that live on detritus. They digest it by extracellular digestion, and then absorb the soluble nutrients. Given time, they can completely break down any organic matter (including cellulose and lignin) to inorganic matter such as carbon dioxide, water and mineral ions.
    • Without saprotrophs, butriets would remain locked permanently in dead organic matter and organsisms that need nutrients would die out.

The carbon cycle is one way nutrients are recycled.

4.1.14 Draw the carbon cycle to show the processes involved.

As this diagram shows, there are really many carbon cycles here with time scales ranging from minutes to millions of years. Microbes play the major role at all stages.

  • Far more carbon is fixed by microscopic marine producers (algae and phytoplankton) from CO2 dissolved in the oceans than by terrestrial plants from CO2 in the air.
  • During the Earth's early history (3000 MY ago) photosynthetic bacteria called cyanobacteria changed the composition of the Earth's atmosphere by fixing most of the CO2 and replacing it with oxygen. This allowed the first heterotrophic cells to use oxygen in respiration.
  • A large amount of the fixed carbon is used by marine zooplankton to make calcium carbonate shells. These are not eaten by consumers and cannot easily be decomposed, so turn into carboniferous rocks (chalk, limestone, coral, etc). 99% of the Earth's carbon is in this form.
  • The decomposers are almost all microbes such as fungi and bacteria. Most of the detritus is in the form of cellulose and other plant fibres, which eukaryotes cannot digest. Only a few bacteria posses the cellulase enzymes required to break down plant fibres. Herbivorous animals such as cows and termites depend on these bacteria in their guts.
  • Much of the CO2 that was fixed by ferns during the carboniferous era (300 MY ago) was sedimented and turned into fossil fuels. The recent mining and burning of fossil fuels has significantly altered the carbon cycle by releasing the carbon again, causing a 15% increase in CO2 in just 200 years.

4.1.15 Explain the role of saprotrophic bacteria and fungi (decomposers) in recycling nutrients.

See above.