B4 ecosystems

1. Organisation in ecosystems

An ecosystem is made up of all the living organisms and physical conditions in an area.  The organisms within the ecosystem are called the community and the area in which they live is the habitat.  The total number of organisms of each species is known as a population.

The organisms in a community and can be divided into three groups:

• Producers: organisms that make their own food by photosynthesis, e.g. green plants and algae.

• Consumers: organisms that cannot make their own food.  They eat other organisms to gain their energy. All animals are consumers.

• Decomposers: Consumers that feed on dead or decaying material to gain energy e.g. bacteria and fungi.

2. Energy Transfer

Light energy from the sun is converted into chemical energy in green plants (producers) during photosynthesis.  The glucose produced in photosynthesis can then be converted into carbohydrates, fats and proteins, which are used as energy stores, and for growth and repair.  As an organism grows it increases its biomass.  This is the mass of living material present.

Consumers then eat producers and the energy is transferred to the consumers.  The energy transferred can be used by the consumers for growth and their biomass increases.

3. Food chains

A food chain shows the feeding relationships between several different living things.  The arrows in a food chain point from the food to the feeder and show the direction of energy transfer (and therefore transfer of biomass).  Each step in the food chain is a trophic level.

Consumers are further classified by their position in a food chain.

A green plant (producer) is eaten by a herbivore.  The herbivore is

known as the primary consumer.  The herbivore can be eaten by a

carnivore.  The carnivore is called a secondary consumer, it is also

a predator.  Predators are animals which feed on other animals.

Prey are the animals eaten by the predators.

4. Food webs

There are many different species in an ecosystem, and lots of possibilities for food chains. These can be shown by a food web.  A food web is a number of interconnected food chains in the same ecosystem.  All the species within a food web are interdependent so, if there is a change in one species all the others are affected.

5. Factors affecting ecosystems

The factors that affect an ecosystem are divided into two groups.

• Biotic factors: living factors, e.g. predators and availability of food.

• Abiotic factors: non-living factors, e.g. temperature, light intensity, moisture level and pH of soil.

6. Effect of abiotic factors on communities

• Temperature: has its greatest effect on the enzymes that control metabolic reactions.  Plants and cold-blooded animals (e.g. lizards) develop more rapidly in warmer temperatures.

• Light intensity: light is required for photosynthesis.  The greater the light availability, the greater the growth and therefore success of a plant.

• Moisture level: a lack of water leads to death for most plant and animal species.  Most plants wilt as water is required to keep their cells turgid, which keeps plants upright.  Water is also required for photosynthesis.

• pH of soil: affects biological activity in soil (e.g. growth of roots, germination) and the availability of certain minerals.  Some plant species grow better in acidic soils while others grow better in alkaline soils.

7. Effect of biotic factors on communities

Biotic factors refer to the interactions between living organisms.  Competition is the most common biotic factor as organisms compete for many resources.

Plants compete for light, water, carbon dioxide, minerals and space.

Animals compete for food, water, space (territory), shelter and breeding partners.

If materials are limited, plant and animals have to compete for these resources.  This may result in weaker competitors dying or leaving that area.  Weaker plant species will often die.

Competition has a direct effect on the size of a population.  For example, if there is a large amount of food available there will be less competition for food and the population size will increase.

8. Interdependence

Interdependence is the ways in which different organisms depend on each other within a community.  There are three main types of interdependent relationships: predation, mutualism and parasitism.

• Predation: relationship between a predator and a prey species.  The size of the predator population directly affects the size of the prey population.  For example, Canadian lynx (predator) and snowshoe hare (prey).

• Mutualism: both organism benefit from the relationship.  For example, oxpeckers are small birds that live on buffalo.  They eat ticks and fleas living on the buffalo’s skin.  They gain food, while the buffalo is free from irritation and potential disease.

• Parasitism: only one organism (the parasite) gains.  The organism it lives off (the host) suffers.

9. Pyramids of numbers and biomass

Pyramids of numbers show the number of organisms at each trophic level of a food chain.

Sometimes pyramids of numbers are not pyramidal in shape.

The “caterpillar” bar in this diagram is longer than the “tree” bar on which they feed because one tree can feed many insects. However, when drawn as a pyramid of biomass the pyramidal shape is restored.

A pyramid of biomass shows the mass of living material at each trophic level of a food chain.  The producer is placed at the base, with the next trophic levels placed above.  The bar width represents the biomass of organisms present.

The biomass will decrease as you move up each trophic level of a food chain therefore they are almost always pyramids

10. Energy flow and loss

Every time one organism eats another, energy is transferred from the food to feeder, as shown by the arrows in a food chain. However, very little energy (10%) is passed on to the next trophic level. Only energy that is used for growth (i.e. producing new cells and thereby increasing biomass) is available to the next trophic level in the food chain.

Most energy available at each trophic level of the food chain is not passed on to the next trophic level. Some of the energy is lost through egestion (undigested waste), excretion (loss of waste products e.g. urea in urine) and respiration (e.g. energy used for movement).

11. Efficiency of biomass transfer

The efficiency of biomass transfer is so low, it limits the number of trophic levels in a food chain.  Very few food chains have more than four trophic levels as not enough energy (biomass) can be transferred to sustain essential life processes at a fifth trophic level.

12. Calculating efficiency of biomass transfer

Use the following formula:

Efficiency of biomass transfer (%) =biomass available after the transfer

biomass available before the transfer

Ensure the biomasses are both in g or both in kg.

Example:

A lamb gains 12 kg in mass after consuming 150kg of grass.  Calculate the efficiency of biomass transfer.

Efficiency of biomass transfer (%) = 12kg

150 kg

= 8%

13. Nutrient cycles

Materials pass between the biotic (living) and abiotic (non-living) components of an ecosystem in a constant cycle.  As living things grow they take in materials, including carbon and nitrogen, to use in their bodies.

14. Carbon cycle

The process of carbon moving between the biotic and abiotic world is called the carbon cycle.  All the major molecules in organisms contain carbon, e.g. carbohydrates, proteins, fats and DNA.

• Photosynthesis

Carbon dioxide is absorbed by plants from the atmosphere and built into carbohydrates (e.g. sugars), fats, proteins and DNA, which it uses to grow.

• Respiration

Both plants and animals respire.  This returns some carbon dioxide back to the atmosphere.

• Decomposition

When plants and animals die, soil bacteria and fungi known as decomposers digest their bodies in the process of decay, and carbon dioxide is released back into the atmosphere through respiration.

• Fossil fuels

Not all plants and animal bodies will decay.  Some are buried under layers of silt and over millions of years begin to fossilise.  This forms fossil fuels, e.g. coal, oil and gas.  Fossil fuels are extracted and burnt to release energy.  Burning fossil fuels releases the carbon stored for millions of years as carbon dioxide.

15. Nitrogen cycle

Another element cycled between the biotic and abiotic world is nitrogen.  It is important to living things because it is used in two major groups of molecules – proteins and DNA.  Nitrogen as an element is very unreactive, but organisms can use nitrogen in compounds such as nitrates.

• Absorption

Nitrogen exists as nitrates in the soil, dissolved in water. These dissolved nitrates are absorbed by plant roots and used to make proteins.

• Decomposition

Decomposers return the nitrates found in dead plants and animals to the soil in the form of ammonia.

• Excretion

Animals also put nitrogen back into the soil in faeces and urea (in urine).

17. Importance of the water cycle

The water cycle is particularly important because it:

• determines the physical characteristics of a habitat, e.g. polar bears need water as ice to hunt and capture seals, fish need water to live.

• brings fresh water to people, animals and plants

• transports nutrients, which helps replenish them when they get used up.

18. The role of microorganisms in nutrient cycles.

A group of micro-organisms called decomposers play a vital role in the cycling of materials through an ecosystem.  These bacteria and microscopic fungi break down (decay) dead organisms and animal waste, including faeces and urine.  Through decomposition materials are released which can then be recycled.

19. Factors affecting rate of decomposition

Decay happens faster at certain times of the year, when decomposers have the right conditions to survive.