OCR A-Level Biology: Ecosystems

What is an ecosystem?

All the interacting living organisms and the non-living conditions in a defined area and also the physical factors present in that area.

What is an abiotic factor?

The non-living conditions in a habitat.

What is a biotic factor?

The living components of an ecosystem e.g. presence of shrews and hedgehogs in a habitat and the size of their food source population.

What are five examples of abiotic factors?

1. Temperature

2. Light intensity

3. Edaphic (soil)

4. Water availability

5. Oxygen availability

How does temperature effect an organism?

It has an effect on the enzymes controlling metabolic reactions.

Plants will develop more rapidly in warmer temperatures, as will ectothermic animals.

Can cause changes in the seasons, e.g. trigger migration in some animal species, and hibernation in others or trigger leaf fall, dormancy and flowering.

How does water availability effect an organism?

Lack of water leads to water stress, which can lead to death.

Plants wilt as water required to keep cells turgid.

Also required for photosynthesis.

Xerophytes deal well with water stress.

How does oxygen availability effect an organism?

In aquatic ecosystems it is beneficial to have fast-flowing cold water as it contains high concentrations of oxygen.

If water becomes too warm or the flow rate too slow, drop in O2 concentration can suffocate aquatic organisms.

In waterlogged soil, air spaces between the soil particles are filled with water, reducing oxygen available for plants.

How do edaphic factors effect organisms?

- Clay: fine particles, easily waterlogged and forms clumps when wet

- Loam: this has different sized particles, retains water but does not become waterlogged.

- Sandy: coarse, well separated particles, allows free draining - sandy soil does not retain water and is easily eroded.

What are ectothermic animals?

An animal that is dependent on external sources of body heat.

What are endothermic animals?

An animal that controls its internal temperature and so are less effected by the external environment.

How is energy transferred to ecosystems?

Sun is the source of energy for almost all ecosystems on Earth.

Photosynthesis - light energy converted to chemical energy in photosynthesis.

Chemical energy transferred to other non-photosynthetic organisms as food.

What is a food web?

This shows us the transfer of biomass, and therefore energy, through the organisms in an ecosystem.

Each stage is known as trophic level.

What is the first trophic level?

Producer - an organism that converts light energy into chemical energy by the process of photosynthesis.

What composes the subsequent trophic levels?

Consumers, which obtain their energy by feeding on other animals.

Second trophic level is occupied by a primary consumer that eats a producer.

After this, a secondary consumer (eats a primary consumer), then a tertiary consumer and a quaternary consumer.

Why are food chains rarely longer than quaternary consumers?

As there is not sufficient biomass and stored energy left to support any further organisms.

What are decomposers?

Organism that breaks down dead organisms releasing nutrients back into the ecosystem. It turns organic compounds into inorganic ones available to photosynthetic producers in the ecosystem.

How can food chains be represented diagrammatically?

As a pyramid of numbers, with each level representing the number of organisms at each trophic level.

The producers are always placed at the bottom.

What is biomass?

Mass of living material present in a particular place or in particular organisms.

To calculate the amount of biomass at each trophic level, you multiply the biomass present in each organism by the total number of organisms in that trophic level - measured in grams per square metre for land and per cubic metre for areas of water.

How can biomass be represented diagrammatically?

As a pyramid of biomass.

This represents the biomass in an ecosystem at a particular time and does not take into account seasonal changes.

How do you measure biomass?

The easiest way is to measure the mass of fresh material present but the water content must be discounted and the presence of varying amounts of water in different organisms makes this unreliable unless very large samples are used.

Scientists usually calculate the 'dry mass' of organisms present but the organism has to be killed to be dried as it is placed in an oven at 80°C until all the water has evaporated - this point indicated by two identical mass readings.

To reduce harm to organisms, only a small sample is taken but this may not be representative of the population as a whole.

How is biomass measured?

In grams per square metre g m⁻² for land or grams per cubic metre g m⁻³ for water.

How does the efficiency of biomass and energy transfer differ between trophic levels?

Biomass in each trophic level is nearly always less than trophic level below.

When animals eat, only a small proportion of the food they ingest is converted into new tissue.

It is only this part of the biomass, energy, that is available for the next trophic level to eat.

How is the energy at each trophic level measured?

In kilojoules per metre squared per year, to allow for changes in photosynthetic production and consumer feeding patterns throughout the year.

What is ecological efficiency?

Efficiency with which energy or biomass is transferred from one trophic level to the next. The amount of energy or biomass converted to new biomass by each trophic level is represented by a pyramid of energy (= (energy or biomass available after the transfer/energy or biomass available before the transfer) x 100)

How can ecological efficiency be measured diagrammatically?

As a pyramid of energy.

Why do producers only convert 1-3% of the sunlight they receive into chemical energy?

- As not all of the energy available is used for photosynthesis - 90% is reflected, some is transmitted, and some is of an unusable wavelength.

- Other factors e.g. temperature, may limit photosynthesis.

- A proportion of the energy is used in photosynthetic reactions

What is gross production?

The total solar energy that plants convert to organic matter.

What is net production?

The remaining energy that is converted into biomass after gross production that is available to the next tropic level.

How is net production calculated and measured?

Net production = gross production - respiratory losses.

It is measured in g m⁻²yr⁻¹.

What is primary production?

The generation of biomass in a producer.

What is secondary production?

The generation of biomass in a consumer.

Why do consumers only convert about a tenth of the biomass in their food into their own organic tissue?

1. Not all of the biomass of an organism is eaten e.g. plant roots or animal bones.

2. Some energy is transferred to the environment as metabolic heat (due to movement and respiration).

3. Some parts of an organism are eaten but are indigestible (digested as faces).

4. Some energy is lost in excretory materials e.g. urine.

How can human activities manipulate biomass through ecosystems?

Agriculture - manipulating the environment to favour an animal or plant species that we can eat.

They are given the abiotic conditions they need to thrive.

Competition from other species is removed.

As is threat from predators.

How is minimum energy lost by agriculture in food chains?

It removes any intermediates between producer and consumer. Minimum energy is lost as there are fewer trophic levels present than in the natural ecosystem, so more energy is transferred to biomass for humans to eat.

Why does energy have to be recycled throughout ecosystems?

They are used up by living organisms and there is no large external source constantly replenishing nutrients in the way the sun supplies energy.

What is decomposition?

Chemical reaction in which a compound is broken down into simpler compounds or its constituent elements

How can organic material be converted into a more useable form by plants?

C and N cannot be used directly in the organic form it is in, so it must be processed into inorganic elements and compounds, which are a more useable form and returned to the environment by decomposers.

What are some examples of decomposers?

Microscopic fungi and bacteria, and some larger fungi.

Why is a decomposer a saprotroph?

As they obtain their energy from dead or waste organic material (saprobiotic nutrition). Through this process decomposers release stored inorganic compounds and elements into the environment.

They digest their food externally by secreting enzymes onto dead organisms or organic waste matter.

The enzymes break down complex organic molecules into simpler soluble ones that and the decomposers can then absorb these molecules.

What is the name given to another class of decomposers and how are they involved in decomposition.

Detritivores.

They are organisms which speeds up decay by breaking down detritus into smaller species, increasing the surface area for decomposers to work on e.g. woodlice that break down wood.

How is a detritivore different from a saprotroph?

Detritivores perform internal digestion, saprotrophs secrete enzymes onto dead matter to break them down and then absorb the more soluble products.

Why is nitrogen an essential element?

It is needed to make amino acids, and consequently proteins, and nucleic acids in plants and animals.

How can nitrogen be used by living organisms?

In its form in the air, nitrogen cannot be used by plants, so it needs to be combined with other elements like oxygen and hydrogen by bacteria to make it into a form that is useable by plants.

Without bacteria, nitrogen would quickly become a limiting factor in ecosystems.

What is nitrogen fixation?

The conversion of nitrogen gas to ammonium compounds, by combining nitrogen gas with hydrogen.

Which bacteria carry out nitrogen fixation?

Azotobacter (free living soil bacteria) and Rhizobium (live inside root nodules - mutualistic relationship), which contain the enzyme nitrogenase that allow for this conversion.

What are root nodules?

Growths on the roots of leguminous plants e.g. peas.

How is the relationship between Rhizobium and leguminous plants a symbiotic mutualistic one?

The plants gain amino acids from Rhizobium, which are produced by fixing nitrogen gas in the air into ammonia in the bacteria.

The bacteria gain carbohydrates produced by the plant during photosynthesis, which they use as an energy source.

What is nitrification?

Conversion of ammonium compounds into nitrites and nitrates. It is an oxidation reaction that occurs in two steps.

What are the two steps of nitrification?

1. Nitrifying bacteria (Nitrosomonas) oxidise ammonia compounds into nitrites (NO₂⁻)

2. Nitrobacter, another genus of nitrifying bacteria, oxidise nitrites to nitrates (NO₃⁻).

Nitrate ions are highly soluble and are the form in which most nitrogen can enter plants.

What is denitrification?

Conversion of nitrates to nitrogen gas e.g. in water logged soil by denitrifying bacteria (Pseudonomas denitrificans). The bacteria use the nitrates as a source of energy for respiration and nitrogen gas is released.

It is an anaerobic reaction.

What is ammonification?

The name given to the process by which decomposers convert nitrogen containing molecules in dead organisms, faeces and urine into ammonium compounds.

What is an example of a non-living process that can cause nitrogen fixation?

Lightning and the Haber process.

What is carbon an important component of?

All major molecules e.g. fats, carbohydrates and proteins.

What is a major source of inorganic carbon for plants?

Carbon dioxide in the atmosphere dissolved in the seas and oceans.

Carbon cycle

Carbon is a component of all organic molecules and as such is recycled through the environment by the processes of photosynthesis, feeding, respiration and decomposition.

Consumers feed on producers to obtain carbon macromolecules.

Both of them are broken down by decomposers at death, releasing carbon into the atmosphere as carbon dioxide when the decomposers respire.

Carbon can become trapped in oceans or bogs if it accumulates in areas where there are no decomposers, which over time forms fossil fuels, which when burned (combust) release carbon back into the atmosphere.

Why are atmospheric carbon levels higher at night than during the day?

As plants are not using carbon dioxide in photosynthesis but all organisms continue to respire.

Carbon dioxide levels also fluctuate with the seasons so that they are lower on a winter than summers day when rates of photosynthesis are higher.

Why have global atmospheric carbon dioxide levels increased?

Due to:

1. The combustion of fossil fuels - which has released carbon dioxide that was trapped for millions of years below the Earth's surface.

2. Deforestation - Removing significant quantities of photosynthesising biomass from Earth, so less carbon dioxide is removed from the atmosphere.

Why is carbon dioxide called a greenhouse gas?

As released carbon dioxide traps more thermal energy in the atmosphere contributing to global warming.

Describe the positive feedback loop between carbon dioxide emissions and carbon banks.

The higher the temperature, the less carbon dioxide dissolved in the seas and oceans, releasing more carbon iodide into the atmosphere - a positive feedback system.

What is succession?

The progressive replacement of one dominant type of species or community by another in an ecosystem, until a stable climax community is established

What is a climax community?

Final stage of succession, whereby the community is said to be in a stable state

Describe the two parts two succession.

1. Primary succession - in an area of land that has been newly formed or is exposed, such as bare rock. There is no soil or organic material present to begin with. Occurs due to volcanoes erupting, depositing lava, creating igneous rock, or when sand is blown by the wind or deposited by the sea to create new sand dunes, when silt and mud are deposited at river estuaries, and when glaciers retreat, depositing rubble and exposing rock.

2. Secondary succession - Occurs in areas where soil is present but it contains no plant or animal species e.g. bare earth that remains after a fire.

What are the stages of succession?

Each stage is called a seral stage.

At each stage, a new key species can be identified that change the abiotic factors, e.g. soil, to make it more suitable for the subsequent existence of another species.

1. Pioneer community

2. Intermediate community

3. Climax community

Describe primary succession

Begins by the colonisation of an inhospitable environment by pioneer species.

They arrive on the land by wind from nearby land masses or in bird or animal droppings.

e.g. algae and lichen.

What is a pioneer species?

The first organisms to colonise an area.

What adaptations do pioneer species have?

- Ability to produce large quantities of seeds or spores.

- Seeds that germinate rapidly

- Ability to photosynthesise to produce their own energy

- Tolerance to extreme conditions

- Ability to fix nitrogen from atmosphere.

What is an intermediate community and how is it formed?

1. When organisms die and decompose, organic products are released into the soil and this organic component is called humus.

2. Soil can now support the growth of new species of plant, secondary colonisers.

They arrive as spores or weeds e.g. mosses.

3. As environmental conditions continue to improve, new species of plant arrive like ferns. These are tertiary colonisers, with waxy cuticles to prevent water loss but they need to obtain most of their water and mineral salts from the soil.

4. At each stage, rock is eroded and mass of organic matter increases as more organisms decompose, allowing more soil water retention.

5. Abiotic conditions now favourable for small flowering plants, grass, shrubs then trees.

Multiple seral stages are involved here until a climax community is reached and at each stage, plants and animals are better adapted.

These organisms outcompete many of the species that were previously present and become the dominant species.

They are the most abundant species by mass present in the ecosystem at a given time.

Describe the biodiversity of the climax community.

The community has reached a stable state. There are a few dominant animal and plant species.

The species that make up the community depends on the climate.

It is often not the most diverse as it tends to reach a peak in mid succession, which decreases as dominant species out compete pioneer and other species, resulting in their elimination.

The more successful the dominant species, the less biodiversity in an ecosystem.

What is animal succession?

Primary consumers first colonise an area, moving in from neighbouring areas, so it is a slow process.

Secondary consumers arrive when a suitable food source is established and existing plant covers will provide them with a suitable habitat. These species also must move in from neighbouring areas.

What is a plagioclimax?

Stage in succession where artificial or natural factors prevent the natural climax community from forming.

What is a plagioclimax often caused by?

Agriculture:

1. Grazing and trampling of vegetation by domesticated animals so areas largely remain as grassland.

2. Removing existent vegetation like shrub land to plant crops - crop becomes the final community.

3. Burning as a means of forest clearance leads to an increase in biodiversity due to nutrient rich ash.

What is the distribution of organisms?

Refers to where individual species are found within an ecosystem. More are found where biotic and abiotic conditions favour organisms, so survival rate is high as all resources available and predation/pressure from consumers is low.

How is distribution measured?

By line transect - A line marked along the ground and samples are taken at specified points.

By belt transect (more information) - Two parallel lines are marked along the ground and samples are taken of the area at specified points.

Both are forms of systematic, non random sampling.

Allows for the study of how differing abiotic factors in different areas of the habitat affect the distribution of a species e.g. successional changes of plant species moving inland from sea in sand dunes.

How does the abundance of organisms fluctuate?

Immigration and births increases numbers.

Emigration and death decreases numbers.

Abundance refers to the number of individuals of a species present in an area at any given time.

What is a population?

A group of similar organisms living in an area at a given time. They are estimated but the sample is never entirely representative of the organism present in the habitat.

To increase accuracy, larger sample size used, reducing probability that chance will influence the result. A random sample will reduce effects of sample bias.

How is plant abundance measured?

Using quadrats placed randomly in an area:

Estimated number in population (m⁻²) = number of indiviuals in sample/area of sample (m²)

How is animal abundance measured?

In capture-mark-release-recapture technique:

1. Capture as many individuals as possible in a sample area.

2. Mark or tag each individual

3. Release the marked animals back into the sample area and allow time for them to redistribute themselves throughout the habitat.

4. Recapture as many as possible in the original sample area.

5. Record the number of marked and unmarked individuals present in the habitat (release all individuals back to their habitat)

Use Lincoln index to estimate population size.

How do you calculate Lincoln index?

Estimated population size = (Number of individuals in first sample × Number in second sample)/number of recaptured marked individuals.

Once abundance has been determined, what can be used to calculate biodiversity?

Simpson's Index of Diversity (D):

D = 1 - sum of (n/N)²

This gives a value between 0 and 1: 0 = no diversity, 1 = infinite. Higher value = more diversity.