OCR A-Level Biology: Ecosystems

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.

Carbon cycle

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.

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

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 can biomass be represented diagrammatically?

As a pyramid of energy.

How can ecological efficiency be measured 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.

How can food chains be represented diagrammatically?

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 can human activities manipulate biomass through ecosystems?

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.

How can nitrogen be used by living organisms?

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.

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

- 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.

How do edaphic factors effect organisms?

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 do you measure biomass?

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 does oxygen availability 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 temperature effect an organism?

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

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 water availability effect an organism?

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

How is a detritivore different from a saprotroph?

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

How is biomass measured?

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.

How is energy transferred to ecosystems?

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.

How is minimum energy lost by agriculture in food chains?

Net production = gross production - respiratory losses. It is measured in g m⁻²yr⁻¹.

How is net production calculated and measured?

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

How is the energy at each trophic level measured?

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.

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

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.

What are decomposers?

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

What are ectothermic animals?

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

What are endothermic animals?

1. Temperature 2. Light intensity 3. Edaphic (soil) 4. Water availability 5. Oxygen availability

What are five examples of abiotic factors?

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

What are root nodules?

Microscopic fungi and bacteria, and some larger fungi.

What are some examples of decomposers?

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 are the two steps of nitrification?

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.

What composes the subsequent trophic levels?

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 is a biotic factor?

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 a food web?

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

What is a major source of inorganic carbon for plants?

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

What is ammonification?

The non-living conditions in a habitat.

What is an abiotic factor?

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 ecosystem?

Lightning and the Haber process.

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

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.

What is biomass?

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

What is carbon an important component of?

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

What is decomposition?

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 denitrification?

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)

What is ecological efficiency?

The total solar energy that plants convert to organic matter.

What is gross production?

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

What is net production?

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

What is nitrification?

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

What is nitrogen fixation?

The generation of biomass in a producer.

What is primary production?

The generation of biomass in a consumer.

What is secondary production?

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

What is the first trophic level?

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.

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

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

Which bacteria carry out nitrogen fixation?

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

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

Why are food chains rarely longer than quaternary consumers?

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.

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

- 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

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

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

Why does energy have to be recycled throughout ecosystems?

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 have global atmospheric carbon dioxide levels increased?

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.

Why is a decomposer a saprotroph?

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

Why is carbon dioxide called a greenhouse gas?

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

Why is nitrogen an essential element?