What you'll learn

• What an ecosystem is, from a tiny rock pool to a woodland.
• How biomass is transferred between trophic levels, and why transfer is inefficient.
• How carbon and nitrogen are recycled by organisms and microorganisms.
• How succession happens, and how you can sample distribution and abundance in the field.

Ecosystems: the big picture

A species is a group of organisms that can breed together to produce fertile offspring. A population is all the organisms of one species in an area. A community is all the populations of different species living together.

A habitat is the place where an organism lives. An ecosystem includes the community and the non-living environment it interacts with.

Ecosystems vary hugely in size. A rock pool, a playing field, a large tree, a pond, a hedgerow, or an entire forest can all be studied as ecosystems.

Ecosystems are dynamic, meaning they change over time. Populations fluctuate, seasons alter conditions, species interact, and long-term processes such as succession can change the whole community.

Biotic and abiotic factors

A biotic factor is a living factor that affects organisms. Examples include:

• predation
• disease
• competition for food, light, water, or mates
• grazing or herbivory
• availability of food
• presence of pollinators or parasites

An abiotic factor is a non-living factor. Examples include:

• temperature
• light intensity
• soil pH
• water availability
• mineral ion concentration
• oxygen concentration
• salinity
• wind exposure

Biomass transfer through ecosystems

Biomass is the total mass of living biological material in an organism, population, or trophic level. It is usually measured as dry mass, because water content varies and would make comparisons unreliable.

A trophic level is a feeding level in a food chain. For example:

• producers make organic molecules using light energy, usually by photosynthesis
• primary consumers feed on producers
• secondary consumers feed on primary consumers
• tertiary consumers feed on secondary consumers
• decomposers break down dead organisms and waste

The diagram shows why biomass decreases along a food chain: not all eaten biomass becomes new biomass in the next trophic level.

Biomass is lost between trophic levels because:

• some material is not eaten, such as roots, bones, or fur
• some eaten material is not digested and is egested as faeces
• some biomass is excreted as nitrogenous waste, such as urea
• much energy is released during respiration and transferred to the surroundings as heat

Biomass transfer can be measured by estimating dry biomass at each trophic level, often using samples. In a practical context, organisms or representative samples may be dried to constant mass so water does not distort the data.

Efficiency of biomass transfer

The efficiency of biomass transfer tells you what percentage of biomass intake becomes biomass in the next trophic level.

$$efficiency=\frac{biomass\; transferred}{biomass\; intake}\times 100$$

Human manipulation of biomass transfer

Humans can manipulate ecosystems to increase biomass transfer, especially in agriculture.

Examples include:

• using pesticides to reduce biomass loss to pests
• using herbicides to reduce competition from weeds
• using fertilisers to increase plant growth
• keeping livestock in warm conditions so less energy is used maintaining body temperature
• restricting animal movement so less energy is used in muscle contraction
• feeding animals high-energy or high-protein diets
• selectively breeding organisms for faster growth or higher yield

These methods can increase food production, but they may also raise concerns about biodiversity, pollution, antibiotic resistance, eutrophication, and animal welfare.

Recycling within ecosystems

Materials must be recycled because ecosystems do not receive a constant new supply of elements such as carbon and nitrogen.

Decomposers, including bacteria and fungi, break down dead organisms and waste. They release enzymes onto organic material, digest it externally, and absorb the soluble products. This returns mineral ions to the environment.

The nitrogen cycle

Nitrogen is needed to make amino acids, proteins, nucleotides, ATP, DNA, and RNA. Although nitrogen gas makes up much of the atmosphere, most organisms cannot use nitrogen gas directly.

The nitrogen cycle converts nitrogen between forms that organisms can use.

Key processes include:

• nitrogen fixation: nitrogen gas is converted into ammonium compounds
• ammonification: decomposers convert organic nitrogen in dead organisms and waste into ammonium ions
• nitrification: ammonium ions are converted into nitrate ions
• assimilation: plants absorb nitrate ions and use them to make proteins
• denitrification: nitrate ions are converted back into nitrogen gas by denitrifying bacteria

Important microorganisms include:

• Azotobacter, free-living nitrogen-fixing bacteria in soil
• Rhizobium, nitrogen-fixing bacteria in root nodules of leguminous plants
• Nitrosomonas, which converts ammonium ions to nitrite ions
• Nitrobacter, which converts nitrite ions to nitrate ions

The carbon cycle

Carbon is essential because it is found in carbohydrates, lipids, proteins, nucleic acids, and many other biological molecules.

Carbon is cycled through ecosystems by organisms:

• photosynthesis removes carbon dioxide from the atmosphere and fixes carbon into organic molecules
• feeding transfers carbon-containing compounds through food chains
• respiration releases carbon dioxide from organisms back into the atmosphere
• decomposition releases carbon dioxide when decomposers respire while breaking down dead material

Physical and chemical processes also affect carbon cycling. Carbon dioxide can dissolve in oceans, carbon can become locked in shells and carbonate rocks, and combustion of wood or fossil fuels releases carbon dioxide.

Primary succession

Succession is the gradual change in a community over time. Primary succession begins on a surface where no soil exists, such as bare rock left by a retreating glacier or newly formed volcanic rock.

A pioneer species is the first species to colonise a new or bare area. Pioneer species, such as lichens and mosses, are adapted to harsh conditions.

The sequence usually goes:

1. bare rock has no soil and few nutrients
2. pioneer species colonise and start breaking down rock
3. dead pioneer organisms add organic matter, forming humus
4. thin soil develops, allowing grasses and small plants to grow
5. deeper soil supports shrubs
6. trees establish as biomass and biodiversity increase
7. a stable climax community develops

A climax community is the final, relatively stable community that develops under the environmental conditions in that area.

Deflected succession

Deflected succession happens when succession is prevented from reaching the expected climax community. Human activity or grazing may maintain an earlier stage.

For example, regular mowing or grazing can maintain grassland instead of allowing shrubs and woodland to develop.

Measuring distribution and abundance

Abundance means how many individuals of a species are present, or how much of the area they cover. Distribution means where organisms are found within the habitat.

Because it is usually impossible to count every organism, biologists use sampling.

Quadrat sampling

A quadrat is a square frame of known area, such as 0.25 m² or 1 m². It is placed in the habitat and the organisms inside are recorded.

Quadrats can be used to measure:

• density: number of individuals per unit area
• frequency: percentage of quadrats in which a species is present
• percentage cover: estimated percentage of the quadrat covered by a species

Random sampling reduces bias. You can mark out a grid, use random coordinates, place quadrats at those coordinates, and calculate a mean.

Transects

A transect is a line placed across a habitat, usually across an environmental gradient such as increasing distance from the sea, changing light intensity, or changing soil moisture.

A line transect records organisms touching the line. A belt transect uses quadrats placed along the line, either continuously or at intervals.

Transects are useful for studying distribution, because they show how species change across a habitat.

Sampling mobile animals

Mobile animals can be sampled using methods such as pitfall traps, sweep nets, pooters, kick sampling in streams, or capture-mark-release-recapture.

In capture-mark-release-recapture, a first sample is captured, marked harmlessly, and released. Later, a second sample is captured. The proportion of marked animals in the second sample is used to estimate population size.