Ecosystems (OCR A Level Biology): Flashcards

A hoverfly is harmless, yet has evolved body colouring like that of a wasp.

This deters potential predators, which behave as though it could deliver a harmful sting, so the hoverfly is less likely to be eaten.

• Availability of water

• Light

• Temperature

• Humidity

• pH

• Salinity

• Soil composition

• Atmospheric composition

• Turbidity of water

A biotic factor arises from another organism's activity, whereas an abiotic factor is a physical or chemical, non-living influence on a community.

'Water' covers several distinct aspects, each of which can affect organisms differently, so you should specify which you mean, e.g.:

• Salinity

• Flow rate

• Mineral content

• Turbidity

• Depth

• Dissolved oxygen concentration

• Simple: a desert

• Complex: a tropical rainforest

True — organisms from one ecosystem are often linked to those in another, e.g. birds feeding across multiple ecosystems.

False — energy flows through an ecosystem, while nutrients are recycled.

The mass of living material in an organism or tissue, and the chemical energy stored within that organism or tissue.

• The dry mass of an organism or tissue in a given area

• The mass of carbon the organism or tissue contains

• The chemical energy content released when the sample is burned in pure oxygen

The mass of an organism or tissue after all the water has been removed (i.e. after it has been fully dehydrated).

The mass of carbon that a sample contains is generally taken to be 50% of the dry mass of the sample.

The water content of an organism or tissue can vary and does not represent living material or stored chemical energy. Removing all the water gives a reliable, comparable measure of the actual living material present.

Multiply the dry mass of one organism (or of a given sample area) by the total number of organisms (or the total area).

For example, if one daffodil has a dry mass of 0.1 kg, then 200 daffodils have a biomass of 0.1 x 200 = 20 kg.

Because biomass can change over time (for example, deciduous trees lose biomass over autumn and winter as they shed leaves). Including a time unit shows the average biomass within a given area over that time period.

A technique used to estimate the chemical energy stored in dry biomass by burning the sample and measuring the heat released.

• The dried sample is burned inside the calorimeter

• The heat released is used to warm a known volume of water

• The change in temperature of the water is measured

• This temperature change is used to estimate the chemical energy stored in the sample (measured in joules or kilojoules)

• Weigh the crucible without the sample

• Place the sample in the crucible and put it in an oven set to a low temperature (to avoid burning the sample)

• Remove and weigh the crucible and sample at regular intervals

• Continue until the mass stops decreasing and becomes constant (the sample is now fully dehydrated)

• Subtract the original mass of the empty crucible from this final constant mass to find the dry mass

If the temperature is too high, the sample may burn, which would cause it to lose biomass and give an inaccurate dry mass measurement.

When the mass of the crucible and sample stops decreasing and becomes constant, all the water has been removed and the sample is fully dehydrated.

• Drying a sample fully can take a long time (several days), because it must be heated at a relatively low temperature to avoid burning

• Precise equipment (e.g. a highly precise balance and thermometer) is needed and may not be available

• A simple, basic calorimeter gives a less accurate energy estimate because heat is lost and not transferred efficiently to the water

A bomb calorimeter ensures that almost all the heat energy from the burning sample is transferred to the water (minimal heat loss), giving a highly accurate estimate of the chemical energy content.

True

False — a bomb calorimeter is more accurate because almost all the heat is transferred to the water, with minimal heat loss.

• Light falls away from plants (misses them)

• Light passes through leaves or is reflected away

• Only certain wavelengths of light stimulate photosynthesis

They convert light energy into chemical energy, which is stored in biological molecules.

The storing of chemical energy as plant biomass makes a certain amount of energy available to the next trophic level.

• Not all of the plant's biomass is eaten by the primary consumer

• Not all of the consumer's biomass intake is digested (e.g. cellulose is lost in faeces)

• The consumer converts much of the chemical energy to movement and heat, leaving only a small amount for new biomass

The efficiency is low, typically around 10%.

Efficiency of transfer = (biomass transferred ÷ biomass intake) × 100

Where:

• Biomass transferred = biomass passed to the higher trophic level

• Biomass intake = biomass of the lower trophic level that has been consumed

Step 1: Check units match

Both values are in kg, so no conversion is needed.

Step 2: Substitute into the formula

(4.1 ÷ 35) × 100 = 11.7%

The net productivity of producers: the chemical energy stored as biomass that is available to the next trophic level, calculated as NPP = GPP − R (gross primary productivity minus respiratory losses).

N = I − (F + R)

Where:

• I = chemical energy store in ingested food

• F = chemical energy lost to the environment in faeces and urine

• R = respiratory losses to the environment

Step 1: Calculate net productivity of the toads

N = I − (F + R) = 10,000 − (2,000 + 7,000) = 1,000 kJ m⁻² yr⁻¹

Step 2: Calculate % efficiency

% Efficiency = (1,000 ÷ 10,000) × 100 = 10%

• Providing artificial light in greenhouses on overcast days

• Optimising planting distances between crops

• Irrigation in dry weather and use of fertilisers

• Selective breeding for fast growth

• Use of fungicides/pesticides and herbicides to reduce competition/pests

• Fencing to exclude grazers

• Use of good quality feeds / food supplements

• Use of antibiotics and vaccines to reduce disease

• Controlling predation with fencing or indoor husbandry

• Reducing competition for grazing (e.g. rabbits, deer)

• Indoor husbandry to reduce energy loss from movement or from getting cold

Because the biomass of an organism is effectively a measure of how much chemical energy is stored within it.

True

False — much chemical energy is lost as movement and heat, and undigested biomass is lost in faeces.

The total chemical energy that producers fix as biomass through photosynthesis, before respiratory losses are subtracted (NPP = GPP − R).

A true decomposer (mainly from the fungi and bacteria kingdoms) that obtains nutrients from dead or waste organic matter by secreting enzymes onto it and digesting it externally.

The method of obtaining nutrients from dead or waste organic matter via extracellular digestion, followed by absorption of the soluble products.

• They secrete enzymes onto their food (dead organisms and waste material)

• The material is digested externally (extracellular digestion)

• The soluble products are then absorbed by the saprobiont

They recycle the nutrients locked up in dead and waste matter, making them available to producers again.

Without them, nutrients would remain locked in dead/waste matter and producers such as plants would not have access to sufficient nutrients.

• Ammonium ions

• Phosphate ions

(Not all products are absorbed by saprobionts — many remain in the soil and are available to other organisms such as plants.)

The symbiotic (mutually beneficial) relationships between fungi and the roots of plants.

• Fungal filaments (hyphae) greatly increase the surface area of the roots

• This increases absorption of water and mineral ions (e.g. nitrates and phosphates)

The fungi in a mycorrhiza are composed of long, thin filaments known as hyphae, which interact with the roots of the plant.

They convert gaseous nitrogen (from the atmosphere) into ammonium ions (NH₄⁺).

Examples: Azotobacter and Rhizobium species.

They nitrify ammonium ions into nitrite (NO₂⁻) and then nitrate (NO₃⁻), which are highly soluble and can be absorbed by plant roots.

Examples: Nitrosomonas and Nitrobacter species.

They use nitrates for respiration and return gaseous nitrogen to the atmosphere.

Example: Thiobacillus denitrificans.

Ammonium ions are converted into nitrite and nitrate by nitrifying bacteria in a process called nitrification.

Nitrogen is the most abundant atmospheric gas but is relatively inert (unreactive), so most organisms cannot use it directly.

True

False — nitrogen-fixing bacteria convert gaseous nitrogen into ammonium ions; nitrifying bacteria convert ammonium ions into nitrite and nitrate.

Carbon is constantly recycled around the biosphere.

Carbon atoms are not created or destroyed; they merely swap from one compound to another via the various processes of the carbon cycle.

• In the atmosphere (as CO₂)

• In sedimentary rocks

• In fossil fuels (coal, oil and gas; coal is almost pure carbon)

• In soil and other organic matter

• In vegetation (e.g. as cellulose)

• Dissolved in the oceans (as CO₂)

An organism that uses the energy of sunlight to 'fix' carbon dioxide, converting its carbon into sugars and other organic molecules.

Autotrophs use the energy of sunlight to 'fix' CO₂, turning its carbon into sugars and other organic molecules.

This removes carbon from the atmosphere.

CO₂ is fixed in the Calvin cycle.

• Terrestrial plants use gaseous CO₂ directly from the air

• Aquatic organisms use CO₂ dissolved in water

As much CO₂ is fixed from ocean microorganisms as from terrestrial plants.

Dead plants are not fully decomposed by saprobionts; their bodies form layers of sediment that accumulate over millions of years, locking carbon into the ground.

All organisms respire, including autotrophs, and heterotrophs rely on respiration for all their energy needs.

Respiration releases CO₂ into the atmosphere, the opposite direction to photosynthesis.

CO₂ is released in the Link Reaction and the Krebs Cycle.

Anaerobic respiration also releases CO₂ into the atmosphere, via fermentation by yeast, moulds and bacteria.

Carbon is passed from autotroph to heterotroph, and from primary consumer to secondary consumer, during feeding.

Biomass transfer always includes the transfer of carbon, as it is the main element in biomass.

Dead plants and animals are fed upon by detritivores and decayed by saprophytes, which:

• Releases carbon into the surroundings

• Supplies carbon to the detritivores and saprophytes

Waste matter such as faeces and urine is also used by decaying saprobionts, and these processes can release CO₂ back into the air.

Since the mid-19th century, humans have extracted and burned increasing amounts of fossil fuels.

CO₂ is returned to the atmosphere faster than it can be absorbed by plants and aquatic producers. Warmer temperatures also mean less CO₂ dissolves in the oceans, so more is released into the air, driving climate change and changing many species' habitats.

The enzyme Rubisco catalyses the fixation, by carboxylating RuBP.

It can form fossil fuels such as peat and coal.

Dead aquatic organisms form sediments on the sea bed that can form oil and gas, while shells and other calcium-containing parts form sedimentary rocks such as limestone.

False — respiration releases CO₂ into the atmosphere whereas photosynthesis removes it, so they act in opposite directions.

True

The process by which an ecosystem changes over time, typically from a very simple community to a relatively complex one, as both the biotic (living) and abiotic (non-living) conditions change.

Because they are constantly changing over time, with both biotic and abiotic conditions altering as succession proceeds.

The process that occurs when newly formed or newly exposed land, with no species initially present, is gradually colonised by an increasing number of species.

• Magma from erupting volcanoes cools to form new rock surfaces or rocky islands\n\n- A drop in sea level or the drying up of a lake exposes areas of bare rock

The first species to colonise newly formed or exposed land (often mosses and lichens), which are adapted to survive in hostile abiotic conditions.

• They have deep roots to reach water other plants cannot access\n\n- They can tolerate the salty environment (high concentrations of sodium and calcium ions from sea spray)

• Wind-carried seeds and spores land on the rock; pioneer species (moss and lichens) grow\n\n- Pioneers die and decompose, forming humus and a basic soil\n\n- Small plants and grasses colonise the shallow, nutrient-poor soil\n\n- These die and decompose, making the soil deeper and more nutrient-rich; roots help hold the soil in place\n\n- Larger plants, shrubs and small trees grow in the deeper soil\n\n- Finally, the soil supports large trees, which become the dominant species of the climax community

The final, stable community formed at the end of succession, containing all the plant and animal species that have colonised the land, dominated by the largest species the environment can support (e.g. large trees).

As pioneer species die and decompose, the dead organic matter, known as humus, forms a basic soil.

They help to break apart the top surface of the rock, and when they die they leave behind humus. Together the fragmented rock and humus form a basic soil, making conditions less hostile so species such as mosses can colonise.

Each colonising species changes the local environment so that it becomes less suitable for the previous species. For example, once a thin soil layer covers the rock, lichens can no longer grow and disappear; later, trees may out-compete shrubs by blocking light.

When human activities (or other factors) prevent or interrupt succession, stopping a climax community from developing.

• Regular mowing prevents larger, woody plants from establishing, so only grasses survive\n\n- Grazing by livestock (sheep, horses, cattle) removes new plant shoots, so grasses remain the dominant plants

• Introducing grazing animals temporarily, so they eat the shoots of shrubs and trees and stop them establishing\n\n- Managed (controlled) burning, which burns away shrubs and trees; species such as heather regrow quickly, resetting succession

Intermediate stages of succession often hold a distinct diversity of plant species of conservation importance, which provide food and habitat for a high diversity of animal species (e.g. rare species or important pollinators). These would be lost if the climax community developed. Maintaining both climax and earlier-stage areas gives a higher overall species diversity.

True

False — lichens cannot grow on soil, so they disappear from the ecosystem once soil covers the rock.

It is impossible to find, identify and count every organism in large, complex ecosystems, and cataloguing every species would be extremely time-consuming.

Instead, samples of the area are taken and used to make an estimate of total species numbers.

How a species is spread throughout an ecosystem.

The number of individuals of that species in an area.

In random sampling, the positions of the sampling points are completely random (due to chance), which avoids sampling bias.

In systematic sampling, the positions of the sampling points are deliberately chosen by the person carrying out the sampling.

When the area is reasonably uniform or has no clear pattern to the distribution of species.

When there are changes in physical (abiotic) conditions across the area, e.g. changes in altitude, soil pH or light intensity.

It removes sampling bias from the person carrying out the sampling, so the results are more representative of the whole area.

• Quadrats — for non-motile or slow-moving species

• Transects — for non-motile or slow-moving species

• Mark-release-recapture — for motile species

• Convert the sampling area into a grid and label each square with a number

• Use a random number generator to select the sample points (co-ordinates)

• Lay the quadrat at each chosen point and record the abundance of the species present

Species frequency is the probability that a species will be found within any quadrat in the sample area.

Divide the number of quadrats the species was present in by the total number of quadrats, then multiply by 100.

E.g. bluebells found in 18 of 50 quadrats: (18 ÷ 50) × 100 = 36%.

Percentage cover is an estimate of the proportion of a quadrat occupied by a species. The quadrat is divided into 100 smaller squares, and the number of squares the species is found in gives its percentage cover.

E.g. grass in 89 of 100 squares = 89% cover.

When it is difficult to count individual plants or organisms.

• Lay a measuring tape in a line across the area (e.g. where abiotic factors change)

• Place quadrats at regular intervals along the tape

• Record the abundance of each species within each quadrat

This produces quantitative data showing how distribution changes with physical conditions.

• Take a first large sample; count and mark individuals in a way that won't affect survival

• Release the marked individuals and allow them to mix randomly with the population

• After sufficient time, take a second large sample and count the marked and unmarked individuals

• Use the proportion of marked to unmarked individuals to estimate the total population size

N = (n₁ × n₂) ÷ m₂

• N = population estimate

• n₁ = number of marked individuals released

• n₂ = number of individuals in the second sample (marked and unmarked)

• m₂ = number of marked individuals in the second sample

• Marked individuals have sufficient time to disperse and mix back into the population

• The marking does not affect survival rates (e.g. does not increase predation)

• The marking remains visible and does not rub off

• The population size stays constant — no significant births, deaths or migration during the study

The ACFOR scale (Abundant, Common, Frequent, Occasional, Rare) is used by biologists to estimate abundance in habitats that are difficult to sample quantitatively, such as lichens on tree branches.

False — mark-release-recapture is used for motile species; quadrats and transects are used for non-motile species.

True