Populations in Ecosystems (A Level only) (AQA A Level Biology): Flashcards

Three North American warbler species coexist in the same conifer habitat by feeding at different heights in the trees, an example of niche differentiation.

True.

Adaptations are structural, physiological or behavioural traits that allow survival under specific conditions, and these determine a species' niche.

The maximum stable population size of a species that an ecosystem can support.

The point at which the growth curve flattens out (plateaus).

At this point, abiotic and biotic factors prevent the population from growing any further.

Because abiotic and biotic factors (limiting factors) limit the survival and reproduction of individuals.

This prevents all individuals from surviving and reproducing, so the population cannot keep growing.

Any three of:

Light availability

Water supply

Temperature

Amount of space available

Soil pH

Individuals use more energy to maintain their optimum body temperature via homeostasis.

This leaves less energy for growth and reproduction.

Fewer individuals reach reproductive age, so population growth is limited.

Competition between different species for the same resources (e.g. food, space or nesting sites).

Competition between individuals of the same species for the same resources.

When resources are plentiful, the population increases.

More individuals then compete for food and shelter, so resources become limiting.

This limits further growth, so the population stabilises at the carrying capacity.

Grey squirrels eat a wider range of food.

They are also larger and have better fat storage in winter.

This gives them greater survival and reproduction, so the red squirrels are outcompeted.

Prey numbers rise, providing more food for predators.

Predator numbers rise, so more prey is eaten.

Prey numbers fall, so there is less food for predators.

Predator numbers fall, reducing the pressure on the prey.

Prey numbers rise again and the cycle repeats.

False.

Predation is a biotic factor, as it involves an interaction between living organisms.

Competition between two different species for the same resource is called interspecific competition, whereas competition between members of the same species is called intraspecific competition.

In large, complex ecosystems it is impossible to find, identify and count every organism.

Instead, samples of the area are taken and used to estimate the total numbers present.

In random sampling, the positions of the sampling points are chosen due to chance.

This means there is no bias from the person carrying out the sampling that could affect the results.

Quadrats - for non-motile or slow-moving species.

Transects - for non-motile or slow-moving species.

Mark-release-recapture - for motile species.

When the sampling area is reasonably uniform or has no clear pattern in the way the species are distributed.

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

For example, bluebells found in 18 out of 50 quadrats = (18 / 50) x 100 = 36%.

The quadrat is divided into 100 smaller squares.

The number of squares the species is found in is equivalent to its percentage cover in that quadrat.

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

A belt transect shows how the distribution or abundance of a species changes along the gradient.

A line is set up using a measuring tape across the area.

Quadrats are placed at regular intervals along the tape.

The abundance of each species in each quadrat is recorded, producing quantitative data.

Population size = (number in first sample x number in second sample) / number marked in the second sample.

Substitute the values into the formula:

N = (n1 x n2) / m2

N = (236 x 244) / 71

N = 811 leafhoppers

A first large sample is caught, counted and marked in a way that will not affect survival.

The marked individuals are released and allowed to mix randomly with the population.

After sufficient time, a second large sample is caught.

The number of marked and unmarked individuals is counted and used to estimate the population size.

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

The marking does not affect survival rates (e.g. does not make individuals more likely to be predated).

The marking remains visible and does not rub off.

The population size stays constant, with no significant births, deaths or migration.

The gradual change in an ecosystem over time, from a simple to a more complex structure.

Succession that occurs on newly formed or exposed land with no initial life (e.g. bare rock from cooled lava or a dried-up lakebed).

The first species to colonise newly exposed land (often mosses and lichens).

Their death and decay forms basic soil (humus).

Pioneer species (e.g. mosses and lichens) colonise the bare rock and grow.

When they die and decay, they form basic soil (humus).

This allows small plants and grasses to grow, whose roots hold the soil in place and further increase soil depth and nutrient content.

The final, stable, complex ecosystem formed at the end of succession, containing a variety of plant and animal species and dominated by the final species (e.g. large trees).

Each stage changes the abiotic and biotic conditions, for example by building deeper, more nutrient-rich soil.

This makes the environment less hostile and more suitable for the next community.

At the same time, it may become less suitable for the previous species.

True.

As succession progresses, the environment becomes less hostile and can support a greater variety of species, so biodiversity increases.

They interrupt succession, maintaining the ecosystem in an earlier stage.

Mowing prevents shrubs and trees from establishing, so only grasses persist.

Grazing livestock eat new shoots, halting succession and maintaining grass-dominated areas.

Because intermediate stages (e.g. grassland, heathland) can support many species not found in climax communities.

In a climax community, dominant species may outcompete others, or changed abiotic conditions may no longer suit some species.

Intermediate habitats can also be important for rare or threatened species, including pollinators like bees.

Naturally, the moorland would progress to spruce forest via succession.

However, the moorland supports unique biodiversity not found in spruce forests.

Management maintains both climax forest and earlier moorland to maximise species diversity.

Grazing - introducing animals to eat tree and shrub shoots, halting succession.

Managed burning - using controlled fires to remove woody plants, allowing species like heather to regrow and resetting succession.

The first species to colonise bare rock are called pioneer species, and the final stable stage of succession is known as the climax community.

A measure of the cloudiness of a suspension (i.e. how much light can pass through it).

As the microorganisms reproduce and their population grows, the suspension becomes more turbid (cloudy).

Measuring how much light passes through the suspension at fixed time intervals therefore estimates the number of cells.

Any of:

A turbidity meter

A light sensor

A colorimeter (connected to a datalogger)

The microorganisms are grown in a broth culture.

After the initial inoculation of the nutrient broth, the turbidity is measured at fixed time intervals.

The results are used to plot a population growth curve showing how the population changed over time.

Bacteria can grow at rapid rates, producing very large numbers within hours.

This gives a wide range of very small and very large numbers, making a suitable linear scale hard to choose.

A logarithmic scale allows these large changes to be shown on a compressed axis.

Each interval increases by powers of 10.

For example: 10^{2 = 100 and 10}3 = 1000.

The intervals on the axis are not evenly spaced in terms of the actual values.

The spacing between powers of 10 is even, but the numbers in between are not linear.

True.

The pH scale is logarithmic; the concentration of hydrogen ions varies massively between each pH level.

False.

You will not be expected to convert values into logarithms or create a log-scale graph.

You may instead be asked to interpret results that use logarithmic scales or explain the benefit of using one.

As microorganisms reproduce in a broth culture, the suspension becomes more turbid, meaning that less light can pass through it.

A logarithmic scale increases by powers of ten, and can be recognised because its intervals are not evenly spaced.

Humans use Earth's resources (e.g. land, water, wood and fossil fuels) for needs such as buildings, agriculture, fuel and electricity.

As population and economic development increase, demand rises, causing environmental damage to ecosystems, climate and biodiversity.

The sustainable management of species and habitats, meeting present needs without compromising the future.

Managing resources so that present needs are met without compromising the ability of future generations to meet their own needs.

They protect habitats with legal restrictions on access, development and hunting.

This balances biodiversity with controlled land use.

Through captive breeding, which helps restore species populations and supports research.

This reduces the pressure on wild populations.

They store plant diversity safely.

This allows future crop restoration and species recovery, offsetting habitat loss.

It generates income through tourism and provides local jobs and funding for services.

This increases community support for conservation.

One study does not provide sufficiently certain conclusions.

Results from multiple similar studies are normally required; if these results agree, a more certain conclusion can be drawn.

A control site (where no intervention occurs) allows other variables (abiotic factors like water quality and biotic factors like predation) to be discounted.

This increases the validity of the results.

So that the data is not biased.

This gives a more accurate estimate for the whole area.

A decline over a short period could simply be a natural population fluctuation that may reverse over a longer time period.

A smaller study area means population size estimates may be less accurate.

Conservation involves the sustainable management of species and habitats, meeting present needs without compromising the future.

Abiotic factors (e.g. light intensity, nutrient concentration, temperature).

Biotic factors (e.g. intraspecific or interspecific competition, predators or pathogens).

Random sampling (frame quadrats) - when there are no clear or obvious changes in the physical conditions across the area.

Systematic sampling (belt transects) - when there is an obvious change in an abiotic factor (e.g. altitude or light intensity) across the area.

Soil temperature - a digital thermometer.

Soil pH - a digital pH meter.

Light intensity - a photometer.

Set up two tape measures as x and y axes to form a 5m by 5m grid.

Use a random number generator to generate 10 sets of random coordinates.

At each coordinate, place the quadrat and record the percentage cover of the plant species and a measurement of the abiotic factor.

Place a tape measure as a transect running from one extreme of the abiotic factor to the other (e.g. from under the tree canopy to open ground).

At regular intervals (e.g. every 1m), place a quadrat.

Record the percentage cover of the plant species and a measurement of the abiotic factor at each point.

To select the coordinates for the quadrats randomly.

This avoids sampling bias, giving a more representative estimate of the area.

The number of the 100 small squares in which the species is found is equivalent to its percentage cover in that quadrat.

Plot a graph of the percentage cover of the species (dependent variable) against the abiotic factor (independent variable).

A correlation indicates the effect of the abiotic factor on distribution.

A statistical test (e.g. Spearman's Rank Correlation) can test whether the correlation is statistically significant.

False.

Correlation does not always mean causation.

The correlation could be due to chance, or to other abiotic or biotic factors influencing the results.

A change in species abundance (e.g. moving from woodland to open field) may not be due to the investigated factor (e.g. light intensity) alone.

Other factors, such as a change in soil pH or the presence of a specific pollinator, could also be responsible.

A belt transect is used for systematic sampling when there is an obvious gradient in an abiotic factor, whereas frame quadrats placed using a random number generator are used for random sampling.