populations in ecosystems

biotic factors

the living components of an ecosystem that affect the survival and reproduction of organisms

abiotic factors

the non-living components of an ecosystem that affect living organisms,

eco system

a community and its interactions with the non-living factors in the environment

features of ecosystems

• There is a flow of energy within an ecosystem and the nutrients within it are recycled

  • E.g. in the carbon, nitrogen and phosphorus cycles

• Ecosystems vary greatly in size and scale

  • E.g. both a small pond in a back garden and the open ocean could be described as ecosystems

populations

a group of organisms of the same species living in a particular space at a particular time that can potentially interbreed

communities

multiple populations living and interacting in the same area

features of communities

• Within a community, each species depends on other species, e.g. for food, shelter and pollination

• If one species is removed it can affect the whole community; this is called interdependence

habitat

the local environment in which a species normally lives

niche

• A niche is the role an organism plays in its ecosystem, including its:

  • use of resources

  • responses to abiotic factors

  • interactions with biotic factors

• Each species has a unique niche – only one species can occupy a particular niche

If the niches of two species overlap then the species compete with each other; this can result in either:

1. one species outcompeting the other; the less successful species may adapt to a new niche or may become locally extinct

2. both populations continue to exist, but with smaller population sizes than they would have in the absence of competition

• A species’ niche is determined by its adaptations:

  • Adaptations are structural, physiological or behavioural traits that allow survival under specific conditions

carrying capacity

• The maximum stable population size of a species that an ecosystem can support is known as the carrying capacity

• Although all individuals can potentially reproduce, abiotic and biotic factors limit survival and reproduction

• This ensures the population size of each species is limited at some point (i.e. the carrying capacity of that species is reached)

• Most species remain below or at carrying capacity due to these limiting factors

  • Humans are a possible exception, as we've overcome many natural limitations

interspecific competition

• Occurs when different species compete for the same resources (e.g. food, space, nesting sites)

• Can limit both populations if species are similarly adapted, as resources are shared

• If one species is better adapted, it may outcompete the other:

  • The weaker competitor declines in number or is excluded

intraspecific competition

• This is competition for the same resources between individuals from the same species

• For example: grey squirrels compete with grey squirrels

• When resources are plentiful, grey squirrel numbers increase

  • More individuals compete for food and shelter

  • Resources become limiting, so the population stabilises at the carrying capacity

predation

• Producers are eaten by primary consumers, which in turn may be eaten by secondary consumers who are themselves eaten by tertiary consumers

• Predators are consumers that kill and eat prey

• In a stable community, predator and prey populations cycle as follows:

  1. Prey numbers rise → meaning there is more food for predators

  2. Predator numbers rise → so more prey is eaten

  3. Prey numbers fall → therefore there is less food for predators

  4. Predator numbers fall → meaning less pressure on prey

  5. Prey numbers rise again → cycle repeats

estimating the size of a population

• Measuring all the different levels of biodiversity within an ecosystem could be very time consuming

• Finding out which species live in an ecosystem and the size of the populations requires the identification and cataloguing of all organisms present to build a species list

• This is possible for areas that are very small or where the species are very large like trees

• However, for larger and more complex ecosystems like rainforests, it is simply impossible to find, identify and count every organism that exists there

• When this is the case different samples of the area can be taken and used to make an estimate for the total species numbers in the area

random sampling

• In random sampling the positions of the sampling points are completely random or due to chance

  • This method is beneficial because it means there will be no bias by the person that is carrying out the sampling that may affect the results

• When a sampling area is reasonably uniform or has no clear pattern to the way the species are distributed then random sampling is the best choice

systematic sampling

• In systematic sampling the positions of the sampling points are chosen by the person carrying out the sampling

  • There is a possibility that the person choosing could show bias towards or against certain areas

  • Individuals may deliberately place the quadrats in areas with the least species as these will be easier and quicker to count

  • This is unrepresentative of the whole area

sampling methods

• There are three main sampling methods used when trying the estimate the size of a population:

  • Quadrats (for non-motile or slow-moving species)

  • Transects (for non-motile or slow-moving species)

  • Mark-release-recapture (for motile species)

primary succession

• Succession is the gradual change in an ecosystem over time, from a simple to a more complex structure

• It involves shifts in both biotic and abiotic conditions

  • This makes the environment less hostile and more suitable for new species

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

  • As a result, biodiversity changes continually

• Primary succession occurs on newly formed or exposed land with no initial life (e.g. bare rock from cooled lava or dried-up lakebeds)

the stages of primary succession

1. Firstly, seeds and spores that are carried by the wind land on the exposed rock and begin to grow

   • These first species to colonise the new land (often moss and lichens) are known as pioneer species

   • Their death and decay form basic soil (humus)

2. Seeds of small plants and grasses, carried in the wind, in bird faeces etc, land on this basic soil and begin to grow

   • Their death and decay further increases the depth and nutrient content of the soil

   • Their roots help to hold the soil in place and prevent it from being washed away

3. Larger plants, shrubs and small trees can now begin to grow in the less hostile conditions (deeper soil, more nutrients and more water)

4. Finally, the soil is able to support growth of large trees

   • The final, dominant species form part of a climax community– a stable, complex ecosystem with a variety of plant and animal species

human activities and succession

• Succession is the natural process where ecosystems change over time, often leading to a climax community

• Human activities such as mowing and grazing interrupt succession, maintaining ecosystems in earlier stages

  • Mowing: prevents shrubs and trees from establishing; only grasses persist

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

managing succession for conservation

• Conservation often involves halting succession to protect species diversity

  • This could be necessary where Intermediate stages of succession (e.g. grassland, heathland) support many plant and animal species not found in climax communities

    • This is because dominant species in climax communities can outcompete other species, or changes in abiotic conditions lead to an environment not suited to some species

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

• For example, the Scottish moorlands

  • Naturally would progress to spruce forest via succession

  • Moorlands support unique biodiversity not found in spruce forests

  • Management involves maintaining both climax forests and earlier moorland to maximise species diversity

methods to prevent succession

• There are a few different ways that succession can be deliberately prevented for conservation purposes. For example:

  • Grazing: This involves introducing animals to eat tree/shrub shoots, halting succession

  • Managed burning: Where controlled fires are used to remove woody plants, allowing species like heather to regrow and resetting succession

Investigating growth rate using turbidity measurements

• The population growth rate of microorganisms, such as bacteria or yeast, can be investigated by growing the microorganisms in a broth culture

• The turbidity of the suspension can then be used as a way of estimating the number of cells (i.e. the population size) of the microorganisms in the broth culture

  • Turbidity is simply a measure of the cloudiness of a suspension (i.e. how much light can pass through it)

• As the microorganisms in the broth culture reproduce and their population grows, the suspension becomes progressively more turbid (cloudy)

• This changing turbidity can be monitored by measuring how much light can pass through the suspension at fixed time intervals after the initial inoculation of the nutrient broth with the microorganisms

  • A turbidity meter, a light sensor or a colorimeter (connected to a datalogger) can be used to take these measurements

• The results can then be used to plot a population growth curve to show how the population of microorganism grew over time

conservation and human need

• Humans use Earth’s resources, such as land, water, wood, and fossil fuels for various needs:

  • Buildings

  • Agriculture

  • Fuel

  • Electricity

• As population and economic development increase, so does the demand for these resources

  • This leads to environmental damage, affecting ecosystems, climate, and biodiversity, creating a conflict between human needs and conservation

• Conservation involves managing species and habitats sustainably, meeting present needs without compromising the future

• Conservation of habitats frequently involves management of

  succession

  • Some oppose this due to short-term economic impacts, but careful resource management is essential to balance current use with long-term sustainability

national and marine parks

Protects habitats with legal restrictions on access, development and hunting, balancing biodiversity with controlled land use

public engagement

Generates income through tourism; provides local jobs and funding for services, increasing community support for conservation

zoos (captive breeding)

Captive breeding helps restore species populations and supports research, reducing pressure on wild populations

botanic gardens

Conserves rare plants using lab techniques and enables reintroduction; supports research and education to maintain biodiversity

frozen zoos

Preserves genetic material for future reintroduction; reduces pressure on wild populations and supports long-term planning

seed banks

Stores plant diversity safely; allows future crop restoration and species recovery; offsets habitat loss

considering conflicting data

• The data from just one study is not normally enough to draw sufficiently certain conclusions on which to base conservation actions

  • For example, although the results of the investigation outlined above seem to suggest that signal crayfish are causing the decline of white-clawed crayfish, it is unlikely that this one study would lead to conservation action to remove signal crayfish all across the UK

• Instead, the results from multiple similar studies are normally required, and if these results appear to agree, then a more certain conclusion can be drawn

• Sometimes, however, two very similar studies may give different results that do not appear to agree

• Being able to consider this conflicting evidence and its implications is an important skill