Populations & Communities

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Populations in Ecosystems

A population is a group of organisms of the same species living in an area at one time.
Members of a population interact with each other and can breed together.
A population can be isolated from other populations of the same species due to living in a different area.
Isolation means that members of separate populations cannot breed together and gene exchange cannot take place between them.

Estimating Population Size

Sampling involves measuring small samples of a population that act to represent the whole population.
Random sampling selects sampling points at random, avoiding bias by the person carrying out the sampling.
Systematic sampling places sampling points at fixed intervals throughout the site, avoiding accidentally missing out sections of habitat due to chance.
A transect is a line along which samples are taken, often used to investigate the effect of environmental features on species distribution.
When a sampling area is reasonably uniform, random sampling is the best choice.
Random sample sites can be selected by laying out a grid, generating random number co-ordinates, and placing sample sites in the grid squares that match the random number co-ordinates.
A sampling error is the difference between an estimated population size and the true population size; it occurs when a sample is not truly representative of the whole population.
Sampling error can be minimised by good investigation design, such as carrying out the right type of sampling and taking a large enough sample size.

Random quadrat sampling

Random Quadrat Sampling

A frame quadrat is a square frame placed within the area to be studied to provide a sample.
Quadrats are used to study the distribution of sessile organisms.
Quadrats can be different sizes depending on the species being studied; a 1 m² quadrat can be used for small organisms such as herbaceous plants, while a 400 m² quadrat can be used for large organisms such as trees.
Frame quadrats can be placed randomly, e.g. using random co-ordinates, or systematically, e.g. along a transect.
Data recorded from a frame quadrat can include presence or absence of a species, species frequency, species abundance (using the ACFOR scale), or percentage cover.
Quadrats can be divided up into smaller squares to allow percentage cover to be assessed more easily.
A mean value describes the average value of a data set, and standard deviation is a measure of the spread or dispersion of data around the mean.
A small standard deviation indicates that results lie close to the mean, so there is little variation; a large standard deviation indicates that results are more spread out around the mean, so there is a lot of variation.

Estimating Population Size: Motile Organisms

The mark-release-recapture method is used to estimate the number of individuals in a population of motile organisms.
The first large sample is taken; as many individuals as possible are caught, counted and marked in a way that won’t affect their survival.
The marked individuals are returned to their habitat and allowed to randomly mix with the rest of the population.
After a sufficient time period, another large sample is captured and the number of marked and unmarked individuals within the sample are counted.
The proportion of marked to unmarked individuals is used to calculate an estimate of the population size using the Lincoln index: Population size = (M × N) / R, where M = number of marked individuals in the first sample, N = total number of individuals in the second sample, and R = number of marked individuals recaptured in the second sample.
The Lincoln index assumes that marked individuals disperse and mix back in fully with the main population, marking doesn't affect survival rates, marking remains visible throughout sampling, and the population stays the same size during the study period (no significant births, deaths, or migrations).

Limiting Population Size

The maximum number of individuals of a species that an ecosystem can support is known as its carrying capacity, represented by the letter K.
Carrying capacity is reached when the growth of a population starts to level off.
Abiotic factors that affect carrying capacity include light availability, temperature, and soil mineral availability.
Biotic factors that affect carrying capacity include competition for resources, predation, and disease.
Population density is the number of individuals present per unit area of habitat.
Density-dependent factors have a different effect at different population densities; examples include pathogens and pests, competition for resources, and predation.
Density-independent factors have the same effect on a population at any population density, e.g. a natural disaster such as a flood.
Density-dependent factors tend to act to keep a population at or below its carrying capacity; this is a negative feedback effect.

Carrying capacity

Limiting Population Size: Examples

Predators are consumers that kill and eat other animals; the animals that are eaten are known as prey.
In a stable community, predator and prey population sizes rise and fall in a predator-prey cycle that limits the population sizes of both predators and prey.
In a predator-prey cycle: the number of predators increases when there is more prey available; the number of prey decreases in response to an increase in predators; the number of predators decreases in response to a decrease in prey; the number of prey increases in response to a decrease in predators; the cycle repeats.
The relationship between the Canada lynx and the snowshoe hare is a famous example of the predator-prey interaction.
A population that is limited by predators is controlled by a top-down control; plant populations being limited by herbivory is another example.
A population that is limited by the availability of resources is controlled by a bottom-up control; plant populations being limited by light intensity is also a bottom-up control.
Allelopathy is a strategy that involves damaging the survival of a competing species by secreting harmful chemicals known as secondary metabolites into the environment.
Antibiotic secretion in some bacteria is a well-known example of allelopathy; antibiotics kill bacteria by, e.g., preventing cell wall formation or inhibiting protein synthesis, reducing interspecific competition.

Population Growth Curves

Populations of living organisms tend to follow a set growth pattern over time, giving rise to a population growth curve.
A sigmoid (s-shaped) growth curve contains three phases: exponential phase, transition phase, and plateau phase.
Exponential phase (also known as the logarithmic phase): there are no factors that limit population growth, so the population increases exponentially; the number of individuals increases, and so does the rate of growth.
Transition phase: limiting factors start to act on the population, e.g. competition increases and predators are attracted to large prey populations; the rate of growth slows, though the population is still increasing.
Plateau phase (also known as the stationary phase): limiting factors cause the death rate to equal the birth rate and population growth stops; this plateau occurs at the carrying capacity.
The population size often fluctuates slightly around the carrying capacity.
Population growth is exponential when the speed of growth is proportional to the number of individuals, i.e. a population of 20 individuals will reproduce at twice the rate of a population of 10 individuals.
An exponentially growing population plotted with a log scale on the y axis will appear as a straight line.

The sigmoid population growth curve

Populations: Intraspecific Relationships

Intraspecific relationships involve interactions between individuals of the same species.
Intraspecific relationships can involve cooperation (both members benefit) or competition (one member outcompetes the other and is more successful).
Intraspecific cooperation examples: orcas show cooperative hunting behaviour, working together to catch specific types of prey and then sharing the food; meerkats divide roles in their groups; many species of ants work together in large groups to build nests and provide food for developing young.
Individuals of the same species have the same needs, and so they are frequently in competition with each other for the same resources.
Plants compete with members of the same species for light, minerals, water, and space.
Animals compete with members of the same species for food, mates, and territory (which will increase access to food and mates).
Examples of intraspecific competition include male red deer fighting for access to females, robins being aggressive towards other robins to defend territory, and oak trees growing close to each other competing for light, water, and minerals.
Not all examples of intraspecific competition involve visible conflict; individuals with overlapping territories will be consuming the same resources.

A predator-prey relationship

Community: Interspecific Relationships

A community is multiple populations of different species living and interacting in the same area.
Communities include populations from all groups of living organisms, e.g. plants, animals, fungi, and bacteria.
Living communities interact with their abiotic environment to form an ecosystem.
Interspecific relationships are the interactions between different species that occur within a community.
Types of interspecific interaction include herbivory, predation, interspecific competition, mutualism, parasitism, and pathogenicity.
Herbivory: an organism feeding on a plant, e.g. cattle graze on grass, sea turtles feed on sea grass, honeybees consume nectar and pollen.
Predation: an organism catching and consuming an animal, or consuming a recently dead animal, e.g. dolphins catch and eat fish, lions hunt and eat zebra, red kites eating roadkill.
Mutualism: organisms of different species work together for the benefit of both, e.g. pistol shrimp share their burrows with goby fish, oxpecker birds remove parasites from large mammals.
Parasitism: a parasite organism lives in or on a host organism, causing its host harm, e.g. mistletoe plants grow in the branches of trees, fleas live on the bodies of mammals.
Pathogenicity: an infectious microorganism (pathogen) lives inside a host organism, causing disease, e.g. Mycobacterium tuberculosis bacteria cause tuberculosis in human hosts, Dutch elm disease is caused by a fungal pathogen.

Interspecific Competition

Interspecific competition occurs when organisms of different species compete for the same resources.
Examples of interspecific competition include oak and beech trees competing for light and minerals, lions and hyenas competing for prey, and red and grey squirrels competing for food and territory.
Some species have strategies which increase their ability to outcompete other species, either by increasing their own survival chances or by decreasing the survival chances of a competing species.
Camouflage increases a species' survival chances.
Secretion of harmful chemicals into the environment decreases the survival chances of a competitor; such harmful chemicals are known as secondary metabolites.
Allelopathy is an example of a strategy that involves damaging the survival of a competing species.
Organisms that carry out allelopathy secrete secondary metabolites that harm other organisms into their surroundings, e.g. in plants: secreting harmful chemicals via roots into the soil, releasing harmful gases via the stomata into the air, or storing harmful chemicals in the leaves which are released when the leaves fall and break down.
Examples of plant species that carry out allelopathy include garlic mustard (produces sinigrin which reduces seed germination and root growth in other plant species), bracken ferns (release toxins into the surrounding soil), and Himalayan balsam (secretes allelochemicals into the surrounding soil that limit the growth of other plants).

Interspecific competition

Chi-Squared Test: Skills

The chi-squared test is used to determine whether or not there is a significant association between the distributions of two species.
Ecologists would collect data by sampling, e.g. using quadrats, and recording the presence or absence of each species in each quadrat.
The calculated chi-squared value is compared to values in a critical values table at a chosen probability level (usually 0.05).
If the calculated chi-squared value is smaller than the critical value at the 0.05 probability level, there is no significant association between the two species.
If the calculated chi-squared value is larger than the critical value at the 0.05 probability level, there is a statistically significant association between the two species.
The degrees of freedom for the chi-squared test is calculated as (number of rows - 1) × (number of columns - 1).

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Practice questions

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1.Which of the following best defines a population?
Easy
- AA group of organisms of the same species living in an area at one time
- BAll of the organisms living in an area together with their physical environment
- CMultiple populations of different species living and interacting in the same area
- DA group of organisms of different species living in the same area at one time
2.Which term describes all the interbreeding foxes in a woodland?
Easy
- ACommunity
- BPopulation
- CEcosystem
- DSpecies
3.Which of the following correctly defines a species?
Easy
- AA group of organisms that can interbreed to produce fertile offspring
- BA group of organisms that live in the same area
- CA group of organisms that share the same habitat
- DA group of organisms that compete for the same resources
4.A sampling error is the difference between an estimated population size and the true population size.
Easy
True or false?
5.Which of the following statements about estimating population size in motile organisms is correct?
Easy
- AAny marking method can be used to mark organisms in the first sample.
- BPopulation size can be estimated using quadrat sampling.
- CMarked individuals from the first sample must be allowed to randomly mix with the rest of the population before taking a second sample.
- DSimpson's index is used to calculate population size.
6.Why does exponential growth occur in the early stages of population increase?
Easy
- AThere is competition for resources and increasing predation.
- BDeath rate is equal to birth rate.
- CBirth rate is higher than death rate.
- DResources are readily available and there is little or no predation.
7.In a woodland habitat, blue tits and other small birds feed on caterpillars and beetles, which feed on leaves of trees and herbaceous plants. Which statement accurately describes a population within this example?
Medium
- AAll of the organisms living in the habitat, together with their living and non-living interactions.
- BAll of the small birds living in the habitat.
- CAll of the blue tits living in the habitat.
- DAll of the living organisms in the habitat.
8.Which of the following statements about quadrat sampling are true? (select all that apply)
Medium
- ASampling must be random to avoid bias.
- BRandom samples can be achieved by closing your eyes and throwing a quadrat.
- CQuadrat sampling is only useful for small organisms.
- DQuadrat sampling can be used to study sessile organisms.

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