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What makes a population?
Two groups of different species that live in the same area cannot be considered a single population; because they are different species, they should be considered two different populations. Likewise, two groups of the same species that live in different areas are considered two separate populations.
So a single population is:
A population is a group of individuals of the same species that occupy a particular space at a particular time, whose members can potentially interbreed and produce fertile offspring.
Populations can be very small or very large, depending on the organism. Many endangered species now have very small populations around the world, while the global human population now consists of about 7.8 billion individuals. Bacteria and other microorganisms typically also exist in very dense populations.
Population must not be confused with species, which is a totally different definition.
Species in a population
Several factors must be taken into consideration when defining a species, including similarities in morphology (observable features), genetic material, and reproductive viability. This can be very difficult to do, particularly when different species converge on very similar phenotypes.
A species is a group of similar organisms that are able to reproduce and create fertile offspring.
Why can’t members of different species produce viable offspring?
Most of the time, members of different species cannot produce viable offspring. Members of closely related species can sometimes produce offspring together; however, these offspring are sterile (can't reproduce). This is because different species have a different diploid number of chromosomes, and organisms must have an even number of chromosomes to be viable.
For instance, mules are the sterile offspring of a male donkey and a female horse. Donkeys have 62 chromosomes, while horses have 64; thus, a sperm from a donkey would have 31 chromosomes, and an egg from a horse would have 32. Summed together, this means that mules have 63 chromosomes. This number does not divide evenly during meiosis in the mule, which makes its reproductive success unlikely.
However, there are some cases where interspecies crosses produce fertile offspring. For example, ligers are the offspring of male lions and female tigers. Both parents are relatively closely related felids, and both have 38 chromosomes - as such, ligers have actually been known to produce offspring with other felids!
Fig. 1 - Species versus population
Populations in Ecosystems
An ecosystem consists of all the organisms and non-living elements in an environment. The organisms within an environment are very much influenced by the abiotic and biotic factors in the area. Each species has a role to play in its environment.
Here are some definitions to help you work through the article:
Abiotic factors: The non-living aspects of an ecosystem e.g. temperature, light intensity, moisture, soil pH and oxygen levels.
Biotic factors: The living components of an ecosystem e.g. food availability, pathogens and predators.
Community: All of the populations of different species living together in a habitat.
Ecosystem: The community of organisms (biotic) and non-living (abiotic) components of an area and their interactions within a dynamic system.
Habitat: The region where an organism normally lives.
Niche: Describes an organism's role in its environment.
Variation in population size
Population size fluctuates a lot. Initially, there are no limiting factors so a population may grow rapidly. Despite this, over time, many abiotic and biotic factors may come into play.
The abiotic factors that affect population growth are:
- Light - This is because the rate of photosynthesis increases as light intensity increases.
- Temperature - Each species will have its own optimum temperature that it is best able to survive at. The bigger the difference in temperature from the optimum, the fewer individuals that are able to survive.
- Water and humidity - Humidity affects the rate at which plants transpire and therefore, in areas where water is scarce, only small populations of adapted species will exist.
- pH - Each enzyme has an optimum pH at which it works, therefore pH affects enzymes.
The biotic factors that affect population growth include living factors such as competition and predation.
Carrying capacity: The size of a population that an ecosystem can support.
The number of individuals per unit area of chosen habitat is known as the population density. This can be affected by a number of factors:
Birth: The number of new individuals born to a population.
Immigration: The number of new individuals joining a population.
Death: The number of individuals within a population that die.
Emigration: The number of individuals leaving a population.
Competition
Members of the same species will compete for:
- Food
- Water
- Mates
- Shelter
- Minerals
- Light
Intraspecific competition: competition occuring within species.
Interspecific competition: competition occuring between species.
It's easy to mix up the terms intraspecific and interspecific. The prefix intra- means within and inter- means between so when you break the two terms down, "intraspecific" means within a species, while "interspecific" means between them.
Intraspecific competition is usually more intense than the interspecific competition because the individuals have the same niche. This means that they are competing for the same resources. Individuals that are stronger, fitter and better competitors will have a greater chance of surviving and therefore reproducing and passing on their genes.
An example of an Interspecific competition is red and grey squirrels in the UK.
Predation
Predator and prey have a relationship that causes the population of both to fluctuate. Predation occurs when one species (the prey) is eaten by another (the predator). The predator-prey relationship occurs as follows:
The prey is eaten by the predator so the population of the prey falls.
Predator population grows as there is a plentiful supply of food, however means that more prey is consumed.
Therefore the prey population reduces so there's increased competition for the prey
between the predators.
The lack of prey for the predators to eat means that the population falls.
Less prey is eaten due to there being fewer predators so the population of the prey recovers.
The cycle repeats.
Population changes can be studied using population graphs.
Fig. 2 - Exponential curve for population growth
The graph above shows an exponential growth curve. Although this type of population growth is theoretically possible, it only occurs under ideal conditions and is rarely seen in nature. Some bacterial colonies are able to double their numbers with each reproduction and therefore show an exponential growth curve. Normally the limiting factors talked about above will prevent uncontrolled exponential growth by limiting factors.
Most populations will adhere to a sigmoid growth curve as shown below.
f
Fig. 3 - Different phases of a sigmoid growth curve for populations
The phases that make up a sigmoid growth curve are as follows:
- Lag Phase - Population growth begins slowly and starts from a few individuals.
- Log Phase - Exponential growth occurs as conditions are ideal so maximum growth rate is reached.
- S-Phase - Growth rate begins to slow down as food, water and space become limiting.
- Stable Phase - Carrying capacity for the population is reached and the population size becomes stable.
- Decline phase - If the environment can no longer support the population, the population will crash and the whole process begins again.
Estimating population size
The size of a population can be estimated using randomly placed quadrats, or quadrats along a belt transect, for slow-moving or non-motile organisms.
The abundance of different species can be measured by:
Calculating population growth rate
The population growth rate is the rate at which the number of individuals in a population increases over a specific period of time. It is expressed as a fraction of the initial population.
It can be calculated by the following equation.
For example, let's say that a small town has a population of 1000 in 2020 and by 2022 the population is 1500.
Our calculations for this population would be:
- 1500 - 1000 = 500
- 500 / 1000 = 0.5
- 0.5 x 100 = 50
- Population growth = 50%
Populations - Key takeaways
A species is a group of similar organisms that are able to reproduce and create fertile offspring.
Most of the time, members of different species cannot produce viable or fertile offspring. This is because when parents do not have the same number of chromosomes, the offspring will have an uneven number of chromosomes.
A population is a group of individuals of the same species that occupies a particular space at a particular time, whose members can potentially interbreed and produce fertile offspring.
Both abiotic and biotic factors affect the size of a population.
Interspecific competition is between species whereas interspecific competition is within a species.
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Frequently Asked Questions about Populations
How do you calculate the population size in biology?
It can be estimated using either percentage cover, frequency or the mark-release-recapture method.
What is the definition of population?
A population is a group of individuals of the same species that occupy a particular space at a particular time, whose members can potentially interbreed and produce fertile offspring.
How do you calculate population growth rate?
Using the equation: ((New population - original population)/ original population) x 100
What are the different types of population?
Lag phase, Log phase, S-Phase, Stable Phase and Decline Phase
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