Population Size

Population Size

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Population size can change all the time, with varying numbers of males, females, juveniles and adults. We can track these changes by plotting a population growth curve, which shows the number of individuals in a population over time. Exponential growth happens when a population increases in proportion to its current numbers, resulting in a J-shaped curve on a graph. However, using a logarithmic scale is necessary when there is a wider range of values to be displayed. Unlike a linear scale, the logarithmic scale does not have equal intervals between the numbers on the y-axis. To convert numbers from a linear to a logarithmic scale on a calculator, type in your value and press the 'log' button. The base number is usually 10 on a scientific calculator. For example, if you enter log10(10000) into your calculator, you get 4. To get your original number, 10000, you need to multiply your base number, which is 10, by itself four times.


The linear scale and logarithmic scale
The linear scale and logarithmic scale

How does the carrying capacity affect population size?

Organisms can't reproduce continuously since their populations would grow infinitely, but there are limitations that prevent this in the real world. Let's say there's a herd of cows in a field. If we start with four cows, they will have enough space and grass to eat, but as they reproduce, they will need more food and space. However, the size of the field and the amount of grass won't change. Additionally, the cow population might interact with other organisms in the ecosystem, increasing or decreasing their numbers. For instance, parasites may transmit diseases to cows, killing them or preventing reproduction. On the other hand, there may be and insects that benefit the soil and promote grass growth, providing them with plenty to eat. This combination of conditions in an ecosystem can only maintain a specific maximum stable population size known as the carrying capacity. Once the population reaches the carrying capacity, population growth slows down due to limiting factors in the ecosystem.


The carrying capacity of a population over time
The carrying capacity of a population over time

What abiotic factors affect population size?

Abiotic factors involve the non-living parts of the ecosystem. Many of these factors can influence the population size in an ecosystem.


Temperature is one example of a factor that can affect an ecosystem. Different species have different ideal temperatures, and if an ecosystem's temperature is too far from what a species needs, fewer individuals will survive. Temperature can significantly affect an organism's biochemistry. For example, in plants, cold-blooded animals (poikilotherms/ectotherms), and other organisms whose body temperatures are externally regulated, enzymes work slower when the temperature is sub-optimal, leading to a reduction in metabolic rates. At higher temperatures, enzymes may undergo denaturation, reducing their efficiency. Denaturation happens when a protein's structure and function change, usually enzymes, because the substrate no longer matches the enzyme's active site. Homeostasis is the regulation and maintenance of a constant internal environment, achieved through mechanisms like negative and positive feedback loops.


Other important factors, among others, include the availability of light, humidity, and soil pH.

What biotic factors affect population size?

Biotic factors refer to the living components that influence population size in an ecosystem. The main biotic factors include competition, predation, disease, food availability, and waste accumulation. Competition occurs when two or more organisms compete for the same limited resources, such as food, water, or shelter. Predation refers to the act of one organism (the predator) hunting and killing another organism (the prey) for food. Disease can spread through a population and impact its size, as it can cause fatalities or limit reproduction. Food availability can also affect the population size of a species, as a lack of food can lead to malnourishment, starvation, and death. Finally, waste accumulation can cause environmental problems that can impact populations. For example, excess waste can lead to pollution, which can harm the health of organisms and even cause death.


Competition can be split up into:

Interspecific competition refers to competition that occurs between members of different species, while intraspecific competition occurs between members of the same species. Ecosystems have limited resources like water and shelter, and species that can obtain these resources more efficiently have a competitive advantage over other species. Over time, species with a competitive advantage will cause the population size of other species to reduce drastically. This can lead to changes in the ecosystem's structure and can have ripple effects on other species in the ecosystem. The outcome of competition can vary depending on the species involved, the amount and type of resources available, and the environmental conditions. Some species may be able to coexist, while others may be driven to extinction. Interspecific and intraspecific competition are important ecological concepts that help us understand how species interact with each other in ecosystems.


Predation is a biological interaction where one organism, the predator, consumes another organism, the prey. In this relationship, the prey is a limited resource for the predator. The predator-prey relationship displays a fluctuating pattern. This pattern occurs because when prey is available to feed on, the predator population increases. However, as the predator population grows, the prey population reduces due to increased predation pressure. As a result, the predator population also reduces as there is less food available. This creates a cycle of population fluctuations, where the predator and prey populations rise and fall in response to each other's abundance. These fluctuations can have significant impacts on the ecosystem as a whole, affecting the abundance and diversity of other species that depend on the predator and prey populations. Overall, the predator-prey relationship is an important ecological concept that helps maintain balance and stability in ecosystems.


Pathogens, such as bacteria and parasites, can cause infections that reduce the health and fitness of organisms, negatively impacting their rate of reproduction and survival. This can lead to a decrease in the population size of species that are susceptible to disease. Pathogens can spread between individuals through various means, including direct contact, exposure to contaminated surfaces, or through vectors like insects or other animals. The effect of disease on populations can be significant, as it can result in reproductive failure, increased mortality, and decreased genetic diversity. In some cases, diseases can even cause local extinctions of vulnerable species. Understanding the dynamics of disease transmission and the factors that influence susceptibility to infection is important for managing and conserving populations of vulnerable species. Measures such as vaccination and quarantine can help prevent the spread of disease, protecting both individual animals and entire populations.

Food availability

In an ecosystem, food is a limited resource, and when it becomes scarce, species will compete to obtain it. There are two types of competition that can occur: intraspecific and interspecific competition.

Intraspecific competition occurs when members of the same species compete for resources. For example, if there is a limited amount of food available, individuals within a species will compete for access to that food. In this type of competition, the organisms that are best suited to obtain the food will outcompete the others. This can result in differences in growth rates, survival, and reproduction among individuals within the same species.

Interspecific competition, on the other hand, occurs when different species compete for the same resources. For example, if two species of birds feed on the same type of insect, they will compete for access to those insects. In this type of competition, the species with the competitive advantage will outmatch the other. This can lead to changes in the distribution and abundance of different species within an ecosystem.

Competition for resources is an important ecological concept that influences the dynamics of ecosystems. Understanding the factors that influence competition and the outcomes of competition is important for managing and conserving ecosystems and the species that depend on them.

Waste accumulation

When population numbers are high, organisms tend to produce more waste as a byproduct of their daily activities. For example, in human populations, high population densities can lead to the accumulation of sewage and other waste materials. This accumulation of waste can reach toxic levels, which can reduce the air and water quality of habitats. Reduced air and water quality can have significant negative impacts on the reproduction and survival rate of organisms. For example, pollutants in the air can cause respiratory problems or damage plants, which can reduce the availability of food or shelter for wildlife. Similarly, pollutants in water bodies can reduce the availability of oxygen, making it difficult for aquatic organisms to survive. In some cases, the accumulation of waste can lead to environmental disasters, such as oil spills, which can have significant negative impacts on wildlife populations and their habitats. Therefore, managing waste production and disposal is an important aspect of protecting ecosystems and the species that depend on them. Adopting sustainable practices, such as reducing waste production, recycling, and properly disposing of waste, can help minimize the negative impacts of waste accumulation and ensure the long-term health and survival of ecosystems.

How can we calculate the population growth rate?

Imagine a closed population that does not interact with the outside world. There would only be two ways for the population size to change: it will increase if new individuals are born to the population, and it will decrease if individuals die. Thus, the essential factors determining the growth of the population are the birth rate and the death rate.

If we combine these elements, we can get the basic equation for population growth. Births and immigration add to the population size, while deaths and emigration decrease the population size, giving us:

Population size   = births + immigration - deaths - emigration

Over time, the population growth rate is calculated as the population change during the period divided by the population at the start of the period, times 100.

Let us calculate the growth of a mice population from January to March. In January, the population consisted of 40 mice. Ten mice died, and 40 more mice have been born since then. Assume that there is no migration. What is the population growth rate?

Let's start by calculating the current number of mice. Ten mice died, and 40 were born, which gives us 40 - 10 + 40 = 70 mice.The population change is equal to the current population - the starting population. This gives us 70 - 40 = 30.Divide this by the population at the start of the period. 30/40 = 0.75. Multiply this by 100 to get the percentage. The population growth rate is 75%; in other words, the population increased by 75%.

How can we investigate population sizes in biology?

Sampling techniques are necessary to estimate the abundance and distribution of populations within ecosystems, particularly in large and complex ecosystems where identifying and counting every individual is impractical. Sampling involves taking measurements or observations from a subset of the population and using statistical methods to estimate the characteristics of the entire population.

There are various sampling techniques used in ecology, such as random sampling, stratified sampling, and systematic sampling. The choice of sampling technique depends on the research question and the characteristics of the population being studied.

Random sampling involves selecting individuals from the population at random, without any bias. This technique is useful for estimating the average population size or density.

Stratified sampling involves dividing the population into different subgroups, or strata, based on specific characteristics (e.g., age, sex), and then selecting individuals from each subgroup. This technique is useful for comparing population characteristics within different subgroups.

Systematic sampling involves selecting individuals from the population at regular intervals, such as every 10th individual. This technique is useful when there is a regular or repeating pattern in the distribution of individuals within the population.

Overall, sampling techniques are essential tools for estimating population characteristics and understanding the dynamics of ecosystems.

What are the types of sampling in biology?

Just one minor suggestion: you could add a sentence to clarify how systematic sampling can be used to capture patterns in the environment. For example, you could say:

Systematic sampling can be used to capture patterns in the environment because the researcher can select sampling points that are spaced evenly along the gradient or pattern. This ensures that the data collected is representative of the entire gradient, rather than just one specific area.


The difference between random sampling and systematic sampling
The difference between random sampling and systematic sampling

What are the sampling methods used to estimate the size of a population?

There are three main methods used to sample population size:


There are two frequently used quadrats: the point quadrat and the frame quadrat.

Quadrats are very useful sampling methods for plant and fungi species and non-motile or slow-moving animals that do not move away when approached.

The abundance results from the quadrats can be used to determine the frequency or the likelihood that a particular species occurs in a given area. This is normally used for species that are easily countable.

To calculate frequency, the number of squares in which the species is present is divided by the total number of squares multiplied by 100. The formula is as follows:

Species frequency = (number of squares species is present in ÷ number of squares) x 100

If a species of insect were found in 20 out of 50 squares, the species frequency would be ((20/50) x 100 =) 40%.

Quadrats can also be used to determine the percentage cover or portion of a given area that a particular species occurs in. This can be useful when it is difficult to identify individual organisms, such as grasses, mosses, and fungi.

The quadrat is divided into 100 smaller squares, and the number of squares the species occurs in is equivalent to its percentage cover To calculate this,  . Expressed as a formula, this gives us:

Percentage cover = number of squares species is present in ÷ 100

If moss is found in 15 out of the 100 smaller squares within the quadrant, and grass is found in 30 of the squares, the percentage cover of moss is 15% and the percentage cover of grass is 30%.

Belt transects

Some areas show a distinguishable pattern of change in their physical conditions. In these cases, systematic sampling methods like belt transects are more appropriate to use.

A transect is a line along which samples of the population are taken. Quadrats are placed at regular intervals along the line - for instance, at every ten-meter increase in altitude or at every 0.5 increase in pH - and the number of species in each square is recorded. This provides us with continuous samples that we can use to determine if changes in the distribution and abundance of species correspond to changes in the physical conditions.


A transect line displaying quadrats at even intervals corresponding to soil pH
A transect line displaying quadrats at even intervals corresponding to soil pH

What is mark-release-recapture?

Most animals move too quickly to be counted using the methods outlined above. Mark-release-recapture methods are used to account for these species.

A given number of animals are captured and marked with an identifier (for example, with different colours of animal-safe waterproof paints) and released into the wild to mix with the population. After some time, another sample of individuals in the population is collected, and the number of marked individuals within the recaptured sample is recorded.

This method makes the following assumptions:

From this, we can estimate the population size (N) by multiplying the number of individuals in the first (n1) and second (n2) samples and dividing it by the number of marked individuals recaptured (m). The equation is as follows:

N = (n1 x n2) / m

If we were to investigate the abundance of butterflies in a patch of meadow, we could use nets to capture and mark a sample of butterflies on their wing or abdomen and release them into the wild. One day later, we can recapture another large sample and count how many are marked. Let n1= 200, n2 = 213, and m = 50. Plugging these into the equation above gives us (200 x 213) / 50 = 852. There are approximately 852 butterflies in the meadow.

Population Size - Key takeaways

Excellent! Just one minor adjustment: you could clarify that the mark-release-recapture method is used to estimate population size, rather than simply sampling the size of the population. Here's a revised version of your paragraph:

A population growth curve shows how the size of a population changes over time and can be plotted on linear or logarithmic scale. is by abioticotic estimate of or methods such as quadrats (point and frame) or belt transects for less motile species. However, highly motile species are best sampled using the mark-release-recapture method, which involves marking and releasing individuals and then estimating population size based on the proportion of marked individuals in subsequent samples.

Population Size

How does population size affect an ecosystem?

An ecosystem is comprised of the abiotic and biotic factors in a given area. When a species' population size increases, food availability, competition and predation increases, all of which are biotic factors. Additionally, more space is needed, which is an abiotic factor. 

What is population size in ecology?

Population size refers to the number of organisms present of a particular species. 

What is the formula for population size?

The formula for population size can be measured using the mark-release-recapture method.N = (n1 x n2) / mN = population size n1 = number of individuals in first samplen2 = number of individuals in second sample m = number of recaptured marked individuals 

What is population size and growth?

Population size refers to the number of organisms present of a particular species. Population growth refers to the rate at which the population size increases. This is determined by the rate of reproduction and rate of death. 

What controls population size in an ecosystem?

Population size is controlled by the abiotic and biotic factors of an ecosystem. The abiotic factors that influence population size include temperature, space and light availability. The biotic factors that influence population size include competition, disease and food availability.

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