By now, you're probably familiar with the idea that populations change over time, and these changes are examined through analysis of population size, density, and distribution patterns. The population of organisms rarely grows uncontrolled, though, because certain factors limit it. Now let's delve into population limiting factors!
What are population limiting factors?
Firstly, what exactly are these limiting factors that affect population growth? Let's look at the definition of a limiting factor in population ecology.
Limiting factors are conditions or resources within an environment that restrict population growth.
Population growth is the increase in the size of a population over a certain period of time.
For example, suppose a population has only a certain amount of nutrients available. In that case, it will keep growing exponentially until the nutrients are used up, and the population reaches a carrying capacity.
When the carrying is reached, the population size remains relatively the same.
The carrying capacity is the maximum number of individuals of a given species that an environment can support.
Figure 1. Logistic Growth Model, Isadora Santos - StudySmarter Originals.
The carrying capacity of a system is limited by limiting factors. Population growth can be limited by biotic or abiotic factors, and changing these factors can also impact carrying capacity. During natural disasters, for example, ecosystem resources are destroyed.
As a result, the ecosystem is unable to support a large population, resulting in a decrease in carrying capacity.
- Abiotic factors are nonliving factors in an ecosystem such as temperature, sunlight, nutrients, water, pH, salinity, and humidity.
- Biotic factors are living factors such as competition for resources, predation, and disease.
Did you know that microorganisms are also affected by limiting factors? For example, temperature and pH have the ability to limit the growth of organisms such as bacteria, yeasts, and molds!
Population limiting factors examples
Any factor that affects the population size of a species is a limiting factor. There are many examples of these factors, both biotic and abiotic. Some of these are:
- Disease: outbreaks can devastate a population, often causing severe decreases in population size. Only the strongest individuals can survive, which might dote the population of more resistance to that disease.
There are many examples of disease epidemics in wild animal populations. Some of the most well-known ones are:
- White-Nose Syndrome: This disease affects hibernating bats and has caused significant declines in bat populations in North America since 2007. The disease is caused by the Pseudogymnoascus destructans fungus that grows on the bats' skin and disrupts their hibernation patterns, leading to starvation and death.
Canine Distemper Virus: This virus affects several different wild carnivore species, including wolves, foxes, and raccoons. It can cause respiratory and neurological symptoms and has been responsible for significant declines in some populations.
Ranavirus: This dsDNA virus affects amphibians, and has been responsible for large die-offs in several different species of frogs and salamanders. The virus can cause a range of symptoms, including haemorrhaging, skin ulcers, and internal organ damage.
Chronic Wasting Disease: This disease, also called zombie deer disease affects deer, elk, and other members of the deer family. It is caused by a misfolded protein that can spread from animal to animal through saliva, urine, and faeces. The disease can lead to weight loss, behavioural changes, and death.
- Climate change and natural disasters: climate change affects multiple abiotic factors in an ecosystem, causing a disbalance in the conditions animals were used to or need to survive in that location. Natural disasters can suddenly change the conditions of a habitat too; for example a meteorite crash on Earth caused the extinction of all non-avian dinosaurs and other animal species too.
These are just three examples of population-limiting factors, however, there are many more. Human activities and our contribution to climate change are important population-limiting factors today.
Types of population limiting factors
Population limiting factors can be divided into two main categories: density-dependent and density-independent factors.
Density-dependent factors are those that are influenced by the size or density of a population. As the population size increases, these factors become more important and can limit population growth.
Density-independent factors, on the other hand, are not influenced by population size or density. These factors can affect populations regardless of their size or density.
In the following sections, we will explain in depth the density-dependent and independent factors, and provide some examples of each.
Definition of density-dependent limiting factor
Density-dependent factors include competition, predation, resource depletion, and diseases.
Density-dependent factors are biotic factors whose effects in population size depend on population density.
Figure 2. Examples of density-dependent limiting factors, by Isadora Santos - created with Canva.
The effect of density-dependent factors is subdivided into two types: negative density dependence and positive density dependence.
- Negative density dependence happens when the rate of population growth decreases as population density increases.
- Positive density dependence occurs when the population growth rate increases as population density decreases.
Some textbooks might refer to positive density dependence as inverse density dependence or the Allee effect.
Density-dependent limiting factor: competition
At this point in your biology or ecology course, you probably heard of the term competition. Competition occurs when individuals of the same or of different species start competing for resources. In some cases, an increased population density can put a strain on the availability of food, shelter, and water.
Due to competition for resources, this could eventually result in reduced population growth.
Intraspecific competition is the competition for limited resources between individuals of the same species.
Interspecific competition is the competition for limited resources between individuals of different species.
Let's look at an example.
The intertidal zones of ocean shores are home to sessile animals like mussels and barnacles. The open space they have is, therefore, an essential resource for their population growth. Nevertheless, these animals' population growth declines as rocks become crowded and space becomes less available.
Density-dependent limiting factor: diseases
Diseases and parasitism are considered density-dependent limiting factors because as population density increases, they are able to spread more easily within the population, eventually leading to a decrease in population growth.
Bacillus anthracis is a type of pathogenic bacteria that causes a series of complications such as respiratory infections, GI infections, and cutaneous infections (black-colored lesions). In Africa, an infection of zebras with B. anthracis is becoming a concern. Basically, the pathogen lures zebras into the contaminated area so that they become infected by ingesting the microbe, and spread the infection to other organisms.
This disease can be lethal, causing a decline in the population of zebras.
Parasitism is also a density-dependent limiting factor.
For example, the Cordyceps fungi is a type of fungal parasite that infects insects, leading to "summit disease". Basically, the cordyceps fungi invaded the insect's body, growing inside and affecting the insect's brain, making it walk to a high part of a tree and jump, releasing fungal spores to greater distances. An increase in the population density of insects will make it easier for the cordyceps fungi to parasitize. On the other hand, a decrease in insect population density will also decrease cordyceps infection.
Density-dependent limiting factor: predation
Predation involves the threatening of a prey population by a predator, keeping their numbers low.
A common example of predation as a density-dependent limiting factor is the change in the population of moose and wolves on Isle Royale. But, what are the causes for such dramatic changes in numbers?
According to ecologists, there are many factors that limit population growth. Cold winters can weaken moose, reduce food availability and decrease their population size. Now, when the temperature is mild, food is readily available, and the moose population can grow faster.
However, an increase in the moose (prey) population is followed by an increase in the wolf (predator) population. So, the high predator population causes the prey population to decrease.
Density-dependent limiting factors: examples
Let's take a look at some interesting examples involving density-dependent limiting factors.
African swine fever (ASF) is a very dangerous disease that kills pigs and wild boars, with a fatality rate of 100%. It is considered a density-dependent limiting factor and affects different provinces in Africa.
Another important study involving competition as density-dependent limiting factors was performed by ecologist Joseph Connell to study the interspecific competition between two barnacle species on the coast of Scotland: Chthalamus stellatus and Balanus balanoides. According to the competitive exclusion principle, no two species can occupy the same niche, and this was proved to be true in the case of C. stellatus and B. balanoides.
During this study, Connell removed Balanus from the rocks at several sites to analyze whether the distribution of Chthalamus was a result from competition, and he was right! Conner concluded that interspecific competition makes the realized niche of Chthalamus much smaller than its fundamental niche.
Realized niche is the niche that is actually occupied.
Fundamental niche are all the niches that can be occupied.
Density-independent limiting factor definition
Now, let's look at the definition of density-independent limiting factors.
Density-independent limiting factors are usually abiotic factors that limit a population size regardless of population density.
Density-independent limiting factors include natural disasters, unusual weather, seasonal cycles and human activities such as cutting trees and obstructing rivers.
For example, the effects of temperature on the bark beetle population are density-independent. Ecologists have found that in warm temperatures, beetles are able to develop faster and produce more generations per year. However, once sudden drop in temperature can cause them to die.
Figure 4. Examples of density-independent limiting factors, by Isadora Santos - created with Canva.
Another common example involving density-independent limiting factors is the effect of weather change on the population of aphid insects. From April to June, these insects grow exponentially. Then, changes in weather causes a sudden decline in the number of aphids. This decrease in aphid population tend to cause a decrease in the ladybug beetle population because aphids are a popular food source for them!
Limiting factors can also affect the rate of photosynthesis. For example, decreasing light intensity, lowering the temperature, and decreasing carbon dioxide concentration and water supply will lead to a decrease in photosynthesis!
Human intervention is also a type of density-independent limiting factor. For example, cod death rates have been increasing due to fishing fleets catching more fish every year. Since birth rates cannot surpass cod death rates, cod populations have been decreasing in number.
Difference between density-dependent and density-independent limiting factors
Lastly, let's make a table to review the differences between density-dependent and density-independent limiting factors.
| Table 1. Differences between density-dependent and density-independent limiting factors. |
|---|
| Density-dependent | Density-independent |
| The impact of these factors depends on population size. | The impact of these factors does not depend on population size. |
| Predation, competition, disease, waste accumulation | Weather changes, natural disasters, human disturbances |
Population Limiting Factors - Key takeaways
- Limiting factors are referred to as conditions or resources within an environment that restricts population growth.
- Population growth limiting factors are divided into two categories: density dependent or density-independent.
- Density-dependent factors are biotic factors whose effects in population size depend on population density. Examples include competition, disease, and predation.
- Density-independent limiting factors are usually abiotic factors that limit a population size regardless of population density. Examples include weather changes, and natural disasters.
References
- Livescience, In Africa, Anthrax Lures Animals to Their Death, Oct. 2014.
- BD Editors, Limiting Factor, Biology Dictionary, 15 Dec. 2016.
- Brown, M., Everything you need to ace biology in one big fat notebook : the complete high school study guide. Workman Publishing Co., Inc., 2021.
- Relyea, R., & Ricklefs, R. E., Ecology : the economy of nature, Macmillan Education, 2018.
- Campbell, N. A., Biology, 2017.
- Pack, P. E., CliffsNotes AP biology, Houghton Mifflin Harcourt, 2017.
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