Explain the role that limiting factors play in keeping population sizes in check.

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!

Population limiting factors examples

Firstly, what exactly are these limiting factors that affect population growth? Let's look at the definition ofa limiting factor in population ecology.

Limiting factors are referred to as conditions or resources within an environment that restricts population growth.

Population growth is the change in 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 (Figure 1). 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.

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!

Definition of density dependent limiting factor

Population growth limiting factors are divided into two categories: density-dependent and density-independent. The first type of limiting factor that we will be exploring are density-dependent limiting factors (Figure 2). 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.

Density-dependent factors are subdivided into two groups: 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.

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.

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.

Parasitismis 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.

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 its population growth. Cold winters can weaken moose, reduce food availability and decrease their population size. Now, when 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 wolves (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 boar, with a fatality rate of 100%. It is considered a density-dependent limiting factor and affects different provinces in Africa.

Another important study (Figure 3) involving competition as a density-dependent limiting factors was performed by ecologist Joseph Connell to study the interspecific competition between two barnacle species in 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 humans activities such as cutting trees and obstructing rivers (Figure 4).

For example, the effects of temperature in bark beetle population is 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.

Another common example involving density-independent limiting factors is the effect of weather change in the population of aphid insects (Figure 5). 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 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 (Table 1) to review the differences between density-dependent and density-independent limiting factors.

Population Limiting Factors - Key takeaways

• Limiting factorsare referred to as conditions or resources within an environment that restricts population growth.
• Population growth limiting factors are divided into two categories: density dependent ordensity-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

1. October 2014, C. Q. C. 01. (n.d.). In Africa, Anthrax Lures Animals to Their Death. Livescience.com. https://www.livescience.com/48093-anthrax-lures-zebras-to-their-death.html
2. BD Editors. (2016, December 15). Limiting Factor. Biology Dictionary. https://biologydictionary.net/limiting-factor/
3. Brown, M. (2021). Everything you need to ace biology in one big fat notebook : the complete high school study guide. Workman Publishing Co., Inc.
4. Relyea, R., & Ricklefs, R. E. (2018). Ecology : the economy of nature. Macmillan Education.
5. Campbell, N. A. (2017). Biology. Pearson.
6. Pack, P. E. (2017). CliffsNotes AP biology. Houghton Mifflin Harcourt.

What is a limiting factor and how does it affect population size?

What are the factors that can limit the population size?

Why does population size still increases despite the limiting factors?

Which limiting factor is independent of the number of individuals in a population responses?