Density dependent limiting factor

Density-dependent limiting factor
n., plural: density-dependent limiting factors
[ˈdɛn.sə.ti dɪˈpɛndənt ˈlɪmɪtɪŋ ˈfæk.tɚ]
Definition: A limiting factor of a population wherein large, dense populations are more strongly affected than small, less crowded ones

What Is A Density-Dependent Limiting Factor?

Density-dependent limiting factors are limiting factors, which, depending on population density, may limit or slow down the growth of a population. These factors are essential in regulating population growth and thereby help maintain ecological balance. These factors become more pronounced and impactful as the population density increases. They operate through various mechanisms including competition, predation/herbivory, disease transmission, territoriality/aggression, and migration/dispersal. They act on various interactions — be they intraspecific (within species) or interspecific (between species). Thus, density-dependent limiting factors play a crucial role in preventing resource depletion and shaping population dynamics while maintaining sustainable population sizes in an ecosystem.

To understand this further, let us first understand certain concepts fundamental to the definition of density-dependent limiting factors:

• Limiting factors

Limiting factors are factors or variables in an environment that has the capacity to limit the growth, distribution, or abundance of a population in an ecosystem. Conversely, factors that do not limit or restrict the growth, abundance, or distribution of populations or species are referred to as nonlimiting factors. These factors, though, do not necessarily mean they are never going to be perpetually available or are limitless. Food, water, habitat, and other essential resources may still be depleted but they are at an amount that exceeds the requirements of the population. Nonlimiting factors do not act as primary drivers of population growth but limiting factors do. So while there is ample food, any disturbance that leads to a significant decline in food availability can quickly become a factor of population growth — from nonlimiting to becoming limiting. Limiting factors may be classified into density-dependent and density-independent, based on whether they depend on population density or not.

Here is a table summarizing the differences between nonlimiting and limiting factors (Density-dependent vs Density-independent):

• Population density

Population density refers to the number of individuals in a population within a defined area or volume. It measures how concentrated or crowded a space or area is in terms of population.

Watch this vid about density-dependent limiting factors:

What Are 5 Density-Dependent Limiting Factors?

Population growth is limited by density-dependent factors such as follows:

1. Predators and herbivores

   In predator-prey interactions, an increasing population of prey could “attract” predators as hunting becomes easier in an area where the population density of prey is relatively high. The same goes for herbivore-plant interactions. Herbivores can locate and consume plants easier when the plant density is high. The presence of predators or herbivores is essential in keeping check the ecological balance as they tend to limit the abundance of prey and plant in an area. Without effective predation and herbivory, this could lead to high population densities, which could make a profound impact on the ecological balance, as they would tend to engage in competition.

2. Limited resources (food, water, space, mate)

   Resource limitation refers to the availability of essential resources that are vital for survival, growth, and reproduction. As population density increases, there is a tendency that competition will also increase between or among species. This is particularly seen when limited resources such as food, water, and space are dwindling as the population increases. Limited access to these resources may restrict population growth and abundance.

3. Pathogens and parasites

   The spread of disease is easier in highly populated areas than in areas with sparse populations. This is particularly evident when the disease is highly contagious. The transmission of the pathogen from one infected individual to another is easily facilitated when more individuals come in contact with each other or in close proximity. Parasites, as well, can easily find new hosts in denser populations. Thus, diseases and parasitism can be limiting factors, especially when outbreaks occur. The more individuals that are sick in a population, the higher the possibility of reduced reproductive success or decreased fitness of that population.

4. Territorial and aggressive inhabitants

   Many species exhibit territorial behavior where they defend a specific area against other individuals or against other species. An area teeming with territorial or aggressive dwellers will likely exhibit resource partitioning. Individuals will occupy distinct territories that only the occupants would have access to their respective resources. Intruders will be restricted from accessing them especially when the inhabitants are aggressively territorial. They would readily engage in a “fight”, protecting what they “claim as theirs” to the very end. This exclusionary behavior helps control population size by preventing the influx of additional individuals into the area.

5. Environmental factors

   A question arises after food availability, predators, and disease are all considered: Are density-dependent limiting factors biotic or abiotic factors? Density-dependent limiting factors can include both biotic and abiotic factors. They encompass a range of mechanisms that regulate population size in response to changes in population density.

   So while density-dependent limiting factors mentioned above are primarily biotic factors, abiotic factors can also serve as density-dependent limiting factors, influencing population size in a density-dependent manner.

   Environmental factors, including abiotic conditions such as temperature, humidity, pH levels, light availability, and nutrient availability can influence population growth and distribution. Fluctuations (abrupt change) or extremes in these factors can limit population size and impact the ability of individuals to survive and reproduce.

Mechanisms Of Density-Dependent Limiting Factors

How do density-dependent factors work in limiting high population density? How do they regulate or control a growing population?  Here are some ways. Note, however, that in nature, population regulation is a complex system that involves not only density-dependent factors. It also entails the plausible interactions of the density-independent limiting factors with the dependent limiting factors.

• Resource competition

Depleting resources is a significant trigger in a system of regulatory factors. When the population grows, the demand for food, water, shelter, and mate also grows. More and more individuals will compete for depleting resources. Escalating competition leads to reduced access to resources, lower reproductive success, and increased mortality rates. One possible reason is that such scarcity can result in reduced access to nutrition, thereby, adversely affecting growth and their capacity for reproduction, combatting diseases, and survival.

Competition may occur from the same species or from another species:

Intraspecific competition: Competition of individuals from the same species. Interspecific competition: Competition of individuals from different species.

To give an example, take a look at the graph below. Have you noticed how the number of eggs of roundworm Ascaris lumbricoides dropped as the number of worms increases?  Based on the data, it appears that as the population of the parasite becomes denser, the fewer eggs there are, thus, the lower their fecundity. However, why this occurs still warrants sufficient scientific explanation. (Boundless – Biology LibreTexts, 2018)

Competition for resources can occur both within a species and between different species. Intraspecific competition, though, tends to be more intense, as individuals of the same species have similar resource requirements. This mechanism helps regulate density-dependent growth, preventing populations from exceeding the carrying capacity of their environment and maintaining a balance between available resources and population size.

• Transmission and spread of diseases

Diseases and parasites are transmitted easily when the population density is high. They spread at a rather faster rate when their prospective hosts stay in crowded areas or when their vectors can transmit them to their new hosts with ease. Higher infection (or infestation) rates (outbreaks) especially in a dense population without insufficient access to nutrition or with reduced overall fitness tend to be accompanied by higher mortality rates as well.

• Predation pressure

As population density rises, predation also intensifies. Predators tend to hunt in places where there is a high prey density.  Thus, the risk of encountering predators increases. Hunting activity is heightened as predators would have higher chances of finding and capturing their prey when their prey abounds in an area. The resulting increased predation pressure limits prey population size and growth rate.

• Aggression and territoriality

As population density increases, the availability of suitable territories decreases, leading to increased competition and aggression between individuals. This density-dependent regulation mechanism can result in limited access to resources, reduced reproductive success, and population regulation. In crowded conditions, individuals may have smaller territories, leading to heightened aggression, intraspecific competition, and increased mortality rates due to fights or injuries.

• Migration and dispersal

In some cases, population density may lead to migration patterns and dispersal behavior. As noted earlier, as the population increases, the competition for available resources intensifies. Some species tend to seek new habitats or resources so as to alleviate competition and reduce density-dependent pressures. Thus, some of them would acquire new habitats or a new ecological niche. Migration and dispersal can help maintain population balance and prevent overcrowding in certain areas.

Ecological Importance

The role of density-dependent limiting factors in regulating and maintaining ecological stability is vital to the resiliency of an ecosystem. Therefore, understanding how they carry out such a role is important to researchers wanting to gain more insight into population dynamics, predict population responses to changing environments, and formulate conservation and management strategies for possible implementation.

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Further Reading

• Population Growth
• Population Genetics
• Population Regulation

References

1. Boundless – Biology LibreTexts. (2018). 45.2C: Retrieved https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(Boundless)/45%3A_Population_and_Community_Ecology/45.02%3A_Environmental_Limits_to_Population_Growth/45.2C%3A_Density-Dependent_and_Density-Independent_Population_Regulation#:~:text=Density%2Ddependent%20regulation,-In%20population%20ecology&text=Most%20density%2Ddependent%20factors%2C%20which,as%20those%20caused%20by%20parasites.
2. Dynamics of Density-dependent limiting pfactors in appaernt competion:
3. Holt, R. D. (1977). Predation, apparent competition, and the structure of prey communities. Theoretical Population Biology, 12(2), 197-229. doi: 10.1016/0040-5809(77)90042-9
4. Evolutionary implications of density-dependent limiting factors on species interactions:
5. Thompson, J. N. (1999). The evolution of species interactions. Science, 284(5423), 2116-2118. doi: 10.1126/science.284.5423.2116
6. A comprehensive overview of ecological principles, including density-dependent limiting factors:
7. Krebs, C. J. (2009). Ecology: The Experimental Analysis of Distribution and Abundance. Benjamin Cummings.
8. Disease Transmission::
9. Anderson, R. M., & May, R. M. (1979). Population biology of infectious diseases: Part I. Nature, 280(5721), 361-367. doi: 10.1038/280361a0
10. Predator-Prey Dynamics:
11. Murdoch, W. W., Briggs, C. J., & Nisbet, R. M. (2003). Consumer-Resource Dynamics. Princeton University Press.
12. Mechanisms of Density-Dependent Limiting Factors:
13. 1. Begon, M., Townsend, C. R., & Harper, J. L. (2006). Ecology: From individuals to ecosystems. Blackwell Publishing.
14. 2. Anderson, R. M., & May, R. M. (1992). Infectious diseases of humans: Dynamics and control. Oxford University Press.
15. 3. Amarasekare, P. (2008). Interactions between local dynamics and dispersal: Insights from single-species models. Theoretical Population Biology, 74(2), 291-322. doi: 10.1016/j.tpb.2008.07.002

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