ENVIRONMENTAL RESISTANCE: FACTORS AND EXAMPLES - ENVIRONMENT - 2025

The environmental resistance are the factors which together limit the growth of a natural population. These can be dependent on population density, such as competition, predation, parasitism, or environmental quality. They can also be independent of density such as catastrophes or weather seasonality.

In the absence of environmental regulatory factors, any natural population would grow according to its biotic potential, exponentially. However, the effects of environmental resistance limit population growth, reaching a balance.

Predation Dense-dependent environmental resistance factor. Author: www.flirck.com

The different interactions between the factors that exert environmental resistance in population growth generate highly variable population dynamics.

Populations generally reach a dynamic equilibrium that is graphically represented in curves that oscillate around an equilibrium value.

What is environmental resistance?

The simplest model of the dynamics of a population assumes that, under optimal environmental conditions, the number of individuals increases according to the biotic potential of the population.

In other words, the per capita growth rate (r) is always the same, whatever the population size. Under these premises, population growth would be exponential.

In nature, populations can grow exponentially in an initial phase, but they cannot maintain this dynamic infinitely. There are factors that limit or regulate the growth of this population. The sum of these factors is known as environmental resistance.

Factors that exert environmental resistance act to decrease the per capita growth rate as the population approaches its optimal size, better known as carrying capacity.

This dynamics generates a logistic growth that generally reaches a dynamic equilibrium, with stable periodic fluctuations around the load capacity (K).

Environmental resistance factors

-Densoindependent

When the factors that generate environmental resistance are independent of the density of individuals, they are said to be dense-independent.

Some factors independent of density can occur periodically with the seasons, such as fire, drought, flood or frost. These intervene in the regulation of population size.

By constituting on a recurring basis year after year, they exert a constant selective pressure, which at times has generated specific adaptations in individuals that has allowed them to increase their fitness and survive year after year, despite its regulatory effect.

Other random density-independent effects, such as extreme changes in climate, volcanic eruptions and other natural disasters, can produce erratic changes in populations. They cannot keep the population size at constant levels or at a point of equilibrium.

-Densodependent

If the factors that regulate population growth depend on the density of individuals, then they are called density-dependent. These factors can be abiotic or biotic.

Abiotic factors

Abiotic dense-dependent environmental resistance factors are those that occur when the increase in population size alters the physicochemical conditions of the habitat.

For example, a high population density can generate the accumulation of harmful wastes that decrease the survival or the reproduction rate of individuals.

Biotic factors

Biotic factors are those resulting from the interaction between individuals of a species or of different species. For example, competition, predation and parasitism.

Competition

Competition occurs when the vital resources used by individuals of the same or different species are limited. Some limiting resources can be nutrients, water, territory, shelters from predators, individuals of the opposite sex, light, among others.

As the population increases, the per capita availability of resources decreases, thereby reducing the reproductive rate of individuals and the growth rate of the population. This mechanism generates a dynamics of logistic growth.

Predation

Predation is a type of interaction between species whereby an individual of one species (predator) hunts an individual of another species (prey) to consume it for food. In this type of interaction, the density of each population exerts a regulation on the other.

As the prey increases its population size, the predator's population increases due to the availability of food. But, as the density of predators increases, the prey population decreases due to an increase in predation pressure.

This type of interaction generates population growth curves whose equilibrium is dynamic. A static population size is not reached in the carrying capacity, but the populations are constantly oscillating around this value.

Parasitism

Parasitism is an interaction through which an individual of one species (parasite) benefits from individuals of another species (host), producing a decrease in their probability of survival or reproduction. In this sense, it is also considered as a population regulation mechanism.

The interaction between parasites and hosts can generate dynamics similar to that of predators and prey. However, the diversity of types of parasite-host interactions in nature is infinite, therefore, more complex dynamics can also be generated.

-Interactions

In nature, the dependent and independent effects of density interact in the regulation of populations, producing a great diversity of patterns.

A population can be kept close to carrying capacity by density-dependent factors, and eventually experience a sharp decline due to a density-independent natural catastrophe.

Examples

Bacterial growth

When an inoculum of bacteria is seeded in a culture medium, a growth curve with four phases can be observed. In this curve, the initial exponential growth and the effect of environmental regulation can be clearly appreciated.

A stationary phase is initially evident and finally a decline effect in population size.

During the first adaptation phase, bacteria do not reproduce, but instead synthesize RNA, enzymes, and other molecules. During this phase, no population growth is observed.

Bacterial growth curve. Author: M • Komorniczak -talk-Illustration by: Michał Komorniczak This file has been released into the Creative Commons 3.0. Attribution-ShareAlike (CC BY-SA 3.0) If you use on your website or in your publication my images (either original or modified), you are requested to give me details: Michał Komorniczak (Poland) or Michal Komorniczak (Poland).For more information, write to my e-mail address:, via Wikimedia Commons

In the next phase, cell division occurs. Bacteria reproduce by binary fusion, one cell divides into two daughter cells.

This mechanism generates an exponential growth in which the population size doubles in each consecutive period of time. However, this phase cannot continue infinitely because the nutrients in the environment begin to be limiting.

The third phase of the curve is stationary. The reduction in nutrients and the accumulation of toxins lead to a reduction in the population growth rate until reaching a constant value in the number of bacteria. At this point the rate of new bacteria production is balanced by the rate of bacterial death.

In the final phase of the curve there is an abrupt decrease in the number of bacteria. This occurs when all the nutrients in the culture medium have been depleted and the bacteria die.

Lynx and hares

The typical example of population regulation between predator and prey populations is that of lynx and hare. A decrease in the population size of hares produces a decrease in the number of lynxes.

A smaller number of lynx reduces the predation pressure of hares and in turn produces an increase in the number of lynx.

It is important to consider that the population dynamics of hares is also mediated by the availability of food for them.

Population dynamics generated by environmental regulation between lynxes (predators) and hares (prey). Author: CNX OpenStax, via Wikimedia Commons

Lemmings

An interesting case study occurs with the Lemmings in Greenland. The population of these mammals is regulated by four predatory species: an owl, a fox, a species of bird and the ermine (Mustela erminea).

The first three are opportunistic predators that feed on lemmings only when they are abundant. While the ermine feeds exclusively on lemmings.

This interaction between the different regulatory factors produces periodic oscillations in population growth that generate four-year cycles in lemmings. This dynamic can be explained in the following way.

When lemmings are in low population sizes, they are only preyed on by stoats. As it has a relatively low predation pressure, it rapidly increases its population size.

As the lemmings population increases, opportunistic predators begin to hunt them more frequently. On the other hand, stoats also increase their population size, as there is a greater availability of food. This situation generates a density-dependent limit on the lemmings population.

The increase in the number of predatory species and in the size of their populations generates a very strong predation pressure on the lemmings, causing an abrupt decrease in population size.

This decrease in the prey is reflected in a reduction in the population size of stoats the following year, due to a decrease in food, starting a new cycle.

Difference with biotic potential

Biotic potential is the maximum growth capacity of a natural population subject to optimal environmental conditions.

For example, when food is abundant, the environmental conditions of humidity, pH and temperature are favorable, and their individuals are not exposed to predators or diseases.

Theoretical relationship between biotic potential, environmental resistance and carrying capacity. Modified from: flickr.com/photos/internetarchivebookimages

This population characteristic is determined by the reproductive capacity of individuals (generally females), that is, by how many offspring it is capable of producing throughout its life, which depends on the age of first reproduction, the number of children in each reproductive event and of the frequency and quantity of these events.

The biotic potential of a population is limited by environmental resistance. The interaction between both concepts generates the load capacity.

References

1. Wikipedia contributors. Bacterial growth. Wikipedia, The Free Encyclopedia, 2018. Available at es.wikipedia.org.
2. Hasting, A. 1997. Population Biology: Concepts and Models. Springer. 244 pp.
3. Turchin, P. 1995. Chapter 2: Population Regulation: Old Arguments and a New Synthesis. In: Cappuccino, N. & Price PW Population Dynamics: New Approaches and Synthesis. Academic Press. London, UK.
4. Tyler Miller, Jr. and Scott E. Spoolman. 2009. Essentials of Ecology. 5 ^to edit. G. Tyler Miller, Jr. and Scott E. Spoolman. 560 pp.
5. Wikipedia contributors. (2018, December 11). Biotic potential. In Wikipedia, The Free Encyclopedia. Retrieved 16:17, December 22, 2018, from en.wikipedia.org.