The environmental resistance they are the factors that as a whole 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 exponentially according to its biotic potential. However, the effects of environmental resistance limit population growth, reaching an equilibrium.
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..
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The simplest model of population dynamics assumes that, under optimal environmental conditions, the number of individuals increases according to the biotic potential of the population..
That is, the growth rate per capita (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 growth rate per capita 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).
When the factors that generate environmental resistance are independent of the density of individuals, they are said to be densely 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.
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 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 reproduction rate of individuals..
Biotic factors are those resulting from the interaction between individuals of a species or of different species. For example, competition, predation and parasitism.
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, availability decreases per capita resources, thereby reducing the reproductive rate of individuals and the growth rate of the population. This mechanism generates a dynamics of logistic growth.
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 is an interaction by 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 mechanism of population regulation.
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..
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 due to density-dependent factors, and eventually experience a sharp decline due to a density-independent natural catastrophe..
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..
Initially a stationary phase and finally a decline effect in population size is evidenced.
During the first phase of adaptation, bacteria do not reproduce, but instead synthesize RNA, enzymes, and other molecules. During this phase, no population growth is observed.
In the next phase, cell division occurs. Bacteria reproduce by binary fusion, one cell divides into two daughter cells.
This mechanism generates 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 of nutrients and the accumulation of toxins lead to a reduction in the rate of population growth 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..
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 causes a decrease in the number of lynx.
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..
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 the 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 as follows.
When lemmings are in low population sizes, they are only preyed upon by stoats. By having 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 the size of their populations generates a very strong predation pressure on the lemmings, causing an abrupt decrease in population size..
This decrease in 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..
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..
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 offspring in each reproductive event and 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.
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