Adaptation of living organisms and the structure of natural populations

Morphological, physiological and behavioral adaptations

Adaptation is the evolutionarily developed and hereditarily fixed ability of organisms to survive and develop normally under dynamic environmental conditions. Any species inhabits only those areas where ecological factors meet its requirements. Individuals that lack the necessary level of tolerance or are unable to adapt to environmental changes inevitably perish.

Morphological adaptations are expressed in the structural features of the body. In aquatic inhabitants, these are adaptations for fast swimming (as in cetaceans) or for hovering in the water column (in plankton). Desert plants are completely devoid of leaves to minimize moisture loss. Soil organisms develop tough integuments, a cylindrical body shape, reduced vision, and cutaneous respiration.

Physiological adaptations reorganize biochemical processes. Inhabitants of arid zones are able to meet their water needs through the biochemical oxidation of fats. Photosynthesis allows plants to synthesize organic substances from inorganic ones under conditions of a strictly defined atmospheric gas composition. In animals, the composition and nature of ingested food directly determine the set of enzymes in the digestive tract.

Behavioral adaptations help regulate heat exchange and avoid threats. Predators use specific techniques for tracking and pursuing prey, while victims react by lying still or making sharp, repellent movements, as some insects do. During extreme temperatures, animals actively move in search of optimal conditions or construct protective shelters.

Population structure and population control

In natural conditions, living organisms exist exclusively in the form of populations — groups of individuals of the same species that inhabit a specific territory and freely exchange genetic information (panmixia). Scientific research emphasizes that a population literally directs its own destiny by regulating the physiological state of the individuals within it. Proper consideration of intra-population processes is necessary for the competent use and conservation of natural resources.

Each population possesses a strictly defined structure:

• age (ratio of individuals of different ages);
• sexual (sex ratio);
• spatial (distribution into families, colonies, packs).

The abundance and density of a population depend on the capacity of the ecological niche and regulating environmental factors. In nature, the number of individuals can range from dozens for rare species to tens of thousands. In artificial biosystems — for example, in the activated sludge of aeration tanks and biofilters for wastewater treatment — populations of bacteria and protozoa number in the billions, and the quality of the treatment facilities' operation depends directly on them. In agriculture and forestry, it is the actual abundance of herbivorous species that determines the scale of the damage they cause.

Practical nature conservation and sound resource management consist in maintaining the regimes of ecological factors. Only by preventing the destruction of ecological niches can the normal functioning of populations, their reproduction, and the correspondence of their structure to actual habitat conditions be ensured.

Population density is the population size related to a unit of occupied space, or the average number of individuals per unit area or volume.

Thus, the population density of moose and other warm-blooded animals is determined by the number of individuals per 10,000 hectares, while the population of soil invertebrates is related to a square meter. When characterizing microorganism populations in activated sludge, the number of individuals in 1 cm3 is used.

By knowing the change in density over time or space, one can determine whether the number of individuals is increasing or decreasing. Population density dynamics reflect complex patterns of relationships between various animals, and between animals and plants, as they all serve as biotic factors in relation to one another. Density can also depend on fluctuations in abiotic environmental factors. For each species, there are optimal limits for its population density. Variation in density within each population depends on the state of the entire ecological system.

Population size and density are not random values. They are predetermined not only by the regimes of ecological factors at a given time but also by the entire preceding development of the given population and previous generations in a particular community. It is customary to say that the population volume is determined by the stational (station - habitat) capacity of the ecosystem for representatives of a population of a given species or the capacity of the location of the ecological niche.

Fluctuations in the number of living organisms inhabiting a particular ecological system are of great importance to humans, as many animals and plants are the objects of their economic activity or the cause of damage. Therefore, knowledge of the patterns of population dynamics is necessary for predicting potential undesirable events and, if necessary, making adjustments to this dynamics in order to manage it.

Changes in the number of individuals in a population affect density. If density changes within a virtually constant habitat volume, an increase in the number of individuals is possible only up to a certain limit permitted by the carrying capacity of the ecological niche. At a specific moment in time, the number of individuals in a population reflects its birth rate and mortality rate. Depending on the ratio of these indicators, one speaks of population balance. If the birth rate is higher than the mortality rate, the population size increases, and vice versa.

Population birth rate is the numerically expressed capacity of a population to increase, or the number of individuals born over a certain period.

This capacity depends on a multitude of factors: the sex ratio in the population, the number of sexually mature individuals, fecundity, the number of generations per year, food availability, the influence of weather conditions, etc. Low fecundity is characteristic of those species that exhibit great care for their offspring, and conversely, high fecundity indicates poor survival conditions. For example, the ocean sunfish, which does not care for its offspring at all, lays about 300 million eggs. Such caring parents as the pink salmon and smelt lay 1,500 and 100 eggs, respectively. They protect the eggs and larvae from harmful external influences and from destruction by predators. Some insects are capable of producing 2-3 generations per year, i.e., they lay eggs 2-3 times a year in quantities of several hundred; murine rodents, with a gestation period of about a month and a short period of reaching sexual maturity, produce 5-6 generations; large warm-blooded animals carry their young for several months, reach the ability to reproduce at the age of 3-4 years, and give birth to only 1-2 offspring. Bacteria and protozoa, which reproduce by division, repeat this act many times within a few hours.

Thus, if there are 500 individuals capable of reproduction in a population (Nо = 500) and 50 new ones are born within 10 days (∆Т = 10) (∆Nо = 50), then the birth rate will be Р = 50: 10 = 5, or calculated per individual Р = ∆Nо / (∆Т·Nо) = 50 / (10·500) = 0.01.

Population mortality (С) is the number of individuals that have died over a certain period.

It can be very high and varies depending on environmental conditions, age, and the state of the population. In most species, mortality at an early age is always higher than in adult individuals. However, there are species where mortality is approximately the same at all ages or is prevalent in older individuals.

Mortality factors are very diverse. It can be caused by the influence of abiotic factors (low and high temperatures, heavy rainfall and hail, excessive and insufficient humidity, etc.), biotic factors (lack of food, infectious disease, enemies, etc.), including anthropogenic ones (environmental pollution, destruction of animals, deforestation, etc.).

When considering population size for various practical tasks, one always deals with individuals that have survived up to the current moment. Therefore, the actual characteristic of a population's state is survivorship. Survivorship is understood as the proportion of individuals in a population that have lived to a certain point in time or to the age of reproduction. The survival curves shown in Fig. 2.4 display the natural mortality in each population.

In humans, the percentage of mortality is insignificant in the first half of life, then increases sharply. Something similar is observed in the fruit fly and most insects. In the hydra and the seagull, mortality is constant throughout their entire life. In the oyster, there is a high percentage of mortality among young individuals and a low one among the elderly.

In most species, the life expectancy of females is much longer than that of males.

If a population is placed in a stable environment from which all limiting factors have been artificially removed, the population size will increase according to an exponential law as a function of time.

The natural growth of a population can be represented by the ratio of the number of individuals by which the population increased per unit of time to the initial value of its size.

Ndt where N is the number of individuals in the population at time t;

dN is the number of individuals by which the population increased during time dt;

r is the natural population growth rate. From (1 After integrating this expression, we obtain

N = N 0 e rt, (2)

A graph illustrating dependency (2) is shown in Fig. 2.5.

Such is the case with the human population, the size of which depends on time according to an exponent, at least up to the present day.

Experience shows that humans are an exception—no other species of livestock animal or wild animal follows this law. There are limiting factors that bring the number of individuals of each species to an optimal value compatible with its habitat.

For example, if the population experiences a sudden increase in reproduction under very favorable conditions, competition begins to develop between individuals. It then becomes beneficial for the population if some individuals stop reproducing and the population growth slows down. Such mechanisms work very precisely in nature.

Let's take rodents. At a certain stage of their reproduction, at a specific density within the community, internal relations begin to sharpen—due to territory or females. Aggressive forms of interaction begin to prevail over communicative ones. A stressful environment arises, which can increase the mortality of individuals in the population or block the release of sex hormones into the bloodstream. This does not necessarily happen through stress; there are other mechanisms as well.

The opposite scenario is also common: if predators have proliferated excessively or feed is scarce, the population drops sharply. Then, mechanisms that stimulate reproduction are activated.

A process of self-regulation is underway—a population always strives to reach an optimal level of its size.

We encounter this in real life. For example, we control rodents using poisons. One hundred percent extermination of the pest is never achieved. Someone is hiding in a burrow, someone was outside the application areas. And these few survivors, after some time, restore the population size through intense reproduction.

Consequently, there is always a maximum high (K) and low (M) population size and density that the population cannot exceed or fall below, Fig. 2.5.

In this case, two variants of the further dynamics of the given population are possible: 1. The population size stabilizes, and its dynamics, in general, will be characterized by a so-called logistic (S-shaped) curve.

The rate of population growth in this case is determined by the following expression:

K−N where environmental resistance is the set of factors,

K impeding the unlimited growth of the population). 2. After reaching the limit K, mass mortality of individuals occurs, returning the population size to a certain lower limit, after which growth can begin again. Such fluctuations in numbers around a mean value (the carrying capacity of the environment) are typical for many animals.

Thus, the type of population dynamics reflects the correspondence between the organism's requirements and the actual environmental conditions. Anthropogenic influences can significantly affect population dynamics, deviating the historically established types from the settled norm.