The complex of properties of a population aimed at increasing the probability of survival and leaving offspring is called an ecological strategy of survival.
Ecological strategies are very diverse, their peculiarities are reflected in the peculiarities of population fluctuations. We will distinguish two extreme types:
1. populations of species adapted to existence in stable conditions (elephants, lions, penguins, whales, polar bears, etc.), as a rule, are not capable of rapid changes in numbers. Without human intervention, their numbers change smoothly, without sharp peaks or dips. Such a picture of dynamics is typical for organisms with a long development cycle, the populations of which include many age groups.
Example: in the same water body, for example, the number of pike, the population of which consists of 25 age groups, changes much more slowly than the number of bleak, the population of which includes only 3 age groups.
2. Other species living in temperate climates, especially annual animals (most insects) and plants (some types of grasses), are capable of rapid and dramatic changes in numbers.
These changes are broad in scope. In years of minimum and maximum abundance, the number of such species can differ by tens, hundreds, and sometimes thousands of times. These species are characterized by “population explosions” – sharp, explosive increases in numbers that occur almost suddenly. This happens when the conditions are especially favorable for the reproduction of organisms.
Populations of this type are usually the first to colonize new habitats in communities that are in the early stages of their development.
Example: bacteria, annuals, insect pests, etc.
In mature ecosystems, which include many different species of plants, animals and microorganisms, where biotic connections are developed and there is a strict distribution of resources used, relationships such as competition or predation become the main cause of fluctuations in the number of individual species.
Biotic relationships act as a kind of regulators.
They suppress “population explosions”, convert random changes into the form of regular periodic fluctuations, and in some cases stabilize the number of organisms.
Here we are faced with important properties that are endowed with ecological systems of different levels of organization (communities, populations, ecosystems):
- the functioning of an individual element of the system is determined by its connections with other elements;
- individual elements are interchangeable: the loss of one leads to the fact that another element occupying a similar position in the system begins to perform its functions (this is another type of regulation).
Communities, as it were, regulate the changes taking place in individual populations. Populations, on the other hand, help the ecosystem to preserve its properties even in the event of the loss of certain of its elements.
With the disappearance of one species, its place is taken by another, similar to the first in position in the trophic structure of the community.
Example: changes in the species composition of fish in water bodies where fishing is developing. A decrease in the number of the most valuable species as a result of fishing often leads to an increase in the number of so-called “weedy” fish, which are not of interest to fishermen. The species richness is decreasing, although the total fish population remains unchanged.