BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.7. Population Ecology

10.7.3. Population Growth and Growth Curves

The processes of population growth and decline follow certain laws. Population growth is determined by the reproductive (biotic) potential of organisms and environmental resistance. Maximum reproductive potential is the theoretical rate of reproduction when environmental resources are unlimited. It depends on the age Structure OF THE population and the sex ratio (in sexually reproducing species).

Environmental resistance refers to the total sum of limiting biotic and abiotic factors (i.e., factors that restrict the capabilities of organisms) that prevent the realization of the maximum reproductive potential. This includes both external pressures acting on the population (predation, food availability, Temperature, light, space, etc.) and its internal regulatory mechanisms, such as intraspecific competition and territoriality. All these factors are linked by negative feedback mechanisms. For example, a shortage of a particular resource (such as food) intensifies intraspecific competition, which in turn reduces the population size, ensuring that enough of the resource becomes available again for all survivors.

The balance between biotic potential and environmental resistance corresponds to the so-called carrying capacity of the environment—that is, the number of individuals of a given species that can survive under the prevailing availability of resources.

Growth Curves

There are two main forms of growth curves: purely exponential (J-shaped) and sigmoidal (S-shaped).

The sigmoidal or S-shaped curve describes a situation where, in a habitat new to a population, density initially increases slowly (the lag phase, corresponding to the period of adaptation to conditions) and then rapidly, almost exponentially. After some time, the growth rate slows down and eventually becomes zero: birth rate is completely balanced by death rate (Fig. 10.19, A). The curve is said to reach a plateau. This slowdown in population growth is explained by increased intraspecific competition for resources, such as food or nesting sites. As a result, through negative feedback, the mortality rate of individuals increases and their reproduction slows down (fewer mating animals, an increase in stress-induced miscarriages, etc.). In other words, rising environmental resistance balances the biotic potential.

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Fig. 10.19. Types of Population growth curves. A. Sigmoidal (S-shaped) growth curve of a Yeast culture. A simple case where environmental resistance (here, the accumulation of metabolites excreted by the organisms themselves) is directly proportional to population density. B. Curve corresponding to the population growth of *Daphnia* (Water flea) in culture. This shape corresponds to periods of population density 'booms' and 'busts'.

This type of population growth is termed density-dependent, because for a given set of resources, the growth rate is determined by the number of individuals in the limited space occupied by the population. The Abundance of organisms corresponding to the curve reaching the plateau (zero growth rate) is the maximum carrying capacity of the environment for the given species.

S-shaped population growth curves are characteristic of many microorganisms, plants, and animals in both laboratory and field conditions. A clear example is Bacterial growth in a fresh culture medium (Fig. 12.8). S-shaped phytoplankton growth can be observed in spring in lakes and oceans. It is also observed in insects (such as flour beetles) and mites introduced into a new habitat with abundant food and an absence of predators.

Exponential growth without reaching a plateau (the J-shaped curve) corresponds to a situation where, following an initial adaptation period (the lag phase), the number of individuals increases sharply, but then growth suddenly stops when previously absent environmental resistance begins to manifest (Fig. 10.19, B). Such population growth is termed density-independent, because it is unchecked until the very last moment, which is followed by a mass die-off of individuals. This mortality, in turn, may be triggered either by the onset of an unfavorable season, the end of the breeding season of the organisms themselves or their main prey. Sometimes death is programmed into The life cycle of individuals immediately following reproduction (in annual plants, salmon), or it is caused by external intervention, such as insecticide application on crops to destroy pest insects. Overall, such populations are characterized by cyclic population 'booms' and 'busts' (population explosions and crashes), as seen, for example, in certain voracious phytophagous insects or associated with algal 'blooms'.