A population is defined as a homogenous group of organisms with specific characteristics - typically, we mean a group of organisms that belong to the same species, which live in a specified area. The number of individuals in a population changes over time because of reproduction (birth rate), mortality (e.g., predation, starvation, parasitism, and senescence), immigration, and emigration. Increases and decreases in the numbers of a population are controlled by factors that eventually can limit population growth, and are therefore called 'limiting factors' (see later).
A population will often have interactions with several other populations, besides having intrapopulation interactions. Consider, for example, sticklebacks (tiny spiny fish living in brackish and freshwater habitats) that rely on copepods and cladocerans as food and are therefore sensitive to the variation in numbers of these organisms, which in turn are controlled by the amount of edible food (phytoplankton). The sticklebacks are preyed upon by larger fish like trout but are also sensitive to parasitism and to the density of their own population as well as to environmental factors such as temperature.
Thus, the definition of population dynamics is the variation in numbers, individual biomass, and age composition of a given population over a definite period of time. To understand the variability in the dynamics of a population, it is necessary to know the role of limiting factors (biotic and abiotic ones) as well as the potential interactions within populations in a larger matrix (the aquatic ecosystem).
The discipline of studying population dynamics is founded in the so-called mathematical biology that dates more than 200 years back in time. The earliest principle of population dynamics theory was the exponential law by T. Malthus (English demographer, 1766-1834) who identified the conflict between expanding human populations and the need to produce enough food. In the 1920s, scientists developed mathematical models for the study of populations and their interactions. The so-called Lotka-Volterra equations that were developed independently by A.J. Lotka (1880-1949) and V. Volterra (1860-1940) have since been incorporated in any ecological textbook and are often the foundation for more complex and realistic models as well as experimental studies of the interactions between predators and prey, competitive relationships between species, and the regulation of populations.
A related ecological research branch focused on constructions of energy budgets based on biomasses and processes instead of the dynamics of populations in terms of numbers and development stages. At an early stage the concept of trophic levels was developed by A. Thienemann (German freshwater biologist, 1882-1960) by which the flow of energy (food) is transferred through a series of organism or functional groups (like those in Table 1), from the producer level (i.e., phototrophs up the food chain) through several levels of consumers (i.e., heterotrophs).
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