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A number of individuals of the same species in a given area constitute a population. The number typically ranges anywhere from a few individuals to several thousand individuals. Bacterial populations can number in the millions.
Populations live in a place or environment called a habitat. All of the populations of species in a given region together make up a community. For example, in an area of tropical grassland, a community might be made up of grasses, shrubs, insects, rodents and various species of hoofed mammals.
The populations and communities found in a particular environment are determined by abiotic and biotic limiting factors. These are the factors that most affect the success of populations. Abiotic limiting factors involve the physical and chemical characteristics of the environment. Some of these factors include:
• amounts of sunlight,
• annual rainfall,
• available nutrients,
• oxygen levels and
For example, the amount of annual rainfall may determine whether a region is a grassland or forest, which in turn, affects the types of animals living there.
Each population in a community has a range of tolerance for an abiotic limiting factor. There are also certain maximum and minimum requirements known as tolerance limits, above and below which no member of a population is able to survive. The range of an abiotic factor that results in the largest population of a species is known as the optimum range for that factor.
Some populations may have a narrow range of tolerance for one factor. For example, a freshwater fish species may have a narrow tolerance range for dissolved oxygen in the water. If the lake in which that fish species lives undergoes eutrophication, the species will die. This fish species can therefore act as an indicator species, because its presence or absence is a strict indicator of the condition of the lake with regard to dissolved oxygen content.
Biotic limiting factors involve interactions between different populations, such as competition for food and habitat. For example, an increase in the population of a meat-eating predator might result in a decrease in the population of its plant-eating prey, which in turn might result in an increase in the plant population the prey feeds on.
Sometimes, the presence of a certain species may significantly affect the community makeup. Such a species is known as a keystone species. For example, a beaver builds a dam on a stream and causes the meadow behind it to flood. A starfish keeps mussels from dominating a rocky beach, thereby allowing many other species to exist there.
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