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Given adequate conditions populations will grow. The growth of a population depends on several factors: Birth rate, death rate, immigration, and emigration. There are three basic interactions between species. When species populations grow interactions start to occur. 1.) Competition: The outcome is negative for both species involved 2.) Symbiosis: The outcome is beneficial for both species involved 3.) Predation-parasitism: The outcome is beneficial for one species and negative for the other. Competition The competitive exclusion principle states that two species that have the same exact requirements cannot coexist in the exact same habitat. Exponential Growth: If a population has a constant growth rate (which is determined by birth rate and death rate) that is not limited by food or resources the growth is said to be exponential . Constant exponential growth is not usually seen in nature. The availability of resources and space usually control the population of an organism. What is one species that has exhibited exponential growth throughout its existence? Natural populations usually exhibit something called logistic growth. There is an upper limit to the number of individuals the environment can support. Ecologists refer to this as the "carrying capacity" of the environment. Populations in this kind of environment show what is known as logistic growth. Carrying capacity refers to the maximum, equilibrium number of organisms of a particular species that can be supported indefinitely in a given environment. Organisms that live in stable environments tend to make few, "expensive" offspring. Organisms that live in unstable environments tend to make many, "cheap" offspring. r-selection, for those species that produce many The two evolutionary "strategies" are termed "cheap" offspring and live in unstable environments and K-selection for those species that produce few "expensive" offspring and live in stable environments. The following table compares some characteristics of organisms which are extreme r or K strategists: r K Unstable environment, density independent Stable environment, density dependent interactions small size of organism large size of organism energy used to make each individual is low energy used to make each individual is high many offspring are produced few offspring are produced early maturity late maturity, often after a prolonged period of parental care short life expectancy long life expectancy each individual reproduces only once individuals can reproduce more than once in their lifetime