The upper boundary of abundance (carrying capacity) or abundance in general is controlled by numerous factors; however, determining how these factors control populations can be difficult. Internal factors (e.g., interspecific competition for space, food, or light, life-history traits, and cannibalism) can control abundance as can external factors (e.g., environmental conditions). As an organism becomes abundant it can drive down the abundance of the nutrients or energy that they require. Increasing numbers of plants can decrease the quantity of nitrogen, phosphorus, potassium, and other essential elements. Increasing number of heterotrophs (organisms that ingest other organisms or organic particles) can reduce their food supply. Hence, with increasing abundance these organisms can no longer increase at a rapid rate because their supply of fuel is decreasing. Competition between organisms that are using the same fuel resources or space/shelter that become in short supply will lead to reduction in population growth. The manner in which competition can affect abundance of animals is through the need for food, territories, mates, or space, whereas plants may compete for light, nutrients, water, or space. As organisms become more abundant generally, there is an increase in predation or herbivory, disease, and in animal's antagonistic behavior. All of these factors by themselves or often in combination serve to increase mortality, hence decreasing population growth rates. These were examples of density-dependent controls on population changes but there are examples of density-independent factors. For example, in some instances bycatch of fishes could be considered density independent because the number of fishes caught in bycatch may not be determined by the size of the fish population. Mathematical modelers have predicted that when a population's growth rate is greatly affected by density-dependent factors, its abundance is more likely to fluctuate.
The effects of competition can be amazingly complex. Lotka and Volterra (in the mid-1920s) developed a simple model to predict the competition between two species. The change in abundance for each species was dependent on r, N, K for each species and a competition coefficient (i.e., the effect of one species on the growth of the other species). Depending on the strength and nature of the interactions, the two species could exist with stable populations, one or the other species dominates forcing the extinction of the other, or lastly that they could both exist in an unstable equilibrium. Competition will ultimately affect abundances negatively, by reducing the amount of resources to each of the competitors, and this interaction becomes increasingly complex when there are more than two competitors, as is the norm in nature.
Predation also affects abundance, and is often a density-dependent factor. As abundance of a species increases, organisms that consume that species likely increase. Hence, herbivores respond to an increase in a species they consume, as would a predator population increase with the increase of a prey species. The predator-prey relationship is another mechanism that regulates to some extent the size of populations in a density-dependent manner.
Species, through the process of natural selection, have developed certain life-history traits that affect their abundance. Population change is affected by individual traits, such as reproductive output, age at first reproduction, and survival. Species have evolved these traits to maximize their fitness (i.e., maximize their genetic contribution to future generations), which affects the intrinsic rate of increase of the species. Population change also is affected by characteristics affecting the population, such as many of the density-dependent factors. Abundance is affected by factors that affect metapopulations, such as immigration, emigration, and genetic exchange. The study of abundance is fascinating because it incorporates so many of the important factors of a species: individual traits, population controls, and metapopulation dynamics.
See also: Metapopulation Models; Populations: r- and K-Selection.
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