(credit a: modification of work by Charles R. Petersburg, Russia, and (c) a one-month-old baby mammoth, named Lyuba, discovered in Siberia in 2007. The three photos include: (a) 1916 mural of a mammoth herd from the American Museum of Natural History, (b) the only stuffed mammoth in the world, from the Museum of Zoology located in St. For example, a population of deer affected by a harsh winter will recover faster if there are more deer remaining to reproduce.įigure 2. A dense population that is reduced in a density-independent manner by some environmental factor(s) will be able to recover differently than a sparse population. In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact. The same holds true for cold winter weather. Its chances of survival are the same whether the population density is high or low. An individual deer may be killed in a forest fire regardless of how many deer happen to be in that area. Many factors, typically physical or chemical in nature (abiotic), influence the mortality of a population regardless of its density, including weather, natural disasters, and pollution. Density-independent Regulation and Interaction with Density-dependent Factors In this population of roundworms, fecundity (number of eggs) decreases with population density. The actual cause of the density-dependence of fecundity in this organism is still unclear and awaiting further investigation. This hypothesis was tested and disproved in a 2009 study which showed that female weight had no influence. One possible explanation for this is that females would be smaller in more dense populations (due to limited resources) and that smaller females would have fewer eggs. ![]() Denser populations of the parasite exhibited lower fecundity: they contained fewer eggs. In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source.Īn example of density-dependent regulation is shown in Figure 1 with results from a study focusing on the giant intestinal roundworm ( Ascaris lumbricoides), a parasite of humans and other mammals. For example, during intra- and interspecific competition, the reproductive rates of the individuals will usually be lower, reducing their population’s rate of growth. Usually, the denser a population is, the greater its mortality rate. Most density-dependent factors are biological in nature (biotic), and include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Conservation biologists want to understand both types because this helps them manage populations and prevent extinction or overpopulation. Note that in the former, the effect of the factor on the population depends on the density of the population at onset. These are grouped into density-dependent factors, in which the density of the population at a given time affects growth rate and mortality, and density-independent factors, which influence mortality in a population regardless of population density. Nature regulates population growth in a variety of ways. These factors are also important to understanding how a specific population will grow. They engage in interspecific competition: that is, they share the environment with other species, competing with them for the same resources. ![]() Additionally, populations do not usually exist in isolation. Also, natural events such as earthquakes, volcanoes, and fires can alter an environment and hence its carrying capacity. In many areas, the carrying capacity during the winter is much lower than it is during the summer. The carrying capacity varies annually: for example, some summers are hot and dry whereas others are cold and wet. ![]() Implicit in the model is that the carrying capacity of the environment does not change, which is not the case. The logistic model of population growth, while valid in many natural populations and a useful model, is a simplification of real-world population dynamics. ![]()
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