Abel, Environmental Issues. Measuring, Analyzing and Evaluating. There are four variables which govern changes in population size. Biotic Potential Populations vary in their capacity to grow. The maximum rate at which a population can increase when resources are unlimited and environmental conditions are ideal is termed the population's biotic potential. Each species will have a different biotic potential due to variations in the species' reproductive span how long an individual is capable of reproducing the frequency of reproduction how often an individual can reproduce "litter size" how many offspring are born each time survival rate how many offspring survive to reproductive age There are always limits to population growth in nature.
Populations cannot grow exponentially indefinitely. Exploding populations always reach a size limit imposed by the shortage of one or more factors such as water, space, and nutrients or by adverse conditions such as disease, drought and temperature extremes.
The factors which act jointly to limit a population's growth are termed the environmental resistance. They grow stronger as the population grows. Density-dependent factors include:. Predation and parasitism. Predators and parasites tend to prosper as the numbers of their prey increase.
Density-independent factors that increase the death rate are not influenced by population size. They include:. The carrying capacity is the size of a particular population that can be sustained by a particular environment for a long period of time.
It is important to note that the carryng capacity of an environment for a particular population changes if the envrionment changes. It is not a static parameter. If exponential population growth greatly exceeds the carrying capacity of an environment, a population "crash" results. Exponential growth continues until the population is much larger than the carrying capacity of the environment.
At some point, deaths increase rapidly from density dependent factors and the population drops well below the carrying capacity. In some cases, the population dies out altogether. A test tube of bacteria shows this pattern, getting very dense and then becoming completely clear as the population crashes from accumulated waste.
Populations of deer and other mammals have also shown population crashes. This is a topic of great importance to all of us. At present, the human population of the globe is growing exponentially. Our numbers have tripled in the last hundred years and there are now over 6 billion humans on the planet. The rate of human population growth has accelerated dramatically in the last hundred years because the death rate has declined, owing to the benefits of modern medicine and agriculture.
Birth rate has not yet declined proportionally. As we have seen, biological populations do not grow indefinitely. In both examples, the population size exceeds the carrying capacity for short periods of time and then falls below the carrying capacity afterwards. This fluctuation in population size continues to occur as the population oscillates around its carrying capacity. Still, even with this oscillation, the logistic model is confirmed.
Logistic population growth : a Yeast grown in ideal conditions in a test tube show a classical S-shaped logistic growth curve, whereas b a natural population of seals shows real-world fluctuation. Population regulation is a density-dependent process, meaning that population growth rates are regulated by the density of a population.
In population ecology, density-dependent processes occur when population growth rates are regulated by the density of a population. Most density-dependent factors, which are biological in nature biotic , include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites. Usually, the denser a population is, the greater its mortality rate.
In addition, low prey density increases the mortality of its predator because it has more difficulty locating its food source. An example of density-dependent regulation is shown with results from a study focusing on the giant intestinal roundworm Ascaris lumbricoides , a parasite of humans and other mammals.
The data shows that denser populations of the parasite exhibit lower fecundity: they contained fewer eggs. One possible explanation for this phenomenon was that females would be smaller in more dense populations due to limited resources so they would have fewer eggs.
This hypothesis was tested and disproved in a study which showed that female weight had no influence. The actual cause of the density-dependence of fecundity in this organism is still unclear and awaiting further investigation.
Effect of population density on fecundity : In this population of roundworms, fecundity number of eggs decreases with population density. Many factors, typically physical or chemical in nature abiotic , influence the mortality of a population regardless of its density.
They include weather, natural disasters, and pollution. An individual deer may be killed in a forest fire regardless of how many deer happen to be in that area. Its chances of survival are the same whether the population density is high or low. In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact.
A dense population that is reduced in a density-independent manner by some environmental factor s will be able to recover differently than would a sparse population.
For example, a population of deer affected by a harsh winter will recover faster if there are more deer remaining to reproduce. Privacy Policy. Skip to main content. Population and Community Ecology. See explanation Explanation: It would probably be more accurate to say that exponential growth will eventually exhaust any finite resource no matter how large - and probably sooner than you think.
Related questions What is Exponential Growth? What is the exponential growth formula?
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