Also, when the population is denser, diseases spread more rapidly among the members of the population, which affect the mortality rate. Density dependent regulation was studied in a natural experiment with wild donkey populations on two sites in Australia. The high-density plot was twice as dense as the low-density plot. From to the high-density plot saw no change in donkey density, while the low-density plot saw an increase in donkey density. The difference in the growth rates of the two populations was caused by mortality, not by a difference in birth rates.
The researchers found that numbers of offspring birthed by each mother was unaffected by density. Many factors that are typically physical in nature cause mortality of a population regardless of its density. These factors include weather, natural disasters, and pollution. An individual deer will 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.
The same holds true for cold winter weather. In real-life situations, population regulation is very complicated and density-dependent and independent factors can interact. A dense population that suffers mortality from a density-independent cause will be able to recover differently than 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. Woolly mammoths began to go extinct about 10, years ago, soon after paleontologists believe humans able to hunt them began to colonize North America and northern Eurasia [Figure 4]. A mammoth population survived on Wrangel Island, in the East Siberian Sea, and was isolated from human contact until as recently as BC.
We know a lot about these animals from carcasses found frozen in the ice of Siberia and other northern regions. It is commonly thought that climate change and human hunting led to their extinction. A study concluded that no single factor was exclusively responsible for the extinction of these magnificent creatures. The maintenance of stable populations was and is very complex, with many interacting factors determining the outcome.
It is important to remember that humans are also part of nature. Population ecologists have hypothesized that suites of characteristics may evolve in species that lead to particular adaptations to their environments. These adaptations impact the kind of population growth their species experience. Life history characteristics such as birth rates, age at first reproduction, the numbers of offspring, and even death rates evolve just like anatomy or behavior, leading to adaptations that affect population growth.
K -selected species are adapted to stable, predictable environments. Populations of K -selected species tend to exist close to their carrying capacity. These species tend to have larger, but fewer, offspring and contribute large amounts of resources to each offspring.
Elephants would be an example of a K -selected species. They have large numbers of small offspring. Animals that are r -selected do not provide a lot of resources or parental care to offspring, and the offspring are relatively self-sufficient at birth. Examples of r -selected species are marine invertebrates such as jellyfish and plants such as the dandelion. The two extreme strategies are at two ends of a continuum on which real species life histories will exist.
In addition, life history strategies do not need to evolve as suites, but can evolve independently of each other, so each species may have some characteristics that trend toward one extreme or the other. Populations with unlimited resources grow exponentially—with an accelerating growth rate. When resources become limiting, populations follow a logistic growth curve in which population size will level off at the carrying capacity.
Populations are regulated by a variety of density-dependent and density-independent factors. Life-history characteristics, such as age at first reproduction or numbers of offspring, are characteristics that evolve in populations just as anatomy or behavior can evolve over time.
The model of r — and K -selection suggests that characters, and possibly suites of characters, may evolve adaptations to population stability near the carrying capacity K -selection or rapid population growth and collapse r -selection. Species will exhibit adaptations somewhere on a continuum between these two extremes. Describe the growth at various parts of the S-shaped curve of logistic growth. In the first part of the curve, when few individuals of the species are present and resources are plentiful, growth is exponential, similar to a J-shaped curve.
Later, growth slows due to the species using up resources. Finally, the population levels off at the carrying capacity of the environment, and it is relatively stable over time.
Any interactives on this page can only be played while you are visiting our website. You cannot download interactives. A limiting factor is anything that constrains a population's size and slows or stops it from growing.
Some examples of limiting factors are biotic, like food, mates, and competition with other organisms for resources. Others are abiotic, like space, temperature, altitude, and amount of sunlight available in an environment. Limiting factors are usually expressed as a lack of a particular resource. For example, if there are not enough prey animals in a forest to feed a large population of predators, then food becomes a limiting factor.
Likewise, if there is not enough space in a pond for a large number of fish, then space becomes a limiting factor. There can be many different limiting factors at work in a single habitat, and the same limiting factors can affect the populations of both plant and animal species. Ultimately, limiting factors determine a habitat's carrying capacity, which is the maximum size of the population it can support.
Teach your students about limiting factors with this curated collection of resources. Population density is the concentration of individuals within a species in a specific geographic locale. Population density data can be used to quantify demographic information and to assess relationships with ecosystems, human health, and infrastructure. What are the most densely populated places in the world? Find out with MapMaker, National Geographic's classroom interactive mapping tool. Density is the number of things—which could be people, animals, plants, or objects—in a certain area.
Join our community of educators and receive the latest information on National Geographic's resources for you and your students. Skip to content. Image Rabbits in the Field Female cottontail rabbits Sylvilagus floridanus are especially fertile, able to give birth to seven litters a year. Twitter Facebook Pinterest Google Classroom. Encyclopedic Entry Vocabulary. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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However, many sources of environmental stress affect population growth, irrespective of the density of the population. Density-independent factors, such as environmental stressors and catastrophe, are not influenced by population density change. While the previously mentioned density-dependant factors are often biotic, density-independent factors are often abiotic. These density-independent factors include food or nutrient limitation, pollutants in the environment, and climate extremes, including seasonal cycles such as monsoons.
In addition, catastrophic factors can also impact population growth, such as fires and hurricanes. The quality of nutrients e. The lower the quality of the nutrients, the higher the environmental stress. In the freshwater Laurentian Great Lakes, particularly in Lake Erie, the factor limiting algal growth was found to be phosphorus.
David Schindler and his colleagues at the Experimental Lakes Area Ontario, Canada demonstrated that phosphorus was the growth-limiting factor in temperate North American lakes using whole-lake treatment and controls Schindler This work encouraged the passage of the Great Lakes Water Quality Agreement of GLWQA — a reduction in phosphorus load from municipal sources was predicted to lead to a corresponding reduction in the total algal biomass and harmful cyanobacterial blue-green algae blooms McGuken ; Figure 3.
As annual phosphorus loads decreased in the mid s Dolan , there was some indication that Lake Erie was improving in terms of decreased total phytoplankton photosynthetic algae and cyanobacteria biomass Makarewicz Further improvement continued until the mid s, until an introduced species, the zebra mussel, began altering the internal phosphorus dynamics of the lake by mineralization excretion of digested algae Figure 3; Conroy et al.
C Change in Lake Erie seasonal average phytoplankton biomass in the central. Pollutants also contribute to environmental stress, limiting the growth rates of populations. Although each species has specific tolerances for environmental toxins, amphibians in general are particularly susceptible to pollutants in the environment.
For example, pesticides and other endocrine disrupting toxins can strongly control the growth of amphibians Blaustein et al. These chemicals are used to control agricultural pests but also run into freshwater streams and ponds where amphibians live and breed. They affect the amphibians both with direct increases in mortality and indirect limitation in growth, development, and reduction in fecundity. Rohr et al. These effects limit population growth irrespective of the size of the amphibian population and are not limited to pesticides but also include pH and thermal pollution, herbicides, fungicides, heavy metal contaminations, etc.
Environmental catastrophes such as fires, earthquakes, volcanoes and floods can strongly affect population growth rates via direct mortality and habitat destruction. A large-scale natural catastrophe occurred in when hurricane Katrina impacted the coastal regions of the Gulf of Mexico in the southern United States. Katrina altered habitat for coastal vegetation by depositing more than 5 cm of sediment over the entire coastal wetland zone.
In these areas, substantial improvement in the quality of wetlands for plant growth occurred after many years of wetland loss due to control of the Mississippi River flow Turner et al. At the same time, however, almost km 2 of wetland was destroyed and converted to open sea, completely eliminating wetland vegetation Day et al. More recently the Gulf oil spill in has again impacted the coastal wetland vegetation.
Though human derived, this large-scale environmental disaster will have long-term impacts on the population growth of not only vegetation but all organisms in the wetlands and nearshore regions of the Gulf of Mexico.
Blaustein, A. Ultraviolet radiation, toxic chemicals and amphibian population declines. Diversity and Distributions 9, — Clutton-Brock, T. Sex differences in emigration and mortality affect optimal management of deer populations. Nature , — Conroy, J. Recent increases in Lake Erie plankton biomass: roles of external phosphorus loading and dreissenid mussels. Journal of Great Lakes Research 31 Supplement 2 , 89— Day, J.
Restoration of the Mississippi delta: lessons from hurricanes Katrina and Rita. Science , — Gilg, O. Cyclic dynamics in a simple vertebrate predator-prey community. Makarewicz, J. Phytoplankton biomass and species composition in Lake Erie, to Journal of Great Lakes Research 19, — McGucken, W. Rohr, J. Lethal and sublethal effects of atrazine, carbaryl, endosulfan, and octylphenol on the streamside salamander Ambystoma barbouri. Environmental Toxicology and Chemistry 22, — Schindler, D.
Eutrophication and recovery in experimental lakes: implications for lake management. Sibley, R. Population growth rate and its determinants: an overview. Philosophical Transactions of the Royal Society B , — Turner, R. Wetland Sedimentation from Hurricanes Katrina and Rita. Wauters, L.
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