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 in Figure 1 with results from a study focusing on the giant intestinal roundworm Ascaris lumbricoides , a parasite of humans and other mammals. 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.
This hypothesis was tested and disproved in a study which showed that female weight had no influence. Figure 1. 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, including 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. 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 is reduced in a density-independent manner by some environmental factor s 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. Figure 2. The three photos include: a 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.
Petersburg, Russia, and c a one-month-old baby mammoth, named Lyuba, discovered in Siberia in Was it due to a meteor slamming into Earth near the coast of modern-day Mexico, or was it from some long-term weather cycle that is not yet understood? One hypothesis that will never be proposed is that humans had something to do with it. Mammals were small, insignificant creatures of the forest 65 million years ago, and no humans existed.
Woolly mammoths, however, began to go extinct about 10, years ago, when they shared the Earth with humans who were no different anatomically than humans today. Mammoths survived in isolated island populations as recently as BC. We know a lot about these animals from carcasses found frozen in the ice of Siberia and other regions of the north.
Scientists have sequenced at least 50 percent of its genome and believe mammoths are between 98 and 99 percent identical to modern elephants. It is commonly thought that climate change and human hunting led to their extinction. A study showed 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. While reproductive strategies play a key role in life histories, they do not account for important factors like limited resources and competition.
A female American toad Anaxyrus americanus can lay thousands of eggs every spring. So why are the meadows and forests of the eastern United States not literally hopping with rabbits and toads?
Cottontail rabbits need food to eat grasses and other plants , water to drink, and a safe place to raise their young. American toads eat insects and, though they often live in forest habitat, need ponds or puddles to lay their eggs. Both toads and rabbits have to watch out for predators. But even if they avoid a hungry hawk or snake, they face other potentially deadly dangers, including diseases, forest fires, or drought.
Any of these factors—food, shelter, breeding sites, predators, and more—may serve to limit the growth of a rabbit or toad population. Often, the population is affected by several limiting factors that act together. Density Matters—Unless It Does Not Limiting factors fall into two broad categories: density-dependent factors and density-independent factors.
These names mean just what they say: Density-independent factors have an impact on the population, whether the population is large or small, growing or shrinking.
For example, a wildfire that sweeps through a dense forest in the Everglades has a big impact on every population in the community, regardless of the density of any one population. Wildfire is abiotic nonliving , and most density-independent limiting factors fall in this category. Other density-independent factors include hurricanes, pollutants, and seasonal climate extremes.
Density-dependent limiting factors tend to be biotic —having to do with living organisms. Competition and predation are two important examples of density-dependent factors. Mountain chickadees Parus gambeli compete for a special kind of nest site—tree holes.
These little cavities are excavated and then abandoned by woodpeckers. Scientists who added new nest sites in one expanse of forest saw the chickadee nesting population increase significantly, suggesting that nest sites are a density-dependent limiting factor.
A small furry rodent found in eastern Greenland called the collared lemming Dicrostonyx groenlandicus is a good example of how predation can be a density-dependent limiting factor. The population goes through a boom-and-bust cycle every four years. The lemming population grows to as much as 1, times its initial size, then crashes. The cause is stoats Mustela erminea , a type of weasel that hunts and eats lemmings almost exclusively.
Stoats do not reproduce as fast as lemmings, so after a crash, when both stoat and lemming numbers are low, stoats do not have much impact on the lemming population. But by the fourth year, after the stoat population has had time to grow to greater numbers, the stoats—together with other predators—cause another lemming crash, and the cycle continues. Carrying Capacity If a population is small and resources are plentiful, a population may grow quickly.
Species population growth or decline can be caused by either density-dependent or density-independent factors:. High biodiversity can help to stabilise an ecosystem and reduce the overall impact of density-dependent and density-independent factors.
Biodiversity is a measure of the difference between the living organisms within an ecosystem. With many endemic species and a great range of wildlife, Galapagos has a high level of biodiversity. This is due to the number of very different habitat zones found in Galapagos, and also due to more acute environmental differences between the habitats of each island.
Biodiversity is an important factor in ensuring a healthy ecosystem. For example, an ecosystem with a wide range of producers will provide the primary consumers with a stable and varied food supply. Each species also plays a unique role in servicing the ecosystem, ensuring that it operates smoothly.
Ecosystems with a high level of biodiversity are more able to recover from disasters, whether natural or man-made anthropogenic. A fluctuation in the size of one species population can impact on other species within the ecosystem.
A species that will have a large impact on the ecosystem is known as a keystone species. The more biodiverse an ecosystem, the less vulnerable it will be to fluctuation in keystone species populations.
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