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Subscriber sign in You could not be signed in, please check and try again. Some loon nesting places have been taken over by human development and the loon population has decreased. Pollution can also hurt animal and plant populations. Sometimes hunting can impact animal populations.
Whale populations have been lowered because of overhunting. If the balance between predator and prey is changed, populations are changed. The white-tailed deer population in some areas has grown too large because there are no natural predators.
Mountain lions and wolves are the natural predators of the white-tailed deer. Wolf and mountain lion populations have been lowered due to overhunting and habitat loss. This loss of a natural predator for the white-tailed deer, along with other factors, has led to overpopulation of the white-tailed deer in some areas. One species, the collard lemming Dycrostonyx groenlandicus , is a chubby-looking rodent living in the arctic of North America and Greenland. It is food for a number of vertebrate predators, including the stoat a short-tailed weasel , the arctic fox, the snowy owl, and the long-tailed skua a seabird.
Because of the simplicity of this system, lemming population dynamics make an excellent case study for examining the factors regulating population growth.
Gilg et al. The lemming population increased and decreased in a regular four-year cycle during the study period, — Figure 1. The number of lemmings increased to as many as ten per hectare. The owl, fox, and skua switched to lemming predation as the lemming numbers increased, preventing rapid population growth. As the lemmings provided the stoat with additional food, their reproductive success increased, allowing an increased stoat population. Stoat population expansion eventually overran lemming population growth, and the lemming population collapsed, soon followed by a collapse in the stoat population, and the cycle repeated itself.
Limitations to population growth are either density-dependant or density-independent. Density-dependent factors include disease, competition, and predation. Density-dependant factors can have either a positive or a negative correlation to population size.
With a positive relationship, these limiting factors increase with the size of the population and limit growth as population size increases. With a negative relationship, population growth is limited at low densities and becomes less limited as it grows. Density-dependant factors may influence the size of the population by changes in reproduction or survival.
The red squirrel Sciurus vulgaris is a small rodent inhabiting forests in Europe and Asia. They studied squirrels in both coniferous and deciduous woodlands and investigated how limitations in food resulted in limitations in reproduction as population densities increased. They found that when squirrel densities were high, territoriality relegated some females to poor quality territory, which in turn reduced their reproductive success.
When squirrel densities were low, no females occupied the low-quality territory. Thus, it was not all individuals suffering from reduced ability to reproduce e.
Instead, a greater proportion of the population was living in poor-quality habitat, while those still living in good habitat continued to have success. This in turn led to a decrease in per capita birth rate, a limitation in population growth as a function of population density. Density dependant factors may also affect population mortality and migration. Clutton-Brock et al. Both juvenile and adult mortality was significantly affected by population density, with juvenile mortality more strongly influenced than adult mortality Figure 2.
Furthermore, they found that these differences were stronger among males than females, so that increasing population density caused a shift in the sex ratio of females to males.
This effect was enhanced by decreased male immigration and increased male emigration. Thus, density-dependant controls on population growth not only increased with increasing density, but also differentially affected males and females within the population. 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.
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