What is the difference between density dependant and density independent

Density independent limiting factors are the factors that influence the size and growth of population irrespective of the population density. In contrast, density dependent limiting factors are the biological factors that influence the size and the growth of population depending on the density of the population.

Overview and Key Difference 2. What are Density Independent Limiting Factors 3. What are Density Dependent Limiting Factors 4. Density independent limiting factors are the abiotic factors and environmental factors that regulate the population growth rate.

Generally, they are physical or chemical in nature. These factors affect the birth and death rates of a population. Some of these factors include climate extremes, natural disasters fires, floods, earthquakes and hurricanes and pollution.

Food or nutrient limitation is another density independent limiting factor. 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. 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. The Galapagos giant tortoise is a keystone species and its population has declined as a result of human activity. During the 18th and 19th centuries, whaling ships would stop at the Galapagos Islands and hunt giant tortoises to feed their crew.

Their population was estimated to be around , in the s, when they were first discovered. However, a census recorded just 3, It would be tornadoes that is a density-independent factor in controlling a population.

This is due to the reason that tornadoes are caused by natural causes which cannot be controlled by any human intervention. Physical factors that affect population density include water supply, climate, relief shape of the land , vegetation, soils and availability of natural resources and energy.

Human factors that affect population density include social, political and economic factors. Climatic conditions are perhaps the most important of all the geographic influences on population distribution.

Apart from physical factors several social, demographic, economic, political and historical factors affect population distribution. Begin typing your search term above and press enter to search.

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