Density Independent Factors

Density Independent Factors Definition

Density independent factors, in ecology, refer to any influences on a population’s birth or death rates, regardless of the population density. Density independent factors are typically a physical factor of the environment, unrelated to the size of the population in question. Density independent factors vary depending on the population, but always affect the population the same regardless of its size. There are many common density independent factors, such as temperature, natural disasters, and the level of oxygen in the atmosphere. These factors apply to all individuals in a population, regardless of the density.

However, density independent factors are often confused density dependent factors for a number of reasons. First, density independent factors for one population of organisms is not the same for every organism on the planet. While oxygen is a density independent factor for most oxygen breathing organisms, it may be a density dependent factor for some. Image an obligate anaerobe bacteria, for instance. Oxygen is toxic to these organisms. As they grow in density, the bacteria furthest from the nearest source of oxygen is protected. If these bacteria where to grow thick, oxygen would not affect each bacteria, and the effect on the death rate would be lessened. This would make oxygen a density dependent factor for these particular bacteria.

Analyzing each population specifically allows scientists to identify their unique density independent factors. Below are several examples of common density independent factors and how they affect various species.

Examples of Density Independent Factors

Natural Disaster

Natural disaster is a perfect example of a density independent factor. Consider a hurricane, slamming into the coastline. While we often see the devastation of these storms on the news, we rarely consider the impacts of such a storm on wildlife and vegetation in the area. The fact is, hurricanes increase the death rate for many species, while some species see a highly increased birthrate after the destruction.

During a hurricane, winds increase to dangerous speeds, tearing large trees out of the ground. Trees like the one in the image above would survive any regular storm. For many species, a hurricane drastically increases the death rate, as the trees simply cannot withstand the wind and waves. Many animals, such as fish and amphibians, succumb to rapidly rising and falling tides. Many news images show pictures of fish washed up into roadways. These animals and plants die, regardless of how dense their population was. They could have been the last of their species, or one in a billion.

Yet, hurricanes do not only bring death. Consider the area unearthed by the tree in the above image. New, smaller plants will be given an opportunity to grow where they were previously restricted by the shade cast by the large tree. Fungi and insects living on dead plant matter will be able to feast and reproduce on the dead wood. The standing water left from the hurricane provides many insects, such as mosquitos, ample breeding sites. While this is often a nuisance for humans, it increases the food source of birds and bats, possibly increasing their birthrates as well. Yet, the hurricane affects all species and individuals within its path, regardless of how many there were.

Pollution

Like other density independent factors, pollution is a good example of a density independence. While humans are concentrated in cities around the globe, the emissions and chemicals we create are dispersed into the atmosphere. From here, they are carried globally and affect all organisms. Even organisms in the oceans are affected, as pollutants dissolve from the atmosphere into various water sources.

Therefore, whether you are the last pair of endangered clownfish in the ocean or have a huge population like sparrows, your birthrate is still negatively impacted. Density independent factors like these often cause a slow and steady drag on populations over time. Even the human population sees drastic health effects from pollution, from lead poisoning do to drinking water to increased lung diseases.

Honeybees

Instead of looking at density independent factors in general, let’s turn our view to a population of honeybees and the factors that likely affect the size of their population. Density independent factors for honeybees include things like weather and temperature. Regardless of the current size of their population, bees need the temperature and weather to stay within certain ranges. If the weather does not stick to this pattern, many bees will die. For example, if there was suddenly snowstorm in the middle of summer, the bees would be caught off guard and would die in the cold.

However, the bees also face a number of density dependent factors. For instance, their food source and its effects on their population is directly related to the size of their population. If they have a small population, there will be plenty of food for all and the bees will grow. If the population is larger than the amount of food available, bees will starve and the death rate will increase. Food, and other usable biological resources, are density dependent. Density independent factors will affect the bees regardless of how many bees are present.

Quiz

1. In a small garden patch under a small tree, several species of plant are planted in differing numbers. Consider the sunlight as a resource for the plants. Is sunlight one of the density independent factors, or is it density dependent?
A. Density Independent Factor
B. Density Dependent Factor
C. Neither

Answer to Question #1

B is correct. As the plants grow, they will take up a larger and larger area, competing for the sunlight. As the years go by, the tree will get larger, and take up more of the sunlight. If more plants are added to the area, there will be even less sunlight to go around. Density independent factors, such as weather events, will affect the plants regardless of how many are in the garden. Sunlight will be more beneficial to the plants if there are less of them in the garden.

2. Do density independent factors always limit the population? That is, do they always increase the death rate or lower the birth rate?
A. No
B. Yes
C. Only Density dependent factors do that

Answer to Question #2

A is correct. Any factor, density dependent or independent, can be either beneficial or negative. This all depends on the population being affected. Some density independent factors, such as hurricanes, help some species while they hurt other species. Hurricanes are a density independent factor because they generate the same effect regardless of the current population density.

3. A population of field mice increases after a farmer leaves his field unharvested for a season. Which of the following categories does this factor fall into?
A. Density Independent Factors
B. Density Dependent Factors
C. Increased death rate

Answer to Question #3

B is correct. Food is almost always a density dependent factor, because if the population gets too big the food abundance will quickly turn to a food shortage. Density independent factors would be things like temperature and tornadoes, which would affect the mice regardless of their current or future density.

References

Cain, M. L., Bowman, W. D., & Hacker, S. D. (2008). Ecology. Sunderland, MA: Sinauer Associates, Inc.

Feldhamer, G. A., Drickamer, L. C., Vessey, S. H., Merritt, J. F., & Krajewski, C. (2007). Mammology: Adaptation, Diversity, Ecology (3rd ed.). Baltimore: The Johns Hopkins University Press.

Kaiser, M. J., Attrill, M. J., Jennings, S., Thomas, D. N., Barnes, D. K., Brierley, A. S., & Hiddink, J. G. (2011). Marine Ecology: Processes, Systems, and Impacts. New York: Oxford University Press.