Why do ecologists study populations

In the real world, with its limited resources, exponential growth cannot continue indefinitely. Exponential growth may occur in environments where there are few individuals and plentiful resources, but when the number of individuals gets large enough, resources will be depleted, slowing the growth rate.

Eventually, the growth rate will plateau or level off Figure 8. This population size, which represents the maximum population size that a particular environment can support, is called the carrying capacity, or K. The formula we use to calculate logistic growth adds the carrying capacity as a moderating force in the growth rate.

Thus, the exponential growth model is restricted by this factor to generate the logistic growth equation:. Thus, population growth is greatly slowed in large populations by the carrying capacity K. This model also allows for the population of a negative population growth, or a population decline. A graph of this equation yields an S-shaped curve Figure 8 , and it is a more realistic model of population growth than exponential growth.

There are three different sections to an S-shaped curve. Initially, growth is exponential because there are few individuals and ample resources available. Then, as resources begin to become limited, the growth rate decreases. Finally, growth levels off at the carrying capacity of the environment, with little change in population size over time. The logistic model assumes that every individual within a population will have equal access to resources and, thus, an equal chance for survival.

For plants, the amount of water, sunlight, nutrients, and the space to grow are the important resources, whereas in animals, important resources include food, water, shelter, nesting space, and mates.

In the real world, phenotypic variation among individuals within a population means that some individuals will be better adapted to their environment than others. Intraspecific competition for resources may not affect populations that are well below their carrying capacity—resources are plentiful and all individuals can obtain what they need.

However, as population size increases, this competition intensifies. Yeast, a microscopic fungus used to make bread and alcoholic beverages, exhibits the classical S-shaped curve when grown in a test tube Figure 9. Its growth levels off as the population depletes the nutrients that are necessary for its growth. In the real world, however, there are variations to this idealized curve.

Figure 9. Yeast grown in ideal conditions in a test tube show a classical S-shaped logistic growth curve. Examples in wild populations include sheep and harbor seals Figure In both examples, the population size exceeds the carrying capacity for short periods of time and then falls below the carrying capacity afterwards.

This fluctuation in population size continues to occur as the population oscillates around its carrying capacity. Still, even with this oscillation, the logistic model is confirmed. If the major food source of the seals declines due to pollution or overfishing, which of the following would likely occur?

Populations with unlimited resources grow exponentially, with an accelerating growth rate. When resources become limiting, populations follow a logistic growth curve. The population of a species will level off at the carrying capacity of its environment. The logistic model of population growth, while valid in many natural populations and a useful model, is a simplification of real-world population dynamics. Implicit in the model is that the carrying capacity of the environment does not change, which is not the case.

The carrying capacity varies annually: for example, some summers are hot and dry whereas others are cold and wet. In many areas, the carrying capacity during the winter is much lower than it is during the summer.

Also, natural events such as earthquakes, volcanoes, and fires can alter an environment and hence its carrying capacity. Additionally, populations do not usually exist in isolation. They engage in interspecific competition : that is, they share the environment with other species, competing with them for the same resources.

These factors are also important to understanding how a specific population will grow. Nature regulates population growth in a variety of ways. These are grouped into density-dependent factors, in which the density of the population at a given time affects growth rate and mortality, and density-independent factors, which influence mortality in a population regardless of population density. Note that in the former, the effect of the factor on the population depends on the density of the population at onset.

Conservation biologists want to understand both types because this helps them manage populations and prevent extinction or overpopulation. Most density-dependent factors are biological in nature biotic , and include predation, inter- and intraspecific competition, accumulation of waste, and diseases such as those caused by parasites.

Usually, the denser a population is, the greater its mortality rate. 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 11 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 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. 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. 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 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. The r — and K -selection theory, although accepted for decades and used for much groundbreaking research, has now been reconsidered, and many population biologists have abandoned or modified it.

Over the years, several studies attempted to confirm the theory, but these attempts have largely failed. Furthermore, the theory ignored the age-specific mortality of the populations which scientists now know is very important.

New demographic-based models of life history evolution have been developed which incorporate many ecological concepts included in r — and K -selection theory as well as population age structure and mortality factors. While reproductive strategies play a key role in life histories, they do not account for important factors like limited resources and competition.

The regulation of population growth by these factors can be used to introduce a classical concept in population biology, that of K -selected versus r -selected species. By the second half of the twentieth century, the concept of K- and r-selected species was used extensively and successfully to study populations.

It includes length of life and survivorship factors as well. For this analysis, population biologists have grouped species into the two large categories— K -selected and r -selected—although they are really two ends of a continuum. K -selected species are species selected by stable, predictable environments.

Populations of K -selected species tend to exist close to their carrying capacity hence the term K -selected where intraspecific competition is high.

These species have few, large offspring, a long gestation period, and often give long-term care to their offspring Table 3. While larger in size when born, the offspring are relatively helpless and immature at birth. By the time they reach adulthood, they must develop skills to compete for natural resources. In plants, scientists think of parental care more broadly: how long fruit takes to develop or how long it remains on the plant are determining factors in the time to the next reproductive event.

Examples of K -selected species are primates including humans , elephants, and plants such as oak trees Figure Elephants are considered K-selected species as they live long, mature late, and provide long-term parental care to few offspring. Oak trees produce many offspring that do not receive parental care, but are considered K-selected species based on longevity and late maturation. Oak trees grow very slowly and take, on average, 20 years to produce their first seeds, known as acorns.

As many as 50, acorns can be produced by an individual tree, but the germination rate is low as many of these rot or are eaten by animals such as squirrels. In some years, oaks may produce an exceptionally large number of acorns, and these years may be on a two- or three-year cycle depending on the species of oak r -selection. As oak trees grow to a large size and for many years before they begin to produce acorns, they devote a large percentage of their energy budget to growth and maintenance.

Furthermore, when it does reproduce, the oak produces large, energy-rich seeds that use their energy reserve to become quickly established K -selection. In contrast, r -selected species have a large number of small offspring hence their r designation.

This strategy is often employed in unpredictable or changing environments. Animals that are r -selected do not give long-term parental care and the offspring are relatively mature and self-sufficient at birth. Examples of r -selected species are marine invertebrates, such as jellyfish, and plants, such as the dandelion Figure Dandelions have small seeds that are wind dispersed long distances. Many seeds are produced simultaneously to ensure that at least some of them reach a hospitable environment.

Seeds that land in inhospitable environments have little chance for survival since their seeds are low in energy content. Note that survival is not necessarily a function of energy stored in the seed itself. Dandelions and jellyfish are both considered r-selected species as they mature early, have short lifespans, and produce many offspring that receive no parental care.

Concepts of animal population dynamics can be applied to human population growth. Humans are not unique in their ability to alter their environment. For example, beaver dams alter the stream environment where they are built. Humans, however, have the ability to alter their environment to increase its carrying capacity sometimes to the detriment of other species e. The depletion of the ozone layer, erosion due to acid rain, and damage from global climate change are caused by human activities.

The ultimate effect of these changes on our carrying capacity is unknown. As some point out, it is likely that the negative effects of increasing carrying capacity will outweigh the positive ones—the carrying capacity of the world for human beings might actually decrease. To reach its biotic potential, all females would have to become pregnant every nine months or so during their reproductive years.

Also, resources would have to be such that the environment would support such growth. Neither of these two conditions exists.

In spite of this fact, human population is still growing exponentially. Human population growth since AD is exponential dark blue line. Notice that while the population in Asia yellow line , which has many economically underdeveloped countries, is increasing exponentially, the population in Europe light blue line , where most of the countries are economically developed, is growing much more slowly.

A consequence of exponential human population growth is the time that it takes to add a particular number of humans to the Earth is becoming shorter. Figure 16 shows that years were necessary to add 1 billion humans in , but it only took 24 years to add two billion people between and As already discussed, at some point it would appear that our ability to increase our carrying capacity indefinitely on a finite world is uncertain.

Without new technological advances, the human growth rate has been predicted to slow in the coming decades. However, the population will still be increasing and the threat of overpopulation remains. The time between the addition of each billion human beings to Earth decreases over time. Humans are unique in their ability to alter their environment with the conscious purpose of increasing its carrying capacity.

This ability is a major factor responsible for human population growth and a way of overcoming density-dependent growth regulation. Much of this ability is related to human intelligence, society, and communication. Humans can construct shelter to protect them from the elements and have developed agriculture and domesticated animals to increase their food supplies.

In addition, humans use language to communicate this technology to new generations, allowing them to improve upon previous accomplishments. Other factors in human population growth are migration and public health. Humans originated in Africa, but have since migrated to nearly all inhabitable land on the Earth.

Public health, sanitation, and the use of antibiotics and vaccines have decreased the ability of infectious disease to limit human population growth. The Population Dynamics of Vector-borne Diseases. Global Atmospheric Change and Animal Populations. Semelparity and Iteroparity. Causes and Consequences of Dispersal in Plants and Animals. Disease Ecology. Population Ecology Editor s :. Part of the Topic. Cell Biology. Scientific Communication.

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