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Have you ever wondered how biologists determine the populations of animals in a particular geographic area? Watch this video lesson to find out, and discover how scientists look at the survivorship and life histories of many different species.

Population Density

It is sometimes too difficult to individually count organisms to determine the population density
Population Density

Let’s talk about populations. You may remember that a population is defined as all organisms in a particular geographic area that belong to the same species.

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When biologists study populations, they usually measure them in terms of population density, or the number of organisms per unit of volume or area. In some cases, the population density can be determined by simply counting the number of organisms in an area and dividing by the area. But many populations are too large, or the individuals are too hard to see or locate to directly count. In these cases, biologists must estimate the actual population density using a variety of methods.One method is sampling, where biologists mark off several representative plots, count the individuals in each plot and then calculate the population density based on the sample plots.

Sometimes biologists will estimate population density based on indirect indicators, such as number of nests, burrows, tracks, droppings or markings that can be found.For populations of animals that move around a lot and are difficult to find, biologists often employ the mark-recapture method. In this method, a number of animals are trapped throughout the habitat of the population and marked with a tag, collar, band, etc.

and then released. After a few days or weeks have passed, to give the individual organisms enough time to randomly mix with the rest of the population, animals are again trapped and the percentage of individuals that are marked are used to estimate the entire population. For example, if biologists capture 50 kangaroo rats in a habitat and mark them with tags, then a week later they capture 100 kangaroo rats in the same habitat and 10 of them have tags, the biologists assume that they had marked 10% of the population in the first capture. This would mean that the 50 individuals that were marked the first time represent 10% of the entire population, so the biologists estimate that the entire population would consist of 500 individuals.

Life Histories and Survivorship Curves

One of the biggest factors that affect populations is life history, or the sequence of events in an organism’s life that relate to its survival and reproduction.

An organism’s life history includes factors such as number of offspring produced, frequency of reproduction, amount of care and resources dedicated to offspring and what kind of survivorship curve the organism exhibits. A survivorship curve is a graph of the number of individuals still alive at each age.

Graph showing the survivorship curves for humans, songbirds and frogs
Survivorship Curve

Here is an example of the survivorship curves for three animals with three different life histories. Let’s start with the curve for humans.

Humans display a type I survivorship curve, which means that humans have low death rates in the younger and middle age groups and high death rates in the oldest age groups. The low death rate in the younger age groups is due to the fact that humans invest a lot of resources, care and protection into their offspring, which gives each individual a very high probability of surviving to adulthood. The low death rate in the middle age groups is due to the adaptability and resiliency of humans and the lack of predation. And the high death rate in the oldest age groups reflects the human life span as defined by physiological limitations of age and accumulated damage.

Other organisms that display a type I survivorship curve include elephants, gorillas and annual grasses. Although annual grasses have a much shorter lifespan, a relatively high number of offspring survive to adulthood compared to other plants due to the relatively large size of their seeds compared to their mass and their ability to grow back even after most of their mass has been eaten by herbivores. The highest death rates occur at the end of their life span, usually when water in the upper soil levels is all used up at the end of the growing season.Songbirds display a type II survivorship curve, with a relatively steady death rate throughout their lifespan.

Songbirds invest a lot of energy and care into their offspring, which helps survivorship at the beginning of their life. However, predation, disease and lack of resources are all causes of mortality that are always present for songbirds of all ages, which causes the death rate to remain relatively steady throughout their life history.Most frog species display a type III survivorship curve, where the death rate is very high early in life and much lower in the middle and older age groups. This reflects a very high death rate for tadpoles mostly due to a high rate of predation.

Frogs compensate for the high death rates by producing a very large number of offspring each breeding season, and for the tadpoles that do survive to become adults, the adult frogs can use camouflage, their terrific jumping ability and their swimming ability to avoid predators. The key hallmarks of species with type III survivorship curves are the production of very large numbers of very small offspring with little parental investment in each offspring. Most insects have type III survivorship curves.

It should also be noted here that not all frogs display a type III survivorship curve. Some poison dart frogs lay only a few eggs at a time and then carry the eggs and tadpoles on their backs, investing a lot of parental care in each offspring. These species have much lower death rates for their offspring, and because they have poisons that protect them from predation throughout their adult lives, they display a type I survivorship curve similar to that of humans.

Defining Life Histories

Life histories are affected by survivorship curves, offspring, parental care, and predation rates
Factors that Affect Life Histories

Ecologists have tried to categorize and define the wide variety of life history strategies because, in addition to the survivorship curves, number of offspring produced, amount of parental care and rates of predation, there are still other factors, such as lifespan, homeostatic capability, time to maturity and number of reproduction cycles, that also affect life histories.One theory that attempted to explain why organisms had such different life histories is the r/K selection theory.

This theory was popular in the 1970s and 1980s and basically categorized organisms as being either r-selected species, which are short-lived species with a high growth rate that produce a large number of offspring, each of which has a low probability of survival to adulthood, or K-selected species, which are long-lived species with a slower growth rate that produce a small number of offspring into which the parents invest a lot of resources to ensure a high probability of survival to adulthood. You can think of r-selected species as those that exhibit a type III survivorship curve and K-selected species as those that exhibit a type I survivorship curve.However, r/K selection theory didn’t stop there. It further theorized that r-selected species were better suited to survive in an environment that was subject to frequent, dramatic changes and that K-selected species were better suited to survive in a stable environment. But the theory had several flaws. First, the theory failed to explain why both r- and K- selected organisms are found in pretty much every environment regardless of stability.

Second, the theory failed to explain why some organisms displayed both r- and K- selected features. For instance, giant sequoias are among the world’s largest and longest-living species, both of which are extreme cases of K-selected traits, but giant sequoias produce millions of very tiny seeds and have one of the lowest survivorship to adulthood of all species on Earth, which is an extreme r-selected feature. Finally, the theory was abandoned by most ecologists after fruit fly experiments in the 1980s and 1990s showed experimentally that r-selected traits were not selected for in unstable environments as the theory hypothesized. However, although r/K selection theory is no longer used by most ecologists, you may still come across it in some situations.Instead of r/K selection theory, most ecologists now recognize that the range of life histories do not fit neatly into just two or three precise categories. Each species’ life history is uniquely adapted to maximize its reproductive success based on all of the factors that affect its fitness.

The factors that have the greatest effect on one species may not be very important to another species, even if it shares the same habitat.There are also fundamental differences between plants and animals when it comes to life histories. For animals, generally speaking, the larger, longer-living animals tend to have fewer offspring and invest more in each one, while smaller animals tend to have large numbers of offspring that require little investment.

For plants, the situation tends to be reversed. Large trees that live a long time produce the most offspring, but very few of the offspring survive. Annual grasses, however, that only live a few months, put a relatively large proportion of their resources into each seed, and this results in a higher survival rate for their offspring.

Lesson Summary

In review, a population is defined as all organisms in a particular geographic area that belong to the same species. All populations are limited by a number of factors, which can include the size of the habitat, the amount of resources available, the population size of other competing species, predation and life history. When biologists study populations, they usually measure them in terms of population density, or the number of organisms per unit of volume or area.

Population densities can be determined by several methods, including direct counting, estimation by indirect indicators, sampling and the mark-recapture method.One of the largest factors affecting populations is life history, or the sequence of events in an organism’s life that relate to its survival and reproduction. An organism’s life history includes factors such as number of offspring produced, frequency of reproduction, amount of care and resources dedicated to offspring and what kind of survivorship curve the organism exhibits. A survivorship curve is a graph of the number of individuals still alive at each age.

If the death rate of a species is highest when it reaches old age, it is said to have a type I survivorship curve. If the death rate of a species is relatively constant over all age groups, it is said to have a type II survivorship curve. And if the death rate of a species is highest before the individuals reach maturity, it is said to have a type III survivorship curve.The r/K selection theory attempted to explain differences in life history based on the type of habitat an organism lived in and categorized species into two groups: r-selected species, which are short-lived species with a high growth rate that produce a large number of offspring, each of which has a low probability of survival to adulthood, and K-selected species, which are long-lived species with a slower growth rate that produce a small number of offspring into which the parents invest a lot of resources to ensure a high probability of survival to adulthood. However, this theory didn’t hold up to scientific scrutiny.

Now it is thought that each species has a specific life history custom-tailored to maximize the fitness of individual organisms based on the factors that play the biggest roles in the survival and reproduction of those individuals.

Lesson Objectives

After watching this lesson, you should be able to:

  • Define population density and describe the methods used to measure it
  • Define life history and survivorship curve, and explain the three different types of survivorship curves
  • Describe the r/K selection theory and understand why most ecologists no longer accept it

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