Population Ecology

The purpose of this blog post is to further inform you about the concept of population ecology and its increasing importance as a sub-topic of ecology. Throughout this post I will share with you the most important aspects of population ecology and further explore a very important question: “What are the main factors that affect the growth or decline of a population?”

Jay Withgott and Scott Brennan (1) describe population ecology as a term that “examines the dynamics of population change and the factors that affect the distribution and abundance of members of a population” (p. 62). The main reason that we study population ecology is because it helps us understand why certain populations increase or decrease in size over time. There are certain sub-topics of population ecology that help population ecologists determine whether or not a population will increase or decline in the future such as population density, distribution, sex ratio, age structure, and birth and death rates. Population density is the number of individuals in a given area. If the population density of an area is very high, that means that there are a lot of individuals in that area, which means there can be a lot of competition for food and other resources, but it is easier for them to find mates and help. When the population density is low it can be rather difficult for species to find mates and help, but there is not as much competition, which makes it easier for them to find food and other resources. Population distribution is a term used to describe the arrangement of organisms in an area. There are three different types of distribution: random, uniform, and clumped. Random distribution is when there is no uniform arrangement usually because there are a lot of resources in the area and can be easily located. Uniform distribution is an arrangement where individuals are evenly spaced because of competition for territory, which means resources are more scarce, such as water in the desert. Clumped distribution is when organisms arrange themselves according to the availability of resources they need to survive.

This image shows the different types of distribution arrangements that can be found amongst a population (2).

Sex ratio is the proportion of males to females, which can help show whether or not a population will increase or decrease in size over time.  If there is a 1:1 ratio in the population, it is more likely that the population will increase over time because the population is evenly balanced and there is no one left without a mate.  Age distribution is a term that describes the number of organisms of each age in a population.  Population ecologists can use this information to predict whether or not a population will grow in the future by examining the number of individuals in the reproductive range.  All of these different factors can determine the birth and death rates of a population.  For example, if there are a lot of old organisms in a population it is likely that they will die off soon, but for some organisms, such as the golden toad, it is the opposite because most of the young die after they are born.

The main purpose of this blog is to further examine a very important question that I have generated: “What are the main factors that affect the growth or decline of a population?”

Withgott and Brennan list four main factors that population growth or decline: Births, deaths, immigration, and emigration.  Both births and immigration add individuals to a population, whereas deaths and emigration remove individuals from a population.  An important topic that Withgott and Brennan explore is an unregulated population that grows at an exponential rate.  They state (3) “Each organism reproduces by a certain amount, and as a population gets larger, there are more individuals reproducing by that amount.  If there are adequate resources and no external limits, ecologists theoretically expect exponential growth” (p. 66).  This means that as a population continues to grow there will be more individuals reproducing, resulting in an ever-increasing population as long as there are no limits, such as food or space.

This graph shows exponential growth of the human population because of a lack of regulations (4).

As stated earlier immigration adds organisms to a population.  Typically, populations will immigrate to another area because of the availability of more resources or the lack of competition within the area.  When they immigrate to another area it adds a lot of individuals to the population that was already located in that area.

There are also certain factors that can lead to high death rates, such as overcrowding.  This reduces access to food, which creates a high level of competition and makes it harder to survive.  Also, overcrowding promotes the spreading of diseases because of the large number of organisms within a certain area.  When there is overcrowding in a certain area it is also more likely that predators will come because of easier access to food (5).  Emigration also reduces the population of an area because the organisms leave the area.  Certain factors that may influence a populations reason to emigrate.  Some of the main reasons that a population would emigrate is because of a lack of food, scarcity of space, or habitat destruction (6).  To learn more about certain factors that increase or decrease the size of a population, read more here.

In this blog I have shared with you the main principles of population ecology and their meaning.  I have also explored a question that I generated based on the concept of population ecology.  While researching this topic I have learned a lot about exponential growth and it has made me think about the human population and how we are growing at an exponential rate.  Indulging myself in this topic has caused me to think more about the many negative implications an unregulated population can have on the Earth.  The main thing I have thought about while writing this blog is what will happen if our population continues to grow at an exponential rate.


1. Withgott, J. & Brennan, S. (2010). Environment: The science behind the stories. 4th Ed. San Francisco: Pearson Education.

2. Britanica.  “Ecology: types of population distribution” (2012) Retrieved November, 10th 2o12 from: http://kids.britannica.com/comptons/art-108151/The-way-a-population-is-spaced-across-an-area-is

3.  Withgott, J. & Brennan, S. (2010). Environment: The science behind the stories. 4th Ed. San Francisco: Pearson Education.

4. “Population Growth” (2012). Retrieved November, 10th 2012 from: http://www.bio.miami.edu/dana/106/106F05_7.html

5. Population Ecology. “Crowding” (1998) Retrieved November ,10th 2012 from :http://www.cfr.washington.edu/classes.esrm.450/Lecture11/Harvest.pdf

6. Factors affecting population. “Emigration” (2010) Retrieved November, 10th 2012 from: http://www.tutorvista.com/content/biology/biology-iv/biotic-community/factors-affecting-population.php#e)-emmigration


2 responses to “Population Ecology

  1. Great blog about population ecology Evan! What kind of things can we expect if population continues to grow at an alarming rate? What kind of things will you take from you learnings and incorporate them into everyday life?

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