The Princeton Guide to Ecology

By Simon A. Levin | Go to book overview

Population Ecology

H. Charles J. Godfray

Understanding what determines the average abundance of species, why their numbers fluctuate, and how they interact with each other is a major part of modern ecology often united under the term population ecology. Of course, the boundaries of population ecology are ill-defined and porous: on the one hand the field grades into physiological ecology—how individuals interact with the environment—and on the other hand into community ecology—the study of large assemblages of species. Population ecology is part of the larger subject of population biology that encompasses both the evolutionary and the ecological processes affecting populations.

The human race has always been concerned with the abundance and fluctuations of the plants and animals that share its environment, not least because they provide its food. But the modern study of populations begins with Thomas Malthus (1798), who, in his Essay on the Principle of Population, realized that if birth and death rates remain constant with the former greater than the latter, then population size will grow geometrically until some extrinsic factor comes into play. The conclusions that Malthus, an upper-class English vicar, drew from his insights were of the importance of doing something about the “irresponsibly fecund lower orders” (as well as the need to attend to other “problems” such as “liberal women” and the French!). Fortunately, Malthus is not remembered as a politician, but his writing hugely influenced the first generation of biologists to think about animal populations, and in particular Charles Darwin, who realized that geometric population growth implied massive mortality and hence a huge advantage to any heritable trait that helped individuals in the struggle for survival. Today we use the Malthusian parameter, the population’s rate of geometric growth assuming demographic parameters remain the same, as an index of the state of the population. A closely related parameter, the growth rate of a rare mutation, is intimately connected to notions of evolutionary fitness. Calculating population growth rates (population projection) is quite straightforward for some species, for example, those with discrete generations. It can be much more complicated when there are overlapping generations and where the population is composed of individuals of different classes (differing in age, size, or other variable), issues discussed in chapter II.1.

But demographic rates do not remain the same forever, and in particular, as population densities increase, birthrates go down or death rates go up. It is these density-dependent effects that are critical in determining the typical range of abundance of different organisms, as discussed in chapter II.2. Densitydependent effects may increase smoothly as population size gets larger but may also be much more capricious, cutting in only above a threshold, the latter itself possibly varying from year to year. The chief factor determining observed population densities at any particular time is often a density-independent process such as the weather, and the densities of some populations may fluctuate in a random way for many generations before they become large enough for densitydependent processes to come into play. However, no population can be regulated, that is, persist indefinitely within certain bounds, without density dependence occurring.

Where density-dependent processes act instantaneously and increase gently with population size, the outcome of population regulation will be a stable equilibrium (though in nature random perturbations will mean that an absolutely constant population density is unlikely to be observed). But if there is a time lag between population increase and the impact of density dependence, or if density dependence is very strong, then overcompensation may occur, and the population will show cycles. As was first realized by ecological theoreticians, particularly by Robert May, in the 1970s, stronger density dependence and larger time lags may lead to population fluctuations that are chaotic—purely deterministic yet impossible to predict in detail. Hastings (chapter II.3) explores these issues and discusses recent findings about how deterministic


Notes for this page

Add a new note
If you are trying to select text to create highlights or citations, remember that you must now click or tap on the first word, and then click or tap on the last word.
One moment ...
Default project is now your active project.
Project items

Items saved from this book

This book has been saved
Highlights (0)
Some of your highlights are legacy items.

Highlights saved before July 30, 2012 will not be displayed on their respective source pages.

You can easily re-create the highlights by opening the book page or article, selecting the text, and clicking “Highlight.”

Citations (0)
Some of your citations are legacy items.

Any citation created before July 30, 2012 will labeled as a “Cited page.” New citations will be saved as cited passages, pages or articles.

We also added the ability to view new citations from your projects or the book or article where you created them.

Notes (0)
Bookmarks (0)

You have no saved items from this book

Project items include:
  • Saved book/article
  • Highlights
  • Quotes/citations
  • Notes
  • Bookmarks
Cite this page

Cited page

Citations are available only to our active members.
Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

(Einhorn, 1992, p. 25)

(Einhorn 25)

1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25,

Note: primary sources have slightly different requirements for citation. Please see these guidelines for more information.

Cited page

Bookmark this page
The Princeton Guide to Ecology
Table of contents

Table of contents



Text size Smaller Larger Reset View mode
Search within

Search within this book

Look up

Look up a word

  • Dictionary
  • Thesaurus
Please submit a word or phrase above.
Print this page

Print this page

Why can't I print more than one page at a time?

Full screen
/ 810

matching results for page

    Questia reader help

    How to highlight and cite specific passages

    1. Click or tap the first word you want to select.
    2. Click or tap the last word you want to select, and you’ll see everything in between get selected.
    3. You’ll then get a menu of options like creating a highlight or a citation from that passage of text.

    OK, got it!

    Cited passage

    Citations are available only to our active members.
    Buy instant access to cite pages or passages in MLA, APA and Chicago citation styles.

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn, 1992, p. 25).

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences." (Einhorn 25)

    "Portraying himself as an honest, ordinary person helped Lincoln identify with his audiences."1

    1. Lois J. Einhorn, Abraham Lincoln, the Orator: Penetrating the Lincoln Legend (Westport, CT: Greenwood Press, 1992), 25,

    New feature

    It is estimated that 1 in 10 people have dyslexia, and in an effort to make Questia easier to use for those people, we have added a new choice of font to the Reader. That font is called OpenDyslexic, and has been designed to help with some of the symptoms of dyslexia. For more information on this font, please visit

    To use OpenDyslexic, choose it from the Typeface list in Font settings.

    OK, got it!

    Cited passage

    Thanks for trying Questia!

    Please continue trying out our research tools, but please note, full functionality is available only to our active members.

    Your work will be lost once you leave this Web page.

    Buy instant access to save your work.

    Already a member? Log in now.

    Author Advanced search


    An unknown error has occurred. Please click the button below to reload the page. If the problem persists, please try again in a little while.