Season 1 Episode 4 - Chapter III Struggle for Existence

This podcast episode is dedicated to Chapter III from Charles Darwin's Origin of Species. The title of the chapter is Struggle for Existence.  We discussed Thomas Malthus and how his ideas of population growth influenced Charles Darwin. In particular, Malthus noted that human population growth is geometric which is clearly represented by the graph below that shows global human population growth in the last 300+ years.  Human population on earth is currently over 7 billion with the estimates that in the next 50 years we may level off somewhere between 7-16 billion people, which is a large margin of error. The thought is that cultural norms and female education will decrease individual female fertility which will ultimately dampen global population growth.

If true and human population stabilizes it will be unusual since humans would be exhibiting self-regulation of their population instead of having external mortality factors (disease or predation) determining population size.

Graph summarizing human population growth.


Although Malthus was correct in his view of human population growth, his prediction about the rate of food availability being linear was wrong because modern agriculture has also shown a geometric increase in yield/acre by utilizing evolution (artificial selection in breeding newer varieties of crops) and fossil fuels to operate machinery, irrigate crops and manufacture fertilizers, pesticides and herbicides to increase crop yield. This phenomenon has been named the Green Revolution and it began in western cultures at the end of World War II. The graphs below show how increases in yield/acre for two food crops (corn and wheat) and two other crops (cotton and tobacco) have mirrored the geometric increase of human population growth.

Increase productivity in selected crops

One of the exciting ideas that Darwin introduces in this chapter is the idea of trophic cascades, the interaction between the various organisms in an ecosystem. We discussed Darwin's beautiful example of the complex indirect relationship between cats in a village and the population of clover in the fields. Recently a complex example has come to light when wolves were reintroduced to Yellowstone national park. The reintroduced wolves altered the foraging behavior of elk thereby reducing the elk negative impact on young tree recruitment which increased forest and understory growth. Increase in tree growth along the river banks stabilized the soil and reduced erosion which resulted in improved water quality and river flow.Ultimately the reintroduction of wolves influenced fish communities, beaver abundance and other indirect effects which are documented in detail in a variety of scientific papers which can be found at the Global Trophic Cascades research program website from Oregon State University. The researchers have linked a large number of their original research papers for free download.

From Ripple et al. (2014) Science 343

 If you do not feel like reading, then we recommend you watch the short but wonderful TED talk by George Monbiot as he describes some of the complex trophic cascades uncovered in different ecosystems.

Josh and Sarah brought up the idea of how invasive species can illustrate the importance of predators and disease in controlling population size since many invasive species can dramatically increase their population size when compared to their native population sizes in the land of origin.

We mentioned Josh's favorite study species the Hemlock Wooly adelgid (Adelges tsugae),  kudzu (Pueraria montana var. lobata)  and winter creeper, all of which are invasive in the eastern US.

Wooly adelgid image from http://www2.ca.uky.edu/

Kudzu image from http://www.forestryimages.org

Winter creeper image from http://www.local12.com

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