Population & Community Ecology

Learning Objectives

  1. Define and recognize a population as a group of interacting individuals who are members of the same species, a community as a set of species that interacting with each other, and an ecosystem as organisms in a community interacting with each other and with the non-living components in their environment, such as lakes, rivers, mountains, soil, rainfall, etc.
  2. Know that four factors matter for the growth of a population of members of the same species: births and immigration increase population size, while deaths and emigration decrease population size. Identify examples of each of these factors from the lecture video.
  3. List and explain the four “Serengeti Rules.”
  4. While Carroll’s rules are named after one specific ecosystem, the Serengeti, give examples from other ecosystems that attest to the generality of these rules of ecosystem resilience.
  5. Know the named detrimental (–) / beneficial (+) interactions, including that predation, parasitism, and herbivory are enemy/victim interactions, and recognize examples of them.

Video Lecture

Read the learning objectives above, then view this video lecture (from minute 8:31 to 1:00:23) by Dr. Sean Carroll on The Serengeti Rules. Dr. Carroll spells out four “rules” for how communities of organisms are regulated. Using the learning objectives as your study guide, bring questions to class about the ideas in the video. (Reminder: playback speed can be adjusted using the settings icon in the lower right corner of the video.)

Interspecific interactions can be classified as beneficial, detrimental, or neutral for one or both species in the interaction, like this:




Competition, where two species use the same limited resource and thus are in conflict with each other over that resource, can be confused as an enemy-victim relationship; however, it doesn’t fit the definition because both species are harmed rather than one benefiting.

Enemy-victim interspecific interactions have a negative impact on one species while providing benefits to the other. The three types are predator-prey interactions, parasite-host interactions, and herbivory. These are all win-lose situations.  Parasitism, but not predatory or herbivory relationships, is a type of symbiosis.

In the parasite-host interaction, also called parasitism, one species (the parasite) lives on or in another species (the host) that it harms over time, sometimes even resulting in the death of the host. Parasitism is a win for parasite and a loss for the host.

Another win-lose scenario is predator-prey interactions, or predation. Predation involves one species (the predator) consuming—yes, eating—the other (they prey), which obviously dies in the exchange. A well-studied example is the lynx-hare interaction in the Canadian arctic.

Herbivory is similar to predation in the sense that a grazer eats a plant. However, some grazing interactions can be argued to provide a benefit because grazing can promote the growth of new plant material. To determine if an herbivore-plant interaction is a win-lose, the researcher needs to measure the cost and benefit to the plant.

Finally, mutualism is a type of symbiosis where both species benefit in the interspecific interaction. For example, plants have fungi that live on or in their roots, called mycorrhizae. The fungus gains access to sugars like glucose and sucrose from the root system, and the plants use the phosphorus and nitrogen that the fungi “fix,” or process into a biologically usable form. Without this fairly universal mutualism in almost all plants, we would not have crops to eat or as much oxygen to breathe.