10 Species Interactions

Andrea Bierema


Learning Objectives

Students will be able to:

  • Define niche.
  • Describe types of species interactions.
  • Define competitive exclusion and resource partitioning principles.
  • Use food webs to infer examples of species interaction within a community.
  • Use ecological models to appropriately predict how an abundance of species may impact other species within a community.

Community Ecology

Ecology is studied at different scales and species interactions are part of the “community ecology” scale.

Two-Species Interactions

Community ecology includes the ways in which species interact. Research sometimes focuses on two species of a complex community and the general ways those species interact with one another can be classified by whether the species are positively, negatively, or neutrally impacted. The following video describes the main types of species interactions, with examples. The table below summarizes these types.

For closed captioning or to view the full transcript see the video on YouTube. Or click on the “YouTube” link in the video.

Species Interactions

Interaction Definition Species A Species B Possible Symbiosis (i.e., lives in close proximity to each other)
Mutualism* A long-term, close association between two species in which both partners benefit + + Yes
Commensalism* Species A benefits from the relationship and species B is not affected. + N Yes
Consumption: Parasitism Species A consumes part- but typically does not kill- species B. + - Yes
Consumption: Parasitoidism* A new generation of species A consumes species B. + - Yes
Consumption: Predation Species A consumes species B. + - No
Consumption: Herbivory Species A consumes part- but does not kill- Species B. + - No
Competition Two species “fight” over a resource. Although one species may “win,” it is still negatively impacted by taking part in the competition - - No

A list of ways that two species can interact with each other, including their definitions and how the species are impacted by the interaction. Some of these interactions can be symbiotic.


Test your understanding of species interactions!

Learn more about “aquatic cleaning stations“!

Niches and How They Influence Competition

Resources are often limited within a habitat and multiple species may compete to obtain them. All species have an ecological niche in the ecosystem, which describes how they acquire the resources they need and how they interact with other species in the community. The competitive exclusion principle states that two species cannot occupy the same niche in a habitat. In other words, different species cannot coexist in a community if they are competing for all the same resources.

An example of this principle is shown below with two protozoan species: Paramecium aurelia and Paramecium caudatum. When grown individually in the laboratory, they both thrive. When they are placed together in the same test tube (habitat), P. aurelia outcompetes P. caudatum for food, leading to the latter’s eventual extinction.

Three graphs: P. aurelia growing alone successfully (population grew to 250 cells after 10 days and then leveled off), P. caudatum growing alone successfully (population grew to 70 cells in 10 days and stabilized), and both species growing together in which P. aurelia grows successfully (grew to 230 in 14 days and stabilized) and P. caudatum only grows to 20 cells and dies about after 15 days.
Paramecium aurelia (graph a) and Paramecium caudatum (graph b) grow well individually, but when they compete for the same resources, the P. aurelia outcompetes the P. caudatum. In graph c, the top growth curve is P. aurelia and the bottom growth curve is P. caudatum.


Does competition between two species that share a similar niche always result in one dying off? Try out the Virtual Biology Lab’s simulation of barnacle competition. Once on the website, read through the background information and the tutorial. Then run the experiment- feel free to keep the variables consistent or see what happens when you change them.

Multi-Species Species Interactions

This section refers to food webs. Please see Khan Academy’s Food Chains and Food Webs for a review.

In community ecology, we can examine the interaction of two species or we can think at a larger, more complex scale and examine how many species interact. One way to do so is to start with a food web to identify some of the interactions that are occurring within a community. The following video describes this in more detail using the example of a coastal food web.

For closed captioning or to view the full transcript see the video on YouTube. Or click on the “YouTube” link in the video.


Arrows from kelps & other algae to sea urchins and sessile invertebrates. Arrow from microscopic planktonic algae to sea urchins. Arrows from sessile invertebrates to sea stars and larger crabs. Arrows from sea urchins to sea starts and sea otters. Arrows from sea starts to larger crabs and sea otters. Arrows from larger crabs to sea otters and larger fishes & octopuses.
This is the coastal community food web shown in the video above by Khan Academy.

Species Interaction Model

The food web is a model that illustrates how energy moves. Models can also be created to show how species interact with one another (beyond predator/prey and competition for food). The following model was introduced in the video. Rather than having each line represent energy movement, they can be labeled with different species interactions such as mutualism. This is a visual way to explain the complexity of species interactions within a community.

20 species on the outside of a circle and each species is connected to at least two other species with lines.
A model showing the connections between species within a community.

For instance, the following model is a complete species interaction model using the species from the video’s Pacific coast food web. Notice how species are now connected with lines rather than arrows. Interactions influence both species involved, while an arrow in a food web indicates the direction that energy moves. Each end of a line is also labeled with a positive (+), negative (-), and neutral (N), indicating how the species is impacted by the interaction. These symbols align with the symbols used in the interactions table shown toward the beginning of the chapter. For instance, in looking at the lines/interactions labeled as “competition for food,” both species are labeled as being negatively impacted by placing a negative on each side of the line.

The “predation” and “competition for food” interactions align with the food web. Food webs allude to predation interactions and if species are eating the same species, then they are likely competing for the food source. The rest of the interactions labeled are from outside research, rather than from studying the food web.

See caption and text below the image.
A species interaction model, connecting species with lines that are labeled with the interaction taking place (e.g., predation).

The interactions in the above species interaction model are (in no particular order):

  • Predation of sea otters (+) on sea stars, sea urchins, larger crabs, and larger fishes (-)
  • Sea otters (-) compete for food with sea stars and larger fishes (-)
  • Sea otters (+) hide in kelp & other algae (neutral), which is a commensalism
  • Larger fishes (+) have a mutualism with invertebrates (+) because some invertebrates “clean” the fishes and consume what they clean.
  • Larger crabs (+) are predators of invertebrates (-)
  • Larger crabs (-) and kelps & other algae (-) compete for space
  • Invertebrates (+) are predators of kelps and other algae (-) and microscopic planktonic algae (-)
  • Invertebrates (-) compete for food with abalones and sea urchins
  • Larger fishes (-) and sea stars (-) compete for food
  • Larger fishes (+) lay eggs on kelp (neutral), which is a commensalism
  • Sea stars (+) are predators of abalone (-), invertebrates (-), and larger crabs (-)
  • Abalones (+) are predators of microscopic planktonic algae (-)
  • Sea urchins (-) compete for food with abalone (-) and invertebrates (-)
  • Sea urchins (+) are predators of microscopic planktonic algae (-) and kelps & other algae (-)
  • Larger fishes (+) are predators of larger crabs


This chapter is a modified derivative of the following articles:

“Community Ecology” by OpenStax College, Biology 2e, CC BY 4.0. Access for free at https://openstax.org/books/biology-2e/pages/1-introduction

Interactions in Communities” by Khan Academy, CC BY-NC-SA 4.0.

Share This Book