SUMMARY

CONCEPT 15.1 In positive interactions, no species is harmed, and the benefits are greater than the costs for at least one species.

15.1.1 Compare mutualism and commensalism and give examples of their importance in communities.

Mutualism is a mutually beneficial interaction between species, and commensalism occurs when one species benefits but the other is not affected.

Mutualism and commensalism are ubiquitous interactions that are important in both terrestrial and aquatic communities.

15.1.2 Describe how positive interactions form and evolve over space and time.

Mutualism and commensalism can evolve from other kinds of ecological interactions; for example, over time, a host-parasite interaction may evolve to become a mutualistic interaction.

The costs and benefits of a positive interaction can vary from one place and time to another; as a result, a positive interaction may cease to be beneficial under some circumstances.

15.1.3 Explain how positive interactions can vary in their strength under different physical environments.

Positive interactions may be more common in stressful

environments.

CONCEPT 15.2 Each partner in a mutualistic interaction acts in ways that serve its own ecological and evolutionary interests.

15.2.1 Categorize different types of mutualisms.

Mutualisms can be categorized by whether one partner provides the other with food (a trophic mutualism) or a place to live (a habitat mutualism).

15.2.2 Justify why mutualisms are not altruistic.

The partners in a mutualism are in it for themselves—it is not an altruistic interaction.

Some mutualists have mechanisms to prevent overexploitation by cheaters.

CONCEPT 15.3 Positive interactions affect the abundances and distributions of populations as well as the structure of ecological communities.

15.3.1 Explain the consequences of positive interactions on the distribution and abundance of species.

Positive interactions can provide benefits that increase the growth, reproduction, or survival of one or both of the interacting species.

As a result of such demographic effects, positive interactions can determine the abundances and distributions of populations of the interacting species.

15.3.2 Describe how positive interactions can increase species diversity in communities.

Species that are dependent on positive interactions can be lost from the community, causing species diversity to decline.

If a dominant competitor depends on a facilitator, loss of the facilitator may reduce the performance of that dominant species and increase the performance of other species, thereby increasing species diversity.

15.3.3 Illustrate ways in which positive interactions can affect ecosystem processes.

Positive interactions can provide resources that contribute to the overall production of an ecosystem.

REVIEW QUESTIONS

1. Summarize the key features of positive interactions described in Concept 15.1.

2. Researchers who study mutualism do not think of it as an altruistic interaction. Explain why.

3. High water temperature is one of several stressors that can cause coral bleaching, in which a coral expels its algal mutualists and hence loses its color. If bleaching occurs repeatedly, coral death may result. Some corals are more sensitive than others to high water temperatures. If a reef containing a mixture of coral species were exposed to increasingly high water temperatures over a series of years, how might the community change over time?

HONE YOUR PROBLEM-SOLVING SKILLS

The survival rates of small seedlings of the Scotch pine tree (Pinus sylvestris) growing in open areas and shaded beneath Salvia shrubs are given in the table below, as are mean values for certain abiotic conditions.

Source: J. Castro et al. 2002. Restor Ecol 10: 297-305.

LIST OF KEY TERMS

arbuscular mycorrhizae cheaters Commensalism ectomycorrhizae facilitation habitat mutualisms Mutualism mycorrhizae positive interactions symbiosis trophic mutualisms