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Positive interactions, when at least one species benefits from an interaction and none are harmed, play a key role in enhancing coexistence among species. Positive interactions are more common in stressful environments, where physical conditions can limit the population growth of some species. Positive interactions can have dramatic effects on populations and communities, especially by providing habitat for other species.
learning outcomes
You should be able to:
Give reasons why the types of mutualistic interactions, from generalized to specialized, and from obligate to facultative have evolved.
Explain, with examples, why stressful environments are more likely to give rise to positive species interactions.
List examples in which one species serves as habitat for other species. Explain whether each situation represents mutualism or commensalism, and justify your answer in each case.
Offer reasons for why insect pollinators and animal dispersers might evolve obligate versus facultative mutualistic relationships with plants.
An obligate mutualism would evolve when the two species involved in the interaction benefit from each other more than from other partners. A facultative mutualism would evolve when the two species involved in the interaction benefit from partnerships with multiple species. For example, if multiple species can deliver pollen or seeds to their desired locations, then facultative relationships will be more likely to evolve.
Refer to Figure 55.13. Where in the rocky intertidal might Semibalanus benefit from positive interactions with other species? How about Chthamalus? Explain your answer.
Both species of barnacles are more likely to benefit from positive interactions under the more stressful conditions of the high intertidal where desiccation occurs.
Give an example of how species that provide habitat could affect the number of species in a community.
Examples include trees, coral reefs, and kelp forests, where the habitat provided by these species facilitates many other species that depend on them.