Positive interactions, or facilitations, are ubiquitous on Earth. Consider the simple fact that most vascular plants have beneficial pollinators, seed dispersers, and root-associated fungi and bacteria that are key to their survival. Consider all the species that provide essential habitat for other species. These positive interactions, while common, were often overlooked or underappreciated until recently.

As mentioned in Key Concept 55.1, there are two types of positive interactions: mutualisms, in which both species benefit from the interaction, and commensalisms, in which one species benefits and the other is unaffected. Some positive interactions are symbiotic. In addition, some interactions can be obligate (necessary) while others are facultative (optional).

Table 55.1 lists mutualistic interactions that range from highly specialized obligate symbioses to highly generalized facultative nonsymbioses. Mutualistic interactions allow organisms to access limiting resources and often involve an exchange of food for housing or defense. Plants and their mycorrhizal fungi (see Key Concept 29.2 and Figure 29.9), corals and their photosynthetic endosymbionts (see Key Concept 26.4), and lichens formed from fungi and photosynthetic algae (see Key Concept 29.2 and Figure 29.9) all provide examples of obligate and/or facultative mutualistic interactions in which food is exchanged for housing and nutrients.

Some common mutualisms occur between sessile organisms—particularly flowering plants—and mobile animal species that can pollinate flowers or disperse their offspring. These interactions can be highly specialized or very general and facultative. For example, about three-fourths of the 250,000 flowering plant species on Earth require the transport of pollen by an animal partner. The benefit from the plant’s perspective is clear: the animals move pollen from one plant to another and thereby promote sexual reproduction and thus genetic diversity. The most direct reward for pollinators is the pollen itself, which serves as food. Plant reproduction would not be served, however, if pollinators were to eat all of a plant’s pollen; thus plants have evolved various adaptations to ensure that they benefit from the exchange. For example, some plants have two types of anthers: feeding anthers to produce pollen for pollinators, and fertilization anthers to produce pollen for reproduction.

1201

Plants not only need to attract pollinators, but must also ensure that those pollinators carry their pollen to other members of the same species. Repeat visits by a pollinator to different individuals of a particular plant species increase the likelihood that the pollen will end up on the appropriate stigma; thus some plants have adaptations to encourage repeat visits by a given animal, while discouraging others. The nectar of tobacco flowers, for example, contains trace amounts of nicotine, an insecticidal neurotoxin. Many flower visitors, including hummingbirds, can ingest only tiny amounts of nicotine-laced nectar before moving on to other flowers. To other pollinators, however, nicotine may actually be addictive. Putting small amounts of a potentially addictive substance in nectar may be one way tobacco plants improve their odds of a repeat visit by the right pollinator species.

Many animals that eat fruits (called frugivores) provide a valuable service to the plants that produce those fruits by dispersing seeds. Seed dispersal by animals not only offers plants the advantages of delivery to potential germination sites away from the parent plant (described in Key Concept 37.1), but comes with the bonus of organic fertilizer for the seeds. Interactions between plants and frugivores, however, are not always reciprocal; in many cases, one party benefits more than the other. Whereas the frugivore is paid “in advance” for its transportation services, the seeds may never reach an appropriate destination for germination (your windshield, for example, will not do). From the plant’s perspective, its partnership with frugivores requires a delicate balance between discouraging them from eating fruits before the seeds are capable of germinating and attracting them when the seeds are ready. In addition, the plant must protect the seeds from destruction in the frugivore’s digestive tract and defend them against inappropriate consumers that would damage the seeds or fail to disperse them at all.