15.14: Not all species interactions are negative: mutualism and commensalism.

It’s easy to get the idea that all species interactions in nature are harsh and confrontational, marked with a clear winner and loser. That is largely the case when it comes to competition and predation. However, not all species interactions are combative. Every flower you see should be a reminder that evolution produces beneficial species interactions as well. These types of interactions fall into two categories: mutualism and commensalism.

Mutualism: everybody wins Corals that gain energy from photosynthetic algae living inside their tissues. Termites capable of subsisting on wood, but only with the assistance of cellulose-digesting microbes living in their digestive system. Flowers pollinated by animals that are nourished by nectar. Each of these relationships is an example of mutualism, an interaction in which both species gain and neither is harmed (FIGURE 15-29). Mutualism is common in almost all communities. Plants, particularly, have numerous such interactions: with nutrient-absorbing fungi, nitrogen-fixing bacteria, and animals that pollinate them and disperse their fruits.

Mutualistic interactions are so widespread and ecologically important that they are described in significant detail throughout this book. The relationship between plants and mycorrhizae bacteria, for example, is discussed in Section 12-15. Plants and their rich variety of mutualistic relationships with animal pollinators and fruit dispersers are covered throughout Sections 12-9 and 12-11. Lichens, the symbiotic association of fungi and photosynthetic algae or bacteria, are discussed in Section 12-15.

You need not look farther than your own gut for an important mutualistic relationship. Living within your large intestine are huge populations of E. coli bacteria that synthesize a significant portion of the vitamin K—essential for several metabolic processes and blood clotting—you need each day.

Commensalism: an interaction with a winner but no loser Some species interactions are one-sided. The cases in which one species benefits and the other neither benefits nor is harmed are called commensal relationships, or commensalism. Cattle egrets have just such a relationship with grazing mammals such as buffalo and elephants (see Figure 15-29). As the large mammals graze through grasses, they stir up insects. The birds, which feed near the mammals—particularly near the forager’s head—are able to catch more insects with less effort this way. The grazers are neither helped nor harmed by the presence of the birds.