In addition to interactions that benefit both partners and those in which at least one partner is harmed, there are interactions in which one partner benefits with no apparent effect on the other. These interactions are called commensalisms. For example, Grey Whales in the Pacific Ocean are commonly covered with barnacles. The barnacles benefit from the association, obtaining both a substrate for growth and a free ride through waters rich in planktonic food, but the presence or absence of barnacles doesn’t seem to affect the whales one way or the other.

Some ecologists also recognize another class of interaction, called amensalism, in which one partner is harmed with no apparent effect on the other. A commonly cited example is the coconut palm and Brazil nut trees, tropical trees that produce heavy nuts that harm animals and plants they fall on. The spectrum of costs and benefits associated with interactions among organisms is summarized in Table 47.1.

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Associations are not fixed—they can change over time. A mutualism can in some cases become antagonistic if one of the partners “cheats” by imposing a larger cost than benefit on the other. In fact, mutualisms that are loose associations among changing partners can become one sided rather quickly. For example, many plants have evolved tubular flowers that guide bees past their anthers or stigma on the way to the nectar at the base, taking just a little more time from bees who try to visit as many flowers as possible. Some bees have short-circuited this plant mechanism by nipping the flower base from the outside and then drinking the nectar without pollinating the flower. Most plants have enough successful pollination to ensure seed production, but losses from cheating can still be costly.

Some interactions begin as a benefit to one species, with no benefit or cost to the other. For example, cattle egrets follow water buffalo to pick up the insects stirred up by the buffalo passing. The egrets gain food, but the buffalo are not affected either way—an example of commensalism (Fig. 47.10). However, cattle–egret commensalism can become a mutualism over time if the egrets give early warnings of nearby predators such as lions or eat insects like tsetse flies that carry buffalo diseases.

Quick Check 3 What are some of the costs and benefits to an apple tree and to the honeybee that pollinates it?