Textbook examples of mutualisms usually involve two interacting species; for instance, honey bees and clover. But of course, one host may have many mutualists (e.g., bees, butterflies) and one mutualist may have many hosts (e.g., clover, blueberries). In many cases, the benefits of having multiple possible host species are obvious. For instance, for ants that eat tasty elaiosomes and disperse plant seeds, it would be hard to get by while specializing on the seeds of only one plant species, because those seeds would only be available for part of the year. But how does the host benefit from having many symbiont species?
In ant-plant interactions, one plant species may have individual visitors from many ant species. For instance, the shrub Urera baccifera receives visitors from 22 facultative ant species, which visit to harvest food bodies and/or fruits (Dutra et al. 2006). Similarly, the yellow alder (Turnera ulmifolia) receives visitors from 24 facultative ant species, which visit to harvest seeds with elaisomes and/or to feed at extrafloral nectaries (Cuautle et al. 2005). While they’re on the plant, these ants can protect the plant from herbivores, like caterpillars. And finally, there may be multiple ant species visitors to a given plant species even when the associations are obligate: for instance, Acacia drepanolobium may be inhabitated by one of four ant species at any given time (Palmer et al. 2010).
Ok, but why so many species of ant visitors? Why shouldn’t the plant sanction any partners except the most beneficial species? This is still an active area of research, but at least part of the answer is that different ant species vary in the services that they provide to the plant. For instance, ant species may be more of less aggressive defenders, more or less likely to disperse seeds, more or less likely to farm scale insects, etc. (You might remember that this is true for guard crabs, too, where big crab species are good at protecting corals against starfish, while small crab species are good at protecting against vermetid snails.) Ant species also vary in how costly they are to harbor. For instance, one species may be a good defender, but it may also sterilize the host plant (e.g., Stanton 1999). So, a plant’s lifetime fitness may be determined not only by the guild of ant species that visits the plant during its life, but also on the timing and order in which those species visit the plant (Palmer et al. 2010). For instance, having sterilizing symbionts when you’re young can actually increase lifetime plant fitness. Isn’t that wild?
Cuautle M, Rico-Gray V, Diaz-Castelazo C (2005) Effects of ant behaviour and presence of extrafloral nectaries on seed dispersal of the Neotropical myrmecochore Turnera ulmifolia L. (Turneraceae). Biological journal of the Linnean Society 86: 67-77.
Dutra HP, Freitas VL, Oliveira PS (2006) Dual Ant Attraction in the Neotropical Shrub Urera baccifera (Urticaceae): The Role of Ant Visitation to Pearl Bodies and Fruits in Herbivore Deterrence and Leaf Longevity. Functional Ecology 20(2): 252-260.
Palmera TM, Doak DF, Stanton ML, Bronstein JL, Kiers T (2010) Synergy of multiple partners, including freeloaders,increases host fitness in a multispecies mutualism. PNAS 107(40): 17234–17239.
Stanton ML, Palmer TM, Young TP, Evans A, Turner ML (1999) Sterilization and canopy modification of a swollen thorn acacia tree by a plant-ant. Nature 401:578–581.