The study of mutualism has passed through a number of stages, but has yet to move very far from the descriptive phase (Bronstein 2001). Many early naturalists were skeptical about mutualistic relationships. For example, before Janzen (1966) established the obligatory mutualism between Pseudomyrmex ants and Acacia trees, the ants were said to be of no more use to the plants than "fleas on a dog." Once mutualisms were described they were often pronounced to be mostly confined to the tropics. However, Janzen (1985) has pointed out that every organism is involved in at least one mutualism in its life. Given that the eukaryotic cell evolved as the result of mutualistic relationships, the phenomenon is ubiquitous. And extra-tropical locations do not lack mutualisms. Forest trees with mycorrhizal fungi dominate boreal habitats, deserts are populated with legumes and their nitrogen-fixing bacteria, and tundras are dominated by lichens.
While mutualistic interactions have become accepted as comparable in importance to ecosystems as competition and predator-prey relationships, the view that mutualism represents "cooperation" between species has been challenged. Bronstein (2001) has stressed that mutualisms involve costs for each species, as well as benefits. Costs of mutualism are only now being tabulated, and there is little consistency in how data are gathered. Bronstein (2001) cites the following examples: (i) 20% of the total carbon budget of forest trees may be consumed supporting mycorrhizae (Johnson et at. 1997); (ii) 3% of the energy budget of many plants is devoted to providing floral nectar for pollinators (Harder and Barrett 1992); (iii) extrafloral nectar costs about 1% of the energy budget of the plants involved (O'Dowd 1979, 1980). In the obligatory interaction between figs and their wasp pollinators, Bronstein (2001) estimated that 53% of ovaries of Ficus aurea are lost to the wasps, while yuccas (Yucca spp.) evidently lose 5-20% of their seeds to their moth pollinators. Finally, Wolfe (2001a, 2001b) estimates that nitrogen-fixing bacteria consume 20% of the carbohydrates produced by legumes.
Rather than assuming that species have somehow entered into permanent, mutually agreeable contracts, we need to analyze mutualism with the following points in mind:
1 Mutualism always involves costs as well as benefits.
2 Costs set limits on the evolution of mutualisms.
3 There is a conflict of interest between the mutualistic species.
4 Organisms that "cheat" on the mutualism by reaping the benefits without reciprocation will often enjoy an advantage.
5 Related organisms (such as non-mutualistic ant species) may take advantage of a mutualism and act as parasites on the relationship.
6 Mutualisms may evolve toward a host-parasite relationship from a mutualistic one.
7 Continuous coevolution is necessary to maintain a mutualism.
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