The outcomes ofinterspecific interactions are not simply (+, 0, —), but instead vary along a continuum. Mutualism, like predation and competition, is in many cases not a fixed attribute or outcome of the interacting species. For example, the upper half of Figure 1 shows that mutualism can grade into commensalism (+, 0) and then predation (+, —) as the effect on one of the two partners changes. This variation in the strength and outcome has become known as conditionality or context dependency ofmutu-alism. Mutualistic outcomes can vary depending upon numerous factors, including the abundance of predators and competitors, the supply ofresources such as nutrients, the density and distribution of mutualists, and the size, stage, or age classes of interacting species. All of these factors can lead to spatial and temporal variation in the community and environmental context of mutualistic interactions. Gradation of mutualism into other interaction outcomes arises mechanistically via changes in the relative magnitudes of benefits and costs associated with spatial and temporal changes in these above factors.
Mutualisms are often contingent upon external factors, such as the availability of limiting resources or the presence and/or density of a predator or competitor. The protection mutualism between ants and treehoppers (plant-feeding insects) exemplifies how outcomes can vary with predator density. In a high-predator year or location, treehoppers are decimated by predators if not protected by ants. In contrast, at places and times where predators are few, the interaction is commensal or even parasitic: ant protection is not necessary, yet treehoppers still must pay the cost of providing food resources to the ants. Thus, variation in the magnitude of benefits of the mutualism to treehoppers generates a shift in the outcome of the interaction: it is conditional upon the abundance of predators.
The interaction between plants and root-associated mycorrhizal fungi represents an example of how the outcome of mutualistic interactions can be conditional upon nutrient availability. Mycorrhizal fungi increase the availability of soil phosphorus for the host plants; in turn, the plants provide mycorrhizae with carbon resources (root exudates). When plants are grown in phosphorus-rich habitats, the cost of providing mycorrhizae with carbon can exceed the benefits of the phosphorus obtained from mycorrhizae. Consequently, some plants can reduce their mycorrhizal infections under these conditions, even excluding mycorrhizae from their roots altogether.
In addition to spatiotemporal variation in environmental resources and predators, variation in benefits and costs associated with functional responses can lead to conditional outcomes of mutualism. As shown in the yucca/moth example above, irrespective of the particular species involved, the strength and outcome of a mutualism will vary with the densities of interacting partners. If mutualist densities occur at which costs equal or exceed benefits (Figure 2), then the outcome of an interaction will degrade into commensalism or predation (Figures 1 and 2). Thus, it is feasible for one 'mutualistic' species to have positive net effects on its partner at some population densities, and commensal or parasitic net effects at other densities. These examples demonstrate how complex mutualisms can be, and how dependent their outcomes are on the biotic and abiotic environment in which they occur.
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