Fidelity or love the one you're with? Biotic complexity and tradeoffs can drive strategy and specificity in by-product mutualisms

By-product mutualisms are ubiquitous yet seldom considered in models of mutualism. Most models represent conditional mutualisms that shift between mutualism and antagonism in response to shifts in costs and benefits resulting from changes in environmental quality. However, in by-product mutualisms, benefits arise as a part of normal life processes and do not incur additional costs and do not have antagonistic alternate states. Recognizing that mutualisms are diverse, new, or more flexible, models are needed. We present a conceptual model that differs from traditional models in three ways: (1) partners exchange by-product benefits, (2) interactions do not have alternate antagonistic states, and (3) tradeoffs are allowed among factors that influence environmental quality. We applied this model to bark and ambrosia beetles that associate with fungi and that have developed two distinct pathways to by-product mutualism. We used independent axes for each major factor influencing environmental quality including those that exhibit tradeoffs. For these beetles, tradeoffs in nutrition and defense were key to which mutualism pathway was taken and the degree of fidelity that occurred among partners. This approach can be used to test hypotheses on the relative importance of the drivers underlying the development of by-product, or other, mutualisms.