Resource-exchange mutualisms, where two partners exchange required resources that they are each best at acquiring, often mitigate nutrient limitation and drive productivity in natural ecosystems. Such mutualisms are often evaluated in terms of costs and benefits that assume instantaneous trade between partners. However, many mutualists donate resources to develop significant infrastructure, such as fungal hyphal networks, arthropod foraging networks and coral structures, which allow their partners to acquire a second resource at scale. In this case, the exchange may be better described as an investment with initial negative but a large future return, discounted for risks of "cheating" and loss of invested resource. Here we present an ecological analog of the Capital Pricing Model from finance theory, which is typically used by venture capitalists to decide whether to invest in new companies, to address whether a mutualist should invest in partners that provide immediate returns (trade) or potentially greater discounted future returns (investment). Model predictions were tested with measured microbial abundance and composition from grass roots and rhizosphere soil in the Serengeti Ecosystem, Tanzania. We compared spore biomasses of different arbuscular mycorrhizas and copy numbers of the nitrogen-fixing gene nifH across gradients of annual rainfall, soil nitrogen and phosphorus availability, and soil disturbance.
Results/Conclusions
The Capital Pricing Model approach predicts that investment, rather than trade, will most likely occur when resources donated by the investing species are highly available, the resource returned by the recipient has intermediate availability, and there is low risk of loss of invested tissue through mortality or of "cheating" by the recipient. These predictions were supported by the predominance of "traders," N-fixing bacteria (Azotobacter spp.) at low rainfall, intermediate soil N sites and of fast-growing, small arbuscular mycorrhizas (Glomus spp.) at intermediate rainfall, extremely low soil P, and in tilled soils. Highly beneficial (to grasses) species of mycorrhizas (Gigaspora and Acaulospora spp.) that require significant investment of carbon in hyphal networks, predominated only at sites with high rainfall, intermediate soil P and undisturbed soils. These results suggest that the nature of many mutualist interactions, and perhaps the most mutually beneficial ones, may be time-dependent and thus most subject to disruption by climate trends and variability and disturbances from invasions and human activities.