LB 23-242 - Modelling energy constraints on resource mutualism networks

Background/Question/Methods:

Humans and trees (with the aid of mycorrhizal fungi) are very divergent life forms, yet both have developed the ability to trade resources. Mutualistic bilateral exchange of resources between individuals or groups can benefit both participants and increase the overall utilization of resources in a community by distributing them to where they are needed. However, the formation of these networks requires energy: for trees this is in the form of carbon payments to mycelium, for modern humans this is usually hydrocarbons: petroleum and natural gas. How the energy requirements of resource networks affect their function is poorly understood. Both examples mentioned are highly specialized, complex networks, yet they share similarities, and it is these generalities that we aimed to explore.



Key features of these networks were included in a simplified agent-based model that could be used to examine the role of energy cost on the connectivity between nodes and other network properties. The growth of each node follows Liebig’s law and thus is limited by the growth of its limiting resource. The agents in this network are allowed to trade with each other to increase their carrying capacity and a Pareto efficient solution was found for each bilateral exchange.

Results/Conclusions:

We considered a situation where there is no speciation regarding the efficiency of resource utilization, but where the environment that each node inhabits is given a unique resource distribution. To focus on the properties of the network, the resource properties are generalized to be of equal density. We find that overall community level population deviates from the theoretical maximum in an exponential fashion as the energy cost of trade increases. With an increase in the ratio of energy resources to total resources, the slope of this relationship becomes shallower.



The results of this research are important in several areas. Mycorrhizal networks are important for the resilience of forests in an age of climate variability. Understanding the energy requirements of these networks is essential. For humans, oil-based trade has enabled the human populations to flourish much beyond local carrying capacities. With future declines in the availability of oil, trade networks will decay and the overall carrying capacity will decrease to the ecologically sustainable levels.