American Society of Naturalists

Benedicte Bachelot and Charlotte T. Lee (March 2020)

Disturbances can promote and hinder coexistence of competitors in on-going partner choice mutualisms

Read the Article (Just Accepted)

Most animals and plants rely on positive (mutualistic) associations with microorganisms to meet their dietary needs. Such positive associations are key for plant and animal survival. Yet they can be fragile and environmental disturbances may disrupt them, potentially leading to the death of the partners. Understanding how disturbances influence these mutualistic associations is a very important task in the face of global changes.

A team composed of a theoretical ecologist at Duke University and quantitative community ecologist at Rice University used a mathematical model to simulate positive interactions among a plant and two mutualistic fungi under different disturbance scenarios. During the simulations, the plant was able to maximize its fitness over a long period of time by changing how much it interacted with each fungus.

The study showed that depending on the disturbance characteristics (strength, timing, etc…), survival of the two fungi could be either compromised or promoted. This finding emphasizes the need to understand how the environment influences mutualistic interactions. Since ecosystems are under constant threat from anthropogenic and natural disturbances, further research about the effect of disturbances is needed.

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Abstract

Ecosystems are under threat from anthropogenic and natural disturbances, yet little is known about how these disturbances alter mutualistic interactions. Many mutualistic interactions are highly context-dependent and dynamic due to “on-going” partner choice, impeding our understanding of how disturbances might influence mutualistic systems. Previously we showed that, in the absence of additional known mechanisms of competitive coexistence, mutualistic fungi can coexist in a system where the plant community associates dynamically with two empirically-defined arbuscular mycorrhizal fungal types: a cheap kind that provides low nutrient benefits and an expensive type that provides high nutrient benefits. We built on this framework to ask how disturbances of different types, frequencies, amplitudes, and predictabilities alter on-going partner choice and thereby influence the coexistence of mutualists. We found that the effects of disturbances depend on the type, amplitude, and predictability of disturbances, and to a lesser extent on their frequency. Disturbance can disrupt mutualist coexistence by enabling hosts more efficiently to exclude partners that behave as parasites. Disturbance can also promote coexistence by altering the strength and direction of consumer-resource interactions. Predicting the effects of disturbance on the mutualist community therefore requires us to understand better the consumer-resource relationships under various environmental conditions. We show how, through such context-dependent effects, disturbance and on-going partner choice can together generate relative nonlinearity and investment in future benefit, introducing fluctuation-dependent mechanisms of competitive coexistence. Our findings support a broadening of the conceptual framework regarding disturbances and competition to include fluctuation-dependent mechanisms alongside the spatiotemporal Intermediate Disturbance Hypothesis.