Manipulating trophic interactions to quantify carbon dynamics in marine and freshwater ecosystems

Background/Question/Methods

An extension of trophic cascade theory suggests that predators can alter ecosystem carbon dynamics through their indirect effects on primary producers. These effects on carbon cycling can be mediated through predator-induced changes in the behavior and/or abundance of their prey. Thus, trophic interactions can be an important determinant of the contribution of primary producers to photosynthetic carbon fixation, storage and remineralization. However, the idea that animals can play an important role in regulating carbon cycling and storage dynamics in ecosystems is highly contentious and remains insufficiently discussed in the scientific literature. To illustrate the importance of animals in regulating carbon dynamics in aquatic ecosystems, I will discuss empirical evidence from four study systems (coral reefs, artificial ponds, streams, and bromeliads). Here, I will explore how changes in predator abundance, through either experimental manipulations or natural spatial variations, influence CO₂ dynamics and carbon storage in these systems.

Results/Conclusions

In all four aquatic ecosystems, predators significantly altered the abundance and/or behavior of their prey, which in turn led to cascading effects on primary producers at the base of their food webs. In the artificial freshwater ecosystems, alterations to the abundance of predators significantly influenced the magnitude of CO₂ efflux between the aquatic ecosystems and the atmosphere. However, the direction of the predator effect on CO₂ was influenced by food chain length, with predators in odd-numbered food chains reducing CO₂ emissions, and predators in even-numbered food chains increasing CO₂ emissions. In the coral reef ecosystem, predators helped protect sedimentary carbon stocks by providing a refuge for benthic primary producers from herbivores, subsequently promoting carbon accumulation and storage in sediments. These results add to a growing body of evidence that predators, via trophic cascades, help regulate carbon cycling in a wealth of different ecosystems. We can no longer ignore the mounting evidence that animals can play an important role in carbon dynamics, and it is time for policy and management to reflect this reality.