Sea waters contain a large variety of planktonic species. Image credit: Jay Nadeau, Chris Lindensmith, Jody W. Deming, Vicente I. Fernandez, and Roman Stocker. Image courtesy of David Liittschwager (CC BY-SA 4.0)
The surface waters of the ocean are teeming with microscopic creatures known as plankton, which get carried across vast distances by the currents. In a single ecosystem, thousands of plankton species may coexist, all competing for very few types of food sources. According to the principle of competitive exclusion, this should not be the case. Indeed, this theory states that the population levels of two species competing for the same resource cannot remain steady over time – or more generally, that the number of consumer species in an ecosystem cannot be higher than the number of resource types on which they rely. And yet, the Earth abounds with examples where a limited variety of resources supports a large number of competing yet coexisting consumer species. This is known as the paradox of the plankton.
Many models have been proposed to explain how the limitations set by the competitive exclusion principle can be overcome, yet it is still unknown how to resolve the paradox of the plankton in a steady environment. In response, Kang et al. set out to test whether a phenomenon known as predator interference, which emerges when two or more individuals of the same species compete for the same resources, could help address the paradox of the plankton.
To test this idea, Kang et al. developed a mathematical model of predator interference for multiple species of plankton feeding on a limited variety of food sources. The model put predators of the same species into encountering pairs to simulate predator interference. In this scenario, a wide range of predator species were able to live alongside each other with the numbers of each type of predator remaining steady over time.
These results can be understood as follows: as a species becomes more successful at extracting resources from its environment, its population grows – and with it, the number of individuals engaged in intraspecific interference. Locked in interference, these species become less effective at getting food, creating a negative feedback loop that slows down the expansion of the species, allowing others to occupy the same niche.
These findings may benefit ecologists and conservationists by offering insights into how to maintain biodiversity in ecosystems and protect endangered species. Further work is needed to test how well the model applies to other ecosystems.
