Scientists in Lisbon have now developed a promising mathematical approach to this question, one that could resolve a longstanding paradox and have important implications for biodiversity conservation – one of the most pressing challenges today.
Experts have been trying for decades to explain how limited resources can support the multiplicity of species that live on Earth.
The first theoretical attempts to understand biodiversity led to a conclusion that did not pass the reality test: the theory predicted that the number of species had to equal the number of types of resources available in the environment, something that clearly does not correspond to what we see around us.
The contrast between theory and experimental observation is so stark that it has been called paradoxical.
This paradox is often known as the “plankton paradox” because it is so well illustrated by the properties of plankton ecosystems. In the oceans, there are less than ten resources that support the growth of these organisms, such as light, nitrogen, carbon, phosphorus, iron, etc. However, even under these conditions, hundreds of different species of plankton manage to coexist in a stable way, none of them leading to extinction.
Despite recent advances, this biodiversity conundrum remains to be clarified. But now, scientists at the Champalimaud Center in Lisbon have developed a new mathematical model that could solve it.
Their results were published in the journal Proceedings of the Royal Society B.
According to Andres Laan, the first author of the study, led by principal investigator Gonzalo de Polavieja, classic models of competition for resources predict that each resource will sustain the species that consumes it most efficiently, thus leading to species that compete with extinction. But this univocal correspondence does not happen in nature. On the contrary, the order of magnitude of the number of species that live on Earth is much higher.
Scientists decided to look for a new solution to the plankton paradox. What is new in this study is that, to explain the biodiversity paradox, they used aggression models inspired by an area of mathematics called game theory.
“We started from a theoretical scenario where we only had two 'species': falcons and pigeons”, explains Laan. “Hawks are carnivores and are always ready to fight. Pigeons are peaceful and tend to share resources or flee from the fight. According to game theory, in the end, neither pure hawks nor pure pigeons become dominant – on the contrary, the two 'species' coexist.”
Next, they wanted to know what would happen if several species entered this game of hawks and pigeons, competing for many types of resources at the same time.
For each resource, each species could choose to be a hawk or a pigeon. “This rich set of choices generated a combinatorial diversity that resulted in a large number of potential species. And as had happened in the simple case of two species, the multiple species ended up coexisting and not extinguishing each other”, says Laan.
According to your model, biodiversity actually increases exponentially with the number of resources. “With one resource, two species can coexist; with two features, four species; with four features, 16 species; and with ten features, we get over 1000 species to coexist. The exponential growth is very fast, providing, therefore, a good way to maintain biodiversity”, explains the scientist.
This theory, say the authors, also makes a number of predictions that have been experimentally confirmed. The model accurately estimates the abundance of each species in real ecosystems. In these ecosystems, there are few more abundant species, yet they represent a disproportionately high fraction of the total biomass of the system.
“This is similar to what happens with wealth inequality in human societies, where the rich own a disproportionately large share of the total wealth,” says Laan.
The authors think that solving this paradox could provide the key not only to understanding biodiversity, but also to better understanding species extinction and predicting possible future directions of animal evolution.
“These ideas are still largely theoretical and we need to test to what extent the competition mechanisms proposed in our article correctly describe what goes on in competition between real species, but these first results look quite promising”, concludes Laan.
Author Champalimaud Center
Science in the Regional Press – Ciência Viva
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