Descartes' visual revolution

La Fontaine was right: we have to work together to make it last

HAS In the classical era, those who looked at their century and wanted to speak to their contemporaries often used satire or philosophical stories. A nice example is of course that of Fables from La Fontaine, which every schoolchild in France must have encountered on his or her path, either through a story about cheese or through a bad ant. Among these well-known fables is that of the Lion and the Rat, which states that cooperation between very different animals can change a fate that promises to be disastrous. However, in a very recent paper, a team of physicists from Paris, interested in ecosystem dynamics, have irrefutably demonstrated this important result: La Fontaine was right: “we often need someone smaller than ourselves”.

They studied a series of equations that model the key features of a complex ecological system. Their model assumes the existence of a large number of species that interact with each other and benefit from a local food source (e.g. plants). These interactions can be divided into three main classes: they can be competitive (for example, two species compete for a food source), predatory (one prey species serves as a resource for another species), or cooperative (the flourishing of one promotes that of the other). other and vice versa).

In addition, the model used takes into account the existence of geographical diversity, assuming the existence of several distinct communities of the same species, but where local particularities have created different relationships between them. To this geographical diversity is added a migratory capacity that plays an important stabilizing role, because it allows a species to reinvest an area where it would have become impoverished. Like all work in physics, this model is not used to describe a specific situation involving a small number of actors described in detail, but rather to describe the general trends that can be expected when there are a large number of species in the be a game.

A lesson in physical ecology

And the results of this study are fascinating! In a spatially extensive ecosystem, two characteristics oppose each other to sustain life in the long term: on the one hand, the unavoidable dangers of fertility and mortality negatively impact a species’ chances of survival. On the other hand, migration capacity promotes long-term survival if it is sufficiently efficient. However, this qualitatively quite simple ‘dynamic equilibrium’ becomes much more subtle to analyze when cooperative interactions exist within the species (see definition above). In this case, the ecosystem is more robust and continues to exist even in regimes where migration is too low, thanks to species that express significant mutualism in their interactions. Everything happens as if the latter have developed greater resilience, thanks to a community of destinies born of their collaboration. This greater robustness comes at a price, because the ecosystem then becomes ‘subcritical’: if conditions become even more hostile to its survival (for example, if the migratory capacity of species is reduced even further), its disappearance occurs in a catastrophic manner. through an irreversible tipping point and very difficult to predict, because there is no warning signal in the average numbers of the populations present.

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