2024-03-21 17:00:04
In the classical era, those who looked at their century and wanted to speak to their contemporaries often used satire or philosophical tales. A pleasant example is of course that of Fables by La Fontaine, which every schoolchild in France must have encountered on their path, whether through a story of cheese or a bad ant. Among these very well-known fables, there is that of the Lion and the Rat, which states that cooperation between very different animals can change a destiny that promises to be disastrous. However, in a very recent article, a team of physicists from Paris interested in the dynamics of ecosystems has just irrefutably shown this important result: La Fontaine was right, “we often need someone smaller than ourselves”.
They studied a set of equations that model the main features of a complex ecological system. Their model assumes the existence of a large number of species interacting and benefiting from a local food resource (plant, for example). These interactions fall into three main classes: they can be competitive (two species compete for a food resource, for example), predatory (one prey species serves as a resource for another) or cooperative (the flourishing of one favors that of the other and vice versa).
Furthermore, 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 sculpted different relationships between them. To this geographical diversity is added a migratory capacity which has an important stabilizing role, since it allows a species to reinvest an area where it would have become impoverished. Like all physicist work, this model is not used to describe a specific situation involving a small number of actors described in great detail, but rather the overall trends that should be expected if a large number of species are at play. .
A lesson in physical ecology
And the results of this study are fascinating! In a spatially extended ecosystem, two characteristics oppose each other to maintain life in the long term: on the one hand, the inevitable hazards of fertility and mortality have a negative impact on the chances of survival of a species. On the other hand, migratory 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 the definition above). In this case, the ecosystem is more robust and persists even in regimes where migration is too low, thanks to species expressing significant mutualism in their interactions. Everything happens as if the latter developed greater resilience, due to a community of destinies born from their cooperativeness. This increased robustness has a price, because the ecosystem then becomes “sub-critical”: if conditions become even more hostile to its survival (for example, if the migratory capacity of species is reduced even further), its disappearance occurs catastrophic by an irreversible tipping point and very difficult to predict, because there is no warning sign in the average numbers of the populations present.
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