Why do large ecosystems stay stable?

How can massive ecosystems, such as tropical rainforests, persist despite their complexity? This question touches on a long-standing paradox in ecology: the idea that as ecosystems grow larger and more intricate, they should become increasingly unstable. A recent study from the Centre for Complex Systems Studies (CCSS) and the Mathematical Institute at Utrecht University sheds light on this mystery by showing that stability depends not just on size, but on structure, challenging earlier theories.

In the 1970s, ecologist Robert May made a striking claim:  theoretical work suggested that large ecological systems with many species were inherently prone to instability. This led to the widespread belief that bigger ecosystems are inherently more fragile. But there’s a catch: real-world ecosystems often defy this prediction. Massive networks, like rainforests and coral reefs, can persist for centuries, even in changing environments. What’s going on?

To address this contradiction, researchers revisited a widely used mathematical framework called the Lotka-Volterra system, which models interactions between species. Their findings were insightful: it’s not the size of the ecosystem that determines stability, but the way species interact and balance one another.

Jenga tower

In a game of Jenga, adding new blocks eventually makes the tower tremble and fall. But if you distribute the weight carefully and maintain balance, the tower can grow surprisingly tall. Ecosystems work in much the same way: stability isn’t only about the number of species but about how interactions are structured.

The researchers found that ecosystems remain stable as long as no single species becomes too dominant. When interactions are evenly balanced, a kind of natural equilibrium emerges, allowing even vast systems to endure. This helps explain why diverse ecosystems like rainforests can thrive for so long.

A revised approach to ecological network models

The study suggests that real-world ecosystems likely have built-in mechanisms to prevent interactions from becoming too intense. These subtle regulatory processes help maintain stability even in highly complex networks. Accounting for such mechanisms will provide more realistic models and  hence a deeper understanding of biodiversity.