Environment-dependent diversity-stability relationship in microbial communities

For decades, ecologists have debated a simple but stubborn question: does having more kinds of life make a natural community stronger, or does it make it more fragile? On one side, the idea that diversity acts as a safety net suggests that if many species are present, the loss or wobble of one can be absorbed by the others, keeping the whole system steady. On the other side, mathematical theories suggest that as a community grows more complex with more connections between species, it becomes a tangled web that is easier to shake apart. Real-world observations have offered answers for both sides, leaving scientists without a clear rule for when more diversity helps and when it hurts. The missing piece has been understanding that the environment itself might change the rules of the game, turning a stable community into a chaotic one, or vice versa, depending on the conditions.

A researcher at Shenzhen Bay Laboratory set out to test this idea by watching how bacterial communities behave under different temperatures and dilution rates. They did not look at wild forests or oceans, but at carefully assembled groups of bacteria in a laboratory setting. By growing these communities in a nutrient-rich liquid and then diluting them daily—essentially taking a tiny sample and adding fresh food—they could watch the populations grow, crash, and recover over many generations. They tested these communities across a wide range of temperatures, from a cool ten degrees Celsius to a scorching fifty degrees, and also varied how much they diluted the cultures each day. Their goal was to see if the relationship between how many species were present and how much the population numbers swung up and down changed depending on the environment.

What they found was that the relationship between diversity and stability is not fixed; it depends entirely on the conditions. In the middle range of temperatures, around twenty to thirty degrees Celsius, the communities behaved in one specific way. Here, many different species lived together and fluctuated in sync. When the population numbers went up and down, they did so together across many different types of bacteria. In this scenario, the more diverse the community was, the more unstable it became. The researcher observed that in these conditions, the species that were most abundant would change over time, with different bacteria taking the lead in different days. This suggests that the complex web of interactions between many species was driving the instability, creating a positive link where more variety meant more movement.

However, when the temperature shifted to the extremes, the story changed completely. At forty degrees, the environment became harsh enough that only a few tough species could survive. In these communities, the instability did not come from a complex web of interactions. Instead, it was driven by just one or a few dominant species that would boom and then bust on their own. These single-species crashes happened because the bacteria grew so fast that they altered their own environment, specifically by changing the acidity of the liquid, which eventually killed them off. In this high-temperature regime, having more species did not make the community more unstable, nor did having fewer species make it more stable. The link between diversity and instability simply disappeared. The fluctuations were caused by the physical limits of the environment and the behavior of a single dominant player, not by the number of players in the game.

The researcher also tested what happened when they changed how much they diluted the cultures, keeping the temperature constant. They found that this, too, could shift the community from one type of behavior to the other. When the dilution was weak, allowing more bacteria to survive from one day to the next, the communities tended to show both collective fluctuations and dominant-species fluctuations, with the latter being more frequent under these conditions. When the dilution was strong, washing away most of the population each day, the communities showed smaller fluctuations overall. This confirmed that the environment was the switch that determined which rule applied.

To understand why this happened, the researcher looked closely at how individual bacteria reacted to their own waste. They discovered that as bacteria grew, they changed the pH of their environment, making it either too acidic or too alkaline. At higher temperatures, this change happened faster and was more deadly. A population could grow huge in a single day, only to die off dramatically because the environment became toxic to them. This cycle of rapid growth followed by environmental suicide created the wild swings in population numbers. In the moderate temperatures, many species could tolerate these changes and coexist, leading to the collective fluctuations. In the harsher temperatures, only the toughest species survived, and their own growth became the cause of their instability.

The study used computer simulations to confirm that this mechanism was sufficient to explain the results. By building a model where bacteria competed for food and changed the pH of their shared home, the researcher could reproduce both the stable communities, the collective fluctuations, and the single-species crashes. The model showed that when growth was fast and the environment changed quickly, the system naturally fell into one of two patterns. Either many species would fluctuate together, creating a link between diversity and instability, or a single species would dominate and crash on its own, breaking that link.

This work suggests that the long-standing debate about whether diversity stabilizes or destabilizes nature might be missing the context. The answer is not a single universal rule. Instead, the relationship between how many species are present and how stable the community is depends on the environment. In some conditions, diversity drives instability because many species are interacting and reacting to each other. In other conditions, instability is driven by the environment itself, and diversity plays no role. The researcher showed that by changing the temperature or the dilution rate, they could flip the switch between these two worlds. This means that to understand why a community is stable or unstable, we must look not just at how many species are there, but at the specific conditions that shape how those species grow and die. The rules of the game change with the weather, and the stability of life depends on which set of rules is currently in play.