Lab Microcosms Reveal How Fluctuations and Interactions Shape Invasion Risk in Ecosystems

Researchers test invasion dynamics by growing hundreds of microbial communities made of 20 bacterial species in lab microcosms. By varying nutrient levels and introducing random invaders, they find that invasions are more likely in diverse, fluctuating ecosystems, while strong interspecies interactions tend to repel invaders, though successful invaders can still dramatically boost biomass. The work links Elton's biodiversity ideas with modern ecology, proposing the survival fraction as a potential predictor of invasion risk and showing that simple Lotka-Volterra models can reproduce complex dynamics observed in experiments.

Background: Elton's biodiversity resilience hypothesis

Ecologists have long debated whether ecosystems with more species are inherently more resilient to invasions. Elton suggested that diverse webs leave fewer opportunities for interlopers, while simpler webs are more vulnerable. This episode of Quanta Magazine examines a fresh take on that question by examining ecological dynamics in simplified, controllable systems. "The survival fraction may be a unifying concept that could predict how likely a natural ecosystem is to resist or succumb to an invasive species." - Jeff Gore, MIT

Laboratory approach: microbial microcosms as models of real ecosystems

Jeff Gore and colleagues build hundreds of microbial communities on a rectangular plate with 96 wells, each well acting as an artificial habitat. They source bacteria from MIT campus environments and create networks by feeding different nutrients, thereby altering interaction strengths among species. After allowing communities to stabilize for a week, they introduce a randomly chosen invader and then sequence DNA to track species survival and measure total biomass. This approach enables rapid, repeatable testing of ecological theories without the complications of fieldwork. "ecologists often rely on static species counts when assessing an ecosystem. Gore's more dynamic concept of biodiversity with fluctuations offered a very nuanced approach to what a diverse community really is." - Megan Lee, Swiss Federal Institute of Aquatic Science and Technology

Key findings: fluctuations, diversity and invasion probability

In line with earlier work on phase transitions in microbial ecosystems, the experiments reveal three phases as the number of species or interaction strength increases: a stable phase, a destabilized phase where some species die off, and a highly oscillatory phase with large population swings. In these last two phases, invaders were significantly more likely to establish themselves than in the stable, species-poor communities—invaders survived about eight times more often in the diverse, fluctuating systems. The researchers interpret this as fluctuations creating new ecological niches, potentially opening doors for invaders. Additionally, communities with strong interspecific interactions tended to repel invaders, though successful invaders in such networks could cause large jumps in total biomass. "Invasions were more likely in diverse ecosystems than in ones with fewer species, especially when the populations of individual species rose and fell over time." - Jeff Gore, MIT

Concepts and modeling: connecting theory to data

To interpret their results, the team revisits the Lotka-Volterra framework, a foundational predator-prey model, adapting it to include an outside species. They find that the observed fluctuations and high-diversity dynamics can emerge as a natural property of dynamical systems with multiple interacting species, without requiring new mechanisms. Gore argues that the survival fraction—defined as the ratio of alpha diversity (local ecosystem) to gamma diversity (regional diversity) after the initial stabilization—correlates with invasion outcomes and could serve as a practical predictor for openness to invasion in natural settings. The work also ties into Elton’s ideas by showing that simple mathematical models can reproduce surprisingly complex ecological behavior. "The revelation that intrinsic ecosystem dynamics can affect the success of an invader is exciting and novel." - Jonathan Levine, Princeton University

Expert perspectives and implications

Jonathan Levine notes that the finding challenges a straightforward interpretation of Elton’s diversity-resilience link, suggesting that rapid dynamics in short-lived communities (such as microbes, insects or plankton) may be more relevant than in long-lived plant or animal systems. Megan Lee highlights the value of a dynamic view of biodiversity, which captures fluctuations that static counts miss. William Shoemaker, although not involved in the study, affirms that the demonstration of invader effects in a microbial loop provides a clear illustration of how invasions can reshape ecosystems. The researchers also discuss practical implications for protecting wild ecosystems and, at a microscopic scale, for maintaining gut microbiome health by understanding how invasions occur in fluctuating environments. The study invites future work to unpack the mechanisms behind the fluctuations and to test whether similar dynamics occur in more complex, real-world ecosystems.

"The survival fraction may be a unifying concept that could predict how likely a natural ecosystem is to resist or succumb to an invasive species." - Jeff Gore, MIT