“Unlocking the Secrets of Invasion: A Revolutionary Approach to Predicting Species Success in Ecosystems”

Upon the introduction of a new species into an ecosystem, it may thrive in establishing itself, or it may fail to secure a foothold and perish. Researchers at MIT have now formulated a model that can forecast which outcome is more probable.

The scientists developed their model based on the examination of hundreds of varying scenarios that they simulated using populations of soil bacteria cultivated in their laboratory. They now intend to evaluate their model in larger ecosystems, such as forests. This method could also prove valuable in forecasting whether probiotics or fecal microbiota treatments (FMT) would effectively address infections within the human GI tract.

“Many individuals consume probiotics, yet numerous ones can never penetrate our gut microbiome, because introducing it does not automatically imply that it can develop and inhabit, thereby enhancing your health,” remarks Jiliang Hu SM ’19, PhD ’24, the primary author of the research.

MIT physics professor Jeff Gore is the senior author of the manuscript, which is published today in the journal Nature Ecology and Evolution. Matthieu Barbier, a researcher at the Plant Health Institute Montpellier, and Guy Bunin, a physics professor at Technion, also contributed to the manuscript.

Population Variations

Gore’s laboratory specializes in utilizing microbes to investigate interspecies interactions in a controlled manner, aiming to gain deeper insights into the dynamics of natural ecosystems. In prior research, the team has employed bacterial populations to illustrate how alterations in the environment where the microbes reside influence the stability of the communities they establish.

In this investigation, the scientists aimed to understand what factors determine the success or failure of an invasion by a new species. In natural ecosystems, ecologists have posited that increased diversity leads to greater resistance against invasions, as most ecological niches are typically filled, leaving limited resources for an invader.

Nonetheless, in both natural and experimental frameworks, researchers have noted that this does not hold consistently: While certain highly diverse populations resist invasion, others with similar diversity are actually more susceptible to being invaded.

To investigate the reasons behind these varying outcomes, the scientists established over 400 communities of soil bacteria, all native to the soil surrounding MIT. They formed communities of 12 to 20 bacterial species and six days later introduced one randomly selected species as the invader. On the 12th day of the trial, they sequenced the genomes of all bacteria to verify if the invader had managed to establish itself in the ecosystem.

Within each community, the scientists also modified the nutrient levels in the culture medium on which the bacteria were nurtured. When nutrient concentrations were elevated, the microbes exhibited vigorous interactions, marked by intensified competition for nutrients and other resources, or mutual inhibition via mechanisms like pH-mediated cross-toxin effects. Some of these populations created stable states in which the proportion of each microbe remained relatively constant over time, whereas others formed communities where most species varied in abundance.

The researchers discovered that these variations were the most crucial determinant in the invasion’s outcome. Communities displaying more fluctuations tended to be more diverse but were also more predisposed to successful invasion.

“The fluctuations are not prompted by environmental changes; instead, they are internal variations driven by species interactions. Our findings indicate that fluctuating communities are more easily invaded and also show greater diversity than stable ones,” Hu asserts.

In some populations where the invader established itself, the other species persisted, albeit in reduced numbers. In other cases, some resident species were outcompeted and completely vanished. This displacement was observed more frequently in ecosystems characterized by stronger competitive interactions among species.

In ecosystems exhibiting more stable and less diverse populations, with more robust interspecies interactions, invasions were more likely to falter.

Regardless of whether the community was stable or fluctuating, the researchers found that the proportion of original species that continued to exist in the community before the invasion serves as a predictor for the likelihood of invasion success. This “survival fraction” can be estimated in natural ecosystems by calculating the ratio of the local community’s diversity (measured by the number of species present in that specific area) to the regional diversity (number of species found throughout the entire region).

“It would be fascinating to investigate whether local and regional diversity could help anticipate susceptibility to invasions in natural communities,” Gore remarks.

Forecasting Success

The researchers also identified that under specific conditions, the sequence in which species entered the ecosystem affected the success of an invasion. When species interactions were strong, the likelihood of a species being successfully incorporated diminished when it was introduced subsequent to other previously established species.

Conversely, when interactions are weak, this “priority effect” dissipates, and the same stable equilibrium is attained irrespective of the order of microbial arrival.