Innovative Insights: Assessing Species’ Invasion Potential in Ecosystems and the Human Gut

Upon introducing a new species into an ecosystem, it might either manage to settle successfully or fail to establish itself and become extinct. Physicists at MIT have developed a formula capable of forecasting which of these scenarios is more probable.

The scientists based their formula on the evaluation of hundreds of different scenarios that they simulated with populations of soil bacteria cultivated in their lab. They now intend to validate their formula in larger ecological settings, including forests. This methodology may also aid in anticipating whether probiotics or fecal microbiota transplantation (FMT) will effectively address infections in the human gastrointestinal tract.

“Individuals consume numerous probiotics, yet many cannot invade our gut microbiome at all, as merely introducing it does not guarantee its growth and colonization to enhance your health,” remarks Jiliang Hu, the principal author of the research.

MIT’s professor of physics Jeff Gore holds the position of senior author of the study, which is published today in the journal Nature Ecology and Evolution. Matthieu Barbier, a researcher at the Plant Health Institute Montpellier, along with Guy Bunin, a physics professor at Technion, are co-authors of this publication.

Population Variability

Gore’s laboratory specializes in employing microbes to study interspecies interactions in a controlled setting, aiming to gain deeper insights into the functioning of natural ecosystems. In previous investigations, the team utilized bacterial populations to illustrate how altering the environment inhabited by the microbes impacts the stability of the communities they form.

In this investigation, the researchers aimed to ascertain the factors influencing whether an invasion by a novel species would succeed or fail. Ecologists have proposed that in natural communities, greater diversity in an ecosystem enhances its resistance to invasion, as most ecological niches are typically filled, leaving limited resources for an invader.

Nevertheless, both in natural and experimental contexts, scientists have noted that this is not always the case: While certain highly diverse populations resist invasion, others with similar diversity may be more prone to being invaded.

To delve into why both outcomes can be observed, the researchers established over 400 communities of soil bacteria, native to the soil surrounding MIT. The team formed communities encompassing 12 to 20 bacteria species, then, six days later, introduced one randomly selected species as the invader. On the twelfth day of the experiment, they sequenced the genomes of all the bacteria to verify if the invader had integrated into the ecosystem.

Within each community, the researchers varied the nutrient concentrations in the culture medium where the bacteria were cultivated. In conditions with elevated nutrient levels, the microbes exhibited strong interactions, characterized by augmented competition for nutrients and resources, or mutual inhibition via mechanisms such as pH-mediated cross-toxin effects. Some of these populations reached stable states where the proportion of each microbe exhibited minimal variability over time, while others developed communities where the population numbers of most species oscillated.

The researchers discovered that these variations were the most critical element affecting the outcome of the invasion. Communities exhibiting greater fluctuations tended to be more diverse, but they were also more susceptible to successful invasion.

“The variation is not initiated by environmental changes; instead, it arises from internal fluctuations driven by species interactions. Our findings indicate that the fluctuating communities are more easily invaded and also exhibit greater diversity compared to the stable ones,” Hu states.

In some populations where the invader thrived, the remaining species persisted but in reduced numbers. In other populations, certain resident species were overcompeted and vanished entirely. This displacement occurred more frequently in ecosystems characterized by intense species interactions.

In ecosystems with more stable, less diverse populations and stronger species interactions, invasions had a higher likelihood of failing.

Regardless of whether the community was stable or fluctuating, the researchers identified that the fraction of original species that survived prior to the invasion indicated the likelihood of invasion success. This “survival fraction” may be inferred in natural communities by calculating the ratio of local community diversity (determined by the number of species in the vicinity) to regional diversity (the total number of species found in the broader area).

“It would be intriguing to investigate whether local and regional diversity could serve as predictors of invasion susceptibility in natural communities,” Gore asserts.

Anticipating Success

The researchers also identified that in specific situations, the sequence in which species entered the ecosystem influenced the success of an invasion. When species interactions were robust, the likelihood of a species being successfully assimilated diminished if it was introduced after other species had already settled.

When interactions were weak, this “priority effect” vanished, allowing the same stable equilibrium to be attained regardless of the arrival order of the microbes.

“In a scenario of strong interactions, we determined that the invader faces certain disadvantages as it

arrived later. This is significant in ecology because individuals have consistently discovered that in certain instances, the sequence in which species appear is crucial, while in other situations it is insignificant,” Hu states.

The scientists are now aiming to attempt to replicate their results in ecosystems where species diversity information is accessible, including the human gut microbiome. Their equation could enable them to forecast the success of probiotic therapies, where advantageous bacteria are ingested, or FMT, a novel treatment for severe infections like C. difficile, where beneficial bacteria from a donor’s stool are implanted into a patient’s colon.

“Invasions can be detrimental or advantageous depending on the circumstances,” Hu explains. “In some scenarios, such as probiotics or FMT to address C. difficile infection, we desire the healthy species to invade effectively. Additionally, for soil conservation, individuals introduce probiotics or beneficial species into the soil. In that context, people similarly wish for the invaders to thrive.”

This article is republished courtesy of MIT News (web.mit.edu/newsoffice/), a well-known platform that provides updates on MIT research, innovation, and education.