When a new species is introduced to an ecosystem, it can either succeed in colonizing or fail and go extinct. MIT physicists have devised a formula that can predict which of these outcomes is most likely.
The researchers developed the formula based on an analysis of hundreds of different scenarios they modeled using lab-grown soil bacterial populations. They now plan to test the formula in larger ecosystems, including forests. The approach could also help predict whether probiotics or gut microbiota therapeutics (FMTs) can effectively combat gastrointestinal infections in humans.
“People take a lot of probiotics, but a lot of them fail to colonize the gut microbiome, because just because you put them in your body doesn’t necessarily mean they’ll grow, colonize and have a positive effect on your health,” said Jiliang Hu SM ’19, PhD ’24, lead author of the study.
MIT physics professor Jeff Gore is lead author of the paper, published today in Nature Ecology & Evolution . Mathieu Barbier, a researcher at the Montpellier Institute of Plant Health, and Guy Bunin, professor of physics at the Technion, are also authors.
Demographic Changes
Gore’s lab specializes in using bacteria to analyze species interactions in a controlled way, with the goal of learning more about how natural ecosystems function. In previous studies, the team used bacterial populations to demonstrate how changes in the environment in which bacteria live can affect the stability of the communities they form.
In this study, the researchers wanted to understand what factors determine whether an invasion of a new species will succeed or fail. Ecologists hypothesize that in nature, the more diverse an ecosystem is, the more resistant it is to invasion, because most ecological niches are already occupied, leaving fewer resources for invaders.
However, in both natural and experimental systems, scientists have observed that this is not always true: some highly diverse populations are more resistant to invasions, while other highly diverse populations are more susceptible to invasions.
To explore why this might be the case, the researchers constructed communities of more than 400 soil bacteria taken from soil around MIT. They created populations of 12-20 species of bacteria, and after six days, they randomly added one species as an invader. On the 12th day of the experiment, the researchers sequenced the genomes of all the bacteria to determine whether the invader had become established in the ecosystem.
The researchers also varied the nutrient levels of the media in which the bacteria were grown within each community. At high nutrient levels, bacteria exhibit strong interactions characterized by increased competition for food and other resources, or mutual inhibition through mechanisms such as pH-mediated cross-action of toxins. Some of these populations form a steady state in which the proportion of each bacterium does not change much over time, while others form communities in which the abundance of most species fluctuates.
The researchers found that these variations were the most important factors in invasion outcomes: more unstable communities tend to be more diverse, but also more likely to invade successfully.
“This fluctuation is not driven by environmental changes, but internal fluctuations caused by interactions between species. And we found that fluctuating communities are both more susceptible to invasions and more diverse than stable communities,” Hu said.
In some populations where invasive species have become established, other species are still present, but in smaller numbers. In other populations, some native species have been outcompeted and disappeared entirely. This replacement tends to occur more frequently in ecosystems where competitive interactions between species are stronger.
In ecosystems that are more stable, with lower population diversity and stronger interactions between species, invasions are more likely to fail.
Whether a community is stable or fluctuating, the researchers found that the proportion of original species surviving in a community before the invasion predicts the likelihood of an invasion success. This “survival rate” can be estimated by taking the ratio of the natural community’s diversity (measured by the number of species living in the area) to its regional diversity (the number of species found in the entire area).
“It will be interesting to see whether regional or local diversity can be used to predict invasiveness in nature,” Gore says.
Predicting success
The researchers also found that in certain cases, the order in which species arrive in an ecosystem can affect whether an invasion is successful: if interactions between species are strong, the species are less likely to integrate successfully if they are introduced after the other species has become established.
When interactions are weak, this “preference effect” disappears and a stable equilibrium is achieved regardless of the order in which bacteria arrive.
“We find that in areas of strong interactions, the invaders are at a disadvantage because they come later. This is really interesting in ecology, because we’ve always found that the order in which species arrive can sometimes be important and sometimes not,” Hu said.
The researchers now plan to replicate their findings in ecosystems using data on species diversity, including the human gut microbiome. Their formula may be able to predict the success of probiotic treatments, in which beneficial bacteria are taken orally, or FMT, an experimental treatment for serious infections such as Clostridium difficile, in which beneficialbacteria harvested from a donor’s faeces are transplanted into a patient’s colon
“Invasions can be harmful or beneficial depending on the situation,” Hu said. ” In some cases, like probiotics and FMT for treatingClostridium difficile infections , you want healthy species to become established. And then to protect the soil, people add biological products and beneficial species to the soil. So you’re also hoping the invaders succeed.”
This research was funded by a Schmidt Polymath Award and the Sloan Foundation.
