A mathematical model has been developed to forecast the impact of introducing a new species into an ecosystem, predicting whether the species will thrive or diminish.
The introduction of a new species to an ecosystem could either lead to its successful establishment or to its eventual extinction. Researchers at MIT have created a formula that can accurately anticipate these outcomes.
The scientists based their model on extensive analysis of numerous situations simulated with soil bacteria cultures in the lab. They intend to validate their formula in larger ecosystems, like forests. This framework may also assist in predicting the effectiveness of probiotics and fecal microbiota transplant (FMT) therapies in treating human gastrointestinal infections.
“Many probiotics are consumed, yet a lot of them cannot colonize the gut microbiome effectively. Introducing them doesn’t guarantee growth or health benefits,” explains Jiliang Hu SM ’19, PhD ’24, the primary author of the research.
Jeff Gore, an MIT physics professor, is the senior author of the study published in the journal Nature Ecology and Evolution. Matthieu Barbier from the Plant Health Institute in Montpellier and Guy Bunin, a physics professor at Technion, are also contributors to the paper.
Population changes
Gore’s laboratory focuses on utilizing microbes to examine interactions among species in a controlled environment, aiming to enhance understanding of ecosystem behavior. Their previous research has shown how modifications to the microbial environment can influence community stability.
In this investigation, the team sought to determine the factors influencing the success or failure of species invasions. Ecologists previously theorized that greater biodiversity in ecosystems enhances their resistance to invasions, as most niches are filled and resources are limited for newcomers.
However, both in nature and in lab settings, evidence suggests this is not always the case—while some highly diverse ecosystems do resist invasion, others may be more prone to it.
To unravel why such contrasting outcomes occur, the researchers created over 400 soil bacteria communities sourced from local soils. They formed groups consisting of 12 to 20 bacterial species and then introduced one random species as an invader six days later. On the experiment’s twelfth day, they sequenced the genomes of all bacteria present to check if the invader had taken hold.
Throughout each experiment, the nutrient levels in the bacteria’s growth medium were varied. High nutrient settings enhanced interactions among microbes, leading to increased competition for resources or mutual inhibition due to factors like pH-related toxins. Some communities stabilized, where microbial proportions remained relatively constant over time, while others displayed significant population fluctuations.
The research indicated that such fluctuations were crucial in determining the invasion outcome. Communities marked by greater fluctuations tended to feature higher diversity but also exhibited a greater likelihood of successful invasions.
“These fluctuations arise not from environmental changes but from internal species interactions. Our discovery shows that fluctuating communities are both vulnerable to invasions and more diverse compared to stable ones,” notes Hu.
In communities where the invader thrived, other species persisted but in diminished numbers, while in some cases, resident species were completely outcompeted. This displacement was more common in ecosystems characterized by intense species competition.
Conversely, in ecosystems with stable and less diverse populations, where species interactions were stronger, invasions were more likely to fail.
The researchers found that the proportion of original species surviving before the invasion serves as a predictor of invasion success. This “survival fraction” can be assessed in natural communities by comparing local diversity (the number of species in a specific area) to regional diversity (the total number of species in a larger area).
“It would be fascinating to examine whether local and regional diversity can predict invasion susceptibility in natural ecosystems,” Gore expresses.
Determining success
The study also revealed that under specific conditions, the sequence of species introduction impacts invasion success. When species interactions are strong, late-arriving species face lower chances of establishment after earlier species have settled.
In cases of weak interactions, this “priority effect” is negligible, resulting in a consistent stable state irrespective of species arrival order.
“We found that when interactions are robust, an invader arriving later faces disadvantages. This is intriguing in ecology, as in some instances, the order of arrival significantly matters, while in others, it does not,” Hu adds.
The researchers aim to verify their results in ecosystems with available species diversity data, including the human gut microbiome. Their model could allow predictions regarding the effectiveness of probiotic treatments, where beneficial bacteria are ingested, or FMT procedures for severe infections like C. difficile, which involve transplanting beneficial bacteria from a donor’s stool into a patient.
“Invasions may be beneficial or detrimental depending on context,” Hu states. “For instance, with probiotics or FMT for treating C. difficile infections, we seek successful invasion of beneficial species. Similarly, in agriculture, when probiotics or advantageous species are introduced to soil, we desire their successful integration.”
