Ecosystem attack: Predicting the success of species invasions

When a new species enters an ecosystem, it might establish itself successfully or fail to gain a foothold. Researchers at MIT have recently devised a formula to predict these outcomes. The research is based on extensive modeling and laboratory experiments with soil bacteria. 

The findings, which could apply to ecosystems ranging from forests to the human gut microbiome, offer valuable insights into the dynamics of species interactions and potential applications in medicine and environmental science.

Mechanics of species invasions

MIT physicists, led by Jeff Gore, a professor of physics, analyzed hundreds of experimental scenarios to determine the factors that influence whether an introduced species succeeds. 

The team’s formula, grounded in their research on bacterial communities, could have implications for understanding natural ecosystems, improving probiotics, or enhancing treatments like fecal microbiota transplantation (FMT).

“People eat a lot of probiotics, but many of them can never invade our gut microbiome at all, because if you introduce it, it does not necessarily mean that it can grow and colonize and benefit your health,” noted study lead author Jiliang Hu, a PhD student at MIT.

Experimenting with bacterial communities

The researchers studied over 400 soil bacterial communities, each comprising 12 to 20 species native to soil around MIT. They introduced a new bacterial species into each community and monitored the outcomes over 12 days. 

By sequencing bacterial genomes, the team assessed whether the invader had successfully established itself.

To further understand the role of environmental factors, the team varied the nutrient levels in the bacteria’s culture medium. High nutrient levels often led to intense species interactions, such as competition for food or inhibition through mechanisms like pH-mediated cross-toxin effects.

The researchers observed two types of community behavior: stable populations, where species numbers remained constant, and fluctuating populations, where species abundance varied over time.

Key factors in successful species invasions 

The study revealed that fluctuations in population size were a critical determinant of invasion success. Communities with higher levels of fluctuation were more diverse and more prone to successful invasions. 

“The fluctuation is not driven by changes in the environment, but it is internal fluctuation driven by the species interaction,” Hu explained. “And what we found is that the fluctuating communities are more readily invaded and also more diverse than the stable ones.”

In stable communities with less diversity and stronger interspecies competition, invaders were less likely to succeed. In fluctuating ecosystems, however, the invader sometimes coexisted with the original species, albeit in smaller numbers. In other cases, the invader outcompeted and replaced certain resident species.

Species interactions and the “survival fraction” 

The researchers identified a factor they call the “survival fraction” – the proportion of the original species that remain after an invasion – as a reliable predictor of invasion success. 

In natural ecosystems, this survival fraction could be approximated by comparing local diversity (species in a specific area) to regional diversity (species in a broader region).

Another important finding was the influence of the order in which species arrive in an ecosystem. 

When species interactions were strong, the likelihood of an invader succeeding decreased if it was introduced later. However, when interactions were weaker, the arrival order had little impact, and communities reached similar equilibrium states regardless.

Implications for medicine and ecology

The researchers plan to apply their formula to larger ecosystems and explore its relevance to the human gut microbiome. For example, the formula could help predict the success of probiotics or FMT treatments for infections like C. difficile. 

“Invasions can be harmful or can be good depending on the context,” Hu noted. “In some cases, like probiotics, or FMT to treat C. difficile infection, we want the healthy species to invade successfully.”

In agriculture, the formula could guide efforts to introduce beneficial microbes into soil to enhance plant growth or protect crops from pathogens. 

Similarly, it could inform conservation strategies by predicting how ecosystems might respond to invasive species or restoration efforts.

Future research directions

Future studies will test whether the local-to-regional diversity ratio can predict invasion success in ecosystems where diversity data is available. 

The team also aims to investigate the underlying mechanisms that drive the observed population fluctuations and how these dynamics apply across various ecological contexts.

This study, published in the journal Nature Ecology and Evolution, sheds light on the complex interplay of species interactions, diversity, and environmental factors in shaping the outcomes of biological invasions, with potential applications spanning environmental conservation, agriculture, and human health.

—–

Like what you read? Subscribe to our newsletter for engaging articles, exclusive content, and the latest updates.

Check us out on EarthSnap, a free app brought to you by Eric Ralls and Earth.com.

—–