The approach has been fruitful. In 2022, Gore and colleagues discovered that ecological communities undergo phase transitions — a core organizing principle in physics that describes, for example, water’s change from solid ice to liquid to gas. As the researchers increased either the number of species in their experimental ecosystems or the strength of the interactions between species, the ecosystems might progress through three phases. In phase one, all bacterial populations remained stable. In phase two, some species died out while others survived. And in phase three, the populations of the remaining species started oscillating wildly, revealing a loss of stability.
Gore next wondered what would happen to his microbial communities if he sent in a potential chaos agent — an interloper.
Niche Dynamics
What makes an ecosystem vulnerable or resilient in the face of invasion is not just an academic exercise; it’s one of the most important questions in ecology. Protecting vulnerable ecosystems from harmful invasive species could save at-risk wildlife and prevent billions of dollars in environmental damage each year. At a microscopic level, preventing hostile takeovers of the usually beneficial microbial ecosystems that inhabit our guts could protect many thousands of people annually from serious illness.
“Invasive species are one of the primary drivers of these sorts of problems,” Gore said. He noticed, though, that many studies seemed to focus on what properties make an invader more or less successful, with few asking what makes an ecosystem more or less open to invasion.
To explore this question, Jiliang Hu, then a graduate student in Gore’s lab, went outside and collected soil from a lawn on the MIT campus, as well as leaves from nearby trees and water from the Charles River. Using bacteria isolated from those samples, he established hundreds of communities, each composed of a different set of 20 bacterial species (from a larger pool of 80), and fed them for a week to give them time to stabilize.
If more ‘native’ species can coexist in an ecosystem, it stands to reason that an invader can find a way to coexist with them, too.
To create different kinds of ecological networks, the scientists fed more nutrients to some communities than to others. They knew from past experiments that altering nutrient levels could make the microbes compete more intensely for food and other resources, creating stronger interactions among species.
In all ecosystems, the majority of starting species died off. In roughly half the ecosystems, the remaining bacterial species settled into a stable state in which populations remained steady. In the other half, populations rose and fell wildly. Consistent with the 2022 study uncovering phase transitions, these roller-coaster ecosystems harbored more species diversity, perhaps because the fluctuations created more ecological roles, or niches, for a species to fill in an ecosystem.
The scientists then tried to disrupt the ecosystems. To some wells they added a randomly chosen additional species — an invader. Uninvaded ecosystems served as controls. After another week, the scientists sequenced parts of the bacterial genomes, to see whether the invader had successfully established itself, and tallied up the total biomass in each ecosystem.
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