Mutualistic Partnerships Between Species Abound Across Nature

I then found many more examples of this — the maximum was five different species of ants living in one plant. So I did a wide range of experiments to understand what was going on. The first possibility was that these ant species were simply not very aggressive. To test this, I took workers from one of the species of ants and put them near workers of the other species to see whether there would be conflicts. And yes, there were. But what was interesting was that in field lab experiments, when you kept the nests apart, the ants could share a nearby sugar source without killing each other.

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The ultimate test was to disrupt these compartments. When I removed the wall between two different species of ants, I found they would fight to death. After half an hour, few ants would be left alive, even though they’d been living there for years. The plant suddenly ended up without any ants at all.

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They are rather unique, though species that have bacterial symbionts — humans included — do promote the growth of different bacteria in different areas of the body, which is arguably a kind of compartmentalization. Bodies also have mechanisms to prevent bacteria from entering areas where they can cause disease. When otherwise beneficial skin or gut bacteria enter our bloodstream, for example, they can become very dangerous. This shows that mutualism is always a matter of balance and control.

Many plant roots have similar protections to prevent the fungi that help them absorb nitrogen and phosphorus from growing too far inside, where they can cause damage. They also often host different strains of bacteria or fungi in different compartments, so they can selectively reward or punish them based on what they contribute.

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I think this is the major pathway to evolution of obligate dependence in which species really depend on each other for survival. Both species might lose the ability to perform certain functions, say digesting certain nutrients, producing certain vitamins or defending themselves from predators, because they are provided by the partner. It’s an automatic evolutionary mechanism that makes species dependent — and vulnerable. When one species goes extinct, the other one is likely to disappear as well.

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