Many organisms shape their environments as they are shaped by it – the North American beaver builds dams, social insects construct intricate nests, and the world’s forests are criss-crossed with game trails. Indeed, cooperation between individuals is a common way to modify the environment for the better of all.
However, as humans have become all too aware, transforming the environment can have negative side effects. Human-generated pollution threatens ecosystems and species across the globe and is even shifting the planet’s climate.
But it seems we’re not the only organisms capable of wiping ourselves out through changing the world in which we live.
Christoph Ratke and senior author Jeff Gore of Massachusetts Institute of Technology in the US and Jonas Denk of Ludwig-Maximilians University in Munich, Germany, have just published the results of their recent study into microbial group suicide.
Gore (who is also known for his crusade to eradicate the American penny) and team report that soil bacteria of the genus Paenbacillus often significantly lower the pH of their environment. When population density is high, they make their surroundings so acidic that the swift and complete eradication of the entire community results. Remarkably, this occurred in about a quarter of the pH-modifying bacterial strains tested in their experiment.
The authors call this ‘ecological suicide’ and it’s something of a paradox, as it goes directly against a well-established biological principle known as the Allee effect.
In the 1930s one of the founders of ecology, WC Allee, noted that for many species, fitness increases with population density. However, in the current research the opposite seems to be true – low-density populations thrive while high density leads to ecological destruction.
Even more surprising is the realisation that substances used to kill bacteria – such as antibiotics in medicine, or salt and ethanol in food preservation – may actually save populations of these bacteria and enable them to grow.
But how could evolution lead to such a situation?
Ecological suicide, the authors muse, may be connected to something more ominous – ‘evolutionary suicide’. The extinction of a species is thought to occur when the environment changes and a species is unable to adapt. Evolutionary suicide is an alternative explanation, in which evolution selects for adaptations that are beneficial for individuals but are fatal for the species. The authors suggest that the bacteria may have evolved to metabolise nutrients in a way that is fast but produces acidic byproducts: in an environment where nutrients are scarce, this produces advantages for the individual, but it creates problems for the group when population density increases.