A new study suggests nature's ecological web is so tenuous that it's amazing anything survived this long; even the smell of a predator can have disastrous effects in populations of small size in flies. They spend less time eating, more time being vigilant, have less sex, and produce fewer offspring.
This long-standing biological conundrum related to population size and extinction, known as the Allee effect, after Walter Clyde Allee, the man who first described the phenomenon, deals with how animal populations show what is called "density-dependence". This means that as a population increases, competition for food and mates increases, and reproductive success and survival decline. One might assume that the reverse would also hold true. That is, that the smallest populations would have the highest vital (or growth) rates and would quickly rebound from declines in population. However, paradoxically, in some cases exactly the opposite happens and small populations have the lowest vital rates. This puzzling phenomenon is one aspect of what is known as the Allee effect.
Credit: Ian Dworkin
By looking at fruit flies, rather than social animals, the researchers believe that they have gained a greater understanding of the role that fear may play in the decline and extinctions of various populations.
When scientists from McGill University and the University of Guelph exposed fruit flies to the scent of a praying mantis, a known predator, they found that the risk of extinction increased up to seven fold. The increased risk of extinction occurred because at small population sizes, as the flies spent more time being vigilant and less time eating, populations that declined could not quickly rebound. The varying sizes of populations of fruit flies were exposed to the scent of praying mantises during breeding and non-breeding seasons. The scholars then measured how exposure to this smell affected the body weight and fecundity of the following generation of fruit flies.
They discovered that in small populations, the scent of the predator was enough to significantly reduce both fecundity and offspring growth. This suggests that once a population has reached a limited size, the effect of fear alone may lead to its extinction.
Citation: Kyle Elliott et al., "Fear creates an Allee effect: experimental evidence from seasonal populations", Proceedings of the Royal Society B