Populations of predators and their prey usually follow predictable cycles. When the number of prey increases -- perhaps as their food supply becomes more abundant -- predator populations also grow. When the predator population becomes too large, however, the prey population often plummets, leaving too little food for the predators, whose population also then crashes.But all bets are off if both the predator and prey species are evolving in even small ways, according to a new study published this week in the journal Proceedings of the National Academy of Sciences. When both species are evolving, the traditional cycle may reverse, allowing predator populations to peak before those of the prey. In fact, it may appear as if the prey are eating the predators.
Researchers at the Georgia Institute of Technology have proposed a theory to explain these co-evolutionary changes. And then, using data collected by other scientists on three predator-prey pairs -- mink-muskrat, gyrfalcon-rock ptarmigan and phage-Vibrio cholerae -- they show how their theory could explain unexpected population cycles. The new theory and analysis of these co-evolution cycles could help epidemiologists predict cycles of disease and the virulence of infectious agents, and lead to a better understanding of how population cycles may affect ecosystems.
Evolution is often perceived as an historical event, noted Weitz, who also has a courtesy appointment in the Georgia Tech School of Physics. But organisms are evolving continuously, with certain phenotypes becoming dominant as environmental and other conditions favor them. In organisms such as birds or small mammals, those changes can be manifested in as few as ten generations. In microbial species with brief lifespans, evolutionary changes can happen within days or weeks.
Evolutionary changes can dramatically affect relationships between species, potentially making them more vulnerable or less vulnerable. For instance, if a mutation that confers viral resistance in a species of bacteria becomes dominant, that may change the predator-prey relationship by rendering the bacteria population safe from harm. More generally, co-evolutionary cycles can arise when predator offense is costly and prey defense is effective against low offense predators.