By causing harm, parasites can sometimes help the species they afflict.
Scientists investigated pollinator wasps (Pleistodontes imperialis) and an Australian species of fig (Ficus rubiginosa). The wasps and figs are mutualistic — they cooperate, with each profiting from benefits supplied by the other partner.
The relationship between the wasps and figs — which has lasted more than 60 million years — involves female wasps entering figs and pollinating tiny flowers within. The pollinators then lay their eggs in the blossoms, and wasp offspring feed off the seed that develops in the flower.
One mystery scientists face when it comes to mutualism is how it remains stable. What prevents one partner from exploiting the relationship? For instance, the wasps might easily overwhelm the figs with offspring that devour all the seeds.
"Mutualisms are very pervasive in nature but have proved harder to understand than other interactions between species," said researcher James Cook, an evolutionary ecologist at the University of Reading in England.
It turns out another species of parasitic wasp, once thought detrimental to the mutualistic wasps and figs, may actually help keep their cooperative relationship stable, preventing the wasps from taking advantage of the figs.
The researchers looked at small fig trees that typically grow on large boulders on rocky hillsides in Australia, so researcher Derek Dunn, who did most of the field work, became "something of a mountain goat to get his samples," Cook recalled.
The pollinators lay eggs in flowers near the center of the fruit. The scientists found out the pollinators avoid blossoms near the outer wall because their offspring are at high risk of attack there from different species of parasitic wasps. This leaves the fig trees free to develop seeds from the outer flowers.
The parasitic wasps were generally thought to have a negative impact on the cooperative relationship between the pollinator wasps and the fig trees — "after all, they kill the pollinating wasps," Cook said. "What we have shown is that they actually contribute towards stabilizing the mutualism in the long term by placing pressure on the pollinating wasps to leave a subset of flowers to develop as seeds."
Mutualisms are often exploited by parasites, "but we now have to ask how often parasites actually play important roles in stabilizing mutualisms in general," Cook said. Three may not always be a crowd. "We need to think of mutualisms as being embedded in — and sometimes reliant upon — a wider network of species interactions."