What effect does species extinction have on the evolution of the surviving species? Evolutionary biologists from the University of Zurich have investigated this question using a field experiment with gall flies and their predatory parasites. It was shown that the loss of natural enemies could make it difficult to adapt to future environmental changes.

According to many experts, the earth is at the beginning of a sixth mass extinction, which is already having devastating effects on the function of natural ecosystems. However, it is still unclear what impact this species extinction will have on the future adaptability of the surviving species.

Researchers at the University of Zurich investigated this question in a field experiment in California. They investigated how the characteristics of a tiny fly changed when a group of its natural enemies was removed. From their observations, they drew conclusions about changes in the genetic diversity of flies.

Eliminate predatory parasites

The fly Iteomyia salicisverruca lives on willow leaves in tooth-shaped growths, so-called galls, which it forms in the larval stage. The fly’s natural enemies include several types of parasitic wasps. These lay their eggs in the fly larvae, where they go through their development as predatory parasites. Before the adult wasp leaves the bile, it eats its host, the fly.

Some species of parasitic wasp attack the fly larvae before they form galls, others parasitize the larvae later in development and penetrate the bile from the outside. The researchers specifically targeted the latter group of natural enemies. To do this, they attached fine nets over the leaves with galls before the attack.

After three months, the evolutionary biologists collected around 600 galls and checked whether the fly larvae contained therein had survived. In addition, they measure three characteristics that influence the survival of flies when attacked by predatory parasites: the size of the bile, the number of flies within a bile and the preference of the fly to form their bile on willow trees with certain genetic properties. From the results, they modeled so-called fitness landscapes on the computer, which visually represent the adaptability of a species.

Fewer enemies, less variability

It turned out that when all natural enemies were present, different combinations of these three characteristics contributed to the survival of the fly. “So there are several equally good solutions that ensure the survival of the fly,” says lead author Matthew Barbour. However, if some of the natural enemies were missing, the fly survived only with a very specific combination of the three characteristics. “This suggests that the extinction of natural enemies limits the evolution of the fly towards one optimal solution.” Genetic variations that lead to a different development of the traits could be permanently lost in the flies’ genome.

This loss of diversity could have consequences: “A large number of possible survival solutions serve to maintain the genetic variability for the characteristics of the bile,” said Barbour. And because genetic variation provides the raw material for evolution, the elimination of natural enemies could make it difficult to adapt the bile flies to future environmental changes.

“Overall, our study suggests an insidious side effect of species extinction,” says Barbour. “The extinction of natural enemies could affect the ability of the surviving species to adapt to and survive in a changing world.” Ecosystems would then be at much greater risk than is currently assumed.

Reference: Matthew A. Barbour, Christopher J. Greyson-Gaito, Arezoo Sotoodeh, Brendan Locke, and Jordi Bascompte. Loss of consumers constrains phenotypic evolution in the resulting food web. Evolution Letters. April 20, 2020. DOI: 10.1002 / evl3.170