Glacier Retreat Unveils Truth of ‘Predator First Paradox’

A recent paper published in Molecular Ecology studies ecological evolution in areas exposed by glacial retreat, shedding light on the “predator first paradox,” a phrase used by ecologists to describe the predator-dominated primary succession in glacier forelands. The authors found that predator anthropods such as spiders and beetles can show up as pioneering dwellers on newly exposed land, even before plants colonize the area. The predator first phenomenon shakes up the traditional understanding of a bottom-up ecological pyramid in which plants serve as the basis of the food chain that feeds the predators. Less was known about the prey that sustains these predators in the early stages of succession. By examining the stomachs of insect predators, the researchers determined that spiders and beetles can survive without vegetation on the prey species of local food webs as well as some flying insects.

View from the separating ridge of two of the valleys investigated by the researchers. Gaisbergtal lies to the left and Rotmoostal to the right (Source: Daniela Sint).

Daniela Sint, the paper’s lead author from the University of Innsbruck in Austria, told GlacierHub, “We could show that the amount of local production and the importance to sustain the arthropod predators on those sites was underestimated over many years.” This conclusion is at odds with previous studies that found that flies from other areas, instead of local mites, are the primary food source of the spiders and beetles.

To understand how ecological evolution starts on bare land, the authors selected several glacial forelands in three valleys in the Tyrol region of Austria, namely Gaisbergtal, Rotmoostal and Langtal, which have recently undergone glacial retreat. All three areas have a glacier above them and lie close to each other, with similar climatic conditions. The researchers found that the three glacier toes had retreated 1.5 to 2 kilometers each since 1850, placing these forelands in the early stages of the ecological progress.

Sint and her colleagues pictured as they approach the study sites located close to the edge of the glaciers (Source: Daniela Sint).

Using self-made pitfall traps, the authors collected samples of spiders and beetles from exposed areas to study how the anthropods feed themselves. The paper notes that the authors went so far as to turn over the stones to catch spider and beetle species missed by traps.

“It’s the first time that so many different prey types were molecularly checked for,” Sint explained to GlacierHub. Sint and her team examined the gut of nearly 2,000 spiders and beetles and conducted a DNA analysis on a total of twenty species.

Through the “autopsy” of these captured spiders and beetles, and a DNA detection of prey within their guts, the researchers found only 30 percent of gut content was made up of flies from other places. The rest of the anthropods’ diet comprises mites and other prey found locally.

The researcher’s data shows that the spiders and beetles have dietary preferences toward mites (not flies), regardless of the differences between the sites. Meanwhile, as time passes, the prey options for spiders and beetles increases, providing more food for the predators. Gradually, this positive interaction empowers the substantial development of the food chain and ecological community.

Some dry ground beetles trapped by the pitfall set by the researchers (Source: Daniela Sint).

Although the researchers identified different food sources for the spiders and beetles, resolving important questions about the prey of predators, Sint also discussed with GlacierHub her team’s plans for future research. “We still were not able to cover the whole food web on the study sites. For example, we found out that springtails are very important food for the predators, but we still don’t know what the food for the springtails themselves is,” she said. “There are several options as they might feed on locally produced algae or fungi, but it could also be that ancient carbon and nitrogen released from the melting ice might play a role.” A follow-up study at the University of Innsbruck is currently focused on this question.

Sint says she will continue to research glacier areas, as “glacier retreat is the factor initiating the whole process of primary succession.” When the glacier melts, land that has been covered by ice for thousands of years is “released” and colonization by microorganisms, plants and animals starts immediately.

Sint further described her concerns about global warming-driven glacial retreat worldwide, saying, “This does not only have the local effect of additional land becoming ice-free and being thus available as new habitat exposed to primary succession, but it also has strong influences on numerous other aspects. Many of them will only become obvious once a specific glacier is gone.”