By: Ashley Shaw, PhD student in the Graduate Degree Program in Ecology at Colorado State University. For her dissertation research, Ashley works with the NSF-funded McMurdo Dry Valleys Long Term Ecological Research program.
On first look, the McMurdo Dry Valleys of Antarctica are desolate. This cold desert is among the world’s most extreme environments and forms the largest ice-free area in Antarctica. It is frigid, dry, and windy. It’s dark there for half of the year. During the austral summer, the one long day when the sun shines, the soils thaw and refreeze as the sun warms dark soils despite temperatures remaining below freezing. The UV levels are high (thanks, ozone hole!) and a constant threat due to 24 hours of daylight. Despite these challenges, there is life in the dry valleys.
In the dry valleys, the largest terrestrial animals are soil invertebrates: tardigrades, nematodes, and rotifers, along with a few springtails and mites. The soils they inhabit have their own suite of harsh factors and often have high pH, salt, and nitrogen due to deposition and build up that is not flushed out by precipitation. Not only is there infrequent snow, there is little liquid water available at all. No vascular plants survive here, and there are only trace amounts of organic carbon in the soils. Until now, environmental factors such as temperature, pH, salinity, and water availability were considered to be the only drivers of soil invertebrate communities.
While one tough nematode, Scottnema lindsayae, dominates and thrives in the dry soil that makes up >95% of the landscape, most of the soil invertebrate biodiversity prefers the scarce wetted margins of streams and lakes. These areas often have cyanobacterial mats – orange, black, and green - that sustain the soil food webs. In and around these mats, nematodes, rotifers, and tardigrades prosper. These animals share a habitat and are regularly extracted from the same soils. Could their interactions – such as predator-prey or competition – shape the communities in these soils?
The nematode Eudorylaimus antarcticus prefers wet soils, but is sometimes also found in dry soils with S. lindsayae. E. antarcticus is member of the order Dorylaimida, which contains many omnivore-predator nematode species. Given its close relationship to other predators, E. antarcticus could be a potential predator in the dry valleys. However, previous studies have only found evidence that it eats algae. We set out to test whether E. antarcticus could be a predator in the dry valley soils.
Stable isotopes are a great approach for understanding food webs. They are similar to radioisotopes (but have the benefit of not being radioactive!) and are also traceable. We can understand what an organism’s diet is like, because it picks up a detectable signature from its food sources. We used stable isotopes to test each invertebrate group’s position in the soil food web. To do this, we collected soil from both wet and dry habitats in the dry valleys. We extracted invertebrates from the soil, identified them to species under a microscope, and then collected (by hand, using an eyelash tool) hundreds of individuals by taxa. These invertebrates were analyzed for their 13C and 15N isotopes.
The isotope results showed that rotifers, tardigrades, and several nematode species were grazers – eating bacteria and algae. But E. antarcticus was different: its isotopes put it the top of the food web. This new evidence, combined with previous evidence that it eats algae, showed that E. antarcticus is an omnivore-predator.
This is big news for cold desert ecology. Not only is this the first solid evidence of a nematode predator present, it also opens the door to testing how biotic interactions such as predator-prey relationships might shape communities in these soils. This is especially important because the landscape is predicted to become wetter and more connected in the future. Thus, species ranges may shift, altering interactions or creating new contacts between species. Understanding species’ relationships to one another is the first step in predicting how their interactions might shape soil communities.
Antarctic soils are far from desolate. Life abounds in these soils, where there are still many questions to be explored!