—Written by Liam Heneghan, PhD, professor in the EcoMyths Alliance partner DePaul University Department of Environmental Science.
A short note in which I conjecture on a potentially vast local extinction event of Midwestern soil organisms especially of those inhabiting the leaf litter of woodlands.
In our evolutionary progression humans scrambled from the leafy treetops about half way down the length of the trunk. We now live perched between treetop and root ball on that convenient platform we call the soil. If physicists can give themselves vertiginous shivers by imagining those empty atomic spaces that constitute the seeming sturdiness of ordinary things then it is surprising that soil ecologists ever leave their homes knowing as they do how vastly crenulated, fissured, fractured and porous is the soil.
Ours is the exceptional ecological enterprise since more organisms live in the soil in those porous and interstitial lodgings than on the soil. We are not directly equipped for flight, we rarely burrow, we are condemned to walk upon the dirt until at last we may complete our descent into the ground, toppling into that large furrow excavated for our remains. A soil pore will have us after all.
If we had been just a little smaller and had migrated just a little further down the length of that primordial tree we’d be living in one of the most biologically diverse and ecologically active compartments of the biosphere. The upper ten centimeters or so of soil teems with living things. The organisms living in Earth’s thin and hyperactive rind are phylogenetically diverse, trophically heterogeneous, functionally assorted, highly variable in size, dissimilar in longevity, variegated in morphology, behaviorally divergent, adapted to different soil horizons, disparately pigmented, but are united in their reliance on death. Specifically, soil organisms are all similar in that they feed on detritus (i.e., dead organic matter). As I discussed in a recent column, collectively the action of these organisms within detrital-based food webs results in the breakdown of dead organic matter and the mineralization of organic compounds that makes key nutrient available to the living.
Examine your foot a moment. If it is like mine when shod it measures roughly 30 cm in length (yes, a foot) by about 9 cm wide (your foot, of course, may not be quite so rectangular!). A pair of feet such as these out for a stroll treads minimally upon the bodies as 270,000 protozoa, 135 mites, 3 springtails, and one or more large earthworms with each footfall. In places of high animal density the injury toll would be higher by several orders of magnitude. If you were sallying along a woodland path in the temperate zone these crushed critters will be representative of about 30 distinct and species of which up to half may be previously undescribed by taxonomists. Scaled up there can be as many as 200 species of soil insects and 1,000 species of soil animals in total in every square meter of soil.
These soil animals are drawn from many taxonomic groups: protozoa, nematodes, rotifers, tardigrades, springtails, mites, the preposterously adorable pseudoscorpions, insects from many orders, centipedes, millipedes, and on and on.
Conservationists need to pay more attention to soil organisms because they are a very large component of the biological diversity at many sites set aside for the conservation of species. They also play a role in the regulation of nutrient availability and this in turn exerts a large influence on a site’s biological diversity. So even if one was not as charmed by a soil mite as by, let’s say, a Northern Hairy-nosed Wombat (one of the rarest of our larger mammals), nevertheless, the functional significance of the soil mite should persuade you that it deserves a little of your attention. Soil critters are examples of what biodiversity guru E.O. Wilson once described as the “little things that run the world.”
In the last couple of years my lab has initiated investigations on the diversity of soil organisms and their significance in regional conservation efforts. We are addressing these questions in ongoing restoration projects designed to conserve biodiversity in and around Chicago (see the map below of the 100 one-hectare sites we are examining in collaboration with managers in four counties surrounding Chicago). These sites, in woodland, savanna and prairie habitats, are heir to the typical problems associated with open space in a major metropolitan setting—they are highly disturbed, heavily invaded, eutrophied, and fragmented. We are especially interested in learning how our current best conservation practices influence the composition of these below ground communities and, assuming such practices are altering these biotic communities, we want to know the influence these soil critters have on ecosystem processes.
Our studies are still in their early stages. One thing is clear to us though: There is a high probability that soil organisms are going locally extinct in woodlands around Chicago at rates faster that we can study them comprehensively. This may be true especially of those living in the litter layer of partially decomposed plant material.
If soil animals hummed as the ambled like Winnie to Pooh the sound of their productive murmurs would have noticeably dimmed in recent years. More silent that a bird-less spring is the silence of a habitat from which inconspicuous creatures have imperceptibly slipped away.
This vast dying of tiny things in Midwestern woodlands is a conjecture at this point. We simply do not have enough information on the issue to state it definitively. But the conjecture is nonetheless backed up with some evidence. I review a few relevant points to clarify what is a stake and what the major threats to Midwestern soil biota might be.
Temperate Zone Soil Biodiversity Is “The Poor Man’s Tropical Rainforest”
We know very little about soil organismal diversity in the Midwestern United States. Taxonomic experts admit, for instance, that only a fraction of soil arthropods have been described. For mites it may be as few as 5 percent of all species globally, less than 50 percent in the temperate zone. Most groups of organisms increase in diversity from poles to tropics where life flourishes best. To put it as did Jim White from National University of Ireland to us biology students in the 1980s: life is a tropical affair. We do not know much about these so-called latitudinal gradients of soil animals though the evidence is coming in that for many groups, species diversity peaks in the temperate zone (for example, the diversity of soil mites and free-living nematodes appear to peak at mid-latitudes). The density of many soil critters also peaks in temperate regions. For this reason the community of soil organisms in the temperate zone has been referred to by Michael Usher as the “poor man’s tropical rainforest.” The significance of this is that conservationists working in the mid-latitudes have a special responsibility for the conservation of these species. In Chicago where extensive tracts of open space are set aside for conservation and restoration purposes we need to be confident that our conservation management is protecting cryptic biota below-ground.
Dominant Invasive Plant species and the Creation of Interspersed Denuded Zones (IDZs) in the Forest Preserves
There are many stressors in Midwestern environments that may have a negative impact on the diversity of soil biota. These include fragmentation of habitat, anthropogenic nitrogen deposition from the atmosphere, elevated heavy metal concentration in soils, and altered soil hydrology to name a few. In particular I have been interested in one aspect of change in the woodlands of the Chicago region: Many of the dominant invasive species in lands of conservation concern close to the city can have very high decomposition rates and this, for readily understandable reasons, can have a disproportionate influence of species loss. For example European buckthorn (Rhamnus cathartica), a rarity in its native range, has become the dominant woody plant in Chicago’s Forest Preserves. The leaf of this handsome shrub is easily decomposed and unlike many of the native species that it replaces this litter is fully decomposed before it is replenished in autumn. As a consequence a series of interspersed denuded zones (IDZ) open up intermittently in woodlands. From the perspective of litter dwelling arthropods perspective this is like the mass clearing of a housing project. Leaf litter provides habitat for a vast diversity of species. In addition, the litter modulates the physical conditions of the upper layers of the soil which also harbors a large diversity of organisms. Several years ago undergraduate researcher Brad Bernau examined the abundance of diversity of soil microarthropods (mites and springtails) in standardized samples of litter (255 cm2 grabs) in several woodlands and found that diversity and abundance was lower in IDZs and moreover diversity stayed low even after litter was replenished. Bernau’s study needs to be conducted on a much grander scale to assess this phenomenon. In recent years PhD candidate Basil Iannone (University of Illinois, Chicago) has been developing the most comprehensive observational database yet on buckthorn and although he is not looking at soil arthropods, his work will give us unprecedented insight into the implications of this species on the environment of woodlands in our region.
Invasive Earthworms Accelerate Breakdown of Woodland Floor
In addition to changes in the dynamics of the woodland floor as a consequence of shrubby invasion, these woodlands are also invaded by non-native earthworms. Worms are titans in the kingdom of decay and they contribute to the breakdown of the woodland floor and to the creation of denuded zones. The significance of worm-work is accented when one recalls that the ecological systems of the Midwest developed in the absence of these animals.
Loss of Litter-Dwelling Species
Putting this together we can say that conservationists in the U.S. Midwest have a global responsibility for protecting the diversity of soil animals whose numbers peak in the temperate zone. Areas set aside for protecting nature need to designed and managed in ways that achieve this aim alongside other priority species and processes. Although the evidence that the vast diversity of Midwestern soil critters is undergoing a mini local extinction event is indirect, it is enough to warrant serious investigation.
A thought that haunts me: In the 1990s I worked on the diversity of soil arthropods in Costa Rica, Puerto Rico, Hawaii, and in the Southern Appalachians. The leaf litter at Coweeta Hydrologic Laboratory in North Carolina was thick and was home to an almost unimaginably vast diversity—larger than at the tropical sites. Small samples of the litter in a single 100 m2 patch of forest floor at Coweeta yielded well over a hundred species of soil mites alone. In contrast graduate student Claire Gilmore from DePaul recently surveyed mites at 11 sites throughout the Chicago region as part of our 100 Sites project found about half that number. Though the studies are not directly comparable they should give us pause.
Humans migrated from ancient canopies, a habitat of unparalleled species diversity, to the soil surface. Now below our feet is what Belgian taxonomist Henri André called the “other last biotic frontier.” Assemblages of soil arthropods are exceptionally diverse, functionally significant and vastly understudied. For those of us who see the challenge of biodiversity conservation as saving all the pieces, our challenge has become a little muddier than before. Our new motto: Ad terram—to the soil!
Thanks to Vassia Pavlogianis, who collaborated in coining the term Interspersed Denuded Zones. Funding for some of work on soil biodiversity comes from The Gaylord and Dorothy Donnelley Foundation, Chicago.
Photo Credits: Soil microarthropods by FAO, Interspersed Denuded Zone under buckthorn and 100 Sites for 100 Years by Lauren Umek and Alex Ulp.