Tracing Landscape Change through Dung Beetles
As part of the ongoing series Silent Spring Continued: A World without Insects, environmental anthropolgist Olea Morris recounts how fieldwork in Mexico introduced her to the world of dung beetles. This post follows on from Olea’s Insect Profile on dung beetles.
*Featured Image: Dung beetle, by Olea Morris
By Olea Morris
In some ways, the dung beetles and I had a lot in common! Working as a volunteer on a farm in the highlands of Veracruz, Mexico, I was assigned the very unglamorous but important role of tending to the manure of the animals raised there. Every morning, I would put on my knee-length, white plastic boots, hike up a misty hillside in the heart of the cloud forests, and clean the stables of the sheep after they had been moved out to pasture for the day. It was monotonous work, using coarse brushes to roll the waste into heaps and push it through the gaps in the wooden floor to the compost piles below. I often thought of the dung beetles, which the farmers I worked with had pointed out to me, even though I never saw much of them other than the traces of their burrows in the pastures. I wondered how many of them there must have been, toiling on their Sisyphean tasks, spending their nights moving mountains, only to have them replaced the following day.
Whether they know it or not, local farmers rely on the work of dung beetles—the coprophagous (“feces-eating”) insects from the Scarabaeinae family—perhaps more than they realize. As they make their homes in and digest livestock waste, dung beetles integrate organic material back into the soil, making it available to microorganisms. They destroy the eggs of parasites, which constantly threaten farm animals with infection if left unchecked. Parasitic diseases are a major problem: despite regular treatments, health checkups, and pasture rotations, at least three lambs died during my fieldwork from parasitic infections. Dung beetles also tunnel and bury, and in the process move seeds about in the layers of soil—not only those left behind in deposits of animal waste, but seeds interred long before; a process which allows native vegetation to regenerate. The ecological services provided by dung beetles in these tropical montane forests are so great, in fact, that their presence (or absence) has served as an indicator of levels of forest destruction and ecosystem health.
Despite this close interrelationship with increasing rare forest habitats, the loss of old growth forest doesn’t always mean a loss of dung beetles—or at least, not a kind of loss that can be succinctly articulated. The regular presence of livestock animals means a steady supply of the very stuff that dung beetles live on—the substrate they consume and in which they reproduce and live out their lives. Unexpectedly, one study has shown that cattle grazing actually promotes biodiversity of dung beetle species assemblages (1). However, this particular study was based on comparisons not between farm and forest ecosystems, but between the species richness of pasture land and scrubland (scrubland being defined as an intermediate ecological zone transitioning between grazing area and forest). Generally speaking, it seems from this that the presence of cows, and hence, manure, usually means beetles—or to be more accurate, there is at least no reduction in species richness in grazed areas (2).
But looking in more detail at the community composition of dung beetles in different habitats can shed light (quite literally) on other aspects of dung beetle behavior. In a study measuring beetle diversity, for which cow pats were laid as traps for beetles in various spots in a pasture, researchers found that manure placed in the shade was significantly more attractive to some dung beetles than manure placed in open areas (2). Tree shade, it turns out, was important for protecting native and local beetle species from the sun. It seems that, within these promising numbers of “high diversity,” a story of loss can still be read: these researchers have shown that as forests are modified to pasture, local species die out and are replaced by more broadly distributed ones, as well as non-native species brought to the region decades ago. Tracing the beetle’s story this way seems circuitous: Are pastures then good or bad for beetle life? If forests make for better, shadier, habitats for native species, but scrubland in transition to forest appears to be worse for beetle diversity than grazing land, how should farmers interpret this information and plan their livestock rotations?
Studying dung beetles, especially from an anthropologist’s perspective, always seemed to leave me one step away—I learned about them from the traces they left behind, or more precisely, the traces of their interactions with the traces of other animals. The people I spoke with during my fieldwork were deeply concerned about dung beetles: shepherds, scientists, and sometimes shepherds-turned-scientists. In the laboratory at the nearby Institute of Ecology in Xalapa, shelves of plastic buckets, seemingly filled only with soil, actually contained whole worlds within them. Opening them up revealed the lives hidden below the surface.
To see such creatures in the field, though, required hiking up through the forest to the hilly pastures, and using a keen eye to locate potential beetle homes in places where the cows had been the previous day. One shepherd, whom I worked with on a daily basis, made a study of the cow pats left in his fields, leaving some where they lay and burying others. Manure left in fields was a bad thing, both for him and his livestock: it meant higher risks of parasitic infections, less grass growth, and overall less productivity. Burying it, or hauling it away to the compost piles, might be a better solution, he reasoned, but he was curious: What would this mean for dung beetle populations? After all, less beetle labor (digesting and moving the byproducts of human agriculture) meant more human labor. A few days spent in the stables, I came to realize, will have you wishing for more beetle helpers in no time.
At the end of my fieldwork, dung beetles left me with more questions than answers: Where does ethnography get us if we are always a few steps behind our (elusive) subjects? How far down the dung beetle burrow must we delve in order to get a clearer picture of the human effects on these ecosystems? Are we measuring their losses—in species richness, or total individual count, or beta diversity—by our own measures, or theirs?
seeingthewoods.org 
Fantastic piece. Glad to see the social sciences featured.
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