Quick look
An Iowa State University ecologist was part of a research team that provided new evidence for co-extinction theory, findings that relied on his long-running research plots designed to simulate the extinction of large animals such as elephants.
AMES, Iowa – Jake Goheen first traveled to a Kenyan savanna about 25 years ago, but part of him never left. The jaw-dropping sight of iconic large mammals struck him intensely, permanently recalibrating his career trajectory.
“I couldn’t help but be utterly astounded. It’s everything you’ve seen on Animal Planet that you were never sure you’d witness with your own eyes. It turns out they’re all real,” said Goheen, an Iowa State University assistant professor of natural resource ecology and management. “And then I’d have questions about these animals that would keep me up at night. I was just so curious about them.”
Up to that point, Goheen’s focus as a graduate student had skewed smaller. His master’s thesis was about squirrels. Seeing the likes of elephants and giraffes in the wild sent him down a path marked with far bigger footprints, leading him to establish a research program in areas densely populated with charismatic megafauna.
That doesn’t mean he moved on entirely from more diminutive creatures. Goheen’s work often spans the size spectrum, exploring how large herbivores and predators affect their ecosystems – including their impact on far smaller animals.
“One of the things that we learn in freshman year ecology class is that everything is connected to everything else. While that’s sometimes true, the idea is repeated so frequently that we believe without the data to support it,” Goheen said.
A study he co-authored in Science this summer added to the evidence supporting co-extinction, a core ecological concept that predicts a cascading chain reaction of biodiversity loss when host animals die off. A nine-person research team showed that the number and types of dung beetles fall sharply in the absence of elephants, a collapse that demonstrates their outsized influence as a keystone species.
Without elephants, beetles collapse
The study led by researchers at Princeton University incorporated field surveys and computer simulations, but the core experimental data came from the exclusion zones Goheen has maintained for 18 years at the Mpala Research Centre, a wildlife conservancy of nearly 50,000 acres in the high-altitude savannas of north central Kenya. The center is jointly managed by three Kenyan research and conservation agencies, along with U.S. partners Princeton and the Smithsonian Institution.
The 27 enclosures run by Goheen and colleagues at Princeton and the University of Florida, each a square with 100-meter sides, are designed to keep out varying sizes of animals, depending on the height of the electrified fencing. One-third have an opening about 6 feet high at the perimeter’s base, excluding only elephants and the area’s less prevalent giraffe population. Another nine enclosures keep out all but the smallest animals. The remaining nine are a middle ground, preventing a broader group of large mammals from entering while providing access for most smaller creatures.
The exclusion zones are a tool for running ecological experiments that simulate extinction in a controlled way that reduces variables like weather and soil conditions, Goheen said. The enclosures have played a role in more than 50 research papers.
“If you’re studying elephants, it allows you to isolate the effect they actually have as opposed to looking at an area where there haven’t been elephants for a long time, but where there are a dozen other confounding factors,” he said.
In the Science study, researchers tested how dung beetles fare without elephants by comparing the populations in control plots to those in zones that fenced out either most herbivores or just elephants and giraffes. In each area studied, they served up a buffet line of petri dishes with cattle, zebra and elephant dung, then collected the traps after 24 hours to count how many dung beetle specimens and species were in each set of samples.
In areas excluding just elephants and giraffes, total dung beetles fell 23% and species diversity dropped by 67%. Zones that excluded all sizable herbivores had similar beetle losses to the tracts excluding only elephants and giraffes, suggesting the effects were mostly due to missing elephants. The downturn matched what the researchers’ modeling predicted, based on data they collected that indicated elephant feces is by far dung beetles’ most-preferred meal.
Co-extinction consequences
A fall-off in the quantity and variety of dung beetles has consequences. In plots without elephants, researchers found a decline in two important services dung beetles provide for the broader ecosystem: seed dispersal and dung decomposition.
Researchers tested the decomposition rate of dung without beetles by using mesh-covered petri dishes – tiny versions of Goheen’s fenced enclosures – and found that excluding elephants without any restriction on beetles slowed decomposition nearly as much as excluding beetles in an area where elephants roam.
“Counterintuitively, you need elephants to support enough beetles to decompose the other animals’ dung,” Goheen said.
Breaking down dung has both ecological benefits and, in livestock settings, economic value. Decomposition provided by dung beetles annually delivers an inflation-adjusted $1.6 billion in value to the U.S. and U.K. cattle industries combined, according at a 2006 estimate cited by the researchers in the Science paper.
Those sorts of effects are difficult to pinpoint, but the combination of his long-running exclusion experiments with additional field research and computer simulations strengthened the dung beetle study, Goheen said.
“Co-extinction is challenging to document. Its impacts can be both huge and really well-hidden,” he said. “But the bottom line is that if you can help it, don’t get rid of your big mammals because there can be unanticipated consequences. That’s the practical implication.”
Research team
Co-authors of the paper, “Importance of elephants for dung beetle biodiversity and ecosystem functions,” include:
- Finote Gijsman, Princeton University (lead author)
- Jake Goheen, Iowa State University
- Matthew C. Hutchinson, University of California, Merced
- Fathiya Khamis, International Centre of Insect Physiology and Ecology, Nairobi
- Scott E. Miller, National Museum of National History
- Ciara Nutter, Princeton University
- Fidelis Ombura, International Centre of Insect Physiology and Ecology, Nairobi
- Todd M. Palmer, University of Florida
- Robert Pringle, Princeton University
