In recounting his journeys to Mount Kilimanjaro, ecologist Marcell Peters remembers how difficult it was to breathe at more than 3,500 meters above sea level. Up in the subalpine forests on the upper flanks of Africa’s highest mountain, with a pinnacle that reaches nearly six kilometers above sea level, the air gets thin, and hiking, let alone conducting an ecological study, can be exhausting, says Peters, a researcher at the University of Würzburg in Germany.
Yet the mountain’s ecology was exactly why Peters undertook the six-day trek on multiple occasions between 2011 and 2016. He, fellow Würzburg ecologist Ingolf Steffan-Dewenter, and a team of nearly 50 other scientists from Europe and Africa collectively made hundreds of trips up Kilimanjaro during that period in an effort to survey the mountain’s animals, plants, insects, and microbes. Their goal was almost as lofty as the mountain itself—to untangle the invisible connections between human land use, regional climate conditions, and local biodiversity.
Kilimanjaro is a natural laboratory for researchers who want to study how these three factors interact. The mountain is among the most biodiverse places on the planet, transitioning from arid savanna at its base, to rainforest, to subalpine forests and alpine meadows over a distance of less than 30 kilometers as the crow flies. Tropical mountains such as Kilimanjaro are great environments for studying how humans and climate influence biodiversity, says Peters, “because they host these huge climatic gradients in a very small area.”
Land use varies widely across the Mount Kilimanjaro region, too. The local human population has more than doubled in the past 30 years, and agriculture has expanded at a similar pace. Cornfields have cropped up throughout the savanna, industrial-scale coffee plantations have been cut out of the rainforest, and clear-cuts and burn areas dot once-pristine forest as a result of logging.
Theoretically, if you could establish enough organoids, you could test a lot of different drugs.—Noah Shroyer, Baylor College of Medicine
Starting in 2011, the team of scientists went to work cataloging the animals, plants, and microbes living in or passing through each of 60 2,500-square-meter study sites. Peters collected ants by hiding cups filled with soapy water in holes he dug in the dirt; Steffan-Dewenter did the same with colorful bowls to attract pollinating insects. Meanwhile, botanists meticulously identified every plant and measured leaf area, fruit abundance, bark density, and many other characteristics of the flora.
Other researchers identified birds by their calls and used mist nets to capture and classify bats, while wildlife biologists set up camera traps to spot and count terrestrial mammals. To measure the biodiversity residing in the ground itself, microbiologists collected soil samples and sequenced bacterial DNA back at the lab.
In addition, Peters and Steffan-Dewenter investigated the broader ecological context around each of the study sites. They devised a formula to quantify the degree to which human land use had altered the environment, accounting for deforestation, fertilizer inputs, livestock grazing, and the extent of agriculture in the landscape around each study area. They also installed sensors at each site to measure temperatures over a 2-year period and used a 15-year record from rain gauges around Kilimanjaro to estimate precipitation.
Piecing together all of these data, the team found that the degree to which human land use was associated with biodiversity loss on Kilimanjaro depended on the extremity of the climate (Nature, 568:88–92, 2019). In the dry, hot savanna, maize fields had 50 percent lower plant diversity and 30 percent lower animal diversity than undisturbed savanna. Similarly, at cold, subalpine elevations, the diversity of animals was lower in areas affected by logging and grazing relative to more-natural ecosystems.
On the other hand, coffee plantations at wet and warm mid-elevations didn’t show any differences in biodiversity compared with areas of pristine rainforest—despite the fact that coffee plantations typically require broader deforestation and heavier fertilizer use than the lower-elevation maize plantations.
The results indicate that human agriculture is affecting plants and animals differently across climate zones, says Peters, with natural communities at hot and dry or cold and wet elevations suffering more from land use than those in more-temperate areas of the mountain. Further supporting this idea, the team found that a statistical model incorporating interactions between climate and land use better predicted the observed patterns of biodiversity on Kilimanjaro than models based on temperature, precipitation, or land use alone.
To Mark Urban, an ecologist at the University of Connecticut who was not involved in the study, this result suggests that, to understand how humans influence biodiversity, “we need to understand not just [climate and land use], but their joint impacts.”
While it’s not yet clear what’s driving these effects, the team has some theories. In the savanna and subalpine zones, clearing grass to grow maize or trees for lumber and grazing can leave native organisms exposed to extreme heat or cold. In contrast, plants living in the more forgiving rainforest climate are able to regenerate more quickly. Microorganism diversity, which showed no significant relationship with land use at any elevation, may depend more on soil chemistry than on either climate or land use, the authors suggest.
Steffan-Dewenter says that sampling so many different locations makes for a robust analysis of the effects of land use and climate on biodiversity. However, he acknowledges that unmeasured elevation?related gradients such as changes in the dominant crop type or natural forest composition could also play a role. For example, coffee plantations may encourage foraging among birds and insects, and they typically aren’t cleared of vegetation after harvest as maize fields are. “We need many more detailed studies to really dig into the mechanisms behind all these patterns,” he says.
Forest Isbell, an ecologist at the University of Minnesota who was not involved in the study, agrees that researchers will need to do more to rule out the influence of confounding factors such as crop type—work that he acknowledges will be challenging. But he commends the current study for its scale. “They’re looking at a wider range of species—plants, animals, and microbes—than we’ve looked at in most areas, and especially in these understudied mountain areas. It’s a really comprehensive look at what’s going on across a fairly big area.”
The study could also aid research beyond Kilimanjaro, as ecologists can use data such as those collected by the Würzburg team to better understand how climate change will affect global biodiversity over the coming years. Using different climates within a small region as a proxy for past and future climates is known as a space-for-time substitution and constitutes “the best [approach] we have to predict what climate change and land use change might mean in the future for biodiversity,”
Steffan-Dewenter says. The implication that hotter, drier climate conditions amplify the negative effects of agriculture doesn’t bode well for the planet’s biodiversity, he adds, especially given that global food demand is expected to increase by more than 60 percent by 2050.
Steffan-Dewenter is hopeful that this study will serve as a benchmark for tracking the biodiversity of the African mountain itself during the next few decades. “In five or ten or twenty years, someone can go back to Mount Kilimanjaro and do the same sampling again and compare what has changed.”