This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
The Devil is coming for Ethiopia’s zebras.
Only about 140 Grevy’s zebras (Equus grevyi) remain in Ethiopia, where they share the Allideghi Wildlife Reserve with thousands of humans and their livestock. It’s not an easy balance; the zebras—the rarest and largest equid species—face continual threats from poaching, habitat fragmentation and competition with livestock for food and water.
Now a new threat has emerged, and it’s one that threatens not only the zebras but the people who live near them. The local Afar people call it devil tree. We know it by a different name: Mesquite.
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Yes, the same wood that provides that delicious smoky flavor on our barbeques here in the U.S.
Mesquite (Prosopis juliflora) first came to Ethiopia from Mexico about 30 years ago in a misguided government-led attempt to reduce soil salinity and, therefore, increase crop production. The woody tree grows quickly and without much water, so it also provided a perfect source of firewood, fencing, building materials and shade from the hot African sun.
But mesquite grew much more quickly than expected, in part because it is so adaptable but also because the North American species that eat it and normally control it are not present in Africa. As it grew it began to choke out native grasses and other plants, block access to water and soak up nutrients from the soil. Its deep roots make it almost impossible to eradicate. Its prodigious seeds—spread through the feces of the very livestock it threatens—allowed it to expand its range throughout the country.
Mesquite first turned up in Allideghi Wildlife Reserve around 1997 and started to take off around the year 2004. Now, according to a paper published July 29 in the journal Rangeland Ecology & Management, it has invaded much of the Allideghi Plain, the grassland portion of the reserve on which the zebras live. Researchers from Utah State University calculated that 16 percent of the plain is now covered by high levels of mature mesquite forest. Another 50 percent has moderate cover.
The invasion has had a cascade of environmental effects. In regions where the mesquite does not yet grow, the ecosystem has seven types of grass and 13 species of flowering plants. That diversity plummets when mesquite appears. In areas of moderate mesquite cover, native species declined to three grasses and nine flowers because the mesquite outcompetes other plants for light, water and nutrients. Plant diversity was even worse in the areas with the worst mesquite levels, where just two grass species and eight flower species remained.
This is all particularly bad news for Grevy’s zebras, which rely on two grass species—Cenchrus ciliaris and Chrysopogon plumulosus—for the majority of their nutrition. Both of those species were completely eliminated in the areas with the most mesquite.
The authors warn that it took less than 10 years for mesquite to take over more than half of the reserve. They wrote that it might not take another decade before Grevy’s zebras are extirpated from their last habitat in Ethiopia. It won’t drive the species into extinction—another 2,000 or so Grevy’s zebras live in neighboring Kenya—but it will rob the species of an important population segment. Grevy’s zebras have lost about 86 percent of their total population since the 1970s, so every individual counts if the species is to successfully recover.
There’s some small amount of hope. The Afar people, whose livelihood depends on livestock, celebrated mesquite 20 years ago because it provided firewood, shade and building material. Now, the Utah State researchers found that they view mesquite in a negative light. For one thing, livestock rely on the same disappearing grasslands as the zebras. The Afar also blame mesquite for pushing them out of their traditional territories into the reserve in the first place.
Unfortunately the people living in the reserve don’t have a good record of controlling mesquite invasions—it’s just too hard to kill—and according to the researchers the Ethiopian government has a hands-off attitude when it comes to Allideghi. They said it’s time for that to change. A multi-tiered effort by government agencies and NGOs, the researchers wrote, could make mesquite eradication an achievable goal.
Hopefully someone will hear the warnings, because otherwise the mesquite invasion will continue and the devil may claim its due.
Previously in Extinction Countdown:
Photo by Kevin Walsh. Used under Creative Commons license