This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
It’s almost barbeque season, which means that for many of us it’s just about time to break out the mesquite.
But mesquite isn’t all that popular when it isn’t providing a nice, smoky flavor to our burgers and chicken. Many people actually hate it. In fact, mesquite trees are some of the worst invasive species on the planet.
Mesquite is originally from the Americas, but humans have spread the trees (several species in the Prosopis genus) worldwide over the past half century, often in order to provide firewood or protection against erosion. But wherever mesquite goes, local species suffer. Mesquite’s prodigious seeds, deep roots, thick branches and ability to grow in dry environments make it a formidable opponent and a dangerous exploiter of natural resources.
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Mesquite’s stranglehold is so bad that some people in Africa refer to it as the “devil’s tree.”
Now the devil has come to India.
Mesquite has actually been in India for decades but its environmental impact is just now becoming clear. According to a paper published last week in National Academy Science Letters, mesquite plants are rapidly invading the only habitat for the endangered Indian wild ass (Equus hemionus khur), threatening to push the horselike animals out in the process.
Ironically, mesquite would not have such a choke hold on the nearly 5,000-square-kilometer Wild Ass Wildlife Sanctuary if it weren’t for the very animals for which it is named. Wild asses, it turns out, find mesquite to be quite edible, if not as tasty or nutritious as native grasses. The asses eat seed pods and then pass the seeds through their digestive systems. This process–combined with damp, nutrient-rich dung–creates the perfect conditions for mesquite seeds to germinate. Without the wild asses’ help the seeds cannot grow in the sanctuary’s saline desert, which has little water and inhospitable soil.
During the wetter 1990s mesquite’s territory expanded as many as 25 square kilometers each year. Today, with dryer weather, it doesn’t spread as quickly. The paper estimates that mesquite is currently invading 1.95 square kilometers of the sanctuary every year.
That might not seem like a lot, but remember that this is a desert. There’s already little room for the vegetation on which both the asses and numerous other species depend. The authors of the new paper wrote that over the past 10 years mesquite has taken over large areas of grassland that formerly fed the wild asses. In the process mesquite’s thick branches have completely blocked access to watering holes birds and mammals use as well as sunlight that ground plants need to grow.
Over the course of the mesquite invasion Indian wild ass populations have actually increased. A census released this past February found that the number of wild asses rose from 4,038 in 2009 to 4,451 this year. That’s a fantastic success story for a species that numbered just 362 animals back in 1960.
But that success has come with a cost. The sanctuary can only hold and feed so many wild asses. As their numbers grow–and as mesquite limits the amount of accessible water and food available to them–the asses expand their territory. The new census had to cover an additional 15,000 square kilometers beyond what it covered five years ago. This spread puts the animals into direct conflict with humans, especially when the animals start eating crops. It also leaves them susceptible to other dangers; earlier this month two asses were found dead, probably from pesticide poisoning.
Government officials will have to recapture some of the sanctuary land to reduce this conflict and protect the asses in the future. Reclaiming territory taken by the trees won’t be easy–mesquite is hard to kill because its roots thrive so well deep underground–but it can be done. In 2006 filmmaker-turned-naturalist Pradip Krishen reclaimed a 70-kilometer stretch of the Thar Desert in Rajasthan State from a similar mesquite invasion, slowly but surely chopping mesquite’s long roots out of the local rocks using chisels.
Whether or not a similar project would work at Wild Ass Sanctuary remains to be seen, and none are currently in the works. Meanwhile both the asses and invading trees will likely continue to expand their territories–to no one’s advantage but the mesquite’s.
Photo by Navin Sigamany. Used under Creative Commons license