Feral Horses' Effects on Pine Trees and Grasses

Feral Horses' Effects on Pine Trees and Grasses

The results of a recent study by Argentine environmental researchers suggest that, at least in their South American study areas, pine trees thrive and multiply where feral horses roam. However, where more pine trees grow, fewer grasses survive.

Photo: Ana de Villalobos, PhD

The results of a recent study by Argentine environmental researchers suggest that, at least in their South American study areas, pine trees thrive and multiply where feral horses roam. However, where more pine trees grow, fewer grasses survive.

"Selective grazing by feral herbivores makes grasslands less abundant and less competitive against more grazing-tolerant species like pine trees," said Ana de Villalobos, PhD, researcher for the National Scientific and Technical Research Council (CONICET) at the Universidad Nacional de Sur, Argentina. Her study, which took place in the Ernesto Tornquist Provincial Park near Buenos Aires, revealed that field areas protected from wild horses had more grass, a larger variety of grass species, and fewer pine trees at the end of the experimental period as opposed to those occupied by feral equids.

While horse manure is often thought to contribute to soil fertility for grasslands, the horses' consistent weight on the surface actually undoes any of these benefits, de Villalobos added. Trampled ground has compacted soil with reduced qualities, and fragile grass seedlings have difficulty pushing through to the surface.

"This could eventually lead to desertification with drastic consequences for horses and other animals," she said.

Meanwhile, pine trees survive well in such conditions and can grow abundantly. The trees' shady coverage might seem nice at first, but horses can't survive on pine forests alone, according to de Villalobos. And although her study was on feral horses, she said the same effect could occur on any large population of unmanaged herbivores.

It's important to note that the presence of horses-wild or domestic--doesn't necessarily result in the destruction of all green pastures, she added. It just emphasizes the need for pasture management.

"To avoid negative consequences, grazing management such as herd rotation and overpopulation prevention is necessary," she said.

De Villalobos' next step is to find out what kind of management methods are necessary for optimum co-existence of feral horses, grasses, and pine trees.

"The good news for (wild) horse fans is that we are currently in the process of obtaining the best ecological tools for adequate and sustainable ... management, so that we can create the best habitat for them," she said.

The study, "Pinus halepensis invasion in mountain pampean grassland: Effects of feral horses grazing on seedling establishment," was published in the October 2011 issue of Environmental Research. The abstract is available online.

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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