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