Horses and donkeys engineer water for desert ecosystems

Well-digging horses and donkeys could provide a valuable service for plants and animals in arid ecosystems, according to a study published in the journal Science.

The wells, dug to two metres or more to access the water table, substantially increased water availability in US deserts of Arizona and helped to facilitate diversity of several native species.

This surprise discovery sheds new light on invasive species, according to lead author Erick Lundgren from the University of Technology in Sydney, Australia.

“Feral donkeys and horses have long been studied from one perspective: that of invasion biology,” he says. “Studying them as wildlife instead of as mythical ‘invaders’ shows unexpected resilience and wildness in modern ecosystems.”

When Lundgren first noticed the behaviour, he says he was most surprised to find nothing on well digging in the literature.

So, he and a team of researchers from Denmark and the US investigated how wild donkeys (Equus africanus asinus) and horses (E. ferus caballus) influenced water availability in four Sonoran Desert streams every few weeks over three summers.

They surveyed well-dug water in the streams, comparing it to existing water and dry areas, and used camera traps to monitor other animals and their activity around the wells.

Results revealed that equid wells brought more than two thirds of groundwater to the land surface during dry periods, increasing water density by more than 300% on average and up to 1450% overall.

The extra water decreased the isolation of water features, which can cause several problems for wildlife including competition, predation and disease transmission. At times they were the only water source where intermittent streams had completely dried up.

They also found 59 vertebrate animal species at the wells, and species richness was 64% higher than at dry sites. This compares to 51% greater animal diversity at already existing water holes than dry areas.

Animals found drinking from the equid wells included a mule deer (Odocoilus hemionus), javelina (Pecari tajacu), bobcat (Lynx rufus) and Woodhouse’s scrub-jay (Aphelocoma woodhouseii).

Abandoned wells supported the growth of riparian seedlings (mostly Populus fremontii and Salix gooddingii), pioneer trees with local and global conservation priority. It appears the wells provide a unique opportunity for the trees to grow without competing vegetation, the authors say, functioning as “flood-mimicking nurseries”.

They note that well-digging animals in other dry regions contribute to water supply, including African (Loxodonta Africana) and Asian (Elephas maximus) elephants, Gemsbok (Oryx gazella), zebras (E. zebra, E. grevyi) and khulans (E. hemionus). Feral horses have also been observed providing this service in other regions of North America and Australia.

As the planet faces widespread megafauna extinctions, Lundgrem and colleagues suggest introduced animals could provide important ecological services that have been lost.

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