Many large herbivores may have important roles in dryland ecosystems. Equids such as donkeys and horses, as well as elephants, have been reported to dig wells of a maximum depth of two meters, enhancing water availability for a variety of animals and plants. Noting that this subject has received limited research attention, the authors carried out a study for three summers at the Sonoran Desert of North America to survey changes in groundwater-fed streams and “equid well” water, and the associated effects on the ecosystem.
Effect on animals
They found that the equid wells “provided up to 74% of surface water by accessing the water table” at one of the four groundwater-fed streams they studied [Lundgren 2014: 1]. The wells were especially important at the intermittent stream (unsteady stream that occurs at irregular intervals), providing 100% of available surface water when all other water was lost.
The wells reduced the distance between neighboring water features significantly, thus reducing the distance that animals needed to travel to reach water. The water resources created by the equids also prevented some species from resorting to eating extra plant foods simply to extract its water content, as they are observed to do in the absence of available surface water. Using camera traps, the researchers observed 59 vertebrate species (limited to organisms weighing over 100g and excluding equids) at equid wells, 57 of which they recorded drinking. “Daily species richness was 64 and 51% higher on average at equid wells and background waters [other surface water, such as the streams], respectively, than at dry controls” [Lundgren 2014: 1].
Effects on vegetation
The presence of equid wells enhanced the growth of pioneer trees. The survey showed that the seeding density was higher in equid wells, which function as germination nurseries, than in the riverbank zone. Riverbanks were usually covered by herbs, which reduced the density of trees. Equid wells, on the other hand, provide a non-competitive environment for the small-seeded pioneer trees.
The feral donkeys that dug the equid wells are not native to this dryland ecosystem study site, and yet they proved to mitigate the effects of water reduction and high temperature on biodiversity and ecosystem function. Thus, the ecological roles once played by large native mammals that have since become extinct, can in some cases be filled by non-native substitutes (which are typically viewed as threats to conservation).