Feral donkeys in Death Valley. Photo by NPS (W. Sloan), Public Domain
(Click on the following link to listen to an audio version of this blog … Three Invasive Species
Death Valley has a lingering legacy from the 19th Century: wandering herds of feral donkeys (also referred to as “burros”). It is estimated that there are 4000 of these invasive donkeys living within the boundaries of Death Valley National Park, and despite the Park Service’s attempts to eliminate them, they are persisting, thriving and reproducing!
The burros (whose scientific name is either Equus africanus asinus or simply E. asinus) are a species from North Africa that was domesticated as a beast of burden some five to seven thousand years ago. They were brought to the Americas by Columbus (and also by subsequent conquistadors and settlers) starting in the late 15th Century and widely used throughout new continent as pack animals and in agriculture and mining. Many feral populations have been established over their 500+ years of residence in the Americas.
The Death Valley feral donkeys are thought to be escaped descendants of the mining burros brought into the valley in the 1800’s. They are tough, resilient generalists who can survive in the harsh conditions of Death Valley deserts. The Park Service’s desire to eliminate these animals is based on observations that they trample and degrade potential grazing vegetation, break down stream beds and spring banks, and force native, grazing species (like big horn sheep (Ovis canadensis) out of prime grazing areas. Recent research, though, indicates that the ecological impacts of these feral donkeys may be more complicated than first realized.
For example, the donkeys dig “wells” in the dry stream beds throughout Death Valley in order to reach ground water that might be up to five feet below the surface. It has long been assumed that these excavations damage the streams’ hydrological properties and also accelerate streambed erosion. Closer observations of these “donkey wells,” though, show that many of them serve as foci for growth for native plants and animals. These small, donkey wells can be thought of as “donkey oases” and may significantly increase the overall quality and productivity of the Death Valley ecosystems.
Mountain lion. NPS. Public Domain
When the donkeys inhabit a well area for a short period of time, there is little trampling of vegetation or damage to the banks and beds of the stream. Apparently, a major factor that stimulates the donkeys to move on from a well site is the presence of mountain lions (Puma concolor). Wildlife cameras across Death Valley have recorded mountain lions hunting and killing feral donkeys, and active lion dens have been found near streams that have donkey wells. Mountain lions are not an endangered species, but their numbers are declining all across the Americas. The feral donkeys represent a much needed prey species for these important predators!
The streams located near the lion dens are in remarkably pristine condition indicating the very transient presence of the donkeys. Streams located near campsites or other parts of the park heavily utilized by people, though, are often very degraded by persistant donkey grazing and trampling. Mountain lions avoid the park areas with a strong human presence. These areas, then, represent a lion safe-zone for the donkeys and allow the donkeys to do significant damage to the Death Valley ecosystem.
It is not clear if the rate of predation on the donkeys by the mountain lions will lead to a stable and sustainable donkey population. It is clear, though, that the ecology of these invasive burros is more complex than anyone assumed!
Feral pigs. Photo by NPS. Public Domain
In Australia, another invasive species may have become the preferred dinner for a recently endangered, native predator. Pigs (Sus domesticus) were brought into Australia by European settlers in the 19th Century. Many of these pigs escaped their styes and pens and established robust, feral populations all across the Australian continent. It is estimated that there are currently 24 million feral pigs in Australia, and that they are distributed across 40% of the continental land mass!
Saltwater crocodile. Photo by fvanrenterghem. Wikimedia Commons
Saltwater crocodiles (Crocodylus porosus) are found on ocean shores and tidal rivers in northern Australia, Southeast Asia, the east coast of India, Indonesia and the islands of Micronesia. According to the fossil record, these crocodiles have lived in Australia for millions of years. The saltwater estuaries of the Northern Territories of Australia are a rich habitat for these reptiles. Saltwater crocodiles are the largest, living reptile on Earth capable of reaching lengths of 21 feet and weights just under 3000 pounds!
It was estimated in the late 1940’s that there were 100,000 of these giant crocodiles living in the Northern Territories. By the early 1970’s, though, there were only 3000 of these crocodiles left, and they were teetering on the verge of local extinction.
In 1971, the Northern Territory banned the hunting of these crocodiles, and their population numbers began to increase. Availability of suitable prey species for these large predators, though, was a critical factor in their growth and survival. In places where there were significant populations of feral pigs, the saltwater crocodile populations grew most rapidly.
Examination of the isotopes in the bones of an animal can reveal a great deal about what that animal was eating throughout its life. Isotopic analysis of saltwater crocodile bones collected 50 years ago and bones of saltwater crocodiles collected today reveal profound differences in the nature of their diets. Saltwater crocodiles used to eat almost entirely aquatic prey while today they eat almost entirely terrestrial prey (especially feral pigs!). As one researcher put it: why would a crocodile chase really fast swimming aquatic food around when it can sit near the shore of the estuary and just wait for a pig to come down to the water for a drink?
American alligator. Photo by Rivaddvia.vila, Wikimedia Commons
All across the Gulf Coast of the southern United States, the American alligator (Alligator mississippiensis) had a near-extinction experience similar to that of the saltwater crocodile of Australia. American alligators were listed as an endangered species in 1973, but conservation efforts and hunting bans led to a spectacular recovery of their populations. In fact, the species rebounded so extensively that it was removed from the endangered list in 1987. One of the factors that stimulated this recovery was the local availability of an alien, invasive prey species all across the Gulf Coast states: the nutria (Myocastor coypus).
Nutria. Photo by Rufus46, Wikimedia Commons
Fur farmers in Louisianna had begun raising nutria (a large, semiaquatic rodent from Argentina) in the 1930’s. Some of these nutria escaped from captivity and established rapidly growing and dispersing populations all across the American south. The nutria were quite destructive and caused extensive damage especially to coastal vegetational communities. They also out-competed (or killed) many native, animal species. Nutrias, though, were the perfect food for the resurging populations of the American alligator! In places where these two species were abundant, nutrias quickly became the preferred food for the alligators and the alligator populations grew rapidly
Florida panther. Photo by USFWS, Public Domain
And, finally, just a short note to end this “invasive-species-as-dinner” discussion: The Florida panther (Puma concolor couguar) is an isolated and endangered subspecies of the mountain lion. It is estimated that these are only 150 of the Florida panthers still living in the wild. Most of these remaining wild panthers are found in the swamps of southern Florida where there is also a substantial population of feral pigs. The panthers actively hunt and consume these very abundant feral pigs! One wildlife biologist with the Florida Fish and Wildlife Conservation Commission put it very well: “Hogs may have saved Florida panthers from extinction!”
So, invasive species like donkeys, pigs and nutrias cause a great deal of ecological disruption, but they also do some very good things in their already damaged ecosystems. A number of ecologists are suggesting that we take a longer look at invasive animals and plants in order to appreciate their long-term impacts on our climate-changed and extinction-ravaged ecological communities.