Xenocyon – that great mystery canid of the Pleistocene. There’s continued debate about what genus Xenocyon actually fits into: many consider it to be a canid, while others classify both X. falconeri and the earlier X. lycanoides to be the true ancestor of today’s ferocious yet beautiful African Hunting Dog, Lycaon pictus.
It may be that our understanding of the genus Canis is a little too simplistic, and needs a little revision to include creatures such as Cynotherium, Lycaon and of course Xenocyon, which in appearance was likely somewhere between a hyena and a wolf. Weighing between 30 and 40 kg, with a wide and broad head, Xenocyon was not to be messed with. The fangs alone indicated a robust, hypercarnivorous creature roughly the size of a wolf, perhaps a bit bigger in some cases, and a superb predator. If they were indeed the ancestors of the African Hunting Dog, then their hunting pack techniques would have been savage, relentless – and successful. Sadly, we don’t know if these muscular dogs were beautifully speckled and painted like L. pictus, or if they had coats of one colour, more akin to a wolf or dhole.
It is not surprising these canids reached pretty much the top of the food-chain and were found across Eurasia, Europe and even North America during the Mid-Pleistocene. Fossils have been found as far east as Japan, at the Tama River just outside Tokyo, with specimens also found at Westbury in Britain. However, more plentiful fossils have been found in Untermassefeld in Germany and across Italy and Spain. There are even a few specimens in North America, although some believe they never quite got a grip on the landscape due to competition from the larger and heavier Canis dirus.
So, how did Xenocyon spp manage to be so widespread that they ruled Europe and Eurasia with a paw of iron? Well, most of it is probably due to climate fluctuations on the boundary of the Pliocene and Pleistocene Epochs. We know now that there have been regular periods of glaciation, thaws and reglaciation, long before humans came and made it ten times worse with carbon emissions. The beginnings of the Palaeolithic period are often placed around 1.7 million years ago, when hominids such as Homo erectus were striding out across Africa seeking new vistas. They were not the only ones. Another species of predator was also on the move, in a bio-event known rather catchily as the ‘Wolf Event’. Dogs went walkies from Africa to Eurasia and further.
These periods of intercontinental migration usually occur due to climate changes and the Wolf Event is probably no different (another example of course is the Great American Biotic Interchange, when northern megafauna crossed the land bridge at Panama to access south America). Sometimes Xenocyon is referred to as the African Wolf, as there certainly was movement of the earliest species, X. lycaonoides, from Africa around the same time as other great predators following the movements of herd animals. Nothing remains static in environmental histories. Temperatures in Europe were cooling, starting the story of glaciation and extinction we now call the Ice Age. Herds of herbivorous creatures would follow availability of good grazing, and right behind them would be the hunters, looking for juicy antelopes and equids. In the case of the packs of Xenocyon, they most likely would have welcomed a juicy H. erectus into their a la carte menu, as hominids would have been relatively defenceless against packs of creatures who hunted like African Dogs.
Considering that at least on one occasion, Xenocyon contributed to the creation of a different canid, we can revisit the delicious research carried out by numerous geneticists such as Wayne, Larson and Hulme-Beaman, whose collective findings were recently published in Science journal. The conclusion most geneticists are reaching is that all varieties of modern dogs can trace their ancestry to extinct wolf-like creatures. Random interbreeding, environmental changes and then human involvement in selective breeding for specific qualities means that nature just doesn’t stop dead. What we perhaps have thought of as outright extinction sometimes is really just evolution doin’ its thing- changing and adapting until new responses are needed to new environmental challenges.
[Source: Twilight Beasts]
The origin of domestic dogs is poorly understood [ 1–15 ], with suggested evidence of dog-like features in fossils that predate the Last Glacial Maximum [ 6, 9, 10, 14, 16 ] conflicting with genetic estimates of a more recent divergence between dogs and worldwide wolf populations [ 13, 15, 17–19 ].
Here, we present a draft genome sequence from a 35,000-year-old wolf from the Taimyr Peninsula in northern Siberia. We find that this individual belonged to a population that diverged from the common ancestor of present-day wolves and dogs very close in time to the appearance of the domestic dog lineage. We use the directly dated ancient wolf genome to recalibrate the molecular timescale of wolves and dogs and find that the mutation rate is substantially slower than assumed by most previous studies, suggesting that the ancestors of dogs were separated from present-day wolves before the Last Glacial Maximum. We also find evidence of introgression from the archaic Taimyr wolf lineage into present-day dog breeds from northeast Siberia and Greenland, contributing between 1.4% and 27.3% of their ancestry. This demonstrates that the ancestry of present-day dogs is derived from multiple regional wolf populations.
[Source: Current Biology]
Increasingly, the restoration of large carnivores is proposed as a means through which to restore community structure and ecosystem function via trophic cascades. After a decades-long absence, African wild dogs (Lycaon pictus) recolonized the Laikipia Plateau in central Kenya, which we hypothesized would trigger a trophic cascade via suppression of their primary prey (dik-dik; Madoqua guentheri) and the subsequent relaxation of browsing pressure on trees. We tested the trophic-cascade hypothesis using: (1) a 14-year time series of wild dog abundance; (2) surveys of dik-dik population densities conducted before and after wild dog recovery; and (3) two separate, replicated herbivore-exclusion experiments initiated before and after wild dog recovery. The dik-dik population declined by 33% following wild dog recovery, which is best explained by wild dog predation. Dik-dik browsing suppressed tree abundance, but the strength of suppression did not differ between pre- and post-wild dog recovery. Despite strong, top-down limitation between adjacent trophic levels (carnivore-herbivore and herbivore-plant), a trophic cascade did not occur, possibly because of a time lag in indirect effects, variation in rainfall, and foraging by herbivores other than dik-dik. Our ability to reject the trophic-cascade hypothesis required two important approaches: (1) temporally-replicated herbivore exclusions, separately established before and after wild dog recovery; (2) evaluating multiple drivers of variation in the abundance of dik-dik and trees. While the restoration of large carnivores is undoubtedly a conservation priority, our results suggest that indirect effects are mediated by ecological context, and that trophic cascades are not a foregone conclusion of such recoveries.
Based on field research, my Oregon State University co-author Robert Beschta and I documented the impact of cougars and wolves on the regeneration of forest tree stands and riverside vegetation in Yellowstone and other national parks in western North America. Fewer predators, we found, lead to an increase in browsing animals such as deer and elk. More browsing disrupts vegetation, reduces birds and some mammals and changes other parts of the ecosystem. From the actions of the top predator, widespread impacts cascade down the food chain.
Similar effects were found in studies of Eurasian lynx, dingoes, lions and sea otters. For example in Europe, absence of lynx has been closely tied to the abundance of roe deer, red fox and hare. In Australia, the construction of a 3,400-mile dingo-proof fence has enabled scientists to study ecosystems with and without dingoes which are closely related to gray wolves. They found that dingoes control populations of herbivores and exotic red foxes. The suppression of these species by dingoes reduces predation pressure, benefiting plants and smaller native prey.
In some parts of Africa, the decrease of lions and leopards has coincided with a dramatic increase in olive baboons, which threaten crops and livestock. In the waters off southeast Alaska, a decline in sea otters through killer whale predation has led to a rise in sea urchins and loss of kelp beds.
[Source: The Conversation]
In Australia, the culling of dingoes is a relatively common practice to protect livestock. However, these culls are often strongly opposed on the grounds of the ecological effect they may have on the trophic cascade. According to the mesopredator theory, culling a top predator such as the dingo will result in an increased abundance of mesopredators – feral cats, red foxes and goannas – which in turn increases predation in lower trophic levels.
Allen and colleagues conducted a series of manipulative experiments at nine sites spanning five ecosystem types across the Australian continental rangelands to investigate the responses of mesopredators to contemporary poison-baiting programs. They show that culling dingoes within conventional limits does not result in an increased presence of mesopredators, and therefore contradicts the idea that this effect can result in conservation issues for smaller threatened Australian species.
The researchers suggest that careful planning of dingo culls, such as around the peak cattle calving season, provides livestock producers with a window of opportunity to reduce livestock predation during high-risk times while still maintaining ecological diversity of the trophic cascade.
Allen, who led the study, explained, “Dingo populations recovered to pre-control levels within months, which means that baiting does not create the conditions required for mesopredators to increase. This helps us to understand why, despite years of control measures the numbers of dingoes in Australia is at an all time high.”
On television and in scientific journals, the story of how carnivores influence ecosystems has seized imaginations. From wolves in North America to lions in Africa and dingoes in Australia, top predators are thought to exert tight control over the populations and behaviours of other animals, shaping the entire food web down to the vegetation through a ‘trophic cascade’. This story is popular in part because it supports calls to conserve large carnivores as ‘keystone species’ for whole ecosystems. It also offers the promise of a robust rule within ecology, a field in which researchers have yearned for more predictive power.
But several studies in recent years have raised questions about the top-predator rule in the high-profile cases of the wolf and the dingo. That has led some scientists to suggest that the field’s fascination with top predators stems not from their relative importance, but rather from society’s interest in the big, the dangerous and the vulnerable. “Predators can be important,” says Oswald Schmitz, an ecologist at Yale University in New Haven, Connecticut, “but they aren’t a panacea.”
“The predator was gone for at least 70 years,” says Marshall. “Removing it has changed the ecosystem in fundamental ways.” This work suggests that wolves did meaningfully structure the Yellowstone ecosystem a century ago, but that reintroducing them cannot restore the old arrangement.
Arthur Middleton, a Yale ecologist who works on Yellowstone elk, says that such studies have disproved the simple version of the trophic cascade story. The wolves, elk and vegetation exist in an ecosystem with hundreds of other factors, many of which seem to be important, he says.
Another classic example of a trophic cascade has come under attack in Australia. The standard story there is that the top predator, the dingo (Canis lupus dingo), controls smaller introduced predators such as cats and foxes, allowing native marsupials to thrive. But Ben Allen, an ecologist at the Department of Agriculture, Fisheries and Forestry in Toowoomba, has compared9 areas where dingoes are poisoned with areas where they are left alone, and found no difference in marsupial abundance. He is quite cynical, he says, about “this idea that top predators are wonderful for the environment and will put everything back to the Garden of Eden”
Ripple is not worried about these debates, which he views as quibbling over details that do not undermine the overall strength of the tropic-cascade hypothesis. In fact, when he published a major review10 this year of the effects that predators exert over ecosystems, he left out studies critical of the wolf and dingo trophic-cascade theories; he says that there was no room for them in the space he had to work with. Ripple is particularly concerned with documenting the impacts of Earth’s top carnivores because so many are endangered. “We are losing these carnivores at the same time that we are learning about their ecological effects,” he says. “It is alarming, and this information needs to be brought forth.”
The debate has been harsh at times, but in quieter moments the different factions all tend to talk in similar terms about the great complexity of ecosystems and the likelihood that the truth lies somewhere in the middle. James Estes, an ecologist at the University of California, Santa Cruz, and one of the fathers of the trophic-cascade idea, says that the evidence for cascades mediated by changes in animal behaviour rather than by changes in animal number is “thin”, at the moment — and that many of the effects that have been documented are spotty and badly need to be rigorously mapped out. Still, he adds, “When all is said and done, and everyone is dead 100 years from now, Bill [Ripple] will be closer to right”.
Although Ripple stresses the role of the top carnivores, he agrees they are not the end of the story. “I believe in the combination of top-down and bottom-up, working in unison,” he says. “They are both playing out on any given piece of ground and the challenge will be to discover what determines their interactions and relative effects.”