Wolves used to be widely distributed throughout Europe, Asia and North America, but persecution and conflict with expanding human populations led to their extermination in many regions. They now only occupy approximately one-third of their original range. However, in recent decades wolves have been reintroduced to wilderness areas in many parts of the US, and their numbers are increasing. This development has not been greeted with enthusiasm by everyone, as these mammals can pose a threat to humans, livestock and pets.
Biologists know that predators help to keep prey population numbers in check by preying on older and more vulnerable individuals. Reducing the numbers of moose, deer or elk, for example, leads to less pressure on vegetation and is therefore beneficial for the ecosystem in general. In addition, carcasses left behind after wolf kills provide food for scavenging animals and birds.
It is also widely accepted that predators keep prey populations healthy by selectively removing individuals that are old, weak, sick or injured. This assumption, however, is seldom tested scientifically. In a new study, published today in the journal Frontiers in Ecology and Evolution, researchers from Michigan Technological University report that predation by wolves may be beneficial to prey populations in another, more unexpected way.
“Wolf biologists have in the past assumed that wolves play an important role in regulating the health of prey populations by selectively removing old or diseased animals,” said Dr Sarah Hoy, of Michigan Technological University. “However, a rigorous assessment of that idea has not been tested until now.”
The researchers assessed the extent to which wolves in the Isle Royale National Park, near Michigan’s border with Canada, select moose (Alces alces) on the basis of their age and the presence of osteoarthritis in their joints. Osteoarthritis is a chronic, non-communicable disease that affects the ability of an individual to move with ease. Its presence can be detected by analysis of skeletal remains.
“Osteoarthritis is a progressively crippling disease caused by deterioration of cartilage on the surfaces of moveable joints (for example, knees and hip joints),” explained Hoy. “As individuals get older, they are more likely to develop osteoarthritis and develop more severe forms of the disease.”
The researchers found, as expected, that wolves preyed preferentially on elderly moose and avoided prime-aged adults. “When it comes to wolves and moose, it makes a lot of sense that wolves would preferentially target moose that are in poorer condition because adult moose weigh between 800 and 900lbs which is between eight and 10 times as heavy as a wolf,” said Hoy.
However, among the prime-aged individuals that were killed by wolves, many showed evidence of severe osteoarthritis. The presence of severe osteoarthritis symptoms, but not mild or moderate symptoms, increased the vulnerability of prime-aged moose to predation.
“But the situation is different for older moose. While older moose are more vulnerable to predation, that vulnerability does not strongly depend on whether an old moose has osteoarthritis,” explained Hoy.
When the researchers examined how temporal variation in kill rates was associated with the subsequent incidence of osteoarthritis in the moose population, they found that, over a 33-year period, osteoarthritis in the moose population decreased following years with higher kill rates. This indicates that predation by wolves was resulting in an overall healthier moose population, in terms of osteoarthritis in particular.
“The decline in osteoarthritis following years with more predation is – we think – because wolves preferentially removed moose with osteoarthritis from the population,” said Hoy.
Furthermore, because osteoarthritis is influenced by genetic factors, these results highlight how wolf predation may be acting as a selective force against genes associated with developing severe osteoarthritis as a prime-aged adult. This finding supports the idea that predation can regulate the health of prey populations, and has implications for the management and conservation of wolf populations.
“The management and conservation of wolves is controversial among the public. Yet our results suggest wolves might be an effective, natural, and more ethical way of regulating the health of deer and moose populations – as opposed to using culls or recreational hunting to reduce the incidence of diseases or parasites of concern,” explained Hoy.
“The results are also relevant for policy-related arguments about reasons to refrain from intensively hunting wolf populations,” continued Hoy.
“When deciding whether to hunt wolves it is important to not only consider issues that may be caused by wolves (e.g., occasional predation of livestock) but to also consider the important ecological benefits that wolves may provide by removing old and diseased animals from the populations.”