A recently published paper in Frontiers of Ecology and Evolution discussed how predators can influence the occurrence of certain diseases in prey populations. The researchers first investigated whether wolves prefer certain ages classes in their prey, as is common for many predators. Since juveniles are inexperienced, while older individuals are usually slower, ill and their senses are impacted through old age, these age groups make easier prey compared to an individual in full strength. Additionally, their weapons (teeth, horns, antlers etc) are either not yet fully grown, or already deteriorating – another reason why prime-age individuals are most dangerous to hunt for a predator.
How can wolves be responsible for arthritis levels in their prey?
For this study, the researchers also looked at whether the presence and severity of osteoarthritis influence which individuals wolves prefer to hunt. Osteoarthritis in ungulates causes the same symptoms to the well-known arthritis in humans – deformations and pain in the joints and associated reductions in mobility of the affected area. While it can be extremely painful and limiting for a person, for a moose running away from a pack of wolves, limited joint mobility can mean death.
For the wolves and moose in North America, it was found that wolves prefer old moose over those in their prime stage of life (confirming the above-mentioned assumptions), but also, that this preference reduces when prime adults suffer from osteoarthritis. In other words: younger moose are less likely to be hunted than older moose, but only if they do not suffer from arthritis – then the trend lessens. Since the likelihood of developing arthritis is dependent on genetics, a selective removal of diseased moose by wolves may influence the overall occurrence of the disease in the prey population. There was some evidence for this, as arthritis cases were reduced in years after which wolves had killed many individuals, possibly because of the selective removal of genes.
So in a roundabout way, wolves may be responsible for how healthy or diseased a population of prey animals is.
Similar effects might be true also for other health conditions, especially those linked to osteoarthritis; however, there are often complex interactions at play and it can be difficult to determine trends for certain.
What this means for predator/prey management
If predators are able to control not only prey numbers, but also the health situation within a prey population, the need for manual invasive management may be much reduced. Currently, prey species in many places are being hunted or culled to benefit the ecosystem (e.g. to avoid overgrazing), but also to control the outbreak of certain diseases within those prey populations. If wolves can take over this role (and possibly, do it better than we can), there will be no need for us to interfere. Due to this, the study also highlights the potential political implications, since the hunt on wolves (and other large predators) is a highly debated topic wherever large predators occur. Showcasing how the presence of wolves can benefit prey populations too, might be a powerful argument against culling predators.
Do other predators influence prey health too?
The answer is: yes! As mentioned before, many predators select prey that is easiest for them to kill, because of age-related or disease-induced reasons. And in doing so, they often influence the prevalence of certain diseases in prey populations. Mountain lions in America for example, prefer prion-infected mule deer over those not infected (prions are a protein-cause neurodegenerative diseases). The same is true for prion diseases in deer – disease presence was suppressed in times or areas when deer were hunted by wolves. And in northern Spain, wolf-predation reduces tuberculosis levels in wild boar.
Many more such examples can be found easily, all of them pointing towards the same results: if left alone, prey populations actually benefit from being hunted by their natural predators. Not only because they keep the populations stable, but also because they can prevent the over-occurrence of diseases.
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