Golden eagles deter foxes, facilitate forest grouse

Wetland ecology group_University of Helsinki_golden eagle

Top predators can have surprising effects on ecosystems. Golden eagle @Sari Holopainen

The effects of top and mesopredators on lower levels of food webs have been researched from many perspectives, but less focus has been given to the roles that avian top predators play on mid-sized mammalian predators. The cascade effects of raptors, which concurrently affect several trofic levels, have also gained little attention. However, researchers at the University of Turku have observed how the golden eagle (Aquila chysaetos) affects pine marten (Martes martes) and red fox (Vulpes vulpes) populations, along with the cascade affects induced on black grouse (Tetrao tetrix) and hazel grouse (Tetrastes bonasia) populations.

Golden eagles hunt black grouse, red foxes, and pine martens. When the opportunity arises they will also catch hazel grouse, but because of their smaller size and habitat preferences (thick forests), hazel grouse are better protected from golden eagles, which prefer open territory when hunting. The researchers initially hypothesized that the golden eagle would locally lessen the numbers of red foxes and pine martens, thereby causing a positive affect on the two grouse species.

However, the truth is not quite as simple. Pine marten and red fox densities actually increase in areas with large numbers of golden eagle. One possible reason behind this surprising result could be the large prey populations available for all three predators in these areas, along with the partially overlapping habitat preferences of pine marten and golden eagle. On the other hand, pine martens avoid open territory, possibly because of the non-lethal deterrent effect that golden eagles exert on pine marten.

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Top predators can have surprising effects on ecosystems. Golden eagle @Sari Holopainen

But the story doesn’t end here: high densities of golden eagles still does have an effect, as larger numbers of young hazer grouse and black grouse are present at these sites. The golden eagle may therefore facilitate the grouse by lessening the numbers of mesopredators in their territories through the deterrent effect. This would lead to less predation and egg eating by the pine marten and red fox. In other words, red fox and pine marten avoid golden eagles so effectively, that the two grouse species benefit from their weakened predation performance. A similar protective effect has also been observed with the goshawk (Accipiter gentilis).

Increasing golden eagle territory and offspring densities cause decreasing numbers of black grouse, but this does not occur with hazel grouse. The small size of the hazel grouse most likely protects it from golden eagle predation. The black grouse, on the other hand, favors open territory. Golden eagles therefore appear to have a protective effect on juvenile hazel and black grouse individuals, while threatening adult black grouse.

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Black grouse lekking @Stella Thompson

The cascade effects directed at these grouse species do not appear to change with fluctuating pine marten and red fox densities. The presence of other mesopredators, e.g. raccoon dogs (Nyctereutes procyonoides) and the American mink (Neovison vison), has been suggested as the reason for this. The effects of these other mesopredators were not assessed during the study.

The golden eagle affects mesopredator behavior without affecting their population densities. A similar deterrent effect has previously been observed from white-tailed sea eagles (Haliaeetus albicilla) on the American mink, and golden eagles most probably also deter minks and raccoon dogs. The eagles additionally deter the movements of other potential egg thieves such as corvids.

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Hunters and conservationists – with shared cause

Do hunters conserve nature? This seemingly controversial issue seems to be a source of never-ending debate. I recently found a text discussing this issue, published in the “Finnish Nature” (Suomen Luonto) magazine as early as 1944. While conservationists and hunters may sometimes be in direct conflict, some shared targets were recognized already in 1944.

Extensive red fox hunting makes it possible for the rare arctic fox to breed successfully in Sweden and Norway. Red fox in an arctic fox habitat in Varanger Peninsula, northern Norway. © Sari Holopainen

Extensive red fox hunting makes it possible for the rare arctic fox to breed successfully in Sweden and Norway. Red fox in an arctic fox habitat in Varanger Peninsula, northern Norway. © Sari Holopainen

O. Hytönen (1944) raised the very same observations that are still apparent. Although hunters kill animals, prey animal populations should not be eradicated by responsible hunting practices. Some hunting actions are straightly connected to nature conservation, such as feeding animals during harsh winters, habitat management and predator control. Currently discussed effect of trophy hunting as an important conservation tool in development countries is an example of an indirect connection: by paying for hunting permits hunters help to maintain local animal diversity. As noted in a recently published paper, banning of trophy hunting can lead to exacerbating biodiversity loss.

In 1953 “Finnish Nature” (Suomen Luonto) published another text on the subject, this time written by G. Bergman. Bergman wrote that the relationships between hunters and conservationists has not always been smooth in Finland, or in other Nordic countries, while no benefits could be gained from these conflicts. Bergman noted that modern game management has several shared principles with nature conservation. He also pointed out that nature conservation and hunting have successfully been managed together in the US. As during Bergman’s times, Europe is still somehow on separate paths, and the situation has become particularly inflamed in some countries. Ironically, Bergman wrote that if we refuse to understand the interests of others, nature conservation aims may be disturbed.

The good old American way

What were the good manners already mentioned by Bergman in America? Maybe he meant the Federal Duck Stamp system established already in 1934. All US hunters must buy a Duck Stamp on a yearly basis, however, whoever can get one. With this small cost the buyers contribute to bird habitat conservation. The US Fish and Wildlife Service advertises that the stamp is “among the most successful conservation tools ever created to protect habitat for birds and other wildlife”. About 1 500 000 stamps are bought yearly, and 98% of the profits are given to the National Wildlife Refuge System for wetland conservation.

Coldwater River National Wildlife Refuge in Mississippi provides great circumstances for wintering ducks. The park is protected, but bird watching is aloud in some parts of the park. Duck hunting is possible outside the park in private lands. © Sari Holopainen

Coldwater River National Wildlife Refuge in Mississippi provides great circumstances for wintering ducks. The park is protected, but bird watching is aloud in some parts of the park. Duck hunting is possible outside the park in private lands. © Sari Holopainen

Another traditional American actor smoothly combining conservation and hunting is Ducks Unlimited (DU), founded by a group of hunters in 1937. DU targets habitat conservation, and is now claimed to be the world’s largest and most effective private waterfowl and wetlands conservation organization according to their website. Most DU members are still hunters.

The land of a thousand lakes

Wetlands have been destroyed for a long time due to differing human interests, also in Finland. Some areas have been lost altogether, while some have lost their value due to e.g. vegetation overgrowth. We still have many lakes left, but shallow eutrophic lakes – the waterbird lakes – are the ones that have been lost most often. Hunters are a group with an interest to construct and restore wetlands. According to a report by the Finnish Wildlife Agency, hunters have constructed or restored about 2 000 wetlands during the past decades. In addition to benefitting game animals, the entire ecosystem benefits. Wetlands also offer several other ecosystem services, including water purification and erosion control.

Many wetlands have been established in Finland to support game animals. Saarikko-wetland in southern Finland is a small, yet very productive duck habitat constructed by the REAH-project (active management of game animal habitats). © Sari Holopainen

Many wetlands have been established in Finland to support game animals. Saarikko-wetland in southern Finland is a small, yet very productive duck habitat constructed by the REAH-project (active management of game animal habitats). © Sari Holopainen

Methods matter

Sometimes hunting itself supports animal populations. For example, hunters can help to maintain animal communities through ecosystem engineering pushed by hunting and the co-evolution of animals and humans. In 2013 a scientific paper showed that in Australia Aboriginal hunting was one of the cornerstones supporting monitor lizard populations. Monitor lizards occur most densely in areas where they are hunted, because of the hunting method used; the burning method creates a patchy mosaic of regrowth in the landscape. In areas with no hunters, occasional lightning strikes burn land in a more homogenous way, and thus also lizards are scarcer. The same practice might also benefit several other desert species. However, in many cases Aboriginals have lost their traditional hunting possibilities, and the loss of these traditional practices sustaining habitats might have contributed to decreasing populations of several desert animal species.

While the debate between hunting and nature conservation has already lasted a long time, and is still on-going, common targets have been raised throughout the process by cooperative actors of both sides. There has always been, and currently still are, differing hunting methods for concerning conservational effects, but it is self-evident that all these practices are not against conservation targets.

Read more:

Enrico Di Minin, Nigel Leader-Williams & Corey J.A. Bradshaw: Banning Trophy Hunting Will Exacerbate Biodiversity Loss. 2015. Trends in Ecology & Evolution. Online.

Connecting Aboriginal Land Use Management Strategies, Mammal Extinction Rates and Shifts in Fire Regimes in a Changing Climate: An Interdisciplinary Approach to Inform Conservation Strategies for Threatened Species in the Australian Western Desert

An interdisciplinary approach to understanding the role of anthropogenic fire in the desert grasslands of Australia