Decline of flying and red squirrels in boreal forests revealed by long‐term diet analyses of avian predators

Abstract

Conservation of species requires knowledge on population changes in time, but achieving such data in proper spatio-temporal scales can often be difficult for rare and vulnerable species. We used long-term diet data of three avian predators (the nocturnal Ural and eagle owls and the diurnal goshawk) from four landscapes to study population changes of two forest-dwelling species, the Siberian flying squirrel Pteromys volans and the red squirrel Sciurus vulgaris in Finland. We also determined the role of vulnerable flying squirrel in the diet of these predators. Both squirrel species were relatively rare in the diets of the three avian predators, with the exception of red squirrel in the diet of goshawks. The Siberian flying squirrel also appears to be a less important prey for avian predators than flying squirrel species (Glaucomys spp.) are in North American forest communities. Numbers of consumed flying squirrels, but not of red squirrels, increased as the abundances of voles increased in the diets of eagle and Ural owls, that is more flying squirrels were consumed during the good vole years than the poor ones. The declining trends in the diets of three avian predators appeared to be quite similar for both studied squirrel species, possibly indicating that the process behind the decline has been the same.

Responses of small mammals to clear-cutting in temperate and boreal forests of Europe: a meta-analysis and review

“…However, the amount of forest cover in the landscape is an important factor related to squirrels' presence (Mortelliti et al 2011) and red squirrels avoid clear-cuts (Hansson 1994). Indirect evidence (changes in the diet of avian predators) suggests that red squirrels are declining in numbers in Fennoscandia where gray squirrels do not occur (Selonen et al 2010). This decline might be caused by forest harvest and resulting habitat loss (Selonen et al 2010).…”

“…Indirect evidence (changes in the diet of avian predators) suggests that red squirrels are declining in numbers in Fennoscandia where gray squirrels do not occur (Selonen et al 2010). This decline might be caused by forest harvest and resulting habitat loss (Selonen et al 2010). Red squirrels strongly rely on tree cones for food; therefore, young stands created by large-scale clear-cutting cannot support populations of squirrels until trees mature and produce seeds (Lurz et al 2003).…”

Competitive exclusion within the predator community influences the distribution of a threatened prey species

“…Since goshawks and Ural Owls both prey on flying squirrels (Selonen et al 2010), a question that inevitably arises here is why do flying squirrels in the Suupohja study area aggregate spatially with one of their natural enemies? Although it was not possible to directly observe, we see no other reasonable explanation than that this pattern must be the result of active movements, i.e., that flying squirrels are heterospecifically attracted by goshawks (Goodale et al 2010).…”

“…Although the Ural Owl and the flying squirrel are nocturnal and the goshawk is diurnal, all three species show the same general preference for mature mixed-spruce forest stands (Mikkola 1983, Hanski 1998, Tornberg et al 2006. Both predators occasionally prey on flying squirrels (Selonen et al 2010), but the Ural Owl is clearly more specialized on small mammals (Korpima¨ki and Sulkava 1987), including the flying squirrel (Hanski et al 2000), than the goshawk, whose diet is dominated by avian prey (Tornberg et al 2006). Although Ural Owls may breed in old goshawk nests (Mikkola 1983), they normally avoid breeding closer than one kilometer from nests that are occupied by goshawks (Solonen 1993).…”

“…By (Hanski et al 2000, Selonen et al 2010, the nationally threatened Siberian flying squirrel Pteromys volans L. (Rassi et al 2010;hereafter, flying squirrel). Because landscape composition in addition to other factors usually limit species distributions (Brown 1988, Andre´n 1994, Watling et al 2011, we further investigated (3) to what extent landscape composition explains prey distribution while simultaneously accounting for potentially confounding effects (vegetation zone shift, small-scale spatial correlations).…”