G5 Article dissertation
Large-scale population dynamics of the Eurasian red squirrel
Authors: Turkia Tytti
Publisher: University of Turku
Publishing place: Turku
Publication year: 2018
ISBN: 978-951-29-7390-3
eISBN: 978-951-29-7391-0
Web address : http://urn.fi/URN:ISBN:978-951-29-7391-0
Self-archived copy’s web address: http://urn.fi/URN:ISBN:978-951-29-7391-0
Abstract
The size of an animal population is determined by the birth rate, mortality, and movement of individuals between populations. These demographic parameters, in turn, are affected by internal and external factors. Internal factors include, for example, the density and the sex ratio of the focal population. External factors can be divided to biotic and abiotic factors. In this thesis, I study the population dynamics of the Eurasian red squirrel (Sciurus vulgaris) and factors affecting it on different scales by utilizing snowtrack data, old hunting data and data on nest box occupancy. The red squirrel is widely distributed and common in boreal coniferous forests, and it acts both as a seed disperser and as an alternative prey for several predator species at the low phases of the vole cycle. It has also been an important game mammal, and red squirrel hunting was popular in Finland until the mid-1900s. The results of my thesis show that red squirrel populations fluctuate synchronously within hundreds of kilometers, and that this synchrony is driven by similarly fluctuating and spatially autocorrelated spruce (Picea abies) cone crop. The main predators, the pine marten (Martes martes) and the goshawk (Accipiter gentilis), on the other hand, do not negatively affect red squirrel numbers on the population level. On a smaller scale, however, both the red squirrel and the flying squirrel (Pteromys volans) more likely occupied nest boxes that were far from the nests of avian predators. Thus, the squirrels and their predators are negatively associated on individual level, but the red squirrel and its predators are positively associated on a larger scale. The variation in red squirrel numbers between census sites and years was most strongly affected by spruce cone crop. However, cone crop did not explain the remarkable variations in the red squirrel sex ratio, which were demonstrated by the re-analysis of old hunting data. Overall, food availability emerges as the most significant determinant of red squirrel population density, exceeding the direct and indirect effects of predators. I also found out that the red squirrel is well adapted to human-altered landscape and may even favor mosaic-like areas, even though it is arboreal and adapted to a life in the forest canopy. Both the snow-track data and nest box data suggest that there are more red squirrels near human settlement and in agricultural areas than in continuous forests. A mosaic-like landscape provides red squirrels with alternative food resources, which makes them less dependent on the highly variable conifer cone crop. The apparent preference for agricultural areas by both the red squirrel and the flying squirrel may be partially explained by the higher productivity of forest edges. Even though a red squirrel is still a common sight in backyards and bird-feeding sites, it seems that the species has declined in forests. The analysis of the snow-track data from Finland and north-western Russia revealed that the red squirrel declined in most parts of the 1000000 km2-study area between 1996 and 2012. This alarming trend is partially due to the global warming. I found that the red squirrel population growth rate was lowest in those regions where winters warmed the most. Other climatic parameters, deforestation, and the simultaneous increase in pine marten populations did not explain the decline of the red squirrel. The exact mechanism of how winter warming is detrimental for the red squirrel calls for further studies. The Finnish snow-track data from 1989 to 2017 shows that the red squirrel increased in Southern Finland but declined in other parts of the country. Drawn together, the results of this thesis show that synchrony occurs between red squirrel populations on a scale of hundreds of kilometers, and that the large-scale population dynamics are driven by conifer seed crop while predators have only a minor role. The red squirrel has adapted well to many anthropogenic changes, such as built areas and agricultural fields fragmenting forests, but the global warming seems to pose a threat to it. Studying the changes in red squirrel populations can help understand ecosystems of the boreal forests and the way anthropogenic changes affect them.
The size of an animal population is determined by the birth rate, mortality, and movement of individuals between populations. These demographic parameters, in turn, are affected by internal and external factors. Internal factors include, for example, the density and the sex ratio of the focal population. External factors can be divided to biotic and abiotic factors. In this thesis, I study the population dynamics of the Eurasian red squirrel (Sciurus vulgaris) and factors affecting it on different scales by utilizing snowtrack data, old hunting data and data on nest box occupancy. The red squirrel is widely distributed and common in boreal coniferous forests, and it acts both as a seed disperser and as an alternative prey for several predator species at the low phases of the vole cycle. It has also been an important game mammal, and red squirrel hunting was popular in Finland until the mid-1900s. The results of my thesis show that red squirrel populations fluctuate synchronously within hundreds of kilometers, and that this synchrony is driven by similarly fluctuating and spatially autocorrelated spruce (Picea abies) cone crop. The main predators, the pine marten (Martes martes) and the goshawk (Accipiter gentilis), on the other hand, do not negatively affect red squirrel numbers on the population level. On a smaller scale, however, both the red squirrel and the flying squirrel (Pteromys volans) more likely occupied nest boxes that were far from the nests of avian predators. Thus, the squirrels and their predators are negatively associated on individual level, but the red squirrel and its predators are positively associated on a larger scale. The variation in red squirrel numbers between census sites and years was most strongly affected by spruce cone crop. However, cone crop did not explain the remarkable variations in the red squirrel sex ratio, which were demonstrated by the re-analysis of old hunting data. Overall, food availability emerges as the most significant determinant of red squirrel population density, exceeding the direct and indirect effects of predators. I also found out that the red squirrel is well adapted to human-altered landscape and may even favor mosaic-like areas, even though it is arboreal and adapted to a life in the forest canopy. Both the snow-track data and nest box data suggest that there are more red squirrels near human settlement and in agricultural areas than in continuous forests. A mosaic-like landscape provides red squirrels with alternative food resources, which makes them less dependent on the highly variable conifer cone crop. The apparent preference for agricultural areas by both the red squirrel and the flying squirrel may be partially explained by the higher productivity of forest edges. Even though a red squirrel is still a common sight in backyards and bird-feeding sites, it seems that the species has declined in forests. The analysis of the snow-track data from Finland and north-western Russia revealed that the red squirrel declined in most parts of the 1000000 km2-study area between 1996 and 2012. This alarming trend is partially due to the global warming. I found that the red squirrel population growth rate was lowest in those regions where winters warmed the most. Other climatic parameters, deforestation, and the simultaneous increase in pine marten populations did not explain the decline of the red squirrel. The exact mechanism of how winter warming is detrimental for the red squirrel calls for further studies. The Finnish snow-track data from 1989 to 2017 shows that the red squirrel increased in Southern Finland but declined in other parts of the country. Drawn together, the results of this thesis show that synchrony occurs between red squirrel populations on a scale of hundreds of kilometers, and that the large-scale population dynamics are driven by conifer seed crop while predators have only a minor role. The red squirrel has adapted well to many anthropogenic changes, such as built areas and agricultural fields fragmenting forests, but the global warming seems to pose a threat to it. Studying the changes in red squirrel populations can help understand ecosystems of the boreal forests and the way anthropogenic changes affect them.
UTU Research Portal