Red squirrels decline in abundance in the boreal forests of Finland and NW Russia

Abstract

Recent global warming and other anthropogenic changes have caused well‐documented range shifts and population declines in many species over a large spatial extent. Most large‐scale studies focus on birds, large mammals, and threatened species, whereas large‐scale population trends of small to medium‐sized mammals and species that are currently of least concern remain poorly studied. Large‐scale studies are needed because on a smaller scale, important patterns may be masked by local variation and stochastic processes. Here, we utilized snow track census data from Finland and NW Russia to estimate population growth rates of the Eurasian red squirrel Sciurus vulgaris for a period of 17 yr in an area of over 1 000 000 km2. We also studied the effects of changes in summer and winter temperatures, winter precipitation, predator abundance, and canopy cover on estimated red squirrel population growth rates. Our results suggest that red squirrel populations have declined in most parts of the study area, the only remarkable exception being SW Russia. These results are in concordance with previous studies suggesting that species that are still common and of least concern may be declining. However, our findings are in contrast to the common pattern of northern populations of boreal species increasing due to global warming. The estimated population growth rates are in synchrony over vast areas, suggesting that the underlying reasons also operate on a large scale. We indeed find that the population growth rate was lower in regions where winters warmed faster during the study period, suggesting that changes in the environment (or biotic changes associated with it) are linked with the decline of red squirrels.

Large-scale spatial synchrony in red squirrel populations driven by a bottom-up effect

“…This study thus suggests that the synchrony is driven by fluctuating resources, and demonstrates how an explicit spatiotemporal approach can improve model performance for fluctuating populations. We also found that the red squirrel has declined in most parts of Finland for 29 years (see also Turkia et al 2018b). This is an important conservation message that highlights the urgent need for understanding the underlying causes behind population dynamics of the species.…”

“…The data suggest a slightly decreasing trend for red squirrels throughout the study area, except for Southern Finland. A recent large-scale study from partially the same snow-track data (Turkia et al 2018b) as well as a diet analysis of red squirrel's predators (Selonen et al 2010) also suggested that the red squirrel has been declining. Turkia et al (2018b) showed that warming of winters is one reason behind the decline, but also that there are other, unknown reasons that were not found.…”

“…A recent large-scale study from partially the same snow-track data (Turkia et al 2018b) as well as a diet analysis of red squirrel's predators (Selonen et al 2010) also suggested that the red squirrel has been declining. Turkia et al (2018b) showed that warming of winters is one reason behind the decline, but also that there are other, unknown reasons that were not found. In Finland, one possible reason that can have partially decreased the density of the red squirrel population in addition to the climate change is too heavy forest management, which has decreased the mean age of forests.…”

“…It may be that the increasing trend in Southern Finland, as opposed to the declining trend in the other parts of the country, is also linked to the location of the field triangles in this part of Finland. The red squirrel seems to be declining in forests (Turkia et al 2018b), but the dynamics in more mosaiclike areas may be different. The movement between close forest patches and from forests to yards in a mixed landscape is also consistent with the view that red squirrels can tolerate moderate habitat fragmentation (reviewed in Selonen and Hanski 2015).…”

Predation risk landscape modifies flying and red squirrel nest site occupancy independently of habitat amount

“…The diurnal red squirrel is a far more common species than the flying squirrel, and it is wide-spread in temperate and boreal forest ecosystems throughout Eurasia, bur recent studies indicate that it also has been declining [ 27 , 40 ]. In boreal coniferous forests, the main food of the red squirrel is seeds of Norway spruce [ 41 , 42 ].…”