Immigration is key to flying squirrel population growth rate

Immigration is main demographic rate for population growth in flying squirrels

This project was based on amazing data collected by Ralf Wistbacka in western Finland. Ralf has followed two (!) populations of nest-box breeding flying squirrels for decades. He has surveyed the populations several times each year, and marked all animals (parents and offspring) in the boxes with eartags. The total individual-based data amassed over the years is impressive. Based on this material, we wanted to estimate population growth rates in these two population as well as getting a better understanding of what drives the fluctuations in growth rate. Siberian flying squirrels are a big deal in Finland from a conservation perspective. Partly this is because Finland is one of few EU countries where the species occurs and it therefore is of special interest from a conservation point of view. In addition, flying squrrel life-history, diet and habitat preferences put it at risk in the heavily managed lansccape of today.

We used a so-called "Integrated Population Model" IPM to estimate how between-year variation in fecundity, juvenile and adult apparent survival (true survival + emigration) and immigration affected annula population growth rates. The cool aspect of the IPM is that it allows estimating immigration without the need to identify immigrants (how? read the paper and references in it!). We find that immigration is the real key demographic rate in this species in both study populations. Intriguingly, immigration numbers were estimated to be approximately the same each year. This meant that especially when population sizes were low, this "fixed" number of immigrants were a really important contribution to population growth. Of course, surviving and producing babies is also important, but - from a demographic perspective - these rates did not explain variation in annual population growth rates.

A second key finding was that none of the demographic rates and not even the population growth rates correlated between the two population, although these were only about 100km from each other. This made us argue that flying squirrels live in a network of local populations which fluctuate in numbers more or less independently from each other but where the movement of individuals between populations is key for the "survival" of any population.

Based on another analysis, we observed that movements of adult individuals did not contribute to lifetime migration patterns. In other words, juveniles were responsible for immigration and redistributing individuals within and between populations. This emphasizes the importance of knowledge on natal dispersal, if we want to understand consequences of movement ecology of the species at the population level.


-Jon Brommer & Vesa Selonen

Immigration and population growth rate of flying squirrels (open access)

Lifetime migration patterns in flying squirrel (open access)