Beavers and beetles: studying how a wetland species influences forest beetles

Window traps are emptied out once a
 month so it is important to
attach them firmly to tree trunks. ©Mia Vehkaoja

My PhD research looks at how beavers affect forest beetle populations. I have several research questions: do beaver-induced flood zones have different beetle species assemblages than other areas, do the increased moisture and sunlight conditions in the flood zone affect species assemblage, and do beaver areas advance or hinder potential forest pests or protected species. I have also looked into the dead wood dynamics that beavers create at the flood zones, concluding that beavers are a primary disturbance agent of boreal wetlands. Their actions produce copious amounts of dead wood in boreal forests that are currently highly managed and have consequently become scarce in dead wood, which is necessary for many species of insects, fungi, birds, and even mammals. The dead wood created by beavers is also highly diverse, which therefore maximizes the number of deadwood-dependent species facilitated by beavers. The flooding caused by the species produce e.g. snags and deciduous dead wood, which are especially scarce in the boreal region.

My research combines a game species with widespread effects on its surroundings, and forest beetles, several species of which have become scarce and require protection. Beaver-induced flooding and the species’ habit of felling tree trunks may locally disturb forest owners, but my study is looking into whether beavers’ actions facilitate or disturb forest pests. Combining game and insect research is cool, and generates new information on which to base decision-making for future protection measures, beaver population management, and even for using beavers as a natural tool for restoring degraded wetlands and forests.

Window traps are good for collecting
forest invertebrates. ©Sari Holopainen

Studying insects is interesting yet challenging. Determining individuals to the species level nearly always requires capturing them first, although some species, such as the birch bark beetle (Scolytus ratzeburgi), can be identified by the unique pattern they leave on tree trunks. During the last three years I have used window traps to gather my insect data. The data have been collected from eight sites located at Evo and Isojärvi National Park. Beavers have previously been present at five of the sites, while three sites are controls that are unsuitable for beaver habitation due to certain environmental factors, e.g. not enough deciduous trees. I have a total of 120 traps spread out at the sites, so every summer I collect about 600 samples.

Window traps are widely used for determining the insect assemblages of sites. They are very simple to use: the trap is attached to a tree trunk or set to hang between two trees. Insects crawl or fly into the plastic plexiglas frame and then fall through the funnel into a liquid-filled container at the bottom. The container is filled halfway with water, dishwashing fluid, and salt. The dishwashing fluid prevents the insects from regaining flight, consequently drowning them. The salt helps preserve the insects until the trap is emptied out, which happens about once a month.

After the trap container has been emptied the gathered sample is sifted through using tweezers and a microscope, to separate the insect groups that I am interest in. Next the individuals are determined to the necessary level. Sometimes determining the family level is enough, but if making conservation decisions or gaining new information on certain species is the goal, it is usually necessary to determine individual insects to the species level. How this is done depends on the order in question, e.g. beetles are often recognized by their ankles and genitals. Species, genera, and families are determined using identification keys. The summer of 2016 was the last summer that I collected data for my PhD, so now I can focus on identifying the beetle samples. Once this is complete I can begin statistically analyzing the data. 

University of Helsinki PhD student Stella Thompson is a 2015 LBAYS grant recipient