Recent expansion of the endangered beetle Plagionotus detritus in the greater Stockholm area

Abstract: Biodiversity is declining worldwide, and the biggest reason for this is habitat destruction. In Sweden one third of all the red listed species belongs in broadleaved deciduous forests. The  most common reason for the endangerment of these species is the destruction of the deciduous forest. Pendulate oak (Quercus robur) hosts the largest number of red listed species in Sweden. The reason is this tree grows to be very old, and with age accumulates many different microhabitats that can harbor a variety of different species. One of these species is the endangered longhorn beetle Plagionotus detritus, which specializes in newly dead, sunlit oak trees. P. detritus has over the last century suffered a great decline in its Swedish distribution, mainly because of the lack of sunlit oaks due to encroachment of spruce, and a lack of succession in oak trees.   The aim of this thesis was to investigate which environmental variables that affect habitat choice in P. detritus in an area in central Sweden. I did this by estimating nine different variables collected from oak trees in and around Stockholm- and Solna municipality and associate these variables with the abundance of exit holes. Exit holes are a good approximation of population abundance. In addition, the age of the exit holes was used to investigate the recent historic spread of the species in the area.   I found that Vitality of the tree, the Openness surrounding each tree, and the amount of Canopy cover had the highest impact on the abundance of P. detritus. These results are consistent with previous research findings. I also found indications of a recent west-wards expansion of P. detritus. My result should be useful information for future conservation of the species in neighboring areas.