Three-toed Woodpecker cavities in trees: A keystone structural feature in forests shows decadal persistence but only short-term benefit for secondary cavity-breeders

Primary cavity-producers like woodpeckers are often considered as keystone species, because they produce nest-sites also for several other cavity-nesting animals and, thus, maintain ecological webs of cavity-breeders. However, the detailed temporal dynamics of cavities and their lifetime occupancy rates and survival are not usually known which makes it difficult to assess the actual significance and full impact of primary cavity-breeders. In this study, we monitored cavities in a large forest landscape, covering the full lifetime of cavities. We focused on a mature and old-growth forest specialist cavity-breeder, the Three-toed Woodpecker Picoides tridactylus. The data include the annual occupancy history of 655 old cavities of the Three-toed Woodpecker in 86 territories in a 170-km2 area in southern Finland during 1987-2017. The study area included both managed and natural forest types. The median survival time of a cavity was 10 years, but there were significant differences between forest area types with a range of 7-13 years. The occupancy in all cavities was 21.3%, and the cavities were available for secondary cavity-breeders each year. There was a significant negative correlation between the occupancy and the age of the cavity. The first five years of a cavity were important for the total occupancy, and 86% of occupancies took place before the median age of the cavities. In cavities older than 15 years the occupancy was only 7%. The pattern was similar in all types of forests. Our results show that cavities made by Three-toed Woodpeckers have rather long lifespan but also that their active use by other cavity-breeding species is restricted mostly to few years only. The result indicates that new, fresh cavities are needed continuously in a forest landscape, in order to maintain the role that Three-toed Woodpecker has as a keystone species.