Beaver, peckers and their interactions: an investigation of interactions between beaver and hairy, downy and pileated woodpeckers.

Abstract

Varied interactions occur between organisms that have overlapping habitats or resource needs within complex ecosystems. In the northern hardwood forest of North America there may be interactions between beaver (Castor canadensis), and three species of woodpeckers (Pileated, downy, and hairy; Dryocopos pileatus, Picoides pubescens, and P. villosus respectively). All of these species utilize trees as feeding sites. We investigated whether beaver or woodpeckers foraged randomly in terms of both tree species and diameter in order to infer whether interactions were occurring between these species. Specifically, we wanted to know if beaver could be affecting the amount of feeding sites available to woodpeckers (both pileated and non-pileated). We also wanted to examine if the different species of woodpeckers affected each other. To explore the predictions of our hypotheses, data on tree species, the diameter of the tree at beaver height, and the amount of foraging executed by each of the species on a tree were collected for each site. This information was compared to what each plot would have been like had the beaver not been there (i.e., hypothetical pristine plot). The hypothetical pristine plot helped determine if beaver activity could potentially affect woodpecker feeding sites. It also allowed us to determine if the beaver had an affect on forest composition. From this data we created model scenarios of the fate of dead standing trees so that we could directly infer whether beaver were increasing or decreasing the number of woodpecker feeding sites in a plot. Chi square tests were used to determine if either organism showed foraging preference for tree species. T-tests were used to establish whether beaver and woodpeckers had size preferences using diameter at beaver height for trees within each site. By modeling scenarios of the fate of dead standing trees, we were able to gain insight about what affects beaver may have on woodpecker foraging. We concluded that at three plots beaver forage non-randomly. They forage for different trees species and different sized diameters at different sites. Woodpeckers also foraged non-randomly at three of four sites. We saw that their feeding sites greatly overlap and therefore Pileated and non-pileated woodpeckers are probably affecting each other. From our modeled scenarios, we deciphered that the number of trees beaver girdle and leave standing will affect the number of woodpecker feeding sites available in the future. Thus our results imply that many interactions are occuring within ecosystems containing beaver, Pileated and non-pileated woodpeckers.