Identifying Suitable Woodpecker Nest Trees Using Decay Selection Profiles in Trembling Aspen

Woodpeckers are the primary tree-cavity producers in North America and, through their process of cavity excavation, they create nesting and roosting habitat for a complex web of cavity-using species. Managing for potential woodpecker nest trees requires an understanding of factors that influence decisions by which woodpecker select trees for nest cavity excavation, including tree decay availability associated with softened heartwood and sapwood in trees. We evaluated woodpecker nest cavity excavation in relation to the nature and availability of decay in trembling aspen (Populustremuloides) using 12 years of nest and tree decay class data from British Columbia. We used a cavity-nest web to visually depict decay selection profiles for six woodpecker species and compared these woodpecker decay selection profiles to the availability of decay in aspen (96.7% of all cavity nests found), identifying the most suitable nest tree stages used by woodpeckers. The suitable woodpecker nest tree stage consists primarily of live unhealthy trees and recently dead trees (93.9% of active nests found in aspen). Compared to non-nest trees (trees within which no cavity nests were found during the study period), live trees that were used for nest cavity excavation had higher annual probabilities of entering the suitable woodpecker nest tree stage than non-nest trees, and they also had a lower probability of progressing outside of this stage. This suggests excavation decisions involve a balance of competing tradeoffs between ease of excavation and tree security. Nest trees remained within the suitable woodpecker nest tree stage for an average of 11.8 years, and almost half of this time involved live trees in an unhealthy state. We suggest that forest management guidelines that focus on retaining only dead trees to provide cavities for wildlife are missing a significant component of available and future tree cavity resources

Data from: Lotic cyprinid communities can be structured as nest webs and predicted by the stress-gradient hypothesis

1. Little is known about how positive biotic interactions structure animal communities. Nest association is a common reproductive facilitation in which associate species spawn in nests constructed by host species. Nest-associative behaviour is nearly obligate for some species, but facultative for others; this can complicate interaction network topology. 2. Nest web diagrams can be used to depict interactions in nesting-structured communities and generate predictions about those interactions, but have thus far only been applied to cavity-nesting vertebrate communities. Likewise, the stress-gradient hypothesis (SGH) predicts that prevalent biotic interactions shift from competition to facilitation as abiotic and biotic stress increase; this model has been hardly applied to animal communities. Here, both of these models were applied to nest-associative fish communities and extended in novel ways to broaden their applicability. 3. A nest web was constructed using spawning observations over 3 years in several streams in south-western Virginia, USA. Structural equation modelling (SEM) was then implemented through an information-theoretic framework to identify the most plausible nest web topology in stream fish communities at 45 sites in the New River basin of the central Appalachian Mountains, USA. To test the SGH, the per-nest reproductive success of ‘strong’ (nearly obligate) nest associates was used to represent interaction importance. Eigenvectors were extracted from a principal coordinate analysis (PCoA) of proportional species abundances to represent community structure. Both of these metrics were regressed on physical stress, a combination of catchment-scale agricultural land use and stream size (representing spatiotemporal habitat variability). 4. Seventy-one per cent of SEM model evidence supported a parsimonious interaction topology in which strong associates rely on a single host (Nocomis), but not other species. PCoA identified a gradient of community structure dominated by Nocomis and associates, to communities dominated by other reproductive groups. Both metrics of interaction importance responded positively to physical stress. 5. This study demonstrates that nest webs can be useful in a variety of systems and that SEM can be a quantitative extension of this framework. Likewise, the SGH can be used to understand positive interactions in animal communities and can be extended to predict proportional representation of facilitating and beneficiary species in communities

Nest web observations

Cyprinid nesting observations, including which species constructed the nest and which species used those nests for reproduction. These data were used for generating the nest webs

Habitat, nest counts and sampling effort

Habitat variables as described in the manuscript. Electrofishing effort (effort) is reported in seconds. The variables "agri.rank" and "wsa.rank" represent ranked values of each raw measurement of %agriculture and watershed area (km2), respectively. These two variables were added to calculate the variable "stress.bin", which served as the independent variable in regressions testing predictions of the stress-gradient hypothesis

Grouped demographic data (for SEM and SGH regressions)

Stream fish data combined into reproductive groups (see Methods section of manuscript) and classified as adult or juvenile based on modal breaks in length frequency histograms (see Methods section of manuscript). These data were used in structural equation modeling, and the juvenile portions of the dataset (divided by nest count) were used as response variables in regressions testing the stress-gradient hypothesis predictions

All spp. counts (for PCOA)

Raw count data of fish species of all ages captured at 45 sites in the New River basin of NC, VA and WV (USA) in summer 2012. These data were used for the principal coordinates analysis