Whitebark pine stand condition, tree abundance, and cone production as predictors of visitation by Clark's nutcracker

Abstract

Background: Accurately quantifying key interactions between species is important for developing effective recovery strategies for threatened and endangered species. Whitebark pine (Pinus albicaulis), a candidate species for listing under the Endangered Species Act, depends on Clark's nutcracker (Nucifraga columbiana) for seed dispersal. As whitebark pine succumbs to exotic disease and mountain pine beetles (Dendroctonus ponderosae), cone production declines, and nutcrackers visit stands less frequently, reducing the probability of seed dispersal.

Methodology/principal findings: We quantified whitebark pine forest structure, health metrics, and the frequency of nutcracker occurrence in national parks within the Northern and Central Rocky Mountains in 2008 and 2009. Forest health characteristics varied between the two regions, with the northern region in overall poorer health. Using these data, we show that a previously published model consistently under-predicts the proportion of survey hours resulting in nutcracker observations at all cone density levels. We present a new statistical model of the relationship between whitebark pine cone production and the probability of Clark's nutcracker occurrence based on combining data from this study and the previous study.

Conclusions/significance: Our model clarified earlier findings and suggested a lower cone production threshold value for predicting likely visitation by nutcrackers: Although nutcrackers do visit whitebark pine stands with few cones, the probability of visitation increases with increased cone production. We use information theoretics to show that beta regression is a more appropriate statistical framework for modeling the relationship between cone density and proportion of survey time resulting in nutcracker observations. We illustrate how resource managers may apply this model in the process of prioritizing areas for whitebark pine restoration.

Figure 1. Geographic locations of study sites (solid circles) in four national parks in the southern and northern study regions (open circles), Rocky Mountains.

(Map reproduced with permission from Cartographics LLC, www.rockymountainmaps.com).

Figure 2. Mean values of stand assessment variables, cone number, and squirrel and nutcracker counts for each region.

Data are summed from two stand assessment plots per transect and averaged across transects. Error bars indicate extent of 95% confidence intervals around the means. Abbreviations: WPB whitebark pine, MP mountain pine beetle.

Figure 3. Observed values for the proportion of observation hours resulting in nutcracker visitation in 2008 and 2009 versus that predicted from cone production values using the model presented by McKinney et al. .

The diagonal is the 1∶1 line that represents perfect prediction from the model. All but one observation occurs below the 1∶1 line, indicating that the McKinney et al. model consistently under-predicted the probability of nutcracker occurrence for the cone production values observed in this study.

Figure 4. Upper panel: Average number of Clark's nutcrackers observed per transect in the two study area regions.

Error bars indicate extent of 95% confidence intervals. Lower panel: proportion of observation hours resulting in an observation of Clark's nutcracker as related to average density of whitebark pine cones on survey plots sampled in this study (squares) combined with those sampled in (circles).