Estimating stream occupancy and detection probability parameters of stream-associated amphibians in commercial forests of Oregon and Washington, USA - CANCELLED

Background/Question/Methods

Potential impacts of timber harvests on stream-associated amphibians (SAA; torrent [Rhyacotriton spp.] and giant salamanders [Dicamptodon spp.] and coastal tailed frogs [Ascaphus truei]) in Oregon and Washington, USA, is a management concern because of widespread commercial management of forests across their geographic distributions.  However, accurate understanding of relationships between SAA occurrence and abiotic and biotic variables has been hampered by a lack of analytical methods that did not adjust occupancy estimates for imperfect detection.  As a result, parameter estimates may be biased, leading to spurious conclusions about SAA/habitat relationships and ineffective management actions.  Our objectives were to determine 1) if SAA occupancy probabilities varied with in-stream biotic variables and forest management activities at the stand scale and 2) if SAA detection probabilities varied with site- and survey-specific environmental variables.  We compiled a single GIS layer of all perennially-flowing, non-fish-bearing streams on managed forests in western Oregon and Washington and randomly selected 141 streams for sampling.  Stream segments 100 m in length were sampled up to 3 times each from July-October 2006 to permit estimation of detection and occupancy parameters. 

Results/Conclusions

Giant salamander occupancy was significantly greater on consolidated than unconsolidated geologies (0.92 (SE=0.04) and 0.54 (SE=0.09), respectively, for stands 40 years old) and was positively associated with stand age.  Torrent salamander occupancy varied by species, was positively associated with channel gradient, and was reduced in the youngest and oldest sampled stands.  Tailed frog occupancy was negatively associated with the presence of crayfish, negatively associated with low and high bankfull widths, and positively associated with stand age, although stands less than 5 years old had occupancy rates > 50% in the absence of crayfish.  Mean detection rates were less than 1 for all 3 genera (0.85, 0.66, and 0.63 for Dicamptodon, Rhyacotriton, and Ascaphus, respectively), indicating that conclusions from previous research about relationships between SAA occupancy, environmental variables, and forest practices have an unknown element of uncertainty, particularly if detection rates co-vary with habitat change.  Our results also suggest that streams occurring on forested landscapes that contain stands ranging in age from 0-~60 years old should retain SAA.  However, these results can differ as a result of within-stream biotic and abiotic characteristics, species differences, and geographic variation that may interact with management actions.  Finally, our results address only occurrence of SAA and do not speak to current or historic population levels on landscapes now dominated by intensively-managed forests.