Amphibians as Metrics of Critical Biological Thresholds in Forested Headwater Streams of the Pacific Northwest
We pooled data from six studies in a meta analysis to determine critical thresholds for three headwater species by calculating the realized niche for each relative to water temperature and fine sediments. We used data from a seventh study to examine relationships with in-stream large woody debris (LWD). We then used multivariate generalized additive logistic regression models (GAM's) to predict the presence of these three amphibians when these three stressors act in concert.
The idea of a species' niche is an ecological concept that indicates the range of a given environmental gradient where a particular species maximizes its fitness, or at least replaces itself over time. Each species exists along multiple environmental gradients. Thus the famous definition of an organism's niche by G. Evelyn Hutchenson as being an n- dimensional hypervolume. The term "realized niche" indicates that portion of a given environmental gradient where a species exists in the real world, or in the presence of predators, competitors, parasites, etc. In our meta-analysis we were interested in water temperature, fine sediments, and coarse woody debris, because these three attributes are the most often impacted by anthropogenic purturbations like logging and road building.
Here are data we used to calculate the realize niche for water temperature for the coastal giant salamander, tailed frog and southern torrent salamander. The sample size (n) refers to the number of stream belts. The scatter plots show animal counts/sq. meters (density) on the vertical axis by water temperature in degrees C (horizontal). The Realized Niche Center and Realized Niche Width are indicated by arrows. Note the zero data (no captures) in yellow.
Here are the realized niche centers and width for the three amphibians and the co-occurring coho salmon. RNC and RNW for coho salmon from south coastal basins of Oregon from (Huff et at. 2005).
Our amphibian sampling in 49 tributaries of the Mattole watershed was coincident with nine streams of 26 that were know to support over summering coho salmon based on an independent research effort. Eight of the nine streams (88%) also contained one or both torrent salamanders or tailed frogs. This is the first direct evidence supporting the hypothesis that the presence of headwater amphibians can be used as biometrics for the ability of streams to support coho salmon. Shared evolutionary histories and similar life requisites means the use of headwater amphibians as biometrics will also indicate the ability of tributary networks to support downstream biota such as salmonids.
While the presence of late-seral forest can often assure the presence of cold water, and possibly the presence of tailed frogs or torrent salamanders, it is clearly the low water temperatures that act as the ultimate determinant as to whether a stream supports these amphibians. A particularly significant aspect of the seral continuum for cold-water-adapted amphibians requiring cool temperatures and high moisture in riparian zones, are changes in microclimates toward cooler, moister, more stable states from young to late-seral forests. These cool, moist, stable microclimates are particularly important for species whose life cycles include time as terrestrial adults in riparian and upland forests.
Here we determine the realized niche center and width of the coastal giant salamander, tailed frog, and southern torrent salamander relative to fine sediments (silt and sand). Of the nearly 400 sampling locations with fine sediments greater than 20%, only 17 had tailed frog larvae and only seven had southern torrent salamanders.
This image gives a comparison of the relative sensitivity of the three amphibians to fine sediments. The larval tailed frog has the lowest and narrowest tolerance.
Large Woody Debris
These are data from ten tributarys in the Smith River drainage of extreme Northwestern California showing the relationship of two amphibians with large woody debris cover. The coastal giant salamander and tailed frog densities track rather closely with the amount of large woody debris cover.
The best multivariate model (GAM) for predicting coastal giant salamander presence showed clear thresholds for each of three environmental variables. The probability of detection was significantly reduced in water temperatures less then 10.5 and greater then 17.0°C, where embeddedness was greater then 75.5% and where large woody debris cover was greater then 5.0%. Confidence intervals for LWD above 30% were too large to determine a significant relationship with occurrence. Furthermore, at this level of LWD we could not distinguish between an actual lack of animals and our inability to detect them if present (large logs and log jams provide cover where field crews are unable to search).
The best GAM for larval tailed frogs consisted of five variables, with LWD cover, water temperature, sand, silt, and embedded, respectively, having the greatest influence on tailed frog presence. It had an adjusted deviance of 0.212. The probability of detection of larvae significantly decreased in water temperatures greater then 14.2°C, sand greater then 9.0% and embedded greater then 76.8%, Probability of detection decreased above 2.0% LWD, above 15% the data were not sufficient to clarify the relationship.
The best GAM for the southern torrent salamander indicated reduced probability of detections at water temperatures greater then 16.0°C, a tolerance for fines between 3.6 and 33.0%, and reduced detections with LWD cover greater then 2.8%. Above 12.0% LWD cover data were insufficient to determine a pattern. This model had an adjusted deviance of 0.164, with silt and sand, water temperature, and LWD cover, respectively, having the greatest influence on the detection of this species. LWD data above 12.0% were insufficient to determine a pattern.
Huff D.D., Hubler S.L. & Borisenko A.N. (2005) Using field data to estimate the realized thermal niche of aquatic vertebrates. North American Journal of Fisheries Management, 25, 346--360.
Welsh, H. H., and G. R. Hodgson. 2008. Amphibians as metrics of critical biological thresholds in forested headwater streams of the Pacific Northwest, USA . Journal of Freshwater Biology, 53: 1470-1488.