Mattole Tributaries -
A Landscape Analysis of the Herpetofauna in a 790 sq km Watershed of Northern California: Detecting Biotic Patterns and Inferring Landscape Processes
Ecosystems are rapidly being altered and destabilized on a global scale, threatening native biota and compromising vital services provided to human society. We need to better understand the processes that can undermine ecosystem integrity (resistance-resilience) in order to devise strategies to ameliorate this trend. We used a herpetofaunal assemblage to first assess spatial patterns of biodiversity and then to discover the underlying landscape processes likely responsible for these patterns.
Reptiles and amphibians are a phylogenetically diverse set of species with documented sensitivity to environmental perturbations. We examined ecogeographic patterns of these taxa in aquatic and riparian environments across the landscape mosaic of the Mattole River watershed of northern California, USA. We analyzed species distributions relative to three primary vegetation types (grassland, second-growth forest, late-seral forest) and two hydrologic regimes (perennial vs. intermittent). We sought evidence for the processes behind these patterns by modeling animal distributions relative to multi-scale compositional, structural, and physical attributes of the vegetation or hydrologic type.
At the macro-environmental scale, the discriminant analysis of 35 environmental variables indicated pronounced differences among the three vegetation assemblages that dominate the Mattole. The mixed grassland sites were distinguishable from the second-growth sites by the presence of more large hardwood logs in the former, and a more closed canopy in the latter. The late-seral sites were distinguishable from the other two types by higher numbers of both small hardwoods and large conifers, higher elevations, fewer stumps, a lower percentage of conifer seedlings, and fewer small conifers.
In addition, we tested for differences in the riparian environment by using automated dataloggers to record hourly water and air temperatures and relative humidity throughout the summer. They were deployed in a transect perpendicular to the stream at 10 meter intervals, at four representative streams for each of three vegetation types. Although the three vegetation types that dominate this landscape each had unique structural attributes, the overlap in plant species composition indicates that they represent a seral continuum. None-the-less, we found distinct microclimates in each type.
We found higher diversity (Shannon Index) of herpetofauna along perennial compared to intermittent stream reaches. Reptile richness, and evenness were significantly greater in the mixed grassland. In contrast, amphibian species richness was higher in the late-seral versus the second-growth forest type, but evenness did not differ among vegetation types (ANOVA Table).
Using ANOVA of individual species, we found two of the three most frequently detected amphibians, the Coastal Giant Salamander (Dicamptodon tenebrosus) and the Black Salamander (Aneides flavipunctatus), were significantly more abundant in the late-seral versus the mixed grassland. These two salamanders were also significantly more abundant along intermittent stream reaches. The other commonly detected amphibian, the Foothill Yellow-legged Frog (Rana boylii), was more abundant along the open, sunny reaches of mixed grassland streams compared with second-growth and late-seral types.
We also identified differences in occurrence by vegetation type for five species detected infrequently in our study area. The Southern Torrent Salamander (Rhyacotriton variegatus) was more often found in the late-seral forest than in the mixed grassland, while the Rough-skinned Newt (Taricha granulosa) was more often found in the second-growth and mixed grassland than in late-seral forest. The Tailed Frog (Ascaphus truei) was found only in late-seral forest. The two most common reptiles, and the only two with sufficient detections to test for differences, the Western Fence Lizard (Sceloporus occidentalis) and the Pacific Coast Aquatic Garter Snake (Thamnophis atratus), were both found more often in mixed grassland compared to late-seral forest, with the garter snake also found more often in second-growth compared to late-seral forest.
In the Mattole we found that the composition of the riparian and aquatic herpetofauna varied greatly across the seral continuum. Four amphibians that were significantly associated with the remaining late-seral environments and three were associated with more open forests/grasslands. While these faunal differences may appear on the surface to be directly related to differences in vegetation structure, the actual underlying process is somewhat more complex, having to do with the modification of climate by forest structure differentially during succession, as temperatures (air and water) and moisture (relative humidity) vary greatly depending upon seral stage (Chen et al. 1999). We also found that at a larger spatial scale, both the stage and the extent of vegetation greatly influenced the aquatic and riparian microclimates within entire sub-basins, particularly water and air temperature. Suitable thermal environments are an essential element determining fitness in populations of ectotherms (Huey and Kingsolver 1989).
Chen, J. et al. 1999. Microclimate in forest ecosystems and landscape ecology. Bioscience 49: 288-297.
Huey, R. B. and Kingsolver, J. G. 1989. Evolution of thermal sensitivity of ectotherm performance. Trends Ecol. Evol. 4: 131-135.
Publications to date:
Welsh, H. H. and G. R. Hodgson. 1997. A hierarchical strategy for sampling herpetofaunal assemblages along small streams in the western U.S. , with an example from northern California. Tran. West. Sect. Wildl. Society 33:56-66.
Welsh, H. H., G. R. Hodgson, and N. E. Karraker. 2005. Influences of the vegetation mosaic on riparian and stream microclimates in a mixed forest-grassland landscape in "Mediterranean" northwestern California. Ecography 28: 537-551.
Welsh, H. H., G. R. Hodgson, and A. J. Lind. 2005. Ecogeography of the herpetofauna of a northern California watershed: linking species patterns to landscape processes. Ecography 28: 521-536.