Stream Amphibians as Bioindicators - A Case Study from California's Redwoods
Amphibians comprise a major component of the biomass in stream systems of the Pacific Northwest, and can exceed fish in numbers and total biomass in small streams. Amphibians also are quite sensitive to environmental degradation, especially sedimentation. As such, amphibians make excellent indicators of the integrity of aquatic ecosystems. We tested the utility of amphibians as bioindicators of stream health (as indicated by relative amounts of fine sediments) in a pristine, late-seral ecosystem.
Road construction of the Redwood National Park highway bypass resulted in a large accidental infusion of fine sediments into pristine streams in Prairie Creek State Park, California, during an October 1989 storm event. This incident provided a natural experiment where we could measure, compare, and evaluate native stream amphibian densities as indicators of stream ecosystem stress. We employed a habitat-based sampling design to assess the impacts of these sediments on the densities of aquatic amphibians in five impacted streams by comparing them with densities in five adjacent, unimpacted (control) streams (map). Three species were sampled in numbers sufficient to be informative: tailed frogs (Ascaphus truei, larvae), coastal (Pacific) giant salamanders (Dicamptodon tenebrosus, paedomorphs and larvae), and southern torrent salamanders (Rhyacotriton variegatus, adults and larvae). The only other species detected in protocol was one sub-adult northern red-legged frog (Rana aurora).
Fine sediment levels in the impacted streams were significantly higher, particularly in habitats with slow-flowing water (e. g., pools) where fine sediments tend to store. We found mean sediment depths in the impacted pools ranged from 0.1 to 25 cm (mean = 1.52 cm) compared with 0.0 to 4 cm (mean = 0.31 cm) in the unimpacted pools. Percent of pool tail embedded ranged from 10 to 100% (mean = 62.6 %) on the impacted streams and from 0 to 85% (mean = 44.2 %) on the unimpacted streams. Comparisons were statistically significant for both pool bowl sediment depth (t = 3.28, P = 0.0300), and pool tail embeddedness (t = 3.64, P = 0.0067).
Amphibians were sampled by using a random systematic design based on stream length and ratios of primary mesohabitat types along each stream (Welsh et al. 1997). This allowed for stream mesohabitat types (pool, riffle) to be sampled in proportion to occurrence. Within each selected mesohabitat unit, we systematically placed one or more amphibian sampling units (belts) based on habitat length, placing one belt for every 10 meters of habitat. We sampled a total of 267 belts in 179 mesohabitat units, with 93 habitat units (137 belts) in the impacted streams and 86 habitat units (130 belts) in the unimpacted streams. We captured a total of 540 amphibians; larval and paedomorphic individuals of the coastal giant salamander were the most common (n = 296), followed by larval tailed frogs (n = 205), and larval and adult southern torrent salamanders (n = 39).
Densities of the three species varied by impact and mesohabitat type. The coastal giant and southern torrent salamanders showed significant differences for impact in all cases the densities in unimpacted streams were greater (F=3.95, P=0.0820 and F=4.93, P=0.0572 respectively), however the tailed frog did not vary significantly by impact (F=2.06, P=0.1888). The tailed frog and southern torrent salamander showed significant differences among mesohabitat types (F=11.38, P=0.0001 and F=2.67, P=0.0519 respectively). There was also a significant interaction between impact and mesohabitat type for the tailed frog (F=3.73 P=0.0145).
During their aquatic phase, amphibians use the interstitial spaces under courser substrates for cover, and forage on invertebrates which feed on plankton and diatoms attached to the substrate. Tailed frog larvae actually feed directly on the diatoms attached to rocks. Stream sedimentation covers or displaces their food sources and fills the interstitial spaces where they seek cover.
The use of streambed interstices by amphibians is a characteristic shared with early life stages of both resident and anadromous fishes, as well as many stream invertebrates. These other taxa, however, are either short-lived, explosive breeders, or subject to seasonal movements, all of which can complicate their use as bioindicators. Many species of stream-dwelling amphibians are highly philopatric, long-lived, and occur in relatively stable populations in undisturbed ecosystems. These attributes can make their relative numbers a useful and reliable indicator of environmental perturbations, both from known causes (Corn and Bury 1989, Blaustein et al. 1994b) and also possibly from causes that have yet to be identified (e.g., Corn and Fogleman 1984, Weygoldt 1989, Drost and Fellers 1996, Laurance 1996, Laurance et al. 1996, Pounds et al. 1997, Woolbright 1997, Lips 1998).
Welsh, H. H., Jr., L. M. Ollivier, and D. R. Hankin. 1997. A habitat-based design for sampling and monitoring stream amphibians with an illustration from Redwood National Park. Northwestern Naturalist 78:1-16.
Corn, P. S., and R. B. Bury. 1989. Logging in Western Oregon: responses of headwater habitats and stream amphibians. Forest Ecology and Management 29:39-57.
Blaustein, A. R., P. D. Hoffman, D. G. Hokit, J. M. Kiesecker, S. C. Walls, and J. B. Hayes. 1994b. UV repair and resistance to solar UV-B in amphibian eggs: a link to population declines? Proceedings of the National Academy of Sciences (USA) 91:1791-1795.
Corn, P. S., and J. C. Fogleman. 1984. Extinction of montane populations of the northern leopard frog (Rana pipiens) in Colorado. Journal of Herpetology 18:147-152.
Weygoldt, P. 1989. Changes in the composition of mountain stream frog communities in the Atlantic Mountains of Brazil: frogs as indicators of environmental deterioration? Studies on Neotropical Fauna and Environment 243:249-255.
Drost, C. A., and G. M. Fellers. 1996. Collapse of a regional frog fauna in the Yosemite area of the California Sierra Nevada, USA. Conservation Biology 10:414425.
Laurance, W. F: 1996. Catastrophic declines of Australian rainforest frogs: is unusual weather responsible? Bio logical Conservation 77:203-212.
Laurance, W. F., K. R. McDonald, and R. Speare. 1996. Epidemic disease and the catastrophic decline of Australian rainforest frogs. Conservation Biology 10:406 413.
Pounds, J. A., M. P. L. Fogden, J. M. Savage, and G. C. Gorman. 1997. Tests of null models for amphibian declines on a tropical mountain. Conservation Biology 11: 1307-1322.
Woolbright, L. L. 1997. Local extinctions of anuran amphibians in the Luquillo Experimental Forest of northeastern Puerto Rico. Journal of Herpetology 31:572- 576.
Lips, K. R. 1998. Decline of a tropical montane amphibian fauna. Conservation Biology 12:106 117.
Welsh, H. H., Jr.; Ollivier, L. M. 1998. Stream amphibians as indicators of ecosystem stress: a case study from California's redwoods. Ecological Applications 8(4):1118-1132.