Richter, Klaus O. Wetland mitigation for amphibians.
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WETLAND RESTORATION, ENHANCEMENT AND CREATION SUGGESTIONS FOR AMPHIBIANS *
Klaus O. Richter, Ph.D.
Water and Land Resources Division
201 S. Jackson St., Suite 600
Seattle, WA 98104-3855
215-5622, 09/08/99
Wetland mitigation should provide conditions for the establishment of self-sustaining amphibian populations. To achieve this goal wetlands and proposed restoration and enhancement activities must first be strategically sited at appropriate locations in the landscape and secondarily designed and constructed to allow amphibians to breed successfully. Site selection criteria must include wetland locations with 1) adequate buffers, and 2) corridors to source populations and upland feeding and hibernation sites. Moreover, they must be within watersheds with minor hydrologic and water quality impacts from development. Essential wetland design conditions for encouraging successful amphibian colonization and breeding include, in order of importance, 1) appropriate hydrology, 2) adequate vegetation structure, 3) good water quality, and 4) the exclusion of predators. Each broad habitat requirement is further subdivided into specific modifiers and performance values of importance to one or more species. I have identified the optimum (not minimum) spawning criteria for these habitat conditions have been identified for the Northwester salamander, Long-toed salamander and Red-legged frog. By providing the recommended spawning habitat conditions and specific values at wetland mitigation sites, habitat for other native wetland breeding amphibians most likely will be provided because other species appear to have broader i.e., less restrictive requirements.
Larval and tadpole habitat requirements are not as restrictive as spawning conditions because larvae are able to swim to favorable locations. However, if wetlands dry up prior to metamorphosis all larvae and tadpoles will die. Moreover, if standing water is reduced to only few locations to which all larvae and tadpoles are constrained, competition, predation and other factors will significantly reduce or wipe out survival.
* This document is abstracted from Richter, K. O. 1997. Criteria for the restoration and creation of wetland habitats of lentic-breeding amphibians of the Pacific Northwest. P. 72-94 In: K.B. Macdonald and F. Weinmann (eds.) Wetland and Riparian Restoration: Taking a Broader View. Contributed Papers and Selected Abstracts, Society for Ecological Restoration, 1995 International Conference, September 14-16, 1995 Seattle, WA, USA. Publication EPA 910-R-97-007 USEPA, Region 10, Seattle, WA.
Many low-elevation King County amphibians only use wetlands for breeding during several weeks or a few months late winter and early spring (mid-January-mid-March). They spend most of the year in upland habitats adjacent to wetlands, or further away in the watershed. Hence, mitigation must include upland habitats in which amphibians can live and not just the wetland in which they breed itself. Similarly, one should only attract amphibians to wetlands if these other upland habitats exists around a wetland.
Upland Feeding and Hibernation Patches
Amphibians prefer cool, wet conditions with the greatest number of species using relatively cool, flat forest stands that are not extremely wet. Older stands are ideal habitats but younger stands with large woody debris, dead and decaying wood and other organic matter on the ground make good feeding and hibernation sites.
Other Wetlands
Ideally other wetlands known to be used by amphibians should be found within 1 km ( 0.62 miles) of the new wetland considered for amphibian breeding habitat. Restoration and enhancement wetlands should also be within 1 km of upland habitats known to be used by amphibians, although determining terrestrial use by amphibians is much more difficult than documenting breeding at a wetland. Regardless, some species (particularly adults) return to their spawned, e.g., “home” wetland to breed and are reluctant to breed at new locations.
Corridors Between Wetlands and between Wetlands and Upland Feeding and Hibernation Patches
Undeveloped flood zones, riparian strips and other shrub and forested lands adjacent to rivers and creeks make ideal migration corridors. Similarly, draws and other vegetated depressions may direct amphibians upslope to feeding and overwintering areas. Corridors should be as wide as possible to protect the core area from wind, temperatures, moisture extremes and thereby stay cool and damp. Moreover, it is important to protect and keep (or bring in) large, coarse woody debris including logs, logs with root wads and stumps in corridors to provide cover (burying places), food (snails, slugs, insects) and shelter (moist, damp areas) for migrating amphibians. Material should ideally be larger than 2-ft wide and 6-ft long (high) to retain moisture during summer droughts.
Wetland Design Criteria
Aspect
In my studies I have seen a clear spawning preference along the northwest and northeast quadrants of wetlands. Hence, restoration and enhancement for amphibians should first be along these shorelines with other shorelines receiving less attention. Optimizing water depth through wetland bathymetry e.g., side-slope, vegetation plantings etc. along the northern shore would be best. Buffer cover providing shade for larvae should be provided along the southern shore.
Hydrology
Current Velocity
Wetlands may be either depressional or flow-thru providing current velocities do not exceed 5 cm/sec for spawning. Optimum spawning velocities should be at 2 cm/sec. or lower.
Larvae also prefer stagnant and slow moving water but are able to tolerate some faster water. Some current velocity is good as it gets rid of water impurities and brings in fresh oxygenated water to habitats.
Water Depths
Water depths to 50 cm should be maximized. Depths exceeding 50 cm will not be used for spawning or by larvae of most species. Optimum depths for long toed-salamander are near shore areas of 5-15 cm with mean at 10, and for Red-legged frog and Northwestern salamander of 25 - 45 cm with mean at 30 cm. Terraced, level habitat at these optimum depths will provide the greatest area for spawning. If slopes are required, gradual slopes of 9-1 or greater will provide the widest strips for shallow and medium depth water spawners.
Larvae also prefer the same depths as those used for spawning. Shallow water is used by small salamanders and tadpoles to escape predation by crayfish, larger salamanders, fish and other animals that may eat them.
Water Permanence
Permanent water is not essential for all wetlands for amphibians. Semi-permanent wetlands may be preferred by many amphibians because they preclude the establishment of fish, Bullfrog and other main predators. Permanent water is required only for the native Northwestern salamander because their larvae take two years to metamorphose and become terrestrial. Red-legged frog, Pacific treefrog, Western toad, Long-toed salamander larvae require water only through August of the spawning year.
Significant decreases in water level and distribution will concentrate larvae to remaining areas and thus result in competition for resources and vulnerability to predation from other amphibians, invertebrates and vertebrates.
Water Fluctuation
Ideally water levels should not fluctuate by more than 10 cm from February through March. Certainly, no more than 10 cm below depths at which they are spawned in early spring because the eggs of many species would then be exposed to desiccation. For most species water levels can become higher, but not shallower. For frogs and toads eggs eventually float on the surface so depths can fluctuate if egg masses are originally spawned in deep water. Frequently, however, eggs spawned in deep water are blown towards the shore where they are vulnerable to desiccation by small water fluctuations.
Water level fluctuations are not limiting to larvae other than through the concentration of their populations to areas that have water during low levels.
Open Water
Open water is essential for spawning. Most spawning takes place in either open water adjacent to vegetation or in open water within the water column above short new spring vegetation. Fifty percent open water and 50 % vegetation cover may be ideal. One hundred percent cover or open water is bad.
Cover is important for larvae to escape predation. Open water will make them vulnerable.
Vegetation Cover
In-Wetland Cover
50 % vegetation is ideal with 25-75 % vegetation cover still OK. A mosaic of vegetation and open water as well as a mosaic of vegetation structural types is optimum.
A mosaic of vegetation types, densities and at several depths is ideal for larvae.
Buffer Cover
Vegetation cover in the buffer is important in maintaining cool water in wetlands during late spring and summer, and is especially important along the edges of small, shallow wetlands which tend to warm up quicker and have dissolved oxygen deficiencies. Buffers should remain as wide as possible so temperatures, moisture and other weather factors remain cool and damp for amphibians. Moreover, it is important to protect, keep or bring in large, coarse woody debris including logs, logs with root wads, stumps that provide cover (burying places), food (snails, slugs, insects) and shelter (moist, damp areas) for amphibians. Material should ideally be larger than 2-ft wide and 6-ft long to retain moisture during summer droughts.
Structure
Flexible, herbaceous, thin-stemmed emergent plants are ideal for oviposition sites of many wetland breeding species. Vegetation stem diameter ranges should include 1-3 mm diameter classes for Long-toed salamander and Pacific chorusfrog, 3-6 mm diameter classes for Northwestern calendar and Red-legged frog. Growth form, including presence in spring and stem diameter, are the important vegetation characteristics and the species that provide these traits are unimportant to most amphibians. Nevertheless, mitigate with only native non-invasive species. Rigid, woody and wide-stemmed species including Spirea, cattail, should be avoided.
Amphibians and Appropriate Spawning Vegetation
Pacific Treefrog, Long-toed Salamander
Eleocharis palustris
Agrostis aequivalvis
Agrostis alba, A. tenuis
Scirpus microcarpus
Juncus acuminatus
Red-legged frog, Northwestern salamander
Juncus bufonius
Carex obnupta
Carex rostrata
Carex athrostachya
Polygonum persicaria
Menyanthes trifoliata
Oenanthe sarmentosa
New habitats can be improved until vegetation is established by adding thin-stemmed material including dead Spirea, conifers and other woody vegetation.
Vegetation provides important cover to larvae. However, don't use water or pond lilies to provide cover.
Water Quality
Water quality parameters are not of immediate concern. For most species under most wetland conditions monitored I have not found water quality to be limiting or responsible for acute mortality although chronic mortality could result from long-term water quality problems. Acute effects can certainly be caused by sediment flushes, toxic spills, etc and chronic effects may occur from continued exposure to low level pollutants or from synergistic effects between pollutants. Created wetlands should be protected from water quality deterioration.
Oxygen depletion in shallow water during summer can be a serious problem for larval survival. Where depths should remain above 20cm with for maximum survivorship. Terrestrial cover adjacent to wetlands is important in providing shade to keep water temperatures low.
Predators
Exotic species including bullfrogs should be eliminated and not allowed to remain in the same wetlands as native amphibians. Stocking wetlands with fish is discouraged. In existing larger topographically and vegetatively diverse wetlands fish, and bullfrogs may coexist in with native amphibians.
Questions?
If you have any questions and suggestions give me a call at 205-5622.
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