Using experiments and long term data for optimal wetland mitigation
Environmental Considerations in Planning
Wetlands are among the world's most ecologically productive and economically important environments, but they are also one of the world's most endangered environments. Approximately 53% of wetlands in the lower 48 United States were lost between 1780 and 1980. Widespread creation of freshwater ponds and other wetlands has at least temporarily reversed the trend of net wetland losses in some areas in the USA, but recent meta-analyses indicate that these habitats do not fully replace natural wetlands (e.g., studies have shown that restored wetlands are 25% less productive than natural wetlands). Furthermore, communities in small, depressional wetlands in cold temperate forests are especially slow to recover to reference conditions. If constructed wetlands do not function similarly to those that were lost, the net result is ecosystem structure and biological communities that are negatively altered. Section 404 of the Clean Water Act and the U.S. Corps of Engineers requires mitigation of habitat loss or alteration from large-scale projects such as road building (USACE, compensatory regulations section 404B-1 guidelines). Even though wetlands mitigation with shallow marsh, with some aquatic bed, typically do not fully replace the function and productivity of the impacted wetlands, there is evidence that many (but not all) local amphibians can persist in these ponds. Wetlands mitigation sites can be constructed to hold some water year-round. This can provide greater diversity and cater to different species, depending on depth and extent; however, this can become a challenge when dealing with obligations to satisfy the Army Corps permit conditions which typically relate to wetland type not wildlife species benefit. The effects of global climate change-including changes in the form, timing, and amount of precipitation-are likely to increase challenges to designing wetlands. Changes in precipitation will likely be compounded by increased summer temperatures that can further reduce water availability and stress animals, creating synergies that magnify negative effects. These changes pose especially serious threats to aquatic biodiversity in arid regions like western North America. Drought is shortening wetland hydroperiods, resulting in greater local extinction rates for amphibians. Based on recognition that wetlands are being lost globally, and that climate change is a severe threat to these sensitive habitats, there is a growing need to determine how to engineer replacement habitats to provide appropriate conditions for local species while also providing resistance to drought. Unfortunately, quantitative data on differences in vital rates between constructed and natural wetlands, and the mechanisms behind the differences, are lacking. This information is critical if we are to design and construct mitigation wetlands that host communities similar to, and function as, natural wetlands. Recent construction of US Highway 287 over Togwotee Pass in Wyoming provides an extraordinary opportunity to test specific features of mitigation wetlands. The Wyoming Department of Transportation (WYDOT) was required by the US Corps of Engineers to compensate for the loss of wetlands on Togwotee Pass resulting from the road construction. WYDOT created a new mitigation wetland complex at the site of a former gravel pit used to supply raw material for road construction. The reclamation of part of the pit is phase one of approximately 5 acres of wetlands mitigation. We have marked (PIT tags) and monitored amphibian richness in adjacent natural water bodies in this area since 2003, and marked animals in the phase one mitigation site since 2010. The phase two mitigation site was completed in the spring of 2014. We are in the process of comparing long-term vital rates, population growth, and community dynamics between these 2 mitigation wetlands and the natural wetlands. The second phase of the Togwotee Pass mitigation complex offers a rare opportunity to experimentally isolate mechanisms underlying the efficacy of these mitigation measures. As part of this multi-agency collaboration, WYDOT allowed us to design 3 sets of experimental small ponds, or depressions, that were constructed in late summer 2013. Per guidelines, WYDOT constructed these in addition to the permanent wetlands. Each set contains 3 ponds of similar size that vary in maximum depth: one pond is approximately 0.5 m maximum depth, one is approximately 1.0 m maximum depth, and one is approximately 1.5 m maximum depth. All ponds have similar amounts of = 0.5 m shallow habitat. We know little about designing mitigation wetlands with temporary hydroperiod that support amphibians, while also meeting broader objectives that focus on wetland function and the recovery of plant communities. One the one hand, temporary water bodies that host few predators is typical habitat for montane amphibians, yet variations in hydroperiod can be a liability. For example, construction of ponds that hold water too briefly could be ecological traps that ultimately reduce growth of local amphibian subpopulations, especially in drought years when larvae do not have sufficient time to metamorphose 10,11. Conversely, construction of ponds that are too deep will result in different predator communities and may not produce the desired management objectives. Thus, there is a fine line between deep enough and too deep. Determining this is challenging and is influenced by several factors, including the amount of developmental plasticity of target species. However, identifying structural characteristics that promote community recovery will bring us closer to meeting the challenge of effective wetland mitigation and the reversal of population declines. Having a series of ponds with replicated characteristics (3 replicates of 3 depth treatments) provides the rare opportunity to identify specific mechanisms (e.g., depth) that improve design of mitigation ponds to provide quality habitat for amphibians and other wetland species. We propose to use the combination of our long-term data and new experiments to test hypotheses related to wetland design and community vital rates, providing information that is directly applicable in meeting management objectives. Objective 1: Determine if there are differences in community establishment and recovery related to depth. Hypothesis: By the end of 3 years, experimental ponds with 1.5 m maximum depth will have greater community richness (invertebrates, plants, amphibians) than mitigation ponds with 0.5 m or 1.0 m maximum depths. Application: The predicted result would provide justification for WYDOT and similar agencies to construct wetlands with small ponds (depressions with 1.5 m maximum depth), rather than focusing on either wetland type, as is currently common. Objective 2: Use observations and experiments to test effects of mitigation pond features on growth, development, and survival of larval amphibians. Hypothesis: Variation (i.e., plasticity) in growth and development will not match a species' propensity to colonize habitats. Specifically, we expect colonization frequency will be ordered Boreal Chorus Frog > Western Toad > Columbia Spotted Frog, but we expect plasticity in growth and development will be ordered Western Toad > Columbia Spotted Frog > Boreal Chorus Frog. Application: Lack of plasticity in development, especially in pioneering species like the Boreal Chorus Frog, will result in mitigation ponds acting as ecological traps that attract adults but produce little recruitment. In the long term, this result could reduce net growth of the local (meta)population. Knowing this, ponds can be designed and built for optimal hydroperiods. Objective 3: Estimate annual production and 2-year survival according to variation in maximum depth of experimental ponds. Hypothesis: Production and survival of metamorphs will be lower in 0.5 m maximum depth mitigation ponds, compared to experimental ponds with 1 m and 1.5 m maximum depth. Application: The predicted result would provide justification for WYDOT and similar agencies to construct wetlands with small ponds (such as depression with 1.5 m maximum depth), rather than focus on only wetland type, as is currently common.
The primary payoff of this research is the identification of substantial cost savings in the design and implementation of mitigation efforts. Understanding how mitigation sites are used by amphibians may affect the degree to which these mitigation sites are designed and constructed - less is likely to be more for many western amphibian species. The project is urgent as little is known about designing mitigation wetlands with temporary hydroperiods that support amphibians, while also meeting broader objectives that focus on wetland function and the recovery of plant communities. Shallow marsh with some aquatic bed are key for amphibian reproduction. Furthermore, increases in the severity and frequency of flooding events and wildfires, influenced by changes in climate, are likely to shift hydroperiod dynamics across the affected landscape. In addition to stressors imposed by landscape changes, amphibian declines are of worldwide note, including species in the western U.S. Additional payoffs include the incorporation of information from our current work at a recent WYDOT mitigation site and a seamless implementation of relate research, taking advantage of with matching dollars from the U.S. Geological Survey's Amphibian Research and Monitoring Initiative.
July 1, 2014
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