Progressive success of a restored forested wetland two decades after mitigation

Background/Question/Methods

Increasing levels of urban sprawl and development require increasing compensatory mitigation to preserve remaining ecological systems and associated biodiversity. In the early 1990s, Maryland’s State Highway Administration began work on 13 highway projects, each generating detrimental impact to wetland systems in the Patuxent River Watershed.  One compensatory project was the 1992-93 reclamation of a 55.85 ha sand and gravel quarry bordered by a contiguous stand of mature forest along the Patuxent River in Harwood, Maryland.  The reconstructed portions of the site were required and designed to replace forested, scrub-shrub, and tidal and non-tidal emergent wetland habitats lost through construction.  A survey of the herpetofaunal colonizing community including description of patterns of use of the site was performed in 1995-96.  The present study was designed to follow up that survey and examine successional progress of its herpetofaunal community.  Survey methods from the original study–drift fencing with pitfall and funnel traps, and linear transects with aquatic dip-netting–were performed in 2010 along with new survey methods–hoopnet trapping, cover boards and area constrained searches in 2011.  Differences in diversity between the original study and the present study were examined.

Results/Conclusions

Habitat changes since initial establishment involve a decrease in open, grass-dominated habitat and an increase in permanent and seasonal surface water.  Forested wetland habitat remains constrained to the pre-reclamation forested stands.  The constructed forested wetland has only shifted to a scrub-shrub stage.  Herpetofaunal biodiversity has increased since the original survey from 26 to 33 species through a gain of ten and loss of three.  Changes occurred primarily through the addition of six reptiles; nine new species against three losses (six-lined racerunner, five-lined skink, and Eastern worm snake).  Of these, the latter two are probably still present, but the racerunner is unlikely due to decline in open habitat.  The arrival of spotted turtles suggests not only the presence of suitable forested wetland habitat, but also interconnectivity with other, nearby habitat sites. The only change to the amphibian community was the addition of wood frogs.  Measures of herpetofaunal diversity increased—Shannon-Weiner from 2.09 to 2.74 and Gini-Simpson from 0.83 to 0.92.  Effective species biodiversity for the site has approximately doubled (8 to 15 and 6 to 12, respectively). Our observations suggest the need for long-term monitoring, as well as the potential for using mitigated sites for controlled ecological studies.