Functional Equivalency of Restored and Natural Salt Marshes

Abstract

Although achieving functional equivalency of restored and natural ecosystems is a desirable restoration goal, direct assessments of function are rare, as are data supporting the use of indicators of function. The issue is how structural attributes can be used to assess ecosystem functioning. A functional equivalency index mixed both structural and functional measures, and an often-cited model of ecosystem degradation and restoration depicted a straight-line relationship between the two variables. Several points need clarification, not only for tidal wetlands but for restoration ecology generally. It is unrealistic to expect linear relationships among structure, function, and time; it is also inappropriate to assume, for natural and restored ecosystems, that equivalent structure means equivalent function. For example, using plant biomass to compare primary productivity rates assumes equal biomass:productivity relationships among sites; restored and natural wetlands are less likely to be similar in grazing, decomposition, and export rates than are two natural sites. Likewise, using soil organic matter (OM) and total Kjeldahl nitrogen (TKN) to indicate nutrient availability may be less appropriate for restored and natural marsh comparisons than for two natural sites. We recommend that comparisons of structural attributes be labeled structural equivalency measures, thereby avoiding misconceptions. Useful structural measures for assessing tidal wetland restoration are: soil texture; soil OM, soil nutrients, vegetation structure (height distributions); invertebrate and fish populations (especially fish size distributions), and topographic complexity.