Seeing the forest as well as the trees: An expert opinion approach to identifying holistic condition indicators for mangrove ecosystems

Holistic ecosystem health indices are urgently needed for conserving and managing rapidly dwindling mangrove ecosystems. The appropriate indices should provide consistent outcomes that enable both the assessment of current conditions and assist future planning. The previous approaches mostly focus on the structural condition of trees, with mangrove ecosystems often treated as terrestrial forests. Structural features are too insensitive as representatives of negative internal creeping changes. Some unique mangrove ecosystem attributes can react to anthropogenic pressures much faster than tree or forest structures do, making them more useful as early warning indicators. The development of holistic indicators requires a multidimensional approach engaging specific functions and services underpinning the characteristics of mangrove ecosystems. We present an approach to identify potential variables ranging from assemblage structure to physiological and biochemical variables for reliably indicating mangrove condition. These indicators could then be tested against a range of ecosystem conditions (e.g. disturbance levels) to reveal their response to different degrees of pressures. These variables were first nominated based on the driver-response model relating changes in environmental conditions and ecosystem responses. The nominated variables were then prioritised through an expert questionnaire survey, in which the experts scored every nominated variable on the efficiency, the number of pertinent ecosystem services and the time required them to reflect the impact of the pressure. Sediment properties and elements of forest structure featured strongly in the survey, followed by variables pertaining to biomass, vegetation status and carbon cycling. With respect to community composition, components of the higher trophic levels (e.g. crabs, other macrofauna) were ranked higher by the experts. The mangrove-tree-related variables (e.g. Importance Value Index, IVI), associated biotic components of insects, annelids and moss were ranked lowest. These results provide the basis for establishing a specific framework for mangrove ecosystem assessment.