The classification of wetlands: integration of top-down and bottom-up approaches and their significance for ecosystem service determination

The typology of wetlands provides important information for both water resource managers and conservation planners. One of the most important aims of allocating wetlands to a certain type or class is to provide information about the ecosystem services that the wetland provides. There are two main approaches towards wetland classification. Firstly, there are top-down approaches whereby wetlands are divided into several categories based on a conceptual understanding of how the wetland functions (mostly with regards to water flows). Secondly there are bottom-up approaches whereby the classification of wetlands is based on the collection of data in the wetland that is then subjected to various clustering techniques (mostly with regards to biodiversity). The most utilized system of top-down classification assigns wetlands into hydrogeomorphic units, which function as a single unit in terms of hydrology and geomorphology. This type of classification is most useful for water resource planning, as it provides information about how the wetland is connected to the drainage network and what are the water inflows, throughflows and outflows of the wetland. The bottom-up classification approach typically focusses on the classification of wetland habitats rather than complete wetlands, where wetland habitat represents a spatial unit delineated on the basis of vegetation, embedded within the (complete) hydrogeomorphic unit, and defined as an area of wetland that is homogeneous in terms of opportunities for plant growth. At a broad scale, most ecosystem services can be superficially derived from the hydrogeomorphic unit type and the way water moves through a wetland, but habitat units and the plant species that define them would have a specific effect on the delivery of ecosystem services, for example, with different assemblages providing different resistance to flow. Some types of ecosystem services are exclusively linked to specific wetland habitats, especially provisioning services. For this reason, it is proposed that a combined approach of hydrogeomorphic classification together with a vegetation map, offers the maximum information value for ecosystem service determination. In order to account for the potential pitfall of “double counting” when combining the top-down and bottom-up approaches, each service needs to be considered individually with reference to the degree to which a service is either: (a) primarily determined by HGM class/attributes and modified by the vegetation class/attributes; or (b) primarily determined by the vegetation class/attributes.