Geographically Isolated Wetlands: Rethinking a Misnomer
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We explore the category “geographically isolated wetlands” (GIWs; i.e., wetlands completely surrounded by uplands at the local scale) as used in the wetland sciences. As currently used, the GIW category (1) hampers scientific efforts by obscuring important hydrological and ecological differences among multiple wetland functional types, (2) aggregates wetlands in a manner not reflective of regulatory and management information needs, (3) implies wetlands so described are in some way “isolated,” an often incorrect implication, (4) is inconsistent with more broadly used and accepted concepts of “geographic isolation,” and (5) has injected unnecessary confusion into scientific investigations and discussions. Instead, we suggest other wetland classification systems offer more informative alternatives. For example, hydrogeomorphic (HGM) classes based on well-established scientific definitions account for wetland functional diversity thereby facilitating explorations into questions of connectivity without an a priori designation of “isolation.” Additionally, an HGM-type approach could be used in combination with terms reflective of current regulatory or policymaking needs. For those rare cases in which the condition of being surrounded by uplands is the relevant distinguishing characteristic, use of terminology that does not unnecessarily imply isolation (e.g., “upland embedded wetlands”) would help alleviate much confusion caused by the “geographically isolated wetlands” misnomer.
KeywordsAdjacency Connectivity gradients Hydrogeomorphic classification HGM Rapanos SWANCC Wetland classification Wetland connectivity
It has been over a decade since the state of scientific understanding of isolated wetlands was synthesized in a special issue of Wetlands (Vol. 23, No. 3, September stal). This comprehensive review was catalyzed by a U.S. Supreme Court (2001) decision in the case of the Solid Waste Agency of Northern Cook County v. U.S. Army Corps of Engineers, 531 U.S. 159 (2001) (SWANCC). The Supreme Court’s decision effectively removed Clean Water Act (CWA) protection of “isolated, intra-state, non-navigable waters” in cases where such protection was based solely on use by migratory birds (i.e., the Migratory Bird Rule). The decision also indicated that such waters could be protected under the CWA if there was a “significant nexus” with downgradient “navigable waters” (Downing et al. 2003). This decision brought the issue of wetland “isolation” to the forefront of discussion in the wetland science community (Nadeau and Leibowitz 2003).
A key theme emerging throughout the Wetlands special issue was that wetlands typically referred to as “isolated” were not, from either an ecological, hydrological, or physicochemical perspective, inherently isolated from other aquatic systems. In an effort to represent the manner of isolation more precisely, and by extension the ways in which they were clearly not isolated, Tiner (2003a) adopted the term “geographically isolated wetland,” defined as a wetland that is completely surrounded by upland at the local scale (hereafter referred to as GIW). Leibowitz (2003) recommended that GIW be used as a replacement for “isolated wetland” in an effort to avoid the associated ambiguities inherent in the word “isolation.” As defined by Tiner (2003a), GIWs consist of multiple wetland types including both natural and created wetlands formed in depressions (e.g., vernal pools, prairie potholes, playas, limesinks, and Carolina bays), on mineral and organic flats (e.g., fens and spruce-fir flats), along slopes (e.g., hillside seeps), within coastal dunes, on inactive river floodplains, and on active floodplains as depressional wetlands in well-drained riparian settings (Tiner 2003a). The common characteristic of GIWs is that they are surrounded by upland, not that they are isolated. In fact, Tiner (2003a, p #495) explicitly argued that many GIWs “are hydrologically connected to other wetlands and waters through subsurface or ground-water connections or by infrequent and/or short duration surface-water connections.”
In addition to reducing confusion associated with the term “isolated wetlands,” the original intent of the wetland science community in defining GIWs was to identify information gaps where additional research was needed to define better the functional role of these wetlands, particularly with regard to their effects on the chemical, physical, and biological integrity of “navigable waters” of interest to regulators and policymakers (Leibowitz and Nadeau 2003). Explorations into wetland function and effects on downgradient systems became increasingly important following a subsequent U.S. Supreme Court (2006) split decision in Rapanos v. United States, 547 U.S. 715 (2006) (Rapanos). In the split decision, Justice Kennedy stated that wetlands can be covered under the CWA “if the wetlands, alone or in combination with similarly situated lands in the region, significantly affect the chemical, physical, and biological integrity of other covered waters.” However, it has become increasingly clear that while the original intent was well-founded, referring to wetlands as “geographically isolated” has done little to alleviate the implications of functional isolation that accompany the GIW terminology, as we present from an analysis of the recent literature in a later section. In fact, we posit that the GIW term and its associated categorization of wetlands served to obscure rather than clarify understanding of the complex relationships among interconnected aquatic ecosystems. We suggest that classification systems that account for functional differences among diverse wetland types (e.g., the Hydrogeomorphic [HGM] approach [Brinson 1993]) offer less ambiguous and more scientifically defensible alternatives that are less prone to misunderstanding. For the rare cases in which being surrounded by uplands is the relevant distinguishing characteristic, development of terminology that does not unnecessarily imply isolation (e.g., “upland embedded wetlands”) would help alleviate much of the confusion caused by the “geographically isolated wetlands” misnomer.
“Geographic isolation” Does Not Acknowledge Connectivity Continua
Connectivity, and therefore isolation, refers to the degree to which entities are joined in a relationship. From a hydrological perspective, connectivity may be viewed as the degree to which water moves between uplands, wetlands, and downgradient waters (e.g., Lissey 1971; Winter and Rosenberry 1995; Wilcox et al. 2011). These movements can be readily observable surface-water flows (i.e., those that are persistent) or more subtle surface-water flows that occur at low frequencies, magnitudes, or durations such that no readily observable indicators are formed (e.g., a stream channel or delineable connection of wetland vegetation and soil). Crucially, water movement can also be along flow-paths that are difficult to observe, such as shallow sub-surface or groundwater flows. From a biogeochemical perspective, connectivity may be viewed as the degree to which chemical integrity of a stream, river, wetland, or other aquatic system of interest are influenced by surface water, groundwater, atmospheric, and biotic processes and functions within and among aquatic systems (e.g., LaBaugh et al. 1987; Goldhaber et al. 2011; Forbes et al. 2012). From an ecologic perspective, connectivity may describe how distance and landscape characteristics within and between aquatic habitats interact with species’ dispersal and life history traits to affect movement, gene flow, or population dynamics (e.g., Newman and Squire 2001; Mushet et al. 2013; O’Connell et al. 2013). From a geographic perspective, connectivity may refer to the distance between wetlands, the presence of an impassable geographic barrier, or the geospatial arrangement of aquatic systems on the landscape (e.g., MacArthur and Wilson 1967; Wilcox 1989; Forman 1995). From each perspective, connectivity, and therefore isolation, occurs along a continuum rather than as a binary condition.
The conceptual placement of wetlands along continua of hydrological and biological connectivity has a long legacy. Leibowitz (2003) described gradients as an “isolation-connectivity continuum,” and Euliss et al. (2004) described a conceptual framework they called “The Wetland Continuum.” In combination, these concepts clarify the varying roles of wetlands within complex natural systems. However, the GIW definition is binary (i.e., either 100 % surrounded by upland or not (Tiner 2003a)) and, thus, implicitly ignores isolation-connectivity continua. This overly simplistic definition of geographic isolation does not take into consideration proximity to other aquatic systems, key ecosystem processes, landscape permeability and leakiness, dispersal abilities of biota, or other factors that contribute to varying degrees of connectivity and isolation. By using a binary definition of geographic isolation, it can be easily misconstrued that GIWs represent one extreme of an isolation-connectivity continuum. Indeed, GIWs span a range of hydrologic positions from precipitation-fed wetlands with little surface-water or groundwater inflow or outflow (e.g., ombrotrophic bogs) to wetlands on floodplains that regularly exchange water with an adjacent river or stream (Leibowitz et al. 2008), to seepage wetlands almost entirely dependent on groundwater flow (Tufford 2011).
“Geographic isolation” Does Not Equal Functional Isolation
Geographically “isolated” wetlands are rarely functionally isolated and, as such, are capable of providing most, if not all, of the functions ordinarily attributed to wetlands, such as water storage, nutrient retention/transformation, and living matter growth (Novitski et al. 1996). The biotic and abiotic processes that underlie these functions accumulate in the landscape because what happens in one wetland typically affects or is affected by processes occurring in other aquatic habitats (e.g., Leibowitz 2003; Euliss et al. 2004; Leibowitz et al. 2008; Smith et al. 2011; Golden et al. 2014). In fact, Leibowitz’s (2003) seminal paper recommending GIW usage includes a section entitled “Are ‘isolated’ wetlands isolated?” wherein numerous examples of functional connectivity between GIWs and both aquatic and terrestrial habitats are provided. More recently, and in a similarly-titled paper (“Are isolated wetlands isolated?”), Smith et al. (2011) expanded on these arguments and offered examples of how “isolated” wetlands are functionally interconnected not only to other aquatic systems but also to society through the ecosystem services they provide. Tiner (2003a, p #494) stated “most, if not all, wetlands scientists would agree that there is no such thing as an isolated wetland from an ecological standpoint.” In an analysis of the recent literature on geographic isolation, we adopt perspectives from hydrology, biology, biogeochemistry, and geography to underscore the caveat that “geographic isolation” in the GIW term was not, and should not be, equated with functional isolation. However, despite the great lengths to which Leibowitz (2003), Tiner (2003a) and others have gone to clarify that geographic isolation does not equate to functional isolation, this linkage often occurs within the wetland sciences (see examples discussed below).
“Geographic isolation” Facilitates Incorrect Generalizations
“Geographically isolated wetlands” in Literature
1. How often does geographic isolation occur in titles, abstracts, or keywords?1
• Search terms: (“geographic isolation” or “geographically isolated”)
• Number also containing (genetic* or evolution*)
111206 (61 %)
2. How many publications contain the term “geographically isolated wetland” (GIW) or “isolated wetland” (IW) in the title, abstract, or keywords?2
• GIW search: (“geographically isolated wetland” OR “geographically isolated wetlands”)
23 (11 %)
• IW search: (“isolated wetland” OR “isolated wetlands”) NOT “geographically isolated”)
205 (89 %)
3. How many publications did we miss by placing quotes around the term GIW?3
• Search: (wetland*) AND (“geographically isolated” OR “geographic isolation”) NOT (“geographically isolated wetlands” OR “geographically isolated wetland”)
• GIW sensu Tiner 2003
2 (10 %)
• Demographic or genetic effects of geographic isolation
15 (75 %)
3 (15 %)
4. How many of the 147 papers that cite four seminal papers about GIWs and are in WoS contain the term GIW or IW in the title, abstract, or keywords?4
• Number containing GIW
21 (14 %)
• Number containing IW
65 (44 %)
5. If we broaden the search, how are concepts of geographic isolation most commonly used in the wetlands literature?5
• Search: (geographic* AND isolat* AND wetland*) NOT (“geographically isolated wetlands” OR “geographically isolated wetland”)
• GIW (e.g., “wetlands termed ‘geographically isolated’ ”)
4 (5 %)
• Demographic or genetic effects of geographic isolation
75 (95 %)
In short, introduction of the GIW term has apparently done little to curb use of the potentially misleading term “isolated wetlands” in the wetland sciences (Fig. 2). Additionally, we found that the terms “geographic isolation” and “isolation” in the broader sense are commonly used interchangeably in the wetland sciences. As an example, in a comprehensive review of hydrological methods to model the influence of GIWs on downstream waters, Golden et al. (2014, p #190) stated, “GIWs are traditionally considered ‘isolated’ because they often exhibit unmeasurable or limited hydrologic connectivity to surface waters: therefore, any wetland systems with these characteristics can be considered ‘isolated.’” Similarly, in a study to remotely map potential GIWs in a 2600 km2 area of central Florida, Frohn et al. (2009, p #931-932) stated, “Tiner … maintains that geographic isolation is the easiest way to determine isolation, because it defines the position of the wetland on the landscape, and defines an isolated wetland as a wetland that is completely surrounded by uplands.” Our use of these two examples is in no way a reflection of the quality of the science these authors presented or their knowledge of GIW concepts and issues. Rather, we use these two examples to illustrate how easily the GIW term can be misconstrued to imply “isolation” in general and, rather than serving to replace “isolated wetlands” in the scientific literature as recommended by Leibowitz (2003), the GIW term has facilitated its continued use.
“Geographically isolated wetlands” in Practice
One of the key arguments used by Leibowitz (2003) in recommending use of the GIW category was its simplicity and ease of use. To explore how the GIW grouping has been used in practice, we examined four papers on mapping GIWs recently published in Wetlands, Frohn et al. (2009), Frohn et al. (2012), Lane et al. (2012), and Martin et al. (2012). Each followed the Tiner (2003a) definition of geographical isolation (i.e., the condition of being completely surrounded by uplands at the local scale). However, all four found that currently available spatial data lacked adequate resolution and accuracy at the scale of the research question to adequately identify wetlands surrounded by uplands. As an alternative, an acceptable distance (i.e., buffer) between mapped wetlands and streams was used to identify wetlands that likely intersected or were proximate to streams and rivers. This buffering methodology was used instead of looking for wetlands that were surrounded on all sides by mapped uplands to mitigate the effect of map limitations on study findings (Lang et al. 2012). The methods developed in these papers are supportive of regulatory and policymaking information needs that often center on issues of proximity/adjacency (i.e., distances as reflected in the authors’ use of buffers). These methods also are consistent with those used by Tiner (2003b), who used proximity to streams and rivers to exclude “non-isolated” wetlands when estimating GIW extent. However, they are not consistent with the GIW definition itself, which is not based on proximity to a stream or river but rather on upland embedment – a data layer unavailable at the scale crucial to many scientific investigations. Ironically, using distance buffers to quantify wetlands potentially losing CWA protection as a result of the SWANCC and Rapanos decisions likely provides better information than would have resulted from methodologies relying on “geographic isolation” because distance more directly relates to issues of adjacency than does the condition of being surrounded by uplands. Additionally, the need to map wetlands using distance based buffers around aquatic features rather than mapped uplands brings into question the “ease of use” argument presented by Leibowitz (2003) in promoting use of the GIW grouping.
Prairie Pothole Wetlands: A Case Study of Connectivity
The dynamic nature of prairie pothole wetlands in response to a climate that cycles between periods of drought and deluge (Winter and Rosenberry 1998) necessitates adaptations by wetland-dependent biota that facilitate repopulation of wetlands following periods when they may dry. For example, many wetland invertebrates have adult forms that fly; others disperse through temporary surface connections that can form when water levels are high or have eggs that are dispersed by mechanisms similar to those of wetland plant communities; still others disperse by clinging to the fur or feathers of animals (Swanson 1984). All of these dispersal mechanisms provide ecological connections among aquatic habitats across the PPR’s landscape (Euliss et al. 1999). Uplands surrounding wetlands also can contribute to wetland connectivity by providing dispersal corridors, nesting habitat, and feeding areas (Gibbons 2003; Batt et al. 1989. Mushet et al. 2011). Thus, the upland habitat between wetlands can be the conduit by which discrete wetlands are “connected” rather than “isolated.”
Geographic distances between prairie pothole wetlands range from neighboring wetlands that often merge during high water years (Leibowitz 2003) to those where distances may actually be great enough to form a barrier to connectivity when viewed from a specific functional perspective (e.g., gene flow; Newman and Squire 2001). Additionally, the geographic position of wetlands along topographic gradients influences connections of wetlands to groundwater (Winter and Carr 1980; Swanson et al. 1988) resulting in effects on water chemistry and biota (Euliss et al. 2004). Being surrounded by upland does not mean that these wetlands lack overland water connections. Leibowitz and Vining (2003) estimated that 28 % of the prairie pothole wetlands in their study region had temporary surface water connections to other wetlands during the year of their study (1995). Similarly, at the Cottonwood Lake Study Area in Stutsman County, North Dakota, ten of the 17 wetlands within this wetland complex regularly contributed water to overland flows (Swanson et al. 2003). Only by the most basic definition (i.e., one based solely on the binary condition of being surrounded by upland or not) are some prairie pothole wetlands “geographically isolated.”
It is important to note here that the originators of the GIW terminology also highlighted the multiple connections of wetlands falling into the GIW grouping. However, given the frequency with which the terms GIW(s) and “isolated wetland(s)” have been used interchangeably in recent literature, our case study serves as a needed reminder of the interconnected nature of these geographically “isolated” wetlands. Additionally, we use prairie pothole wetlands as our case study because these wetlands are commonly used as a classic example of GIWs. However, GIWs throughout the United States are also better described along continua of connectivity. For example, “geographically isolated” vernal pools in the northeastern U.S. and along the Pacific Coast, mid-continental playa wetlands, sinkhole wetlands in karst topographies, desert spring wetlands, Delmarva and Carolina Bay wetlands on the Atlantic coastal plains, and cypress dome wetlands in Florida all vary in their hydrologic, biogeochemical, and ecological connections to other aquatic systems.
The importance of having wetland class terminology that is understood throughout the U.S., if not globally, is that it provides a common language that conveys important generalizations about systems to not only the scientific community, but also to regulators and policymakers (Scott and Jones 1995). One of the most commonly used wetland classifications is the Cowardin classification system (Cowardin et al. 1979). This system groups wetlands based on vegetation type, substrates, hydrology, and water chemistry. However, other properties that are important for assessing wetland function, such as landscape position, are not included (Tiner 2011). Alternatively, Brinson (1993) introduced a system (modified by Smith et al. 1995) that places wetlands into unique functional classes based mainly on hydrology and geomorphology (the HGM approach), and Semeniuk and Semeniuk (1995) proposed a HGM classification designed to be applied globally (Table 1). Although these authors used different terminology for their wetland classes, each emphasized the functional importance of hydrology and geomorphic position. Smith et al. (2011) suggested that adopting a HGM perspective would facilitate consideration of ecosystem processes. Recognizing the value of functionally-based classifications as provided by HGM, Tiner (2011) developed a set of descriptors to bridge the gap between the HGM system of Brinson (1993) and the National Wetland Inventory (NWI) that uses the Cowardin et al. (1979) classification system. Thus, wetland classification systems are already in place to facilitate the ability of the wetland science community to refer to wetlands in a consistent manner, a manner that more accurately reflects the functional role of various wetland types in an interconnected landscape.
Further, wetlands are considered “Waters of the United States” if a “significant nexus” exists between the wetland in question and the chemical, physical, and biological integrity of “traditional navigable waters.” In practice, such a nexus is assumed to exist between certain “adjacent” wetlands and traditional navigable waters, but for other “adjacent” wetlands and wetlands that are “not adjacent,” it must be shown to exist on a case-by-case basis. Therefore, the scientific community can best contribute to this ongoing dialog by focusing efforts on studies related to connectivity between individual wetlands that are “not adjacent,” or classes of wetlands that are “not adjacent,” and downstream waters. Because GIWs occur in both “adjacent” and “not adjacent” settings, only after issues of “adjacency” have been adequately addressed can the scientific community realistically address issues of connectivity/isolation as related to wetlands that are considered to be “not adjacent.” Therefore, for CWA purposes, we propose that wetland groupings based on adjacency, defined by functionally relevant distances from streams, lakes, and coastal waters, would provide information more directly relevant to decision makers than overly simplistic groupings based on “geographic isolation” (Fig. 4). Such groupings have already been partially developed through the spatial distance buffering techniques used in recent mapping efforts (e.g., Frohn et al. 2009; Frohn et al. 2012; Lane et al. 2012; Lang et al. 2012; Martin et al. 2012), representing important steps towards more ecologically realistic assessments of functional connectivity between wetlands and other types of aquatic ecosystems.
For the rare instances in which being surrounded by upland is the relevant distinguishing feature, development of terminology that does not unnecessarily imply isolation (e.g., “upland embedded wetlands”) would help alleviate much confusion. Only with a significant change in how the scientific community refers to wetlands currently termed GIWs will wetland scientists be able to address clearly issues of wetland function relative to connectivity and potential isolation without having to first remedy confusion perpetuated by continued a priori designations of these wetlands as being isolated, geographically or otherwise.
The idea for this paper originated during a “Geographically Isolated Wetlands Research Workshop” convened and co-hosted by the U.S. Environmental Protection Agency Office of Research and Development and the Joseph W. Jones Ecological Research Center and held in Newton, GA, November 18–21, 2013. We thank all participants of this workshop for the lively discussions within which this manuscript was conceived. We also thank workshop participants for providing follow-up input and critiques that contributed to our manuscript’s overall development. Additionally, we thank Ned Euliss, Jr., Scott Leibowitz, Ralph Tiner, and two anonymous reviewers for providing their critical reviews of earlier drafts of this manuscript. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
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