Wetlands are the biological powerhouses of planet Earth, with primary production often higher even than rain forests. These powerhouses produce enormous numbers of wild animals including fish, waterfowl, and aquatic animals. They also produce oxygen and store carbon. These services are provided free, and powered by solar energy. So where do we start our reading to understand them? Here is a guide to your reading, a guide structured by causal factors and their relative importance. Causal factors provide a powerful tool for understanding how wetlands form, why there are different kinds of wetlands, and how they can be wisely managed for production and conservation.
a guide to the scientific literature
Table of contents:
Author’s version of contribution that is published online in Oxford Bibliographies in Environmental Science
Suggested citation: Keddy, Paul A. 2016. Wetlands. Oxford Bibliographies in Environmental Science. Ed. Ellen Wohl. New York: Oxford University Press. Viewed online at www.drpaulkeddy.com, date.
Note that crosslinks (indicated by *) do not work in this version, but they are provided on the OBO website.
Wetlands have always influenced humans. Early civilizations first arose along the edges of rivers in the fertile soils of floodplains. Wetlands also produce many benefits for humans—along with fertile soils for agriculture, they provide food such as fish and water birds, and, of course, freshwater. Additionally, wetlands have other vital roles that are less obvious. They produce oxygen, store carbon, and process nitrogen. Since wetlands form at the interface of terrestrial and aquatic ecosystems, they possess features of both. They are often overlooked in standard books, since terrestrial ecologists focus on drier habitats, while limnologists focus on deep water. Shallow water, and seasonally flooded areas, fall comfortably into neither category. All wetlands share one causal factor: flooding. While wetlands may be highly variable in appearance and species composition, flooding produces distinctive soil processes and adaptations of the biota. Thus wetlands and water are inseparable. This treatment will first introduce you to some basic overviews that explain what a wetland is, what different kinds of wetlands exist, and some key processes that occur within them (*General guides and Introductions*). Then we will turn to causal factors: flooding creates wetlands, so it receives a full section. Then we will consider how nutrient availability modifies wetlands. *Other Causal Factors*, such as salinity, competition, herbivory, and roads, are combined into a third section. Having provided this foundation, we will look at the global distributions (*Geography of Wetlands*). By this point, you will know what a wetland is, where they occur, and the main factors that affect their abundance and composition. We will then explore two more specialized topics. First, monographs are identified that apply to particular regions of the Earth (*Regional Monographs*). Second, we look at aquatic plants; they are a relatively small group with important implications for the understanding of wetlands as a whole (*Aquatic Plants*). We close with a section on conservation of wetlands. Two general obstacles must be met in coming to grips with the scientific literature in this field. First, much of the work on wetlands is scattered across ecological journals and may not even appear under key word searches for wetland; instead, material may appear under a term such as bog, fen, shoreline, lake, floodplain, pothole, playa, peatland, or mire (or a dozen other terms). Second, this discipline seems to have attracted a large number of conference symposia, the findings of which are recorded often in expensive books with a haphazard collection of papers, written by a haphazard collection of people, with no unifying theme whatsoever except that all deal with wet areas. Hence, the need is pressing for a few general principles to structure one’s knowledge. Here we focus on general causal factors and their relative importance.
General Guides and Introductions
To learn about wetlands and communicate with other human beings, we need a common frame of reference. Otherwise, our knowledge is more like a heap of bricks than a properly constructed building. Let us begin with three books that provide this common frame of reference. First, Dugan 2005 is a guide that is accessible to the general reader and useful for the professional. The author begins with two basic topics: What are wetlands and why we need wetlands. He then continues with a two-hundred-page survey of the world’s wetlands, supplemented with maps and beautiful illustrations. Next, Wetland Ecology: Principles and Conservation (Keddy 2010) also begins with a general introduction to wetlands. It then proceeds through a series of causal factors that make wetlands, roughly in the order of their importance: flooding, fertility, disturbance, competition, herbivory, and burial. Each of these chapters begins with general principles and then explores experimental and descriptive work that shows how these principles apply to wetlands around the world. Third, Wetlands (Mitsch and Gosselink 2015) also begins with a general introduction to wetlands. However, unlike Dugan 2005 and Keddy 2010, it then divides coverage into five types of wetland ecosystem, with separate chapters on tidal marshes, mangrove swamps, freshwater marshes, freshwater swamps, and peatlands. Whereas Dugan and Keddy emphasize biological diversity, Mitsch and Gosselink tend to emphasize energy flow and biogeochemistry. If you read these three books, you can consider yourself well informed on wetlands as a whole. You can think of these as the trunk upon which many more branches of knowledge are organized. The interested reader can then proceed in two directions. In the first case, one can deepen one’s knowledge of the causal factors that create wetlands and proceed with topics such as *Flooding and Flood Pulses* and *Nutrients*. Or one can focus on the many kinds of wetlands that arise in a local context and proceed with *Regional Monographs*. Finally, with the above sources as a foundation, one can directly consult specialized journals, such as Wetlands, the journal published by the Society of Wetland Scientists since 1981. Otherwise, much of the specialist work on wetland ecology is scattered across journals that deal with ecology and geography. Further, owing to the commercial importance of animals in wetlands (think ducks, muskrats, fish) many papers can be found in fish and wildlife journals, work that is too often marred by an inordinate emphasis upon production of one or a few species of animals. Many wetlands have been damaged in the name of “wildlife management.”
Dugan, Patrick, ed. 2005. Guide to wetlands.
Keddy, Paul A. 2010. Wetland ecology: Principles and conservation. 2d ed.
Mitsch, William J., and James G. Gosselink. 2015. Wetlands. 5th ed. Hoboken, NJ: Wiley. [ISBN: 9781118676820]
Wetlands[http://www.springer.com/life+sciences/ecology/journal/13157]. 1981–. [class:periodical]
Flooding and Flood Pulses
Flooding makes wetlands. This has three main consequences. (1) Flooding causes reduced oxygen levels in the soil. These changes are generally described in Keddy 2010 and Mitsch and Gosselink 2015 (both cited under *General Guides and Introductions*. For more depth and breadth, one can consult Reddy and DeLaune 2008. (2) Plants and animals have to adapt to reduced oxygen levels. The presence of distinctive plants with channels for transmitting oxygen from the atmosphere to the roots (aerenchyma) is a defining characteristic of wetlands. Aquatic plants offer the most extreme case of plants adapted to flooding, and they are therefore further treated in a separate section *Aquatic Plants*. (3) Sometimes the water is higher than other times. High spring flooding creates extensive areas of wetlands along rivers. High spring flooding makes extensive areas of wetlands along the shores of lakes, and high spring flooding makes extensive areas of wetlands in many other kinds of depressions. Keddy 2010 has an entire chapter on this topic, while other monographs, such as Middleton 2002, describe this as “flood pulsing.” An entire literature can now be accessed under “flood pulsing.” It is particularly important for fish (Welcomme 1979). You can say it a hundred times and write books on the topic—yet people will express shock and dismay that their floodplain property is flooded in the spring, and they will equally complain about low water levels that make it inconvenient to use their boat docks. They will also complain when some authority tells them they cannot build a house or factory in a flood-prone area, expecting, of course, that if anything does happen, an insurance company or government will pay for the damage. Yet, so long as snow melts in the spring and rainy seasons arrive, water levels in rivers will be high. A major impact humans have had on wetlands is the systematic disruption of flood peaks in wetlands and watersheds around the world (Nilsson, et al. 2005). Hughes 2003 shows how the restoration of spring floods is necessary for restoring ecological health to wetlands and watersheds. Wilcox, et al. 2007 illustrates the same principle for large lakes. The importance of flood pulsing is now well documented, yet no doubt individuals will continue to think that rivers and lakes should have stable levels so they can build their houses wherever they care—alas, excellent science does not seem to provide an antidote to ignorance.
Hughes, Francine M. R., ed. 2003. The flooded forest: Guidance for policy makers and river managers in
Middleton, Beth A., ed. 2002. Flood pulsing in wetlands: Restoring the natural hydrological balance.
Nilsson, Christer, Catherine A. Reidy, Mats Dynesius, and Carmen Revenga. 2005. Fragmentation and flow regulation of the world’s large river systems. Science 308:405–408.
Reddy, K. Ramesh, and Ronald D. DeLaune. 2008. Biogeochemistry of wetlands: Science and applications.
Welcomme, Robin L. 1979. Fisheries ecology of floodplain rivers.
Wilcox, Douglas A., Todd A. Thompson, Robert K. Booth, and James R. Nicholas. 2007. *Lake-level variability and water availability in the Great Lakes[http://pubs.usgs.gov/circ/2007/1311]*. US Geological Survey Circular 1311. Washington, DC: US Department of the Interior. [class:report]
Nutrients and Fertility
Two elements, nitrogen and phosphorus, control rates of primary production, and they determine species composition, in wetlands. Alluvial floodplains and deltas have high levels, as nutrients are carried in spring flood waters, and they accumulate in sediment. Here one finds some of the highest rates of primary production in the world, in excess of 1000 gm2yr-1 (Whittaker and Likens 1973). This often translates directly into animals, particularly fish (Welcomme 1979, cited under *Flooding and Flood Pulses*). It is difficult to generalize whether it is nitrogen or phosphorous that limits growth (Verhoeven, et al. 1996). Nutrients are not necessarily beneficial. In shallow water nutrients can generate algal blooms with negative consequences on marsh and aquatic vegetation, while at larger scales, entire lakes or estuaries may become so nutrient enriched that the resulting decay consumes oxygen, producing “dead zones” (Turner and Rabelais 2003). The
Davis, Steven M., and John C. Ogden, eds. 1994.
Kadlec, Robert H. 2009. The
Kadlec, Robert H., and Scott D. Wallace. 2009. Treatment wetlands. 2d ed.
Turner, R. Eugene, and Nancy N. Rabelais. 2003. Linking landscape and water quality in the
Verhoeven, Jos T. A., Willem Koerselman, and Arthur F. M. Meuleman. 1996. Nitrogen- or phosphorus-limited growth in herbaceous, wet vegetation: Relations with atmospheric inputs and management regimes. Trends in Ecology and Evolution 11.22: 494–497.
Whittaker, Robert H., and Gene E. Likens. 1973. Carbon in the biota. In Carbon and the biosphere. Edited by George M. Woodwell and Erene V. Pecan, 281–302.
Zampella, Robert A., John F. Bunnell, Kim J. Laidig, and Nicholas A. Procopio. 2006. Using multiple indicators to evaluate the ecological integrity of a coastal plain stream system. Ecological Indicators 6.4: 644–663.
Other Casual Factors
For each particular wetland, there is a hierarchy of causal factors. The challenge for a scientist or a manager is to identify these causal factors and to determine which ones are the most important at a specific site. In general, it is useful to view the composition of a wetland as arising from these causal factors acting upon the pool of species available in the landscape (Weiher and Keddy 1999). Two factors of overriding importance, flooding and nutrients, have already been discussed. Both of these are partially controlled by the geological setting, which acts as a templet for most wetlands (Warner 2004). Superimposed on these foundations is a long list of other factors. The chapters in Keddy 2010 (cited under *General Guides and Introductions*) are organized in approximate order of their importance: flooding, fertility, disturbance, competition, herbivory, and burial and other factors. Here we will consider just four beyond flooding and fertility: (1) salinity, (2) herbivores, (3) fire, and (4) roads. Salinity is a very important factor near coastlines, with species and communities arranged along salinity gradients created by freshwater inputs (Tiner 2013). (2) Herbivores can have a major impact. The impacts of muskrats in marshes provides a classic case in which high population densities of herbivores can lead to almost total loss of aboveground vegetation (Keddy 2010). Such top-down effects are becoming better understood; when humans remove the top carnivores (such as crabs or alligators), the effects can be dramatic (Silliman, et al. 2009). (3) Wetlands can burn during periods of drought. The chapter on fire in the Everglades in White 1994 is a classic example; here, fire not only removes plant biomass, but also it can even remove peat, thereby producing new areas of open water during the next wet period. (4) Roads can have a significant effect upon the biota of wetlands in populated regions. Not surprisingly, road density is a rather good surrogate for the overall impacts of humans in the landscape (Houlahan, et al. 2006). For a global context of road impacts, consult Laurence, et al. 2014. The most important point when reading about these other causal factors is to keep them in perspective. In each wetland, some are very important while others are less important. Here is a case where wetland ecology is contingent: it is essential to know not only the important general factors that create a wetland, but also how these are modified by local circumstances and other causal factors. While reading the literature, one should make a concerted effort to rank other causal factors in order of relative importance.
Houlahan, Jeff E., Paul A. Keddy, Kristina Makkay, and C. Scott Findlay. 2006. The effects of adjacent land use on wetland plant species richness and community composition. Wetlands 26.1: 79–96.
Laurance, William F., Sean Sloan, Christine S. O’Connell, et al. 2014. A global strategy for road building. Nature 513.7517: 229–232. [doi:10.1038/nature13717]
Silliman, Brian R., Edwin D. Grosholz, and Mark D. Bertness, eds. 2009. Human impacts on salt marshes: A global perspective. Berkeley: Univ. of
Tiner, Ralph W. 2013. Tidal wetlands primer: An introduction to their ecology, natural history, status and conservation.
Warner, Barry G. 2004. Geology of Canadian wetlands [http://journals.hil.unb.ca/index.php/gc/article/view/2751/3210]. Geoscience
Weiher, Evan, and Paul A. Keddy, eds. 1999. Ecological assembly rules: Perspectives, advances, retreats.
White, Peter S. 1994. Synthesis: Vegetation pattern and process in the
Geography of Wetlands
Another way to approach the topic of wetlands is to ask where they occur in the world, how they appear, and the kinds of creatures that are found there. A survey like this is a challenge since the volume of detail is far greater than any single book can cover. The natural world is indeed fractal. Still, having said this, the best guide for beginners is a small book, Dugan 2005 (cited under *General Guides and Introductions*), which can be paired with the online map at Global Wetlands 1993. Another useful online source is the list of Ramsar designated wetlands (Ramsar 2015). The problem with the latter list is that it is heavily biased toward
Dugan, Patrick, ed. 1993. Wetlands in danger: A world conservation atlas.
Fraser, Lauchlan H., and Paul A. Keddy, eds. 2005. The world’s largest wetlands: Ecology and conservation.
Global Wetlands[http://www.unep-wcmc.org/resources-and-data/global-wetlands]. 1993. Cambridge, UK: United Nations Environment Programme (UNEP), World Conservation Monitoring Centre (WCMC).
Gopal, Brij, Jan Kvet, Heinz Löffler, Victor Masing, and Bernard C. Patten. 1990. Definition and classification. In Wetlands and shallow continental water bodies. Vol. 1, Natural and human relationships. Edited by Bernard C. Patten, 9–15. The Hague: SPB Academic Publishing. [ISBN: 9789051030464]
Hughes, Ralph H., and Jane S. Hughes. 1992. A directory of African wetlands.
Ramsar. [http://www.ramsar.org/sites-countries/the-ramsar-sites] 2015. Gland, Switzerland: Ramsar Convention Secretariat.
A knowledge of wetlands should begin with an appreciation of general principles and how key environmental factors structure wetlands overall. Having said that, each general principle needs to be calibrated or refined to each particular environment. Hundreds of different types of wetlands exist, many with local names in different languages (e.g., flark, pan, playa, pocosin, yazoo, etc.). Each of these has a distinctive set of qualities or characteristics created by distinctive combinations of factors, such as climate, bedrock, geological history, and biota. Sometimes one is fortunate to find a monograph that highlights their distinctive features. Because such monographs are so numerous in so many languages, space constraints do not permit providing a list for every part of the world, let alone in other languages. The important point is that such monographs often exist, often written by a local expert. The challenge is to find them. I illustrate here the sort of article you are after by sharing some examples from my ecological region in English that I have found useful. It is up to you to find a similar set for your own ecological region and/or language. Consider it a treasure hunt of sorts. For peat bog ecology in eastern
Dansereau, Pierre, and Fernando Segadas-Vianna. 1952. Ecological study of the peat bogs of eastern
Odum, William E., and Carole C. McIvor. 1990. Mangroves. In Ecosystems of
Peace–Athabasca Delta Project Group. 1972. The Peace–Athabasca Delta Summary Report, 1972.
Richardson, Curtis J., ed. 1981. Pocosin wetlands: An integrated analysis of coastal plain freshwater bogs in North Carolina.
Smith, Loren M. 2003. Playas of the
van der Valk, Arnold G. 1989. Northern prairie wetlands. Ames:
Wilcox, Douglas A. 2012.
Aquatic plants provide a distinctive and instructive situation for all wetland ecologists. Aquatic plants provide an extreme case: they make up probably just 1 percent of the world’s flora. Most of the world’s 350,000 species of plants simply cannot tolerate continual flooding; even short periods of inundation can kill plants by eliminating the oxygen needed for root respiration. The best introduction to this unusual group of plants remains The Biology of Aquatic Vascular Plants (Sculthorpe 1985). It ranges across anatomy, morphology, growth, dispersal, and ecology, and this volume should be on the shelf of any ecologist who encounters wetlands. Hutchinson 1975, a volume on limnological botany, does not replace Sculthorpe 1976, but does add new examples and context. Moreover, it provides one hundred pages dealing with the distribution of macorphytes in lakes. (It may also amuse you to read a Yale professor complaining in 1975 (p. vii) about the “enormous increase in the price of books.”) For more on the historical foundations of aquatic botany, an 1886 German monograph has now been translated as Schenck 2003. Two refinements to these monographs should be noted. First, with regard to flood tolerance conferred by aerenchyma, good evidence now exists of mass flow through leaves and rhizomes (Dacey 1981). Second, with regard to causal factors for plant distributions, biological factors, such as competition and herbivory, may need greater emphasis (see Keddy 2010, cited under *General Guides and Introductions*). If we are trying to restore wetlands, it is important to know how and why aquatic plants are dispersed and assembled into ecological communities; one recent overview of wetland plant traits and consequences is Pierce 2015. Aquatic plants, then, although they constitute a small group, have much to teach us about wetland plants (and wetlands) as a whole. Indeed, although it may seem somewhat circular, one of the best indicators of a wetland is the presence of wetland plants. This is so central that it is used in both scientific and legal definitions. Even the US Army Corps of Engineers maintains a website with an official list of wetland plants (US Army Corps of Engineers 2015). One challenge in reading the older literature is the many changes in plant names that have occurred over the last century, particularly with recent advances in molecular systematics. Scirpus, or Schoenoplectus? Aster or Symphyotrichum? There is no easy solution, except use of online reference works with contemporary nomenclature. For
Dacey, John W. H. 1981. Pressurized ventilation in the yellow waterlily. Ecology 62.5: 1137–1147.
Hutchinson, G. Evelyn. 1975. A treatise on limnology. Vol. 3, Limnological botany.
Pierce, Gary J. 2015. Wetland mitigation: Planning hydrology, vegetation, and soils for constructed wetlands[http://wetlandtraining.com/wp-content/uploads/2015/07/Wetland-Mitigation-excerpts.pdf]*.
Schenck, Heinrich. 2003. The biology of aquatic plants. Translated by Donald H. Les.
Sculthorpe, Cyril D. 1985. The biology of aquatic vascular plants. Königstein, Germany: Koeltz Scientific. [ISBN: 9783874292573]
US Army Corps of Engineers. 2015. http://rsgisias.crrel.usace.army.mil/NWPL/
The conservation of wetlands requires the intelligent application of a few basic principles. The most important of these is maintaining the appropriate water levels, particularly the within-year and among-year variation in water levels (see *Flooding and Flood Pulses*). Dams and reservoirs upstream of wetlands reduce these flood pulses and cause declines in biodiversity and area of the wetlands. It is important to get the water right (Pierce 2015, cited under *Aquatic Plants*). The next challenge is to maintain water quality. We are in an era of growing eutrophication, driven by the application of nitrogen and phosphorous to farmland, and by inputs of human and animal excrement into watercourses. Hence, in many cases, the conservation challenge is to maintain nutrient levels as low as possible (see *Nutrients*). Once one has appropriate water levels and nutrient regimes, much of the work is completed. Of course, other causal factors affect wetland composition and services, and these will need to be addressed on a case-by-case basis (see *Other Causal Factors*). The next step is to ensure that the wetland is designated for conservation within defined boundaries. This core area must be surrounded by a carefully managed buffer zone and connected to other wetlands by corridors. The framework of core areas, buffer zones, and corridors is described in Noss and Cooperrider 1994. A brief overview with specific reference to wetlands is provided in Keddy 2010 (pp. 403–406, cited under *General Guides and Introductions*). A well-conserved wetland is therefore part of a protected network that is managed with reference to the key factors that control wetland composition and maintain wetland functions. Regular monitoring of selected indicators (McKenzie, et al. 1992) is then needed to ensure that the desired composition and the desired ecological services are maintained through time. When monitoring shows that composition is changing, or that functions are declining, one must identify the correct causal factor and take steps to remediate the problem. This process is sometimes called “adaptive management.” One of the fundamental causes of undesirable changes in area and composition of wetlands (indeed for natural ecosystems overall) is expanding human populations (Foreman 2014). **Biosphere Reserves** illustrates the general challenge of integrating protected areas with surrounding human populations.
Foreman, Dave. 2014. Man swarm: How overpopulation is killing the wild world. Albuquerque, NM: Rewilding Institute. [ISBN: 9780986383205]
Holling, C. S., ed. 1978. Adaptive environmental assessment and management.
McKenzie, Daniel H., D. Eric Hyatt, and V. Janet McDonald. 1992. Ecological indicators. 2 vols.
Noss, Reed F., and Allen Y. Cooperrider. 1994. Saving nature’s legacy. Washington, DC:
United Nations Educational, Scientific and Cultural Organization (UNESCO). Biosphere reserves[http://www.unesco.org/new/en/natural-sciences/environment/ecological-sciences/biosphere-reserves/]. Paris: UNESCO.