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.
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.
- General guides and introductions
- Flooding and flood pulses
- Nutrients and fertility
- Other causal factors
- Geography of wetlands
- Regional monographs
- Aquatic plants
Author’s version of contribution that is published online in Oxford Bibliographies in Environmental Science
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.
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. Richmond Hill, ON: Firefly. [ISBN: 9781554071111]
An illustrated guide to the ecology and conservation of the world’s wetlands. Lucid, comprehensive, beautifully illustrated, and affordable. A starting point for everyone who wishes to explore the topic of wetlands further.
Keddy, Paul A. 2010. Wetland ecology: Principles and conservation. 2d ed. Cambridge, UK: Cambridge Univ. Press. [ISBN: 9780521739672]
The causal factors that create wetlands. Focuses more on wetlands as a whole rather than breaking them into five types. Also an emphasis upon natural habitats and biodiversity conservation in a global context.
Mitsch, William J., and James G. Gosselink. 2015. Wetlands. 5th ed. Hoboken, NJ: Wiley. [ISBN: 9781118676820]
A popular book in the United States where there is a complicated regulatory framework for wetland management. Focuses more on energy flow and nutrient cycling and, as the table of contents denotes, traditional wetland management.
Wetlands. 1981–. [class:periodical]
An international journal covering wetland biology, ecology, hydrology, soils, biogeochemistry, management, laws and regulations. Published by Springer on behalf of the Society of Wetland Scientists. Since 2005 a sister publication has been published called Wetland Science and Practice.
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 Europe on the restoration of floodplain forests [http://www.geog.cam.ac.uk/research/projects/flobar2/reports/final/flobar2.pdf]*. Cambridge, UK: Department of Geography, Univ. of Cambridge. [class:report]
A well-illustrated and accessible report on the importance of flood pulses and their role in restoring rivers.
Middleton, Beth A., ed. 2002. Flood pulsing in wetlands: Restoring the natural hydrological balance. New York: Wiley. [ISBN: 9780471418078]
A classic reference work on the importance of flood pulses in wetlands and riparian ecosystems.
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.
A thought-provoking overview of just how much humans have altered wetlands and floodplains by the construction of dams. Over half of the world’s rivers (172 of 292) have dams. The map (Fig. 1) speaks for itself.
Reddy, K. Ramesh, and Ronald D. DeLaune. 2008. Biogeochemistry of wetlands: Science and applications. Boca Raton, FL: CRC. [ISBN: 9781566706780]
Changes in soil chemistry arise from a complex array of microbiological processes. These affect most other plants and animals found in wetlands as well as emitting methane to the atmosphere. An important reference work.
Welcomme, Robin L. 1979. Fisheries ecology of floodplain rivers. London: Longman. [ISBN: 9780582463103]
Wetlands greatly enhance production of fish in rivers. Seasonal flooding is a key factor in maintaining this productivity.
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]
A well-illustrated guide to the importance of natural water level fluctuations on wetlands in the Great Lakes.
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 Gulf of Mexico, Chesapeake Bay, and the Baltic Sea are well-known examples of this phenomenon. Other types of wetlands, such as peatlands and shorelines, may have very low levels of available nutrients. Distinctive and rare wetland species often occupy these nutrient deficient wetlands (Keddy 2010, cited under *General Guides and Introductions*): the rare biota of the New Jersey Pine barrens (Zampella, et al. 2006) and the Everglades (Davis and Ogden 1994) both are classic examples. Hence, it may be useful, while reading about wetlands and nutrients, to think of wetlands arrayed along a nutrient gradient. At one end, infertile wetlands have many rare and unusual species. In these cases, the challenge is to maintain low nutrient levels to protect the unusual biota. At the other extreme, fertile wetlands, the challenge may be to maintain existing elevated nutrient levels, particularly those associated with spring flood pulses (see *Flooding and Flood Pulses*) and wisely manage the sustainable harvest of wildlife. Since eutrophication is a now a global process (with nutrients being released from burning coal, eroding uplands, agriculture, and sewage), we may expect infertile wetlands, and their associated biota, to become increasingly scarce in the future. Dead zones, in contrast, may become more common. A further complication arises in reading on this topic: there is an entirely different perspective on nutrients in wetlands, one focused on the deliberate addition of nutrients to wetlands for waste-water treatment. Unfortunately much of this work has developed, it seems, in isolation from the scientific literature on the negative effects of nutrients on species composition. Caution is therefore necessary, but a good place to start is the series of papers in Kadlec 2009, while Kadlec and Wallace 2009 shows how “constructed wetlands” are designed and built to process wastewater.
Davis, Steven M., and John C. Ogden, eds. 1994. Everglades: The ecosystem and its restoration. Delray Beach, FL: St. Lucie. [ISBN: 9780963403025]
The Everglades are a classic case of a distinctive wetland ecosystem arising out of extremely low levels of phosphorous. Can they be protected from nutrient rich wastewater produced by sugar cane plantations? This book is a foundational text for understanding the huge flow of contemporary scientific publications and popular articles.
Kadlec, Robert H. 2009. The Houghton Lake wetland treatment project. Ecological Engineering 35.9: 1285–1286.
The introduction to five papers reporting the effects of lagoon-treated wastewater on a peatland in Michigan (a special issue of Ecological Engineering 35.9: 1285–1366.) Includes effects on water quality and soils, as well as, refreshingly, plants and breeding birds. Good to compare with Zampella, et al. 2006.
Kadlec, Robert H., and Scott D. Wallace. 2009. Treatment wetlands. 2d ed. Boca Raton, FL: CRC. [ISBN: 9781566705264]
Building wetlands specifically for wastewater treatment is a good way to avoid the harmful effects of nutrients on natural wetlands while it also increases the area of wetlands in a landscape. This volume is a good overview of the field.
Turner, R. Eugene, and Nancy N. Rabelais. 2003. Linking landscape and water quality in the Mississippi River Basin for 200 years. BioScience 53.6: 563–572.
The Gulf of Mexico contains a large dead zone near the mouth of the Mississippi River, the result of nutrients entering from agriculture far upstream. For a global perspective, see Donald Boesch, “Global Warming and Coastal Dead Zones,” National Wetlands Newsletter 30.4 (2008): 11–13, 21.
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 Evolution11.22: 494–497.
This overview of nutrient limitation in wet vegetation provides an introduction to both the data available and the implications of that data for wetlands.
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. Springfield, VA: National Technical Information Service. [ISBN: 9780870790065]
Although many refinements to this data have been made over the decades, the productivity table found here is still a classic reference point and puts all later work into context.
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.
A comprehensive study of the changes in species composition that occur along nutrient gradients, using fish, anurans, and stream vegetation. Figure 5 should be viewed by everyone interested in the effects of eutrophication on wetlands.
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.
Roads and houses can produce negative effects on plants in wetlands. For similar effects on amphibians, see Jeff E. Houlahan and C. Scott Findlay, “The effects of adjacent land use on wetland amphibian species richness and community composition,” Canadian Journal of Fisheries and Aquatic Sciences 60 (2003):1078–1094.
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]
Roads have many negative consequences for wetlands (and for forests). Effects include erosion, eutrophication (see *Nutrients and Fertility*), road kill, and improved access of illegal hunting. Protecting areas from further road construction may be one of the most important tasks for conservation.
Silliman, Brian R., Edwin D. Grosholz, and Mark D. Bertness, eds. 2009. Human impacts on salt marshes: A global perspective. Berkeley: Univ. of California Press. [ISBN: 9780520258921]
Predators removed from wetlands ranges from crabs to alligators. Removing predators is likely to have negative consequences for plants and vegetation. A search for “top-down control” will reveal examples from many other vegetation types.
Tiner, Ralph W. 2013. Tidal wetlands primer: An introduction to their ecology, natural history, status and conservation. Amherst: Univ. of Massachusetts Press. [ISBN: 9781625340221]
An excellent example of scientific principles augmented with much natural history. The focus is on North America, but with selected examples from elsewhere.
Warner, Barry G. 2004. Geology of Canadian Wetlands. Geoscience Canada 31.2: 57–68.
Although the focus is on Canada, this article is a useful contribution on the geological foundations for wetlands overall. There is a fine map of glaciolacustrine and glaciomarine deposits and two cross sections (a peatland, a marsh.). The author considers geological time, and humans, as two causal factors in wetlands.
Weiher, Evan, and Paul A. Keddy, eds. 1999. Ecological assembly rules: Perspectives, advances, retreats. Cambridge, UK: Cambridge Univ. Press. [ISBN: 9780521652353]
Wetlands, like all ecosystems, are assembled from species pools when a set of ecological filters subtract those species unable to tolerate key environmental factors. The general challenge in any situation is to enumerate the pool and identify the key factors.
White, Peter S. 1994. Synthesis: Vegetation pattern and process in the Everglades ecosystem. In Everglades: The ecosystem and its restoration. Edited by Steve Davis and John C. Ogden, 445–460. Delray Beach, FL: St. Lucie. [ISBN: 9780963403025]
A classic paper on the role of fire in the Everglades but also much more: an example of how to explain and illustrate the role of fire in many other ecosystem types. We need more such papers.
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 Europe, and one could easily be led to believe that northwestern Europe has far more important wetlands than all of Africa combined! To remedy that impression served as the incentive for the production of The World’s Largest Wetlands (Fraser and Keddy 2005); this book documents that vast areas of wetland exist in lesser known areas, such as the Congo River Basin and the Magellanic Moorlands. Of course, many wetlands may be smaller in area but extremely important for biodiversity, such as the wetlands of Madagascar, Southeast Asia, and even the tops of wet tepuis in South America. These areas remain to be addressed on a case-by-case basis. Wetlands in Danger: A World Conservation Atlas (Dugan1993)provides both some basic wetland ecology as well as a comprehensive geographic survey. On a smaller scale, there is little choice but to seek out source documents for particular wetland types or for particular regions. The best summary for Africa, for example, is still Hughes and Hughes 1992, and it has a section on Madagascar. Many fine regional monographs contain a wealth of information. They are often overlooked because they are too numerous to mention in short overviews such as this article. See the section *Regional Monographs* for more on finding these monographs. For a classification scheme that includes all the major types of world wetlands in a single figure, see Gopal, et al. 1990.
Dugan, Patrick, ed. 1993. Wetlands in danger: A world conservation atlas. New York: Oxford Univ. Press. [ISBN: 9780195209426]
A beautifully illustrated guide to the flora, fauna, and human inhabitants of wetlands. An online map of world wetlands was published at the same time. The Guide to Wetlands (Dugan 2005, cited under *General Guides and Introductions*) captures much of the same information but in a more compact style.
Fraser, Lauchlan H., and Paul A. Keddy, eds. 2005. The world’s largest wetlands: Ecology and conservation. Cambridge, UK: Cambridge Univ. Press. [ISBN: 9780521834049]
A systematic overview of the world’s eleven largest wetlands. A decade ago it was impossible to find a reliable list of the world’s largest wetlands! Now, after a decade of work, we can rank them by size (subject, of course to certain assumptions).
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).
A map available online that was produced alongside Wetlands in Danger (Dugan 1993). It shows the global distribution of wetlands in eight categories.
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]
How does one combine all kinds of wetlands into one figure? Here (p. 14) are two classic attempts. For a third, see Dale H. Vitt, “An Overview of Factors That Influence the Development of Canadian Peatlands,” Memoirs of the Entomological Society of Canada 169 (1994): 7–20. Compare and contrast these perspectives.
Hughes, Ralph H., and Jane S. Hughes. 1992. A directory of African wetlands. Nairobi, Kenya: United Nations Environment Programme. [ISBN: 9782880329495]
A comprehensive, if technical, guide to the wetlands of the enormous continent of Africa. An additional chapter by G. Bernacsek deals with Madagascar.
Ramsar. 2015. Gland, Switzerland: Ramsar Convention Secretariat.
An online map. Ramsar is not an acronym, but the name of a city in Iran where, in 1971, a consortium of nations came together to agree on the conservation and wise use of all wetlands through local and national actions and international cooperation. The Ramsar website is also available online.
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 North America, I still consult Dansereau and Segadas Vianna 1952. The distinctive shrub-dominated freshwater wetlands along the Gulf Coast of North America are called pocosins, and Richardson 1981 provides an introduction to their distribution and ecology. Grasslands can have extensive areas of wetland, particular in areas where receding glaciers have left large numbers of depressions, in the north these are called northern prairie wetlands (van der Valk 1989) while in more southern and arid areas they are called playas (Smith 2003). Presumably these have their equivalent on other continents, in which case the same principles should apply, modified as necessary for local factors such as distinctive biota. Temporary ponds arise in many landscapes, at times filled by rainfall and at other times filled by surface flow. The former pools are known as vernal pools, since they are usually filled by spring rain or spring snow melt. Vernal pools can arise in grasslands, shrublands, and even forests (Calhoun and DeMaynadier 2008). They may be of great importance to amphibians since the dry periods prevent fish populations from occupying the pools and feeding on young amphibians. Mangroves are another distinctive wetland type, and a good introduction is found in Odum and McIvor 1990. The Peace River study (Peace–Athabasca Delta Project Group 1972) is a classic case study showing the negative effects produced on wetlands by dams that obstruct flood pulses (see more on this in *Flooding and Flood Pulses*). Most lakes, which include shoreline wetlands, may have their own monographs; Wilcox 2012 provides an example for the Great Lakes of North America. These examples omit mention of vast areas of the world. Readers should endeavor to find monographs for their ecological region.
Calhoun, Aram J. K., and Phillip G. DeMaynadier, eds. 2008. Science and conservation of vernal pools in northeastern North America: Ecology and conservation of seasonal wetlands in northeastern North America. Boca Raton, FL: CRC. [ISBN: 9780849336751]
In northeastern North America, melting snow fills depressions directly as well as producing flood pulses, which, in turn, fill other low-lying depressions. Areas with seasonal rainfall will have similar types of pools. They are often small, and therefore easily overlooked, especially if the survey is done during the dry season.
Dansereau, Pierre, and Fernando Segadas-Vianna. 1952. Ecological study of the peat bogs of eastern North America, I. Structure and evolution of vegetation. Canadian Journal of Botany 30:490–520.
A foundational study of peatlands and the process of succession. A follow-up can be found in P. H. Glaser, “Raised Bogs in Eastern North America: Regional Controls for Species Richness and Floristic Assemblages,” Journal of Ecology 80 (1992): 535–554.
Odum, William E., and Carole C. McIvor. 1990. Mangroves. In Ecosystems of Florida. Edited by Ronald L. Myers and John J. Ewel, 517–548. Orlando: Univ. of Central Florida Press. [ISBN: 9780813010229]
Wooded wetlands occur in saline environments only under warm conditions. Florida sits near the northern edge of this ecosystem type. Excellent line drawings.
Peace–Athabasca Delta Project Group. 1972. The Peace–Athabasca Delta Summary Report, 1972. Ottawa, ON: Department of the Environment. [class:report]
Yes, dams have negative effects on wetlands, and we have known about this for decades—at least since 1972. Check to see how many people have read, or cited, this important report. In 2015 the British Columbia government announced approval of another huge dam on the Peace River.
Richardson, Curtis J., ed. 1981. Pocosin wetlands: An integrated analysis of coastal plain freshwater bogs in North Carolina. Stroudsburg, PA: Hutchinson Ross. [ISBN: 9780879334185]
When one hears about evergreen shrublands, one is inclined to think about semi-arid Mediterranean landscapes, including chaparral. But some wetlands are dominated by evergreen shrubs and trees such as Pinus serotina. Less than a third remain in a natural state.
Smith, Loren M. 2003. Playas of the Great Plains. Austin: Univ. of Texas Press. [ISBN: 9780292705340]
Wetlands can occur even in arid landscapes and, hence, can have strong environmental gradients from wet to dry conditions.
van der Valk, Arnold G. 1989. Northern prairie wetlands. Ames: Iowa State Univ. Press. [ISBN: 9780813800370]
There are vast areas of wetland in prairie depressions, and these wetlands have many plants that survive dry periods as buried seeds.
Wilcox, Douglas A. 2012. Great Lakes coastal marshes. In Wetland habitats of North America. Edited by Darold P. Batzer and Andrew H. Baldwin, 173–188. Berkeley: Univ. of California Press. [ISBN: 9780520271647]
Large lakes often have extensive wetlands with many plant species. The diversity is a consequence of factors that include long-term water level fluctuations, wave exposure gradients, and variation in substrate types.
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 North America, this would be Flora of North America Editorial Committee 1993–2014.
Dacey, John W. H. 1981. Pressurized ventilation in the yellow waterlily. Ecology 62.5: 1137–1147.
The movement of air through aerenchyma is driven by more than simple diffusion. In Nuphar, for example, air moves from young leaves into the rhizome and out through old leaves. More species with such bulk flow are being discovered, including Phragmites and Carex.
Flora of North America Editorial Committee. 1993–2014. Flora of North America North of Mexico. 18 vols. New York: Oxford Univ. Press. [ISBN: 9780195057133]
For regions outside North America, you will need to find a similar resource that covers your ecological region. (For example, both Scirpus and Schoenoplectus occur in North America, but the latter make up seventeen species of “leafless” reeds.) Available online
Hutchinson, G. Evelyn. 1975. A treatise on limnology. Vol. 3, Limnological botany. New York: John Wiley. [ISBN: 9780471425748]
One of three volumes. Chapters 27–29 (or the first three chapters in this volume) provide a good overview of the nature and diversity of aquatic plants, at least from the perspective of limnology.
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]*. Glenwood, NM: Wetland Training Institute. [class:report]
Chapter 10, “Adaptations of Plants,” provides an overview of plant traits that are relevant for wetland restoration. Such information leads naturally to chapter 11, “Vegetation Planning and Planting”—assuming you have got the hydrology right (chapters 3–7).
Schenck, Heinrich. 2003. The biology of aquatic plants. Translated by Donald H. Les. Ruggell, Liechtenstein: A. R. G. Gantner Verlag. [ISBN: 9783906166117]
English translation of Heinrich Schenck, Die Biologie der Wassergewächse (Bonn, Germany: Cohen, 1886). German botanists have made significant contributions to knowledge of wetland plants. This is Schenck’s monograph in English; includes an introduction and appendix. For a modern evolutionary framework, see Donald H. Les, Denise K. Garvin, and Charles F. Wimpee, “Molecular Evolutionary History of Ancient Aquatic Angiosperms,” Proceedings of the National Academy of Sciences of the United States of America 88 (1991): 10119–10123.
Sculthorpe, Cyril D. 1985. The biology of aquatic vascular plants. Königstein, Germany: Koeltz Scientific. [ISBN: 9783874292573]
Originally published in 1967. The 1985 reprint of this book keeps it available to younger scholars. Includes fifty-eight pages of references, which remind us of the broad interests in this group of plants.
US Army Corps of Engineers. 2015. http://rsgisias.crrel.usace.army.mil/NWPL/
The official USACE portal for wetland plants in the United States, used for wetland delineation. Of the 8,057 listed species, only 2,230 are obligate to wetlands. These range from Acaena exigua (a bog plant from Hawaii, possibly extinct) to Zostera marina (common in the sublittoral zone of salt marshes).
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]
Human populations are negatively affecting all wild places. Regarding wetlands, human population growth drives the drainage of wetlands or construction of dams, while human excrement, and excrement from domestic animals, increases both nitrogen and phosphorus in wetlands. It seems that we continue to avoid the message in Paul Ehrlich, The Population Bomb (New York: Ballantine, 1968).
Holling, C. S., ed. 1978. Adaptive environmental assessment and management. New York: John Wiley and Sons. [ISBN: 9780471996323]
In any large protected area system, or in any large ecological restoration project, it is necessary to monitor a key set of indicators at regular intervals. When a problem is identified, research can identify the cause of the change and remedial action can be taken.
McKenzie, Daniel H., D. Eric Hyatt, and V. Janet McDonald. 1992. Ecological indicators. 2 vols. London: Elsevier. [ISBN: 9781851667116]
Indicators provide a means to continually monitor natural areas to ensure that composition and function fall within desired limits. The challenge is to find a small number of indicators that are relatively sensitive to change and relatively inexpensive to measure.
Noss, Reed F., and Allen Y. Cooperrider. 1994. Saving nature’s legacy. Washington, DC: Island Press. [ISBN: 9781559632478]
A useful guide to setting up and managing systems of protected areas. Of course, these principles apply not just to wetlands, but also to all types of ecosystems.
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.
Biosphere reserves illustrate the many challenges in protecting core areas in landscapes with human populations. Buffer areas are critical. Biosphere reserves can protect any kind of natural area, but all the general principles apply to wetlands. An interactive global map of biosphere reserves is available online.