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Wetland Habitats of North America
Ecology and Conservation Concerns
By Darold P. Batzer, Andrew H. Baldwin
UNIVERSITY OF CALIFORNIA PRESSCopyright © 2012 The Regents of the University of California
All rights reserved.
Wetland Habitats of North America
ANDREW H. BALDWIN and DAROLD P. BATZER
The goal of this book is to summarize recent literature and current perspectives on the ecology of wetland habitats of the North American continent and primary concerns regarding their conservation. All wetlands, by definition, share certain characteristics, notably the seasonal or permanent saturation or inundation of soils. This waterlogging of soils and the presence of standing water impose ecological constraints on organisms and a geomorphological environment very different from those of nonwetland ecosystems. Beyond the broad umbrella of inundation and soil saturation, however, wetlands differ dramatically between and within climatic and geomorphic zones due to myriad interacting physical, chemical, and biological processes. Environmental studies or restoration projects conducted in one type of wetland in one part of North America may have outcomes that are broadly or narrowly applicable to wetlands elsewhere. This creates a dilemma for wetland scientists, students, managers, and policymakers, who must identify research projects, implement management or restoration, and develop environmental policy based on limited information.
In this book, we chose a habitat and geographical approach to highlight differences among the most important ecological characteristics and threats to the major types of North American wetlands. This book is a companion volume to Ecology of Freshwater and Estuarine Wetlands (Batzer and Sharitz 2006), which addresses ecosystem processes and structural attributes of wetlands in general, across many types of wetlands. Other recent books that provide general information include Wetlands (Mitsch and Gosselink 2007), Wetland Ecology: Principles and Conservation (Keddy 2010), and The Biology of Freshwater Wetlands (van der Valk 2006). The earlier edition of Wetlands (Mitsch and Gosselink 2000) included seven chapters on individual coastal and inland wetland habitats, which were updated and expanded in a separate volume, Wetland Ecosystems (Mitsch, Gosselink, et al. 2009). Individual habitats were also treated in Coastal Wetlands: An Integrated Ecosystem Approach (Perillo, Wolanski, et al. 2009) and in Ecology of Tidal Freshwater Forested Wetlands of the Southeastern United States (Conner, Doyle, et al. 2007). Our book differs from these works in that it takes a systematic, geographic approach to capture a similar set of ecosystem characteristics and conservation concerns for all major wetland habitat types within different regions of North America. In this, our approach is similar to the "Community Profiles" series published by the U.S. Fish and Wildlife Service.
The wetlands of North America are wonderfully diverse. They range from the tropics to the Arctic and are widespread in Canada, the United States, Mexico, and Central America. The chapters in this book are written by experts with deep knowledge and experience in most of the major types of wetlands in different regions of North America (Fig. 1.1). Our coverage is complete, with one important exception: Arctic coastal wetlands. Robert Jefferies, a leader in this area, had agreed to author a chapter on this topic, but, to our great sadness, he passed away before he could do so. We refer the reader to a chapter he coauthored in another book (Martini, Jefferies, et al. 2009). To provide a framework, we asked authors to address several main aspects of the wetlands in their region:
Geology, hydrology, and biogeochemistry
Key ecological processes and controls
Conservation concerns or threats
To reduce repetition between chapters, we specifically asked authors not to include general information on wetlands, but to focus only on studies or information collected in their region. Chapters on coastal wetlands are included in part I of the book. They are grouped separately from nontidal wetlands because of the overriding influence of tidal hydrology on their structure and functioning. Inland wetlands are grouped in part II. There are many more habitat types of inland wetlands than coastal wetlands due to great variation in relative inputs of precipitation, surface water, and groundwater, as well as their greater surface area and richness of plant and animal taxa. Wetlands of the temperate zone have received more scientific attention than those of the tropics and Arctic, and hence we include more chapters on those wetlands, which primarily occur in the USA. The vast areas of wetlands in northern North America and biologically rich tropical wetlands are nonetheless important and ecologically interesting ecosystems deserving of greater study.
In this introductory chapter, we summarize important differences and similarities between wetland habitats in different regions of North America, separating discussion of coastal and inland wetlands and focusing on key ecological processes and conservation concerns. To assess the generality of processes and concerns, as well as their regional importance, we list their occurrence in each chapter in Tables 1.1 (coastal wetlands) and 1.2 (inland wetlands). This approach does not mean that some processes are not important in regions elsewhere, but may reflect a lack of literature on the topic. Differences among the relative importance of dominant or unique plant or animal taxa are also discussed here.
Key Ecological Processes and Controls
The development of coastal wetlands is fundamentally a result of geomorphology and climate, which together create hydrologic characteristics that exert an overriding influence on wetland ecosystem structure and function. All authors of coastal chapters in this book explicitly or implicitly identify these factors as fundamental controls on wetland ecosystems (Table 1.1a). The hydrology of coastal wetlands differs considerably from that of inland wetlands in being dominated by tides and, to a lesser extent, surface water inflows (which control salinity, nutrient supply, and sediment deposition). Groundwater and precipitation are rarely mentioned in the coastal chapters. Because coastal wetlands are located in the depositional zone of rivers in estuaries, it is not surprising that sedimentation and salinity are also uniformly recognized as important. However, several biotic processes are mentioned in all of the coastal chapters, including productivity, peat accumulation, predation, outwelling, and vegetation zonation, which are discussed in more detail in this section.
High primary productivity, coupled with slow decomposition rates due to anaerobic conditions, favors the accumulation of peat in soils, providing a mechanism important for the maintenance of wetland surface elevation relative to sea level. Primary productivity and peat accumulation are highlighted in all of the coastal chapters, and decomposition and anaerobiosis are featured in a majority of them (Table 1.1a). Elevation and salinity gradients within individual wetlands and across estuaries lead to horizontal zonation of emergent and submergent plants, another ubiquitous topic in the coastal chapters. Stress-tolerant species occur where salinity or depth and duration of inundation are greater (and hypoxia more pronounced), such as the lower elevations of salt marshes, but studies have demonstrated that these species are often poor competitors in fresher or higher-elevation sites, such as the higher elevations of tidal freshwater wetlands. Under stressful conditions, one species may facilitate the growth of others by, for example, aerating the soil or shading to reduce salinity. Competition and facilitation are recognized as important processes in about half the coastal chapters. Some salt-marsh soils become acidic when drained (acid-sulfate soils, mentioned in two coastal chapters), but coastal wetland soils are generally pH-circumneutral.
High rates of primary productivity and, in many wetlands, structural, spatial, and species diversity of plants, result in high rates of secondary production. Coastal wetlands support estuarine food webs not only by providing in situ habitat and food sources, but also by exporting or "outwelling" organic matter to estuaries, forming an important foundation of coastal food webs that is emphasized in all of the coastal chapters. Coastal wetlands often serve as nurseries for larval and juvenile stages of aquatic organisms, providing refuge from predation, an ecological process mentioned in all of the coastal chapters. Plantanimal interactions are also a major process in most coastal wetlands; herbivory is most frequently mentioned, both for its role in food-web support and as a disturbance process affecting plant community dynamics. Seed-related processes, including dispersal, predation, seed banks, and seedling recruitment, mentioned in a majority of the coastal chapters, are often important to the regeneration of plant communities following disturbance and are critical to diversity maintenance in wetlands containing a large proportion of annual species, such as tidal freshwater marshes. The particular importance of storms and hurricanes is recognized for those regions experiencing regular major hurricanes, i.e., the southeastern U.S. coasts and parts of the Neotropics. Some plants and animals function as ecosystem engineers, altering the geomorphology and hydrology of coastal wetlands; examples include beavers, crabs, and dominant clonal plants. Thus, the vegetation of coastal wetlands is dynamic, in large part due to continued natural disturbance, seed and seedling-recruitment processes, and competitive interactions.
It is not a surprise that sea-level rise is identified as a major threat to coastal wetlands in all parts of North America (Table 1.1b). Increases in the rate of eustatic sea-level rise, coupled with land subsidence in many regions, will challenge the capacity of coastal wetlands to keep pace with rising sea levels. Eutrophication due to inputs of nitrogen or phosphorus from point and nonpoint sources is also identified as important in all regions except the Pacific coast. Excess nutrients are important in shifting plant species composition and have been implicated as a potential factor in marsh die-back.
Many wetlands have been hydrologically altered since European colonization, but despite environmental statutes regulating activities in wetlands, hydrologic alterations continue to threaten coastal wetlands in many regions of North America. Notably, these are not mentioned as being of overriding importance for south Atlantic wetlands, which are relatively unimpacted physically relative to other parts of the North American coast (except for widespread historic conversion of tidal freshwater wetlands to rice fields), or for Pacific coast wetlands, where the vast majority of coastal wetlands historically were lost due to diking, but which are currently closely protected and restoration efforts are ongoing (Table 1.1b). Neotropical coastal wetlands, in contrast, were primarily used for sustainable harvest of vegetation and animal products until recent decades, when many mangrove forests have been converted to aquaculture ponds or tourist resorts.
Nonnative or invasive species of plants or animals are also identified as a major conservation concern in a majority of coastal wetland chapters (Table 1.1b). Exceptions are the south Atlantic coastal wetlands and the Neotropics. South Atlantic wetlands may not face the invasive species problems of other regions due to lower ongoing physical alteration of coastal wetlands, perhaps due to lower population densities. In the Neotropics, temperate-zone plants and animals may not be able to tolerate elevated temperatures or to compete with tropical plants, reducing their threat to tropical ecosystems.
In addition to causing increases in eustatic sea levels, changes in precipitation and temperature may impact wetlands in several ways and are mentioned in a majority of the coastal chapters. Lower precipitation may result in salinity increases in some regions where flow of freshwater to the coast is already insufficient or decreasing due to human alteration of hydrology, for example in the Pacific coast and south Atlantic watersheds. Similarly, climate change may cause widespread drought, a likely factor in die-back of coastal wetlands. Higher temperatures will differentially affect the growth of plant and animal species, as well as speed up processes such as decomposition that are fundamental to wetland ecosystem function. Increases in atmospheric CO2 (mentioned in only two chapters but likely to be important in all wetlands) are not only a cause of climate change, but also will alter plant community dynamics and potentially other ecosystem processes. Increasing human population density (identified in three coastal chapters) continues to threaten coastal wetlands via watershed development, land-use changes, harvesting of plant and animal resources, and further hydrologic alteration.
These and other conservation concerns are related and linked in complex ways that require novel research approaches to untangle. Manipulative experiments that examine multiple processes together can provide insights into how factors such as CO2, nutrients, salinity, and temperature interact. Such experiments, coupled with observational studies and system modeling, can help us progress toward greater understanding and better prediction of how human activities are affecting coastal wetlands. However, the chapters in this book highlight that coastal wetland research must be multidisciplinary so that interactions between hydrology, climate, geomorphology, biogeochemistry, and organisms can be explored.
Key Ecological Processes and Controls
As for coastal systems, hydrologic variation is identified as an important control on ecology by every author addressing inland wetlands (Table 1.2a). Given that inland wetlands can be fed by various sources of water (precipitation, groundwater, riverine flows), and that water budgets can vary greatly—either spatially among and within wetlands, or temporally among and within seasons, years, and decades—the importance given to hydrology is to be expected. Geomorphic and climatic variation are also identified as being important controls by most authors, primarily because these are the factors that most influence the hydrology of inland wetlands. The chemical nature of the water, including nutrient levels (phosphorus and, to a lesser extent, nitrogen), pH, oxygen level, and salinity (especially in western wetlands), is also considered important. It is clear from Table 1.2a that the abiotic template dictated by the physico-chemical conditions of water is considered the major control on the ecology of inland wetlands.
However, biotic interactions in inland wetlands are also considered important influences by most authors (Table 1.2a). Bottom-up controls are most frequently discussed. Nutrient limitation on plants from P and N, as well as competition for light and space, are common themes. Mutualism, especially in relation to nitrogen fixation by plants/microbes, is discussed in five chapters. Plants as food (herbivory or detritivory) or as habitat are considered crucial bottom-up controls on wetland animals by most authors. Top-down control from predation, either on other animals or on plants, is identified as an important control by about half of the authors of inland wetland chapters. Control of wetlands by keystone animal species is another important top-down impact, with beavers being by far the most influential animal.
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Table of Contents1. WETLAND HABITATS OF NORTH AMERICA: AN INTRODUCTION
Andrew H. Baldwin and Darold P. Batzer
I. COASTAL WETLANDS
2. NORTH ATLANTIC COASTAL TIDAL WETLANDS
Cathleen Wigand and Charles T. Roman
3. COASTAL WETLANDS OF CHESAPEAKE BAY
Andrew H. Baldwin, Patrick J. Kangas, J. Patrick Megonigal, Matthew C. Perry, and Dennis F. Whigham
4. SOUTH ATLANTIC TIDAL WETLANDS
Steven C. Pennings, Merryl Alber, Clark R. Alexander, Melissa Booth, Adrian Burd, Wei-Jun Cai, Christopher Craft, Chester B. DePratter, Daniela Di Iorio, Chuck Hopkinson, Samantha B. Joye, Christof D. Meile, Willard S. Moore, Brian Silliman, Victor Thompson, and John P. Wares
5. MISSISSIPPI RIVER DELTA WETLANDS
Jenneke M. Visser, John W. Day, Jr., Loretta L. Battaglia, Gary P. Shaffer, and Mark W. Hester
6. WETLANDS OF THE NORTHERN GULF COAST
Loretta L. Battaglia, Mark S. Woodrey, Mark S. Peterson, Kevin S. Dillon, and Jenneke M. Visser
7. NEOTROPICAL COASTAL WETLANDS
Karen L. McKee
8. PACIFIC COAST TIDAL WETLANDS
John C. Callaway, Amy B. Borde, Heida L. Diefenderfer, V. Thomas Parker, John M. Rybczyk, and Ron M. Thom
II. INLAND WETLANDS
9. NORTHERN PEATLANDS
Line Rochefort, Maria Strack, Monique Poulin, Jonathan S. Price, Martha Graf, André Desrochers, Claude Lavoie, and Line Lapointe
10. NORTHEASTERN SEASONAL WOODLAND POOLS
Aram J. K. Calhoun, Megan K. Gahl, and Robert F. Baldwin
11. NORTHERN RED MAPLE AND BLACK ASH SWAMPS
12. BEAVER WETLANDS
Carol A. Johnston
13. GREAT LAKES COASTAL MARSHES
Douglas A. Wilcox
14. POCOSINS: EVERGREEN SHRUB BOGS OF THE SOUTHEAST
15. SOUTHEASTERN DEPRESSIONAL WETLANDS
L. Katherine Kirkman, Lora L. Smith, and Stephen W. Golladay
16. SOUTHEASTERN SWAMP COMPLEXES
Darold Batzer, Frank Day, and Steve Golladay
17. THE FLORIDA EVERGLADES
Evelyn E. Gaiser, Joel C. Trexler and Paul R. Wetzel
18. FLOODPLAIN WETLANDS OF THE SOUTHEASTERN COASTAL PLAIN
Sammy L. King, Loretta L. Battaglia, Cliff R. Hupp, Richard F. Keim, and B. Graeme Lockaby
19. TROPICAL FRESHWATER SWAMPS AND MARSHES
Patricia Moreno-Casasola, Dulce Infante Mata, and Hugo López Rosas
20. NORTHERN GREAT PLAINS WETLANDS
21. HIGH PLAINS PLAYAS
Loren M. Smith, David A. Haukos, and Scott T. McMurry
22. WESTERN MOUNTAIN WETLANDS
David J. Cooper, Rodney A. Chimner, and David M. Merritt
23. DESERT SPRING WETLANDS OF THE GREAT BASIN
Mary Jane Keleher and Don Sada
24. RIPARIAN FLOODPLAIN WETLANDS OF THE ARID AND SEMIARID SOUTHWEST
Juliet C. Stromberg, Douglas C. Andersen, and Michael L. Scott
25. WETLANDS OF THE CENTRAL VALLEY OF CALIFORNIA AND KLAMATH BASIN
Joseph P. Fleskes
26. FRESHWATER ARCTIC TUNDRA WETLANDS