Centuries of Human-Driven Change in Salt Marsh Ecosystems

TL;DR: It is concluded that the best way to protect salt marshes and the services they provide is through the integrated approach of ecosystem-based management.

Abstract: Salt marshes are among the most abundant, fertile, and accessible coastal habitats on earth, and they provide more ecosystem services to coastal populations than any other environment. Since the Middle Ages, humans have manipulated salt marshes at a grand scale, altering species composition, distribution, and ecosystem function. Here, we review historic and contemporary human activities in marsh ecosystems—exploitation of plant products; conversion to farmland, salt works, and urban land; introduction of non-native species; alteration of coastal hydrology; and metal and nutrient pollution. Unexpectedly, diverse types of impacts can have a similar consequence, turning salt marsh food webs upside down, dramatically increasing top down control. Of the various impacts, invasive species, runaway consumer effects, and sea level rise represent the greatest threats to salt marsh ecosystems. We conclude that the best way to protect salt marshes and the services they provide is through the integrated approach of ecosystem-based management.

Summary

INTRODUCTION

• Humans have influenced all land, marine, and aquatic ecosystems on earth (Vitousek et al. 1997).
• More than 40% of the world’s population resides on the world’s coasts, which account for just 4% of the land surface (UNEP 2006).
• This hyperconcentration at the land-sea interface strains coastal ecological services, particularly in coastal wetlands, which provide more ecological services than any other coastal environment (UNEP 2006).
• Not surprisingly, salt marshes have experienced intense and varied human impacts that range from reclamation, waste disposal, and livestock grazing to less obvious contemporary impacts, such as restoration efforts that are again changing the face of marshes, reflecting a new appreciation for natural ecological services (Silliman et al. 2008).
• No longer can these systems be viewed as swampy wastelands, used to buffer and ameliorate human impacts along the coast.

Vulnerability of Salt Marshes to Human Impacts and Manipulation

• Salt marshes have long attracted human settlement; documented use of salt marshes for fishing and livestock grazing date to the Neolithic in the North and Wadden Seas (Knottnerus 2005, Meier 2004).
• To build land and protect the coast, people have exploited the connection between salt marsh vegetation, water flow, and sediment accretion by using salt marsh plants, many of which are excellent ecosystem engineers (sensu Jones et al. 1997), to modify water flow, trap sediments, and accrete the marsh foundation.
• People have often employed the most effective engineering plants for this purpose, whether native or non-native.
• F or p er so na l u se o nl y. animal fodder, bedding, thatch, and commercial fiber products.
• Salt marshes now face new regional and global challenges that correspond to the increasing scale of human impacts.

Ecosystem Services of Salt Marshes

• Ecosystem services are the benefits that humans derive from ecological systems and are generated directly from the processes that sustain ecosystems, or ecosystem functions, such as primary and secondary productivity, decomposition, and nutrient transformations (UNEP 2006).
• Left uncontrolled, excess nitrogen causes toxic algal blooms and marine dead zones.
• Salt marshes provide essential refuge habitat for young fish and crustaceans, provisioning coastal fisheries (Boesch & Turner 1984) that account for 90% of the world’s fish catch (UNEP 2006).
• The adjustment was done with the U.S. Department of Labor Inflation Calculator, which uses the Consumer Price Index to correct values through time.
• Reclamation of salt marsh for upland uses provides a vital service to humans in added land, but the cost of reclamation is extraordinary if one accounts for the lost ecosystem services of natural marsh.

IMPACTS

• Large efforts have been made to evaluate and remediate human impacts on marshes.
• The authors evaluate and summarize multiple human threats to marshes in a historical context.
• The interpretation of the term threat is, of course, subjective.
• What has in earlier eras been viewed as the improvement of wasteland (land reclamation) has later been characterized as habitat loss (land conversion).
• The authors discuss human actions in salt marshes and their associated impacts, and they contextualize these impacts in terms of their effect on ecosystem services.

Resource Exploitation and Extraction

• The most common use of salt marshes around the world has been as pasturelands for livestock (e.g., cattle, sheep, goats, and horses) (Knottnerus 2005), a practice that has not notably impacted ecosystem services, but that has in some cases made observable changes to species composition and animal populations.
• This practice is still common in China, Chile, and Europe, although it has become rare in Canada and the United States (Hatvany 2003, Sebold 1992).
• Beyond use as fodder, salt marsh plant products have been used as animal bedding, thatch (Phragmites australis, still used in England; Kiviat & Hamilton 2001), rope (S. patens; Seasholes 2003, Sebold 1992) (Figure 1i ), packing for pottery, metal and icehouse insulation (S. patens), and a nearly weedless mulch (S. patens, wrack, or Ulva; Sebold 1992).
• The continued productivity of grazed and harvested salt marsh vegetation demonstrates the resilience and high productivity of salt marshes.
• Horse grazing can affect the use of marshes by birds and crabs, reducing the number of ground nesting birds, but positively affecting the abundance of bird and crab consumers by trimming away the grass canopy and exposing fish and invertebrate prey (Brewer et al. 1998).

Land Conversion

• The conversion of marshland to upland was historically undertaken for agricultural purposes, but in the past century it has mainly occurred for urban development.
• Another way to evaluate the impact of diking is by the area of reclaimed lands, because most were converted from marsh.
• A precious commodity throughout history, salt has long been extracted in massive quantities from salt marshes, and production and extraction processes have converted large areas of marshland.
• Www.annualreviews.org Human-Driven Change in Salt Marshes 125 A nn u. R ev .
• Invaders that alter the physical structure of the marsh environment, such as S. alterniflora, P. australis, and M. senhousia, have predictably strong effects on infaunal organisms (Brusati & Grosholz 2006, Crooks & Khim 1999, Neira et al. 2006, Zedler & Kercher 2004).

Hydrologic Alteration

• Salt marsh ecology is tied to hydrology in many ways.
• Tidal inundation and flushing govern a fluctuating salinity and oxygen regime that is a critical determinant of plant productivity (Mendelssohn & Morris 1999) and microbial production of sulfide and methane (Bartlett et al. 1987, Magenheimer et al. 1996).
• Beginning in 1904, intensive ditching was employed for mosquito control in the eastern United States (Smith 1904).
• Ditches have caused shifts in salt marsh vegetation that favor high marsh species, which are better belowground competitors when anoxic and saline stresses are reduced (Bertness & Ellison 1987).
• Crain et al. (2008) investigated the spatial extent of tidal restrictions in Maine salt marshes and found that in 3193 ha of tidal marsh, 57 restrictions affected 903 ha (28%) of upriver tidal marsh.

Pollution and Eutrophication

• Salt marshes are depositional environments for suspended particulate matter and associated nutrients and metals (Nixon 1980).
• Though salt marsh plants have proved resistant to metal pollution, there is concern that resuspended metals pollute marine systems and that plant translocation of sediment-bound metals introduces contaminants into estuarine food webs.
• Most salt marsh plants are nitrogen limited under natural conditions, and relief of nitrogen limitation results in an increase in aboveground plant height and biomass, often accompanied by a reduction in belowground biomass ( Jefferies & Perkins 1977, Kiehl et al. 1997, Valiela et al. 1976).
• Removal of upland forest for upland development allows more nitrogenladen freshwater to reach the marsh, conditions that favor P. australis growth (Silliman & Bertness 2004).
• Though this result might seem surprising, increases in consumer control in response to anthropogenic triggers have become increasingly common.

Changes in Consumer Control

• Some of the most important contemporary human impacts in salt marshes are only indirectly connected to human activities.
• These cascading events, leading to system collapse, were initiated by the luxuriant use of artificial nitrogen fertilizer in the temperate zone.
• This release of top-down pressure has potentially led to increased snail densities (Silliman & Bertness 2002).
• Compounding increased consumer densities, drought stress, ostensibly a product www.annualreviews.org Human-Driven Change in Salt Marshes 129 A nn u. R ev .
• Recent crab removal experiments have revealed that bare areas, commonly making up half of the marsh habitat, are created and maintained by crab herbivory (Alberti et al. 2007, Alberti et al. 2008) (Figure 3).

Climate Change

• Like changes in consumer control, climate change is a global and accidental force caused by human activity.
• This evidence suggests that the response of salt marshes to elevated CO2 will be dependent on plant composition and that higher concentrations of CO2 will favor compositional shifts toward C3 plants, as C4 plants are gradually outproduced and outcompeted.
• It has not been well characterized.
• In the longterm, the magnitude of the ecological disturbance caused by storms is less in marshes than in terrestrial habitats (Michener et al. 1997).
• Sea level rise effects manifest in salt marshes in two different ways: 1) landward migration of salt marsh vegetation zones and submergence at lower elevations, and 2) interior ponding and marsh drowning.

RESTORATION, CONSERVATION, AND MANAGEMENT

• After millennia of human exploitation of salt marsh resources and services, a new human impact has gained momentum in recent years: restoration.
• As policymakers and coastal dwellers have gained an appreciation for the natural ecosystem services of marshland, attempts have been made to prevent degradation and reverse historical changes.
• These efforts have taken many forms, and the authors discuss several of them, again in the context of human action and ecosystem services.

Restoration to Preimpact or Reference Conditions

• One approach to restoration holds pristine conditions of salt marshes in the highest regard, with the expectation that many of the ecosystem services the authors depend on, such as carbon and nutrient sequestration, coastal defense, and wildlife habitat, are maximized in natural and healthy marshes.
• This type of restoration includes invasive species eradications and removals of tidal restrictions.
• The PSEG project includes breaching formerly diked salt hay farms and removing invasive P. australis with herbicide to restore natural function and ecosystem services (Balletto et al. 2005).
• In California, more than 6000 ha of commercial salt ponds, formerly owned by Cargill Salt Company, are being restored to tidal marsh with the goal of increasing natural habitat in San Francisco Bay, in part to improve flood protection and water quality (Shoreline Study 2005).
• Marsh is built with dredged material deposited in shallow subtidal areas, and sometimes plant regrowth is initiated with plantings (Posey et al. 1997).

Restoration to Maximize a Single Ecosystem Service

• Where salt marshes can prevent or remediate environmental damage through optimization of a single ecosystem service, restorations are designed for this purpose.
• Small wetlands next to airports, highways, industrial complexes, and landfills have been constructed and managed to intercept polluted runoff water.
• Contrary to other types of restorations, in these marshes P. australis is encouraged to grow for its ability to take up and transform nutrients and heavy metals (Shutes 2001).
• Managed retreat, as this goal is known in England, reverts reclaimed land to intertidal marsh and mudflat by moving dikes or seawalls inland to create a more natural coastal flood buffer (Hazelden & Boorman 2001).
• With a full suite of species and ecosystem functions, salt marshes will be better prepared to cope with the next indirect and unpredicted human impact.

CONCLUSION

• The many ecosystem services, natural accessibility, and productivity of salt marshes have made them attractive ecosystems for exploitation and human use throughout history.
• Management has sought to restore some services in many parts of the globe, but an integrated approach, with simultaneous consideration of all ecosystem services, is needed.
• The current most pressing impacts in marshes are invasive species fundamentally altering salt marsh community structure, unexpected consumer effects ravaging marsh vegetation, and the relatively unexplored and multifaceted effects of climate change.
• Experiments must be used to test the relative importance of driving forces that can cause salt marsh degradation.
• We recommend multipriority management schemes (ecosystem-based management).the authors.

Figures

Figure 2

Full text

ANRV396-MA01-06 ARI 8 November 2008 10:38
Centuries of Human-Driven
Change in Salt Marsh
Ecosystems
K. Bromberg Gedan,
1
B.R. Silliman,
2
and M.D. Bertness
1
1
Department of Ecology and Evolutionary Biology, Brown University, Providence,
Rhode Island 02912; email: Keryn
Gedan@Brown.edu, Mark Bertness@Brown.edu
2
Department of Zoology, University of Florida, Gainesville, Florida 32611; email: brs@ufl.edu
Annu. Rev. Mar. Sci. 2009. 1:117–41
First published online as a Review in Advance on
August 28, 2008
The Annual Review of Marine Science is online at
marine.annualreviews.org
This article’s doi:
10.1146/annurev.marine.010908.163930
Copyright
c
 2009 by Annual Reviews.
All rights reserved
1941-1405/09/0115-0117$20.00
Key Words
ecosystem services, reclamation, eutrophication, consumer control, climate
change, restoration
Abstract
Salt marshes are among the most abundant, fertile, and accessible coastal
habitats on earth, and they provide more ecosystem services to coastal pop-
ulations than any other environment. Since the Middle Ages, humans have
manipulated salt marshes at a grand scale, altering species composition,
distribution, and ecosystem function. Here, we review historic and contem-
porary human activities in marsh ecosystems—exploitation of plant prod-
ucts; conversion to farmland, salt works, and urban land; introduction of
non-native species; alteration of coastal hydrology; and metal and nutrient
pollution. Unexpectedly, diverse types of impacts can have a similar conse-
quence, turning salt marsh food webs upside down, dramatically increasing
top down control. Of the various impacts, invasive species, runaway con-
sumer effects, and sea level rise represent the greatest threats to salt marsh
ecosystems. We conclude that the best way to protect salt marshes and the
services they provide is through the integrated approach of ecosystem-based
management.
117
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Further
ANNUAL
REVIEWS

ANRV396-MA01-06 ARI 8 November 2008 10:38
INTRODUCTION
Humans have influenced all land, marine, and aquatic ecosystems on earth (Vitousek et al. 1997).
Here, we evaluate the global distribution and extent of human impacts on the structure and
function of coastal salt marshes. More than 40% of the world’s population resides on the world’s
coasts, which account for just 4% of the land surface (UNEP 2006). This hyperconcentration at the
land-sea interface strains coastal ecological services, particularly in coastal wetlands, which provide
more ecological services than any other coastal environment (UNEP 2006). Not surprisingly, salt
marshes have experienced intense and varied human impacts that range from reclamation, waste
disposal, and livestock grazing to less obvious contemporary impacts, such as restoration efforts
that are again changing the face of marshes, reflecting a new appreciation for natural ecological
services (Silliman et al. 2008). The perception of marshes in scientific circles as the quintessential
mediators of human impacts is also changing. No longer can these systems be viewed as swampy
wastelands, used to buffer and ameliorate human impacts along the coast. Instead, salt marshes now
must be understood as highly valuable habitats whose worth is generated by a suite of ecosystem
services that are critical in sustaining healthy lifestyles for coastal populations and the natural
resources they depend on.
Vulnerability of Salt Marshes to Human Impacts and Manipulation
Salt marshes have long attracted human settlement; documented use of salt marshes for fishing and
livestock grazing date to the Neolithic in the North and Wadden Seas (Knottnerus 2005, Meier
2004). The cradle of civilization and birthplace of agriculture is thought to be the Mesopotamian
tidal marshes (Sanlaville 2002). In North America, colonists settled harbors with abundant salt
marsh in the St. Lawrence, Massachusetts and Plymouth Bays, Bay of Fundy, and Long Island
Sound (Hatvany 2003, Russell 1976, Sebold 1998) and used ancestral European agricultural prac-
tices to develop them (Butzer 2002).
Many features of salt marshes make them attractive to human populations. Salt marshes
are among the most common and extensive intertidal habitats along many temperate coastlines
(Chapman 1977) and have a low topography, often spanning only a few meters in elevation over
thousands of meters of area. Across this simple topography rocks are rare, making salt marshes
targets for conversion to arable land, coastal development, and harbors. Easy access by land and
water has facilitated the exploitation of natural resources in these key coastal communities.
Owing to the coupling of salt marsh accretion with hydrodynamic processes, human ma-
nipulation of water resources and passages to establish permanent water supplies and navigable
waterways has often had severe consequences, altering sediment supply and dramatically affect-
ing plant distributions and biogeochemistry. For example, dikes and dams, engineered structures
that restrict the flow of water to the marsh, deny the salt marsh sediment and ultimately lead
to salt marsh subsidence (Portnoy & Giblin 1997). To build land and protect the coast, people
have exploited the connection between salt marsh vegetation, water flow, and sediment accre-
tion by using salt marsh plants, many of which are excellent ecosystem engineers (sensu Jones
et al. 1997), to modify water flow, trap sediments, and accrete the marsh foundation. People
have often employed the most effective engineering plants for this purpose, whether native or
non-native. Introduction of salt marsh species for coastal defense has led to large-scale inva-
sions of many grasses, mostly from a single cosmopolitan genus, Spartina (Daehler & Strong
1996).
The plants of salt marshes are readily harvested owing to the remarkable primary productivity
of salt marshes (Mitsch & Gosselink 2000) and their striking plant zonation, forming monocultures
of robust natural plant products. Consequently, salt marsh plants have frequently been used for
118 Gedan
·
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·
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ANRV396-MA01-06 ARI 8 November 2008 10:38
animal fodder, bedding, thatch, and commercial fiber products. Intense environmental stress from
temperature fluctuations, high soil salinity, and low oxygen availability has limited salt marsh
plant diversity to a handful of stress-tolerant, halophytic genera, a high proportion of which are
cosmopolitan (Chapman 1977).Colonists of North America, recognizing the productive salt marsh
plant genera of Old Europe, quickly exploited salt marsh resources in their new home (Butzer
2002).
Other impacts to salt marsh ecosystems have resulted from the proximity of human populations,
initially drawn to salt marsh and coastal resources but later thriving in nonagrarian, industrialized
economies. The open space of salt marshes near cities was first used as a dumping ground for
sewage, trash, and other debris (Seasholes 2003, Tiner et al. 2002). As the final filter of runoff
water entering an estuary, salt marshes near human development have long been subject to water-
borne industrial and agricultural pollution, which can dramatically alter the species composition
of normally nitrogen-limited salt marsh vegetation (Levine et al. 1998, Rozema et al. 2000). Large
expanses of flat and open salt marsh lands near cities have been converted to port and industrial
complexes, resulting in their permanent loss (Pinder & Witherick 1990) and the disappearance of
the ecosystem services they generated.
Salt marshes now face new regional and global challenges that correspond to the increasing
scale of human impacts. Nutrient pollution and other large-scale human disturbances, such as
the overharvesting of top predators and species introductions, have triggered consumer control
of primary productivity in some marshes where consumers were not historically important. This
rampant herbivory is of concern because salt marshes were once considered the paradigm of
bottom-up control, and consumer control represents a major departure from this dogma. Con-
sumers can transform marshes to exposed mudflats, a state that can be persistent (Bertness &
Silliman 2008). Additionally, global climate change is sure to affect salt marsh ecosystems, al-
though the outcomes of specific climate effects remain uncertain. Accelerating sea level rise that is
expected to accompany climate change will be a universal threat to these valuable coastal ecosys-
tems and the services they provide.
Ecosystem Services of Salt Marshes
Ecosystem services are the benefits that humans derive from ecological systems and are generated
directly from the processes that sustain ecosystems, or ecosystem functions, such as primary and
secondary productivity, decomposition, and nutrient transformations (UNEP 2006). These ser-
vices come in many forms—the natural products societies require; the regulation of climate, water,
and disease cycles; the processing of nutrients and soils; and the importance of the environment
for recreation. There is increasing interest in identifying and assessing the value of ecosystem ser-
vices to make ecosystems more resilient to our impacts and provide for our needs, an integrated
management practice termed ecosystem-based management. Salt marshes generate some of the
highest and most valuable ecosystem services among natural ecosystems (Costanza et al. 1997,
2007; Levin et al. 2001) (Table 1), and today one of the primary arguments for protecting salt
marshes is to preserve and increase the quality and quantity of these services.
On coastal margins and riverbanks, salt marshes act as natural sea barriers: Salt marsh grasses
bind soils and prevent shoreline erosion, attenuating waves and limiting flooding of coastal cities
and towns (King & Lester 1995, Moeller et al. 1996). Salt marshes also serve as nitrogen sinks,
filtering runoff water and diminishing nitrogen input to estuaries (Valiela & Teal 1979). Left un-
controlled, excess nitrogen causes toxic algal blooms and marine dead zones. Salt marshes provide
essential refuge habitat for young fish and crustaceans, provisioning coastal fisheries (Boesch &
Turner 1984) that account for 90% of the world’s fish catch (UNEP 2006). Migratory waterfowl
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Table 1 Values of ecosystem services of tidal marshes
Ecosystem service Examples of human benefits
Average value
(Adj. 2007 $
a
ha
−1
year
−1
)
Disturbance regulation Storm protection and shoreline protection $2824
Waste treatment Nutrient removal and transformation $9565
Habitat/refugia Fish and shrimp nurseries $280
Food production Fishing, hunting, gathering, aquaculture $421
Raw materials Fur trapping $136
Recreation Hunting, fishing, birdwatching $1171
TOTAL $14,397
a
Dollar values were adjusted for inflation from original data, presented in 1994 dollars (Costanza et al. 1997). The
adjustment was done with the U.S. Department of Labor Inflation Calculator, which uses the Consumer Price Index to
correct values through time. Please see Costanza et al. (1997) for valuation methods and note that this valuation method is
not universally accepted by economists, see Bockstael et al. (2000).
feed in salt marshes, where recreational hunters and birdwatchers follow their movements (UNEP
2006). As global climate change intensifies, the service of carbon storage performed by salt marshes
gains importance, particularly at higher latitudes where carbon release by decomposition is slower
and salt marshes are effective carbon sinks (Chmura et al. 2003).
Humans have long recognized that salt marshes generate these services and many others,
but marshes have often been misused by managers seeking to exploit nonrenewable services.
For instance, reclamation of salt marsh for upland uses provides a vital service to humans in
added land, but the cost of reclamation is extraordinary if one accounts for the lost ecosystem
services of natural marsh. The result is a net loss for society (Table 1). When managers try to
maximize only one ecosystem service, poor decisions are made, such as the introduction of invasive
species of Spartina for coastal protection, which has compromised the provisioning of natural
Chinese salt marshes (An et al. 2007). Alternatives, opportunity costs, and maximization of value,
including all ecosystem services combined, must now be incorporated into salt marsh management
schemes.
IMPACTS
Large efforts have been made to evaluate and remediate human impacts on marshes. In this
review, we evaluate and summarize multiple human threats to marshes in a historical context. The
interpretation of the term threat is, of course, subjective. From different perspectives, changes
in salt marshes can be seen as positive or negative, and the perception of many human impacts
in salt marshes has changed over time. For example, what has in earlier eras been viewed as the
improvement of wasteland (land reclamation) has later been characterized as habitat loss (land
conversion).
We discuss human actions in salt marshes and their associated impacts, and we contextualize
these impacts in terms of their effect on ecosystem services. For each general category of human
action (resource exploitation and extraction, land conversion, species introductions, hydrologic al-
teration, pollution, changes in consumer control, and climate change), we first describe the human
activity and its history, and then discuss impacts of the activity on plant and animal communities,
marsh biogeochemistry, and surrounding habitats. Finally, we end with a discussion of restoration
and management techniques that attempt to undo or compensate for previous human impacts in
salt marshes.
120 Gedan
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Resource Exploitation and Extraction
The most common use of salt marshes around the world has been as pasturelands for livestock
(e.g., cattle, sheep, goats, and horses) (Knottnerus 2005), a practice that has not notably impacted
ecosystem services, but that has in some cases made observable changes to species composition
and animal populations. This practice is still common in China, Chile, and Europe, although it has
become rare in Canada and the United States (Hatvany 2003, Sebold 1992). In the late 1800s along
the St. Lawrence in Canada, marshland was valuable farmland. Marsh-holding farmers (including
those with reclaimed marshland) supported 45% more livestock, on average, than non-marsh-
holding farmers (Hatvany 2003). Spartina patens was considered highly nutritious fodder and was
harvested annually in the United States and Canada—hence its common name, salt marsh hay
(Sebold 1992). The products of marsh farms have often been considered to be of high quality and
even delicacies; butter made from the milk of cows grazed on salt hay in the St. Lawrence Estuary
enjoyed a gourmet boutique status in the late 1800s (Hatvany 2003), and salt marsh–fed lamb is
still advertised as a French delicacy (LeJemtel Hostle 2004).
Beyond use as fodder, salt marsh plant products have been used as animal bedding, thatch
(Phragmites australis, still used in England; Kiviat & Hamilton 2001), rope (S. patens; Seasholes
2003, Sebold 1992) (Figure 1i ), packing for pottery, metal and icehouse insulation (S. patens),
and a nearly weedless mulch (S. patens, wrack, or Ulva; Sebold 1992). Native Americans used the
reed P. australis for an astounding diversity of products including musical instruments, baskets,
arrowshafts, and cigarette casings (Kiviat & Hamilton 2001).
In the face of these impacts, salt marshes have proved to be surprisingly resilient. In the far
north marsh communities of the Baltic, grazing favors halophytic plant species by allowing short-
statured halophytes to compete for light and by compacting the soil, which results in higher
soil salinities (Dijkema 1990). Despite such notable effects on plant community composition, the
continued productivity of grazed and harvested salt marsh vegetation demonstrates the resilience
and high productivity of salt marshes. Turner (1987) found trampling by grazers to have more
significant effects than biomass removal, and salt marsh sites were able to withstand intense grazing.
Perhaps the seasonality of marsh plants at higher latitudes contributes to this resilience; mid-season
removal of biomass does not seem to disrupt plant re-emergence and apparently does not result
in the buildup of hypersalinities that would impede plant regrowth. The fact that many salt marsh
plants reproduce clonally, with little seed-driven reproduction, is likely of key importance to plant
resilience. Levin et al. (2002) suggested that salt marsh plants may have an evolutionary history
that included grazing by now-extinct Pleistocene mammals.
Impacts of grazing on vegetation, however, can have ripple effects in salt marsh animal com-
munities. Horse grazing can affect the use of marshes by birds and crabs, reducing the number
of ground nesting birds, but positively affecting the abundance of bird and crab consumers by
trimming away the grass canopy and exposing fish and invertebrate prey (Brewer et al. 1998). In
the Netherlands, grazing of livestock in marshes affects plant species composition and reduces
vegetation height, making the marshes more attractive to herbivorous geese (Bos et al. 2005).
Grazing management techniques can have the same effect. In Argentina, salt marshes are burned
to improve cattle forage, which reduces cordgrass height, plant species richness, and bird abun-
dance (Isacch et al. 2004). There is no record of grazing impacts affecting neighboring habitats,
although clearly impacts on transitory birds, fish, and invertebrates may spill over.
Land Conversion
The conversion of marshland to upland was historically undertaken for agricultural purposes, but
in the past century it has mainly occurred for urban development. In some cases, human action
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Human-Driven Change in Salt Marshes 121
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Frequently asked questions

Q1. What are the contributions mentioned in the paper "Centuries of human-driven change in salt marsh ecosystems" ?

Salt marshes are among the most abundant, fertile, and accessible coastal habitats on earth, and they provide more ecosystem services to coastal populations than any other environment. Here, the authors review historic and contemporary human activities in marsh ecosystems—exploitation of plant products ; conversion to farmland, salt works, and urban land ; introduction of non-native species ; alteration of coastal hydrology ; and metal and nutrient pollution. The authors conclude that the best way to protect salt marshes and the services they provide is through the integrated approach of ecosystem-based management. 

Q2. What are the future works in "Centuries of human-driven change in salt marsh ecosystems" ?

1. Further work is needed to determine how and why shifts in regulating mechanisms of salt marshes are occurring worldwide and to identify the drivers. The authors recommend further multistressor experiments be undertaken in salt marsh communities. 

Q3. What is the effect of ditches on aquatic plants?

By increasing tidal flushing frequency, ditches ameliorate anoxic stress and increase plant productivity near ditch banks (Shisler & Jobbins 1977). 

Q4. What is the role of salt marshes in nutrient storage?

Like metals, nutrients are taken up and transformed by salt marsh sediments and plants, and salt marshes can act as long-term nutrient sinks (Valiela & Teal 1979). 

Q5. What is the effect of intensive ditching on the waterlogged marsh?

Intensive ditching affects hydrology at a landscape level, altering channel formation and simplifying drainage networks (C.C. Bohlen, unpublished data). 

Q6. How many wetlands have been constructed to intercept polluted runoff water?

Small wetlands next to airports, highways, industrial complexes, and landfills have been constructed and managed to intercept polluted runoff water. 

Q7. What is the effect of peat removal on the Dutch coast?

On the Dutch coast, peat removal for salt production and combustion fuel contributed to subsidence of the land by 2–5 m behind dikes (Hoeksema 2007). 

Q8. What is the history of the snail densities?

snail densities were largely controlled by predators, such as blue crabs and terrapins, which have been overharvested and now suffer from diseases and other maladies associated with small population size. 

Q9. What are the main factors that influence the control of salt marshes?

salt marshes were thought to be controlled exclusively by physical forces such as temperature, salinity, and nutrients. 

Q10. What are the impacts of salt marshes?

Not surprisingly, salt marshes have experienced intense and varied human impacts that range from reclamation, waste disposal, and livestock grazing to less obvious contemporary impacts, such as restoration efforts that are again changing the face of marshes, reflecting a new appreciation for natural ecological services (Silliman et al. 2008). 

Q11. What is the effect of climate change on salt marshes?

Climate-related changes to the carbon cycle are likely to alter the sequestration service provided by salt marshes, as well as affect long-term rates of salt marsh accretion and the ability of marshes to keep pace with sea level rise in ways that are still unclear. 

Q12. How many ha of Monterey Bay were converted to salt works?

In the early twentieth century, 120 ha in Monterey Bay (Van Dyke & Wasson 2005) and 507 ha near San Diego (Zedler 1996) were converted to salt works. 

Q13. What caused the denuding of extensive areas of Arctic marshes?

snow geese populations nearly tripled in a decade, leading to runaway consumption and the denuding of extensive areas of Arctic marshes (currently >37,000 ha in southern Hudson Bay alone). 

Q14. What was the first method of sped ditching?

The invention in the 1800s of doublebladed saws and in the early 1900s of ditch-digging machines sped ditch construction (Sebold 1998). 

Q15. What are the effects of multiple stressors on salt marshes?

The effects of multiple stressors on salt marsh communities have rarely been examined, but when investigated, have had unexpected and synergistic consequences (e.g., eutrophication exacerbates species invasions, drought and overfishing make S. alterniflora more vulnerable to consumer effects). 

Q16. What is the impact of the burrowing isopod on the marsh?

The burrowing isopod Sphaeroma quoyanum was also unintentionally introduced to the U.S. Pacific coast, where its intense burrowing causes erosion at the marsh edge. 

Q17. How does the evidence suggest that the response of salt marshes to elevated CO2 will be?

This evidence suggests that the response of salt marshes to elevated CO2 will be dependent on plant composition and that higher concentrations of CO2 will favor compositional shifts toward C3 plants, as C4 plants are gradually outproduced and outcompeted. 

Q18. How could climate change affect salt marshes?

Climate change could alter the geographical distribution of salt marshes, which currently span temperate and arctic latitudes from 30◦ to 80◦ (Chapman 1977) (Figure 1), and salt marsh plant species and could also affect ecosystem productivity. 

Q19. What are the main categories of human actions in salt marshes?

For each general category of human action (resource exploitation and extraction, land conversion, species introductions, hydrologic alteration, pollution, changes in consumer control, and climate change), the authors first describe the human activity and its history, and then discuss impacts of the activity on plant and animal communities, marsh biogeochemistry, and surrounding habitats. 

Q20. What is the common term used to describe the restoration of salt marshes?

After millennia of human exploitation of salt marsh resources and services, a new human impact has gained momentum in recent years: restoration. 

Q21. What is the effect of ditching on aquatic animals?

Animal communities have had mixed responses to ditching; subtidal and intertidal animals such as fiddler crabs and fish are generally positively affected by the increase in habitat area and access to prey (Lesser et al. 1976, Valiela et al. 1977), and semiaquatic organisms such as dragonflies and water beetles are negatively affected by the loss of habitat and increase in predators (Bourn & Cottam 1950, Resh 2001), although other studies show invertebrates are unaffected by ditching (Clarke et al. 1984). 

Q22. Why do they estimate that salt marshes sequester more carbon than freshwater wetlands?

Chmura et al. (2003) estimated that on an area basis, tidal wetlands (salt marshes and mangroves) sequester 10 times more carbon than peatlands (210 g CO2 m−2 year−1 versus 20–30 g CO2 m−2 year−1 in peatlands), in part because saline wetlands emit less methane and CO2 than freshwater wetlands. 

Q23. How many ha of upriver marsh did Crain et al. (2008) study?

Crain et al. (2008) investigated the spatial extent of tidal restrictions in Maine salt marshes and found that in 3193 ha of tidal marsh, 57 restrictions affected 903 ha (28%) of upriver tidal marsh. 

Q24. What is the link between invasive species and their non-native ranges?

Engineering capacity has been linked to the transformational impact of invasive species in their non-native ranges for both plants and animals (Brusati & Grosholz 2006, Crooks 2002). 

Q25. How much more livestock did marsh farmers support?

Marsh-holding farmers (including those with reclaimed marshland) supported 45% more livestock, on average, than non-marshholding farmers (Hatvany 2003).