Barrier islands are unique island ecosystems that border ...



In Review, Frontiers in Ecology and the Environment

Running Title: Sustaining barrier island ecosystems

Coastal Barrier Islands: Coupling Anthropogenic Stability with Ecosystem Sustainability

William K Smith1*, Rusty A Feagin2, Norbert P Psuty3, Karl F Nordstrom4, Gregory A Carter5, Jane N Gemma6, James C Gibeaut7, David H Thomas8, Richard E Koske9, Maria L Martinez10, John C Whitehead11, Donald R Young12

Coastal barrier islands provide a host of critical ecosystem services to the heavily populated coastal regions of the world. Without these near-shore islands most of our coastal wetland resources would be lost to tidal surge and wave action. Following extreme episodic events such as hurricanes and tsunamis, coastal shorelines have experienced substantially reduced damage when protected by near-shore barrier islands. Despite these critical benefits, no strategic plan exists for sustaining barrier island ecosystems under the challenge of continued economic development and potentially severe global warming impacts. These ecosystems provide an opportunity for developing land management strategies that blend anthropogenic goals for stabilization with the inherent instability of a natural ecosystem, one that is highly vulnerable to global warming.

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Coastal barrier islands (CBI) are unique ecosystems that border coastal shorelines and physically separate the offshore oceanic province from inshore wetlands, bays, sounds, and estuaries. As their name implies, they form a protective barrier between continental shorelines and the ______________________________________________________________________________

In a nutshell:

• Coastal barrier islands are a prime attraction for human development, but are also vital components of coastal wetlands. The tight coupling between geology, vegetation, and extreme episodic storm events generates one of the most dynamic of all ecosystems

• Although native vegetation enhances substrate stabilization, land management practices have largely ignored any application for ecosystem sustainability

• A land management strategy utilizing native vegetation could enhance substrate stability and enable preservation of a host of ecosystem services

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1Department of Biology, Wake Forest University, Winston-Salem, NC 27109 *(smithwk@wfu.edu); 2Spatial Sciences Laboratory, Department of Ecosystem Science & Management, Texas A&M University, College Station, TX 7784; 3Insitute of Marine and Coastal Sciences, Rutgers University, Sandy Hook, NJ 07732; 4Department of Oceanography, Rutgers University, Piscataway, NJ 08854; 5Gulf Coast Geospatial Center, University of Southern Mississippi, Ocean Springs, MS 39564; 5Biological Sciences, University of Rhode Island, Kingston, RI 02881; 7Bureau of Economic Geology, University of Texas, Austin, TX 78713; 8American Museum of Natural History, New York, NY 10024; 9Biological Sciences, University of Rhode Island, Kinston, RI 02881; 10Departamento de Ecologia Vegetal, Instituto de Ecologia, Xalapa, Mexico 91070; 11Department of Economics, Appalachian State University, Boone, NC 28608; 12Department of Biology, Virginia Commonwealth University, Richmond, VA 23284

powerful wave action that originates offshore, especially during oceanic storms (Barbier 2006) or earthquake-driven tsunamis (Ramakrishan et al. 2005, Danielsen et al. 2006). Thus, CBI provide the structural framework for the formation of an extensive array of coastal wetland habitats that host a large variety of native and migratory species, many of which have high economic value. In addition, these same coastlines include some of the greatest concentrations of human populations and accompanying anthropogenic development in the world (eg Weinstein et al. 2007). The native vegetation and geological stability of these ecosystems are tightly coupled in their dependency on one another, and are highly vulnerable to overwash and erosion events. As a result, CBI are especially vulnerable to global warming impacts such as sea level rise and the predicted increase in the frequency of oceanic storms (Martinez et al. 2004, Webster et al. 2004. Thus, CBI are some of the the most valuable, yet, vulnerable ecosystems on Earth.

Beyond the original Coastal Barrier Resources Act of 1982 that prevented federal assistance for activities supporting commercial development of CBI and which designated certain parklands and national seashores to be preserved, no federal or state framework exists today for sustaining CBI ecosystems. Our goal here is to describe briefly the ecological and geological nature of barrier island ecosystems and suggest the possibility that a change in management practices could lead to the sustainability of biodiversity and other ecosystem services, along with continued socio-economic development. The general essence of this approach has been termed Ecosystem-based Management (EBM) and has been recently identified as applicable to a variety of ecosystem types (Arkema et al. 2006, Weinstein et al. 2007). More specifically, this plan incorporates a strategy whereby anthropogenic development and ecosystem sustainability are blended to slow the natural erosion dynamics characteristic of most barrier islands. In general, the strong human desire to maintain landscape stability contradicts a sustainability strategy that incorporates, fundamentally, the strong degree of instability found within natural ecosystems. In particular, natural instability and transience are likely to increase for CBI under predicted z scenarios of sea-level rise and an increased frequency of intense storms. An ideal solution, based on the concept of the urbanized ecosystem, would be a strategic compromise between anthropogenic development and preservation of the natural ecosystem (Kremen and Ostfeld 2005, Felson and Pickett 2005). Within the conceptual framework of EBM, human requirements for stabilization will have to involve a compromise between sustaining completely natural versus purely urbanized ecosystems (US Commission on Ocean Policy 2004). In addition, a better linkage between science and landscape management will be needed (Arkema et al. 2006), and CBI ecosystems could provide an ecologically unique and important experimental model for multidisciplinary studies involving geologists, hydrologists, ecologists, economists, and cultural sociologists—one that should be pursued with some expediency considering estimates of the rate at which global warming impacts may occur (Felson and Pickett 2005).

■ Ecosystem dynamics

Barrier islands have an extremely dynamic, temporal nature whereby major changes in geomorphology and hydrology can occur over days, or even hours, in response to extreme episodic storm events (EESE) such as hurricanes and northeasters (eg east and gulf coasts of the USA and Mexico). Over these relatively short-term time periods, new oceanic inlets can form and entire islands can be displaced kilometers, entirely disappear, or form anew. Because of this relatively unique geological transience, a large diversity of spatial and temporal factors has generated an equally unique array of endemic and indigenous species highly adapted to such transient conditions (Ehrenfield 1990, Shao et al. 1996). However, little is known concerning the nature of the adaptive mechanisms associated with this high level of transience, especially in the form of EESE, although these species are expected to have unique suites of evolutionary, genetic traits that underlie these adaptations (Gutschick and BassiriRad, 2003). Thus, the genomes of the flora and fauna of CBI may be particularly adapted to rapidly changing conditions, a valuable resource for understanding how species might withstand future amplification in EESE under current scenarios of global change (Paine et al. 1998).

■ Socioeconomics

Blending ecosystem sustainability with the socioeconomics of CBI is a challenging task, as is the case for any natural ecosystem (Scheffer et al. 2002). Martínez et al. (2004) calculated the economic value of goods and services provided by coastal ecosystems and found that, altogether, coastal ecosystems contribute to almost 70% of global ecosystem services. Pérez-Maqueo et al. (2007) estimated the extent to which coastal wetlands reduce the damages from storms and, thus, the economic value of this storm protection service. This model for hurricane damage revealed that wetland vegetation substantially reduced overall impacts. For example, their estimates of damage protection provided by wetlands along the Atlantic coast of the USA were approximately $9,683 US/hectare/year. In a global change scenario, where it is predicted that the frequency of hurricanes categories 4 and 5 will double, the estimated benefits of coastal protection would increase to $11,223 US/hectare/year (Pérez-Maqueo et al. 2007). Further integration of ecology and economics is necessary to completely characterize the costs and benefits of barrier island development. For example, in addition to the market costs of sand, another cost of beach nourishment is habitat degradation. The contingent valuation method can be used to estimate the economic values of barrier island fish and wildlife habitat (Whitehead 1993). Benefit-cost analysis of barrier island development, beach nourishment, habitat protection, and other issues should consider all relevant market values (Cordes and Yezer, 1998), as well as non-market values (Parsons et al. 2001), to determine the economic benefits and efficacies of different management options.

■ Cultural history

The modern CBI landscapes are the result of complex and continuously changing interactions between climate, biota, and humans. Over the past four centuries barrier islands are being degraded at an alarming rate (USCOP). Today, they provide a valuable historical perspective that reveals a coupled archaeological and ecological record (Thomas 2006). For example, the Virginia Coast Reserve study (Long-term Ecological Research program, NSF) is one example that demonstrates the existence of transitional stages of CBI that have occurred over centuries, or even decades. Prior to the arrival of European settlers, the Accomacs and other Native American tribes took advantage of the rich island resources. The town of Broadwater on Hog Island was two miles from the beach and peaked in the early 1900s with more than 300 people. However, the increased storm activities of the 20th century accelerated shoreline erosion and by 1933 beach had reached the town. A major hurricane that year ended all permanent settlements on the Virginia barrier islands, generating the current position of Broadwater which is now nearly two miles out to sea.

■ Global warming impacts

As mentioned above, CBI provide the important ecosystem service of absorbing and dissipating wave and wind energy directed toward the adjacent coastline, especially during EESE (tropical storms, hurricanes, and northeasters). These impacts on human populations have often devastated the socioeconomic infrastructure, the overall human condition, and the ecological communities as well (e.g. Hurricane Katrina, 2005). Moreover, the severity of these EESE impacts may increase dramatically in the decades to come unless major changes are initiated in traditional land management practices (Scheffer et al. 2002). For example, simulations of future atmospheric warming predict that more intense weather phenomena will increase in frequency and intensity in this century (Brown and MacLachlan 2002, Webster et al. 2005), and there is recent evidence that storm frequency has already increased over the past 60 years. In addition, current predictions of the future impacts of global warming include substantial increases in sea level (Holgate and Woodworth 2004, Zhang et al. 2004) that will certainly exacerbate EESE impacts (Webster et al. 2005).

The disruption and potential disappearance of coastal barrier islands has now been recognized as a global problem of potentially cataclysmic proportions (IPCC 2001). Specifically for the USA, the coastal areas of New England, Virginia, North and South Carolina are considered at greatest risk, although the Gulf Coast of Texas, the tip of Florida, and the entire eastern seaboard are all considered at high risk for a major hurricane landfall. Despite this future scenario of more frequent and intense EESE, little is known regarding the long-term ecological and socioeconomic impacts on coastal ecosystems (Gutschick and BassiriRad 2003).

■ Native vegetation and EESE

Traditionally, the emphasis in land management for developed coastlines has focused on artificial storm protection (Figure 1), while the importance of utilizing natural processes for the same purpose is just becoming recognized (Nordstrom 2004, Psuty 2004). Maintaining a zone of energy absorption (wind and water) between the beach and the leading edges of personal property (Figure 2 and 3) can enhance the ecological resistance and resilience (recovery rate) of a developed shoreline to EESE, influence landscaping actions taken by neighbors, and enhance the likelihood that natural features will be a positive factor in the resale of coastal property (Nordstrom 2004).

The transitional dune environment in the seaward portion of private lots can be managed as a conduit for sediments and as native habitat for biota. To function naturally, this ecological boundary could be recognized as independent from any demarcated cultural boundary, and the physical and visual barriers of cultural features should be minimized. In the past, most municipal efforts to manage CBI ecosystems often stopped at the seaward extent of private property, effectively eliminating the public beach and leaving little room for government involvement. These types of efforts have led to linear boundaries of development rather than the more natural land-sea ecotone. However, the state of Texas provides a notable exception in its Open Beaches Act, where the property lines are defined by the ecological reality of the seaward extent of the native vegetation. When this zone shifts landward, so does the definition of the private-public boundary. With this approach, land speculation and development must accommodate the reality of a retreating beach ecotone.

While the above approach has led to several vigorous dune restoration projects, overzealous restoration work has also ensued for the purpose of pushing the vegetation zone, and hence private property, towards the ocean. As a result of overextending the vegetation zone and creating an artificially-steep, land-to-ocean gradient that is too narrow. These projects have subsequently been destroyed by the dynamic forces of EESE. In the future, an emphasis should be placed upon creating spatially wider zones to allow plants to respond in a functional manner over longer time periods (Feagin 2005). By allowing more natural, ecological boundary zones (ecotones) of natural vegetation to replace culturally-demarcated boundary lines in management practices, substantially improved protection from overwash is possible, even during EESEs (Figures 1-3). Maintaining these sorts of natural environments in developed settings has the more far reaching benefit of familiarizing people with natural communities, as well as for instilling the importance of restoration and preservation (Savard et al. 2000).

The potential loss of native barrier island plant species reduces the capability of the ecosystem to withstand major storm events (Feagin et al. 2005). Removal of native vegetation results in an accelerated loss of substrate to a point where the capability for recolonization and stabilization is lost permanently. Native plants of barrier island beaches play a critical role in the initial formation of the embryo dunes closest to the high water limit (Feagin & Wu 2007). Natural dunes are often the result of sand accumulation over a decade or centuries, becoming more resistant and resilient to future storm activity as plant size and root depths increase (e.g. Tsoar 2005).

Linked directly to greater substrate stabilization, primary and secondary dunes are followed by the ultimate formation of maritime forests with high EESE resistance and resiliency. In particular, late-successional maritime forest areas often remain virtually intact following EESE that cause substantial losses of substrate in areas without maritime forest (Figure 3; Tsoar 2005). These same areas also recover more rapidly than other areas where sand erosion is more severe and sand accretion delayed. An important, possibly critical, biotic component of nearly all CBI plant species (from embryo dunes to maritime forest), as well as an important component of restoration, is the benefit of mycorrhizal fungi, mutualistic species whose hyphae act functionally as a fine-root system for support, plus water and nutrient absorption for establishment and growth (Koske et al. 2004).

Several studies have indicated a remarkable resiliency of the native CBI flora to EESE (Young et al. 1995). For example, mature maritime forest now appears essential for the increased stability, persistence, and resiliency of most CBI ecosystems (Figure 3), as well as for protecting adjacent mainland shorelines from damage (Danielson et al. 2005). Overall, the use of native vegetation to provide substrate stability and coastal ecosystem protection is an under-utilized resource.

■ Coupling sustainability with anthropogenics

One of the most conspicuous boundaries in the USA between landscapes modified by humans and those formed by natural processes occurs at the seaward edge of private property adjacent to beaches (Mitteager et al. 2006). In most all coastal areas, the desirability of shorefront property for residential use and the high value of property result in large numbers of private holdings per unit length of shoreline. Much of the problem of managing beaches and dunes in developed areas as natural ecosystems is the conflict between the natural dynamics of these natural systems (including EESE) and the human desire for stabilization that enhances safety, maintenance of property rights, or management for perceived aesthetics. Managing coastal landforms and habitats to achieve both natural and human services along a continuum, rather than as bipolar opposites, lies in separating critical needs from those that are simply desirable (Nordstrom 2004, Psuty 2004). Restrictions to the natural interplay of sediments and biota may not represent threats if human actions can overcome these restrictions by planting and augmenting natural vegetation (eg weeding exotic competing species). Although compromise is inevitable, the more these processes emulate nature and are less dependent on human actions, the more cost effective they will be and the greater their value will be in establishing an appreciation for both the ecological and economical benefits of natural vegetation and accompanying landscapes. Suggestions for barrier island coasts include allowing for a certain degree of dune mobility, but possibly restricting them to a narrower zone and relying more on volunteer actions by residents for maintenance (Nordstrom 2004). More extreme ideas such as using more nature-conforming construction methods, even the possibility of more mobile living structures to accommodate shifting substrates on at least a decadal scale, are possibilities for the future coupling of anthropogenic development with ecosystem sustainability.

■ Restoration

Restoration ecology has received little attention in CBI ecosystems, despite recognition that areas with natural vegetation are more protected and resilient to the impacts of EESE (Figure 1-3). One option is to alter the recovery of shoreline development following EESE so that the preservation of native biota is enhanced, providing greater protection from future EESE. Natural dune vegetation is easy to maintain and the least expensive alternative for generating increased stabilization (Feagin 2005), but owners are often unaware of the advantages of this option, or the necessary knowledge to take action. Conversations with homeowners indicate that they are not aware of (1) the advantages of using natural species for landscaping, (2) the great differences in viability of alternative species in the harsh dune environment, or (3) the high levels of maintenance required for non-native species that are not adapted to coastal stresses (Nordstrom 2004). While there are many useful federal and state guidelines for replanting damaged dune systems and restoring the aboveground community, few have addressed the use of native species, especially the critical belowground community (e.g. soil mycorrhiza) upon which a successful above-ground community depends (Koske et al. 2004).

■ Conclusions

Coastal barrier islands are ecologically, socio-economically, and culturally invaluable ecosystems that are highly vulnerable to impacts from future global warming. Access to these areas is highly desired by humans for personal housing and commercial development, despite the prospect of paying large sums of money for replacement costs (some of which are subsidized by state and federal tax funds) due to severe storm damage. For CBI ecosystems, the benefits of the natural vegetation for resisting and recovering from EESE impacts are pronounced. Because an extreme level of human-managed stability is not compatible with such temporally dynamic ecosystems, compromise solutions are needed. A fundamental and important question is what minimum level of stability is needed for human occupation of these naturally dynamic ecosystems. Is there a compromise solution that will allow for ecosystem sustainability and anthropogenic desires for stability? In this regard, CBI may offer an opportunity to study the potential for coexistence, within the framework of the urban ecosystem. For example, natural CBI habitats may be allowed to extend across the entire width of some barrier islands, but may be restricted to the seaward or bayward portion of others. Both personal property owners and businesses could be landscaped with native vegetation, either preserved or cultivated. Finally, CBI ecosystems are excellent candidates for designing experiments that would test the efficacy of using natural vegetation, rather than expensive and temporary artificial structures, for achieving a compromise between completely natural versus blended natural-urban ecosystems.

■ Acknowledgements

This paper was generated from a workshop (April 14-16, 2006, in Biloxi, Mississippi, USA) that organized the Barrier Island Network (BINET), a multidisciplinary group concerned with the preservation of CBI (). Support was provided by the National Science Foundation, Research Coordination Networks in Biological Sciences.

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Figure 1. Extensive root systems and associated mycorrhizal fungi play a critical role in stabilizing sand substrates of barrier islands, in contrast to expensive artificial structures (right) that require high maintenance and are not aesthetically pleasing. The picture on the left was taken along the south coast of RI (Moonstone Beach) in Sept 1985 just after Hurricane Gloria hit. The dune was vegetated almost entirely with Ammophila breviligulata (American beachgrass). Not visible at this magnification is the hair-like hyphae of mutualistic arbuscular mycorrhizal fungi that grow a few cm from fine roots and bind a large amount of sand. At least six species of mycorrhizal fungi occur in the rhizosphere of this dune ecosystem.

Figure 2. Aerial view showing the strong influence of natural vegetation on storm impacts (Dauphin Island, Alabama) following Hurricane Katrina in 2005. Areas with native vegetation (red color in top images, brown-green in bottom images) and hence dune formation show greatly increased substrate protection (left side images). Areas where the native vegetation has been disturbed by anthropogenic development show severe overwash (right images).

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Figure 3. Barrier islands developed for both commercial and private property can benefit from native vegetation (especially the larger, deep-rooted maritime forest) that provides protection from both wind and overwash impacts during storms. Upper photo without native vegetation shows typical vulnerability to natural beach erosion for beach-front houses that lack a buffer zone of established vegetation.

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