RESEARCH COMMUNICATIONS RESEARCH COMMUNICATIONS
                                                                     359
Coastal erosion, global sea-level rise, and
the loss of sand dune plant habitats
Rusty A Feagin1, Douglas J Sherman2, and William E Grant3

   Much of America’s coastline is threatened by overdevelopment and coastal erosion, driven by global sea-level
   rise, a problem that is attracting the attention of researchers around the world. Although we have now
   acknowledged the impending risks, little is known about the response of spatially dependent dune plant com-
   munities to the loss or restriction of their habitat. In order to study this development, a spatially explicit model
   of sand dune plant succession on Galveston Island, Texas, was created, using sea-level rise as the primary
   mechanism causing local erosion. Simulations of sea-level rise scenarios developed by the Intergovernmental
   Panel on Climate Change demonstrated that beach erosion constrained plants to a narrow area, resulting in a
   breakdown of the successional process. The loss of late-succession plants along coastlines, their dependent fau-
   nal species, and possible solutions are discussed. This model and example serves as a harbinger of the future for
   many of the US's sandy beaches and coastal communities.
Front Ecol Environ 2005; 3(7): 359–364



C   oastal erosion is a problem common to shorelines
   around the world, impacting about 70% of the Earth’s
sandy beach environments (Bird 1985). The causes of ero-
                                   erosion rates, as it has historically attained the relative sea-
                                   level rise rates that are expected to affect much of the
                                   receding coastlines of the US over the next century.
sion can be of local (eg a decrease in sediment supply) or        The Gulf Coast of the city of Galveston is protected
global importance (eg a worldwide change in sea level).        from storm surge by a seawall (Figure 1a) constructed in
Beach erosion in a sediment-rich environment will result       response to the damage caused by a 1900 hurricane that
in a landward displacement of coastal environments          still stands as the US’s worst natural disaster (Pilkey and
(including dunes; Martinez and Psuty 2004) as long as         Frasier 2003). Although the seawall and the groins that
there are no barriers (eg cliffs) to restrict such migration.     front it protect the properties immediately landward,
Where there are barriers, including those commonly intro-       these structures have caused greater down-drift erosion by
duced by human activities (Nordstrom 2004), erosion          disrupting the natural sediment transport system, result-
reduces the area available for plant and animal communi-       ing in the need for other shoreline protection measures
ties (Feagin 2005). In some places, coastal development        such as geotextile tubes (Figure 1b). Along most of
confines the natural community to such a narrow stretch of      Galveston Island, extensive development and the pres-
beach that plants are no longer able to disperse or grow.       ence of non-native lawn vegetation behind the beach
  In order to study this development, a spatially explicit      presents a landward barrier that blocks the migration of
model of species dynamics within a sand dune plant com-        native habitat (Figure 1c).
munity on Galveston Island, Texas, was created. Average         Although it is recognized that continued erosion restricts
rates of shoreline loss on Galveston Island exceed several      the plant community’s habitat within a narrow zone, it is
meters per year. Local processes such as subsidence and        not yet understood what the ecological ramifications to the
reduced along-shore sediment transport have contributed        plant community may be in terms of successional dynamics
to historic coastal retreat in this region (Morton et al.       among species and functional groups. Moreover, there has
2004), although current subsidence rates are minimal         been little research on the impact of a narrowing habitat
(USGS 2002). Erosion rates are already accelerating as a       upon community pattern formation in plant succession in
result of sea-level rise (Holgate and Woodworth 2004)         general. Because of the impending habitat loss of dune
induced by atmospheric warming (Titus et al. 1991; Twilley      plant communities in this area, this issue is ideally suited to
et al. 2001), a problem of global dimensions (IPCC 2001).       investigation through simulation modeling.
Galveston Island is an ideal site for the study of accelerated
1
                                     Methods
Spatial Sciences Laboratory, Texas Agricultural Experiment Station
and Department of Forest Science, Texas A&M University, College    Study area
Station, TX 77843-2120 (feaginr@tamu.edu); 2Department of       A detailed description of the study area and baseline model,
Geography, Texas A&M University, College Station, TX 77843-      minus the sea-level parameters, can be found in Feagin et al.
3147; 3Department of Wildlife and Fisheries Sciences, Texas A&M    (2005). The sand dune plant community, upon which the
University, College Station, TX 77843-2258              model was based, was contained in a research plot (Figure

© The Ecological Society of America                                     www.frontiersinecology.org
   Loss of dune plant habitats                                              RA Feagin et al.


360  (a)                                  (b)




    (c)                                 (d)




   Figure 1. Four views of the rapidly eroding sand dune plant habitat on Galveston Island, Texas. (a) Transitory dunes in front of the
   seawall, just visible beyond the lip of the wall; (b) habitat in front of geotextile tubes during a natural storm event (here, Hurricane
   Lily, the eye of which passed over Louisiana, 400 km to the east); and (c) the maintenance of non-native lawn vegetation, leaving a
   tenuous 2 m strip of habitat. (d) Under natural circumstances, the native plant community in the research plot can “move” with the
   dunes and the back-and-forth migration of the beach.

   1d) adjacent to Pirates Beach on Galveston Island, at lati-      mented in Mathematica (Wolfram 1996). A two-dimen-
   tude 29.21 and longitude –94.92. The climate at this loca-      sional lattice, composed of 100 x 100 individual sites, was
   tion is subtropical, characterized by hot summers and mild      created. Each site represented a 0.5 x 0.5 m (0.25 m2)
   winters. The community extends along a 50 m beach–sand        quadrat in the research plot. The top of the lattice repre-
   dune gradient, stretching from the summer berm on the         sented the landward boundary of the sand dune plant
   open beach to the edge of the upland/coastal prairie com-       community while the bottom of the lattice represented
   munity behind the dune ridge. The sand dune plant com-        the seaward boundary.
   munity, depicted in Figure 1d, can be described in terms of       Each site had four possible states, consisting of bare-
   its three successional stages: colonizers, soil binders, and     ground (0), colonizer plants (1), soil binder plants (2), or
   competitors. Multiple years of plant succession in this com-     competitive plants (3). Because sand dune succession is
   munity have been documented since 1998, when Tropical         based on facilitation by adjacent plants, as well as the
   Storm Francis destroyed the plant cover and subsurface        amelioration of environmental constraints by previous
   growth, removing the soil and leaving only bare sand. The       successional stages, each state represented one step in the
   model was calibrated based on spatial patterns observed        successional sequence. For example, a site could start as
   over a 5-year period at this location, with the beach accret-     bareground and move through the three subsequent suc-
   ing at an average rate and growing conditions that were typ-     cessional stages if an adequate number of other plants
   ical for the subtropical, humid coastal dunes of Texas.        were growing in adjacent locations. The relative effect of
                                      a site upon an adjacent site, through facilitation, was
   Baseline model description                      based on proximity, as detailed in Feagin et al. (2005).
                                       During each time step, all sites were simultaneously
   The sand dune plant community was modeled using two-         updated based on the adjacent sites, with time steps rep-
   dimensional cellular automata, and written and imple-         resenting 6 months. The model was run for a total of

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RA Feagin et al.                                            Loss of dune plant habitats


6 years, as this is the frequency of catastrophic distur-      upon coastal erosion rates at specific locations.           361
bances expected at the research site (Morton et al. 1983).      However, the Bruun Rule could not be used indepen-
Such disturbances often result in the retrogressing of the     dently, as it does not accommodate alongshore variability
site back to its initial bareground state.             in profile response (eg Stive 2004). To address this vari-
  An environmental stress gradient was also constructed,      ability, we used historical erosion rate data, available in
where stress values (–1) were distributed along the gradi-     a spatially explicit layer at 50 m increments in the
ent, with 100% of the sites undergoing stress at the sea-      longshore direction (Gibeaut et al. 2003), and the histor-
ward edge and 0% at the landward edge. Sites could ret-       ical eustatic rise rate from the tidal gauge data, and
rogress to earlier stages of succession if the summation of     applied the Bruun Rule every 50 m along the beach in
adjacent sites was reduced by adding stress. The baseline      order to derive the profile response ratio. We then
model produced plant patterns which approximated the        assumed that the ratio represented a constant function
research plot data, as described in Feagin et al. (2005).      describing the cross-shore equilibrium profile at each
                                  location, and back-calculated the expected erosion rate
Sea-level rise calculations                     for the projected sea-level rise scenarios.
                                   For example, the historical data yields:
One estimate for the global rate of sea-level rise is around
0.20 cm yr–1 over the past 100 years. However, the histori-              s    1.68 m yr–1
cal local rate may differ greatly as the relative rise is a func-             =          = 509
                                           a    0.0033m yr–1
tion of several components, including eustatic (eg global
sea-level rise due to deglaciation, or thermal expansion of
water at higher temperatures), isostatic (eg land elevation     as the ratio at our research plot, the value that we then
changes caused by plate tectonics, or rebound of land after     used for simulations at the plot site. If we assume eustatic
deglaciation), and subsidence effects (caused by with-       sea-level rises at 1.00 cm yr–1, then we calculate:
drawal of water, oil, and gas, or compaction of sediments;
Douglas 1991). The local rate of relative rise at the                 s      x
research plot was 0.65 cm yr–1 between 1909 and 1999, as                 =         = 509
                                           a    0.01m yr–1
recorded by the Galveston tide gauge (Permanent Service
for Mean Sea Level; www.pole.ac.uk/psmsl). During most
of this period, the area experienced dramatic land subsi-      with the projected erosion rate at that spatial location as
dence, at a rate of 0.32 cm yr–1, due to oil and groundwater    x = 5.09 m yr–1. We repeated this calculation every 50 m
extraction (USGS 2002; www.hg.subsidence.org). If we        in the longshore direction, using the local values for the
subtract 0.32 from 0.65, this yields 0.33 cm yr–1 as the rate    Bruun ratio.
of eustatic rise at the research plot. This number exceeds      This method assumes that the rate and direction of long-
the global eustatic rate, but there are two possible explana-    shore sediment transport does not change when the sea-
tions for this: (1) thermal expansion in the warm Gulf of      level rises, yet it accounts for morphologically different
Mexico (Miller and Douglas 2004), which agrees quite        cross-shore profiles at spatial locations in the longshore
well with Leatherman’s (1984) calculations for this area,      direction. Our method assumes sediment transport in the
and/or (2) a slight, natural isostatic change at a much       erosion term, s, by using the Gibeaut et al. (2003) dataset at
larger regional scale than that of the USGS (2002) data.      each longshore location. As a reasonable first approxima-
 Sea-level rise is a dominant force driving widespread       tion in our model, the proportion of s that could be due to
coastal erosion (Leatherman et al. 2000; Zhang et al. 2004).    a negative or positive sediment transport term (often
Since the terrain of the Gulf Coast is quite flat, a small rise   known as q in coastal engineering literature) will propor-
in water level can result in a severe loss of land. Historical   tionately change with the rise, a, based on the empirically
erosion rates exceed 3.3 m yr–1 at the research plot loca-     derived relationship at each spatial location separately.
tion (Gibeaut et al. 2003; www.beg.utexas.edu/coastal/haz-     Another assumption is that sediment particle sizes remain
ard_atlas1.htm). If we partition this erosion rate into a lin-   constant, also subsumed in the relatively high Bruun rule
ear function of its known subsidence and eustatic          ratios that can be found on Galveston’s high clay content
components, then 1.68 m yr–1 of this erosion was due to       beaches. The method also assumes that the cause of histor-
eustatic rise.                           ical, long-term erosion is sea-level rise, with beach profiles
 To simulate the impacts of sea-level rise on coastal       recovering from transitory storm effects; this accords with
retreat, we extrapolated the historical erosion rate into      the most recent evidence (Zhang et al. 2002).
the future, and assumed that the Bruun Rule (essentially
a ratio of the sea level rise rate, a, to the profile migration,  Simulations
or erosion, rate, s) provides a reasonable first approxima-
tion of cross-shore profile response of water-level change     Simulations expanded upon the baseline model by (1) test-
(Leatherman et al. 2000; Zhang et al. 2004). We then pro-      ing hypotheses about plant community response to erosion
jected future sea-level rise scenarios and their impacts      induced by climate change, and (2) adding new parameters

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   Loss of dune plant habitats                                            RA Feagin et al.


362   (a)                                       Several changes were also made to facilitate
                                           investigations into plant community responses
                                           to longshore morphology. First, the scale of
                                           inquiry was broadened and 1500 x 100 m strips
                                           were demarcated along the coastline on an aer-
                                           ial photograph within a geographic information
                                           system (GIS). A grid was created, showing
                                           locations where dune growth was restricted by
                                Relative sea-level  barriers such seawalls, geotextile tubes, mani-
                                   rise risk    cured lawn vegetation, or housing develop-
                                      high   ments. The grid was then fed into the model
                                           and run as a 3000 x 200 site lattice, with each
                                     low     site representing 0.5 x 0.5 m in the GIS. The
    (b)                                      historical erosion rate data were also fed into
                                           the model with values explicitly known at 50 m
                                           increments in the longshore direction. In addi-
                                           tion to the previous four states (0, 1, 2, 3), sites
                                           could now also be categorized as an upland
                                           plant community (4) or a barrier to dune devel-
                                           opment (5). Model output was then geo-refer-
                                           enced within the GIS, producing maps that
                                           detailed plant community responses to the
     (c)            (d)          (e)             much broader-scaled phenomena of longshore
                                           coastal morphology.

                                             Results and discussion

                                             Coastal erosion due to sea-level rise, in concert
                                             with human-erected barriers, confined simu-
                                             lated plants to a small zone at the rear of the
       Sand          Soil binders       Upland/coastal prairie
                                             research plot, altering the sand dune plant
       Colonizers       Competitors       Barrier
                                             community’s characteristic patterns. Although
                                             littoral drift, sediment dynamics, and beach
   Figure 2. Sea-level rise, the risks, and impacts to the sand dune plant composition may differentially affect the risks
   community at multiple scales. (a) The risk of relative sea-level rise at the scale to sand dune habitat at a continental scale
   of the continental US (adapted source data from Thieler and Hammar-Klose (Figure 2a), the landscape-scale GIS output
   2000). At the Galveston Island landscape scale (b), the model was run and demonstrates little deviation in the plant com-
   geo-referenced within a GIS. Here, we show the results for the most munity’s development as a result of the long-
   accelerated rate of sea-level rise, with the circles representing the spatial shore morphology response to sea-level rise, for
   locations mentioned in the text. At the plant community scale, simulations 50 m increments (Figure 2b). Yet the spatial
   were executed under the three IPCC scenarios: the (c) low, (d) moderate, and limitations enforced by the barriers led to a dif-
   (e) high rise scenarios. Plant community scale maps have the ocean at the ferential placement of the community in the
   bottom, a barrier at the top, and are 35 x 35 m.                  longshore direction, leaving some areas com-
                                             pletely barren of dune habitat and others with
   to the model to incorporate longshore morphological char- remnant dune populations.
   acteristics at a broader scale with the help of a Geographic       Strong differences in community pattern development
   Information System (GIS). Simulations tested the effect of resulted in the cross-shore direction under the three IPCC
   sea-level rise on the plant patterns by moving the stress scenarios. The low rise scenario (Figure 2c) allowed the
   gradient in the landward direction along the cross-shore plant community to develop fully in 5 years within the
   profile. The rate at which the rising sea, and the associated research plot area. Dunes were covered by all stages of suc-
   stress gradient, moved landward was represented in three cession, including coastal prairie and upland plants.
   well-known Intergovernmental Panel on Climate Change           Under the moderate rise scenario (Figure 2d), plants did
   simulation scenarios (IPCC 2001): a low rise scenario not grow in the lower section of the gradient due to high
   (0.09 m by 2100), a moderate rise scenario (0.48 m by stress levels. There was a random distribution of colonizers
   2100), and a high rise scenario (0.88 m by 2100). We mod- across the mid-section of the gradient, yet the community
   eled each scenario according to the method outlined in the did not develop further due to the high level of stress and
   previous section.                           the lack of permanence in the distribution of the annual

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RA Feagin et al.                                          Loss of dune plant habitats


colonizers. In the upper section of the gradient, only a few   certain to increase. Dependent animal species, such as        363
embryonic dunes had formed, around which the later suc-     the endangered Kemp’s ridley sea turtle (Lepidochelys
cessional species could coalesce. In these areas, the plant   kempii) and the endangered Choctawatchee beach mouse
distribution was not random, as each successional stage     (Peromyscus polionotus), will be lost without them.
depended on prior amelioration of the location by the col-    Although our simulations are restricted to the conditions
onizers in order to survive.                   at Galveston Island, the basis for the model suggests that
  The high rise scenario resulted in a complete breakdown    similar scenarios, with different retreat rates, will occur in
of the successional process, as only colonizers were ran-    the near future, in most dune systems along developed
domly distributed and located in the upper section of the    coastlines experiencing sea-level transgression.
gradient in the research plot area (Figure 2e). The random     Considerable planning needs to be done in order to
distribution of the annual colonizers, their ephemeral      decide how we want our coasts to look, as well as to pre-
nature, and their restriction to the thin strip of habitat did  serve the species that inhabit them. A small increase in
not provide the ameliorative force (eg windblocks, islands    sea-level rise can result in a large amount of coastal ero-
of fertility, elevated dune structures) that was needed for   sion. It may soon be necessary to reassess the importance
the community to progress to advanced stages of succes-     of sand dune plant communities, in the same way that
sion. In terms of the spatial and temporal mechanics of     coastal wetlands were re-evaluated by the last generation
any one focal site, the summation of its adjacent positively   of coastal zone statutes and laws. As it is our duty to
valued sites (facilitative plants) and its adjacent nega-    maintain these coastal plant communities alongside our
tively valued sites (stress) produced a number that did not   private developments and shoreline protection structures,
exceed the threshold for its graduation to the next stage of   we must publicly consider the impacts of squeezing the
succession. Without strong and spatially explicit facilita-   sand dune plant habitat between the land and the sea.
tion to counterbalance the stress induced by sea-level rise,
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