636
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
2003). It is important to note, however, that logistical considerations have often limited experimental
studies investigating relationships between seagrass
patch size and fragmentation vs. faunal abundance to
utilizing experimental units (such as ASUs) which,
while feasible to manipulate and replicate, may be
of limited use because of their small size. Therefore,
our ability to evaluate the impacts of fragmentation
via experimentation is restricted to small extents at
this time.
V. Landscape Dynamics
A. Disturbance and Growth
Studies on seagrasses have documented that seagrass
distribution is strongly related to physiological tolerances and other growth restrictions including water
depth, light attenuation, and salinity zonation (see
Marb` a et al., Chapter 6; Zimmerman, Chapter 13).
Superimposed upon this autecological template are
biotic factors such as grazing or competitive interactions, which may contribute to the observed spatial
arrangement of seagrasses. One of the challenges
emerging from past studies of seagrass landscapes is
to determine how disturbance may act synergistically
with such abiotic or biotic factors to produce changes
in landscape features.
Disturbance is a major factor determining spatial heterogeneity of landscapes (Baker, 1992) often
changing the substrate, the physical environmental,
and resource availability (Farina, 1998). Disturbance
has a number of attributes including frequency, intensity, and size (see White and Pickett, 1985) that
are all related to the extent of landscape modification.
While disturbance is known to shape and change
seagrass landscapes, few studies have examined the
influence of disturbance on the physiognomy of
landscapes dominated by clonal seagrasses (but see
Marb` a and Duarte, 1995; Bell et al., 1999). Physical
disturbance, whether biological (Townsend and Fonseca, 1998) and/or physical (tidal currents, waves,
sedimentation; Fonseca and Bell, 1998; Fonseca
et al., 1998; Robbins et al., 2001; Koch et al., Chapter 8), may shape the spatial patterns of seagrass
landscapes by causing gaps within seagrass beds
(Bell et al., 1999), and/or by influencing the lateral
growth behavior of the seagrass itself (Marb` a and
Duarte, 1995, Townsend and Fonseca, 1998; Fonseca
et al., 2002). Disturbance on seagrass landscapes
may even leave a spatial signature if seagrasses cannot quickly reoccupy the disturbed area of a seagrass
bed (as many species cannot), providing an opportunity to follow temporal sequences of landscape
development.
Regrowth of seagrass patch margins has been directly measured (e.g. Jensen and Bell, 2001) and
closure of gaps (especially those <10 m
2 ) is common within at least one seagrass landscape (Bell
et al., 1999). With both margins and gaps, closure
can occur via vegetative growth or seedling recruitment. Some seagrass species (e.g. Z. marina) have
an abundant seed supply and are capable of repopulating gaps via seeds and/or reproductive shoots
(e.g. Orth et al., 1994; Inglis, 2000). Other species
often have reduced flowering and, consequently, belowground growth patterns rather than seedling recruitment play a major role in gap closure, regardless of the disturbance mechanism (see Rasheed,
1999; Bell et al., 1999). Gap closure rate via belowground growth varies among species with rhizome elongation rates being highly variable both
across (e.g. Hemminga and Duarte, 2000) and within
species (e.g. Jensen and Bell, 2001). A third mechanism of gap closure may be regrowth of short shoots
from buried rhizomes. Gap closure could occur if
sediments shift sufficiently to allow regrowth from
previously buried belowground components, or if
shoots are able to elongate vertically at a sufficient
rate to outpace sedimentation (Marb` a et al., 1994).
However, the capacity of some seagrasses to persist after burial may be limited (Mills and Fonseca,
2003).
B. Pathogens and Disease
Disease, another example of disturbance in seagrasses, is itself influenced by a host of factors. Seagrass disease may cause impacts to the landscape on
a par with more commonly measured disturbances
(Entel and Hamilton, 1999). When a major component of the landscape is infected by a pathogen
then (as is often true for other forms of disturbance)
the potential exists for loss of that component from
the landscape, leading to gap formation and/or fragmentation of the habitat. Some marine communities
(corals) have been reported to be less likely to recover from disease than from severe physical disturbances such as hurricanes (e.g. Bythell et al., 2000)
but whether this applies at the landscape level remains untested for seagrasses.
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
2003). It is important to note, however, that logistical considerations have often limited experimental
studies investigating relationships between seagrass
patch size and fragmentation vs. faunal abundance to
utilizing experimental units (such as ASUs) which,
while feasible to manipulate and replicate, may be
of limited use because of their small size. Therefore,
our ability to evaluate the impacts of fragmentation
via experimentation is restricted to small extents at
this time.
V. Landscape Dynamics
A. Disturbance and Growth
Studies on seagrasses have documented that seagrass
distribution is strongly related to physiological tolerances and other growth restrictions including water
depth, light attenuation, and salinity zonation (see
Marb` a et al., Chapter 6; Zimmerman, Chapter 13).
Superimposed upon this autecological template are
biotic factors such as grazing or competitive interactions, which may contribute to the observed spatial
arrangement of seagrasses. One of the challenges
emerging from past studies of seagrass landscapes is
to determine how disturbance may act synergistically
with such abiotic or biotic factors to produce changes
in landscape features.
Disturbance is a major factor determining spatial heterogeneity of landscapes (Baker, 1992) often
changing the substrate, the physical environmental,
and resource availability (Farina, 1998). Disturbance
has a number of attributes including frequency, intensity, and size (see White and Pickett, 1985) that
are all related to the extent of landscape modification.
While disturbance is known to shape and change
seagrass landscapes, few studies have examined the
influence of disturbance on the physiognomy of
landscapes dominated by clonal seagrasses (but see
Marb` a and Duarte, 1995; Bell et al., 1999). Physical
disturbance, whether biological (Townsend and Fonseca, 1998) and/or physical (tidal currents, waves,
sedimentation; Fonseca and Bell, 1998; Fonseca
et al., 1998; Robbins et al., 2001; Koch et al., Chapter 8), may shape the spatial patterns of seagrass
landscapes by causing gaps within seagrass beds
(Bell et al., 1999), and/or by influencing the lateral
growth behavior of the seagrass itself (Marb` a and
Duarte, 1995, Townsend and Fonseca, 1998; Fonseca
et al., 2002). Disturbance on seagrass landscapes
may even leave a spatial signature if seagrasses cannot quickly reoccupy the disturbed area of a seagrass
bed (as many species cannot), providing an opportunity to follow temporal sequences of landscape
development.
Regrowth of seagrass patch margins has been directly measured (e.g. Jensen and Bell, 2001) and
closure of gaps (especially those <10 m
2 ) is common within at least one seagrass landscape (Bell
et al., 1999). With both margins and gaps, closure
can occur via vegetative growth or seedling recruitment. Some seagrass species (e.g. Z. marina) have
an abundant seed supply and are capable of repopulating gaps via seeds and/or reproductive shoots
(e.g. Orth et al., 1994; Inglis, 2000). Other species
often have reduced flowering and, consequently, belowground growth patterns rather than seedling recruitment play a major role in gap closure, regardless of the disturbance mechanism (see Rasheed,
1999; Bell et al., 1999). Gap closure rate via belowground growth varies among species with rhizome elongation rates being highly variable both
across (e.g. Hemminga and Duarte, 2000) and within
species (e.g. Jensen and Bell, 2001). A third mechanism of gap closure may be regrowth of short shoots
from buried rhizomes. Gap closure could occur if
sediments shift sufficiently to allow regrowth from
previously buried belowground components, or if
shoots are able to elongate vertically at a sufficient
rate to outpace sedimentation (Marb` a et al., 1994).
However, the capacity of some seagrasses to persist after burial may be limited (Mills and Fonseca,
2003).
B. Pathogens and Disease
Disease, another example of disturbance in seagrasses, is itself influenced by a host of factors. Seagrass disease may cause impacts to the landscape on
a par with more commonly measured disturbances
(Entel and Hamilton, 1999). When a major component of the landscape is infected by a pathogen
then (as is often true for other forms of disturbance)
the potential exists for loss of that component from
the landscape, leading to gap formation and/or fragmentation of the habitat. Some marine communities
(corals) have been reported to be less likely to recover from disease than from severe physical disturbances such as hurricanes (e.g. Bythell et al., 2000)
but whether this applies at the landscape level remains untested for seagrasses.