630
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
patch expansion occurred primarily on the landward
edge of the landscape. They also documented that experimental disturbance had greater impact on patch
mortality when directed at the expanding landward
edges of patches (Ramage and Schiel, 1999). In a
study of Halodule wrightii patches on a shoal in
Florida, USA, Jensen and Bell (2001) noted a correlation between belowground morphological characteristics and aboveground plant dynamics. They
suggested that patch dynamics of seagrasses at the
landscape scale might result from small-scale plastic
behaviors of individual seagrass ramets. These studies offer some insight into how spatial arrangement
of patches within a landscape may be dynamic and
can potentially be influenced by a variety of factors.
One aspect of patch dynamics, which is of critical concern, is the origin of patches within a landscape. Several studies have examined the ability of
seagrasses to colonize unvegetated areas. A possible source of new patches is from seed dispersal
and successful plant germination (see Orth et al.,
Chapter 5). Orth et al. (1994) showed that Z. marina
seeds were capable of only very limited dispersal
under normal conditions, and argued that reliance
on surviving patches to recolonize ‘sink’ populations of this species at even moderate distances was
probably not feasible. In a review of seagrass seed
dormancy, Orth et al. (2000) suggested that studies
of seeds in a landscape context, such as a comparison of the spatial arrangement of the seed bank to the
distribution of seagrass above it, would be instructive for evaluating the persistence of populations of
this species [see also Inglis (2000)]. Hammerstrom
and Kenworthy (2003) documented the existence of
a seed bank for Halophila decipiens in offshore seagrass beds, which serves as the dominant mode of
new patch origin (and therefore landscape spatial
structure) in these coastal sediments on an annual
basis.
2. Asexual Propagation and
Faunal Interactions
Many studies on seagrass growth and/or reproduction suggest that local persistence and long distance
dispersal of seagrasses are often achieved by asexual spreading via rhizome elongation rather than by
seed recruitment (e.g. Marb` a and Duarte, 1998; Rollon et al., 2003; Duarte et al., Chapter 11). However,
information on the contribution of asexual reproduction of seagrasses to the development of new
patches remains limited in scope. Robbins and Bell
(2000) reported the appearance of new patches of
Halodule wrightii over a 2-year period in FL, USA,
and because seeds were never observed on plants
at the site, concluded that these patches must have
arisen from asexual growth alone.
Apart from sexual reproduction via seeds or asexual reproduction by clonal lateral growth, neither of
which appear likely to foster dramatic recovery of
many seagrass species into denuded areas, viable
fragments of seagrass shoots transported over large
distances remains the suggested mode of seagrass recruitment into unvegetated areas in some locations.
Harwell and Orth (2001) reported that reproductive
shoots of Z. marina were present in the tubes of
70% of the polychaete, Diopatra cuprea, examined
in the Chesapeake Bay. If worms in unvegetated
sediments can capture viable reproductive shoots,
which then drop their seeds adjacent to the tubes,
worms could perhaps be a catalyst for the colonization or recovery of seagrasses into available habitat (Harwell and Orth, 2001). However, the relative
contribution of dislodged and transported seagrass
shoots to the establishment of new seagrass patches
remains severely understudied for most species [see
also Harwell and Orth (2002)].
While examples of positive influences of fauna
like tube worms on the formation of seagrass patches
exist, there is also evidence that fauna may negatively modify seagrass structure at the landscape
level. Townsend and Fonseca (1998) expanded on
earlier work showing that bioturbators can prevent
expansion of seagrass beds and studied whether disturbance by stingrays might maintain the existing
spatial heterogeneity and patch arrangement of seagrasses (mainly Z. marina and H. wrightii) in North
Carolina, USA. Stingrays were found to disturb sediments to sufficient depth to disturb rhizome elongation, and the return interval of stingray disturbances
at one metre resolution was estimated at 1.2 years;
thus, stingray disturbance may maintain the spatial
structure of these Z. marina patches (Townsend and
Fonseca, 1998). In a study conducted in Florida,
USA, Valentine et al. (1994) reported that large rays
could damage Thalassia testudinum rhizomes but
only at bed edges. In addition, they noted that significant recession of bed edges was recorded as a
result of stone crab (Menippe spp.) burrowing.
Herbivores of seagrasses such as urchins, turtles,
birds, and marine mammals, may also contribute to
patch structure (see also Heck and Orth, Chapter 22).
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
patch expansion occurred primarily on the landward
edge of the landscape. They also documented that experimental disturbance had greater impact on patch
mortality when directed at the expanding landward
edges of patches (Ramage and Schiel, 1999). In a
study of Halodule wrightii patches on a shoal in
Florida, USA, Jensen and Bell (2001) noted a correlation between belowground morphological characteristics and aboveground plant dynamics. They
suggested that patch dynamics of seagrasses at the
landscape scale might result from small-scale plastic
behaviors of individual seagrass ramets. These studies offer some insight into how spatial arrangement
of patches within a landscape may be dynamic and
can potentially be influenced by a variety of factors.
One aspect of patch dynamics, which is of critical concern, is the origin of patches within a landscape. Several studies have examined the ability of
seagrasses to colonize unvegetated areas. A possible source of new patches is from seed dispersal
and successful plant germination (see Orth et al.,
Chapter 5). Orth et al. (1994) showed that Z. marina
seeds were capable of only very limited dispersal
under normal conditions, and argued that reliance
on surviving patches to recolonize ‘sink’ populations of this species at even moderate distances was
probably not feasible. In a review of seagrass seed
dormancy, Orth et al. (2000) suggested that studies
of seeds in a landscape context, such as a comparison of the spatial arrangement of the seed bank to the
distribution of seagrass above it, would be instructive for evaluating the persistence of populations of
this species [see also Inglis (2000)]. Hammerstrom
and Kenworthy (2003) documented the existence of
a seed bank for Halophila decipiens in offshore seagrass beds, which serves as the dominant mode of
new patch origin (and therefore landscape spatial
structure) in these coastal sediments on an annual
basis.
2. Asexual Propagation and
Faunal Interactions
Many studies on seagrass growth and/or reproduction suggest that local persistence and long distance
dispersal of seagrasses are often achieved by asexual spreading via rhizome elongation rather than by
seed recruitment (e.g. Marb` a and Duarte, 1998; Rollon et al., 2003; Duarte et al., Chapter 11). However,
information on the contribution of asexual reproduction of seagrasses to the development of new
patches remains limited in scope. Robbins and Bell
(2000) reported the appearance of new patches of
Halodule wrightii over a 2-year period in FL, USA,
and because seeds were never observed on plants
at the site, concluded that these patches must have
arisen from asexual growth alone.
Apart from sexual reproduction via seeds or asexual reproduction by clonal lateral growth, neither of
which appear likely to foster dramatic recovery of
many seagrass species into denuded areas, viable
fragments of seagrass shoots transported over large
distances remains the suggested mode of seagrass recruitment into unvegetated areas in some locations.
Harwell and Orth (2001) reported that reproductive
shoots of Z. marina were present in the tubes of
70% of the polychaete, Diopatra cuprea, examined
in the Chesapeake Bay. If worms in unvegetated
sediments can capture viable reproductive shoots,
which then drop their seeds adjacent to the tubes,
worms could perhaps be a catalyst for the colonization or recovery of seagrasses into available habitat (Harwell and Orth, 2001). However, the relative
contribution of dislodged and transported seagrass
shoots to the establishment of new seagrass patches
remains severely understudied for most species [see
also Harwell and Orth (2002)].
While examples of positive influences of fauna
like tube worms on the formation of seagrass patches
exist, there is also evidence that fauna may negatively modify seagrass structure at the landscape
level. Townsend and Fonseca (1998) expanded on
earlier work showing that bioturbators can prevent
expansion of seagrass beds and studied whether disturbance by stingrays might maintain the existing
spatial heterogeneity and patch arrangement of seagrasses (mainly Z. marina and H. wrightii) in North
Carolina, USA. Stingrays were found to disturb sediments to sufficient depth to disturb rhizome elongation, and the return interval of stingray disturbances
at one metre resolution was estimated at 1.2 years;
thus, stingray disturbance may maintain the spatial
structure of these Z. marina patches (Townsend and
Fonseca, 1998). In a study conducted in Florida,
USA, Valentine et al. (1994) reported that large rays
could damage Thalassia testudinum rhizomes but
only at bed edges. In addition, they noted that significant recession of bed edges was recorded as a
result of stone crab (Menippe spp.) burrowing.
Herbivores of seagrasses such as urchins, turtles,
birds, and marine mammals, may also contribute to
patch structure (see also Heck and Orth, Chapter 22).