Chapter 11 Dynamics of Seagrasses
283
and the formation and development of new patches
(Marb` a and Duarte, 1995).
The dynamic properties of seagrass patch formation and subsequent growth and survival are essential
to the recolonization process in denuded areas. The
more than 10-fold span across species in rhizome
elongation rates and reproductive effort, defining an
upper limit for patch formation from seed, suggests
contrasting capacities to recover from disturbances
(Duarte et al., 1997b; Marb` a and Duarte, 1998;
Marb` a and Walker, 1999). Small seagrass species exhibit potential fast patch growth, and clonal growth
of these species is held responsible for much of the
temporal dynamics observed following small-scale
disturbances (Williams, 1990; Duarte et al., 1997b).
Sexual reproduction is, however, still essential for
the recovery of small seagrasses (e.g. Kenworthy,
2000). Nevertheless, some of the larger seagrass
species (e.g. Zostera marina) with slow elongation
rates can achieve high colonization potential by having high reproductive effort (Verhagen and Nienhuis, 1983). In contrast the combination of very slow
clonal growth and poor ability to set seeds in other
large species (e.g. Posiodonia oceanica and P. sinuosa) suggests that these are to slow patch growth
and an extremely slow recovery process (Duarte,
1995).
Small seagrass species also tend to produce more
seeds per ground area than large species and have
the ability to build up persistent seed banks whereas
large species typically produce seed with no or limited dormancy (Kuo and den Hartog, Chapter 3).
However, the rate of patch formation from seeds
does not necessarily bear a simple relationship to
seed production but is also influenced by loss processes acting on seeds and seedlings and by the seed
dispersal capacity (Orth et al., Chapter 5). In a recent
study (Olesen et al., 2004), the importance of contrasting reproductive strategies to recovery dynamics was studied over 2.5 years on a mixed-species
Philippine seagrass meadow by following patch formation, growth, and mortality in a disturbed gap
area (1200 m
2 ). Different species were involved in
sexual vs. colonization as the large species Thalassia hemprichii and Enhalus acoroides with slow
clonal growth but relatively high production of large,
broadly dispersed seeds were the major contributors to colonization in areas devoid of vegetation.
Although seedling turnover was rapid the high frequency of sexual recruitment (T. hempricii 0.052–
1.31 m
−2 year
−1 and E. acoroides 0.043–0.081 m
−2
year
−1 ) allowed the successful formation and development of new patches and subsequent patch extension through clonal growth. In contrast the small fastgrowing species Cymodocea rotundata and Halodule uninervis with limited seed dispersal ensured
rapid clonal extension (>1.5 m year
−1 ) of surviving patches in areas where disturbances had only
removed part of the existing flora. Hence, where
species of both strategies are present, the scale of
area affected by disturbance and its interaction with
the reproductive strategy of the contrasting species
is fundamental to the recovery dynamics of seagrass
communities.
V. Gap Dynamics
In seagrass species that form extensive meadows,
intense but localized disturbances can cause scars
in the meadow that are akin to canopy light gaps
in forests. Gap dynamics is a key component of
seagrass dynamics (Bell et al., 1999), as gaps are
produced often through physical and biological (e.g.
Nakaoka and Aioi, 1999) disturbances. In such gaps,
the death of later-successional, better competitor
species through many different mechanisms can provide small gaps that allow space for the recruitment of new individuals into the forest. As there
is often an inverse relationship between competitive ability and colonization potential, the first colonizers to these gaps are generally species that will,
through time, be replaced by the original superior
competitor (see Pickett and White, 1985 for a detailed treatment of forest light gaps). In Thalassiadominated seagrass beds of the tropical Western
Atlantic, small scale physical disturbances caused
by wave action or herbivory can remove the dense
Thalassia canopy and provide room for calcareous macroalgae and faster-growing seagrasses like
Halodule wrightii and Syringodium filiforme to become established (e.g. den Hartog, 1971; Patriquin,
1973; van Tussenbroek et al., Chapter 18). These features tend to erode at one end and fill in at the other,
thereby slowly moving through space in a direction
determined by the predominant wave and current
regime. At the trailing edge of these ‘blowouts’, the
rapidly colonizing species are replaced by Thalassia testudinum, as new ground for the early successional species is cleared at the leading edge by
continued erosion. Disturbances like this allow for
the coexistence of competitively inferior species in a
283
and the formation and development of new patches
(Marb` a and Duarte, 1995).
The dynamic properties of seagrass patch formation and subsequent growth and survival are essential
to the recolonization process in denuded areas. The
more than 10-fold span across species in rhizome
elongation rates and reproductive effort, defining an
upper limit for patch formation from seed, suggests
contrasting capacities to recover from disturbances
(Duarte et al., 1997b; Marb` a and Duarte, 1998;
Marb` a and Walker, 1999). Small seagrass species exhibit potential fast patch growth, and clonal growth
of these species is held responsible for much of the
temporal dynamics observed following small-scale
disturbances (Williams, 1990; Duarte et al., 1997b).
Sexual reproduction is, however, still essential for
the recovery of small seagrasses (e.g. Kenworthy,
2000). Nevertheless, some of the larger seagrass
species (e.g. Zostera marina) with slow elongation
rates can achieve high colonization potential by having high reproductive effort (Verhagen and Nienhuis, 1983). In contrast the combination of very slow
clonal growth and poor ability to set seeds in other
large species (e.g. Posiodonia oceanica and P. sinuosa) suggests that these are to slow patch growth
and an extremely slow recovery process (Duarte,
1995).
Small seagrass species also tend to produce more
seeds per ground area than large species and have
the ability to build up persistent seed banks whereas
large species typically produce seed with no or limited dormancy (Kuo and den Hartog, Chapter 3).
However, the rate of patch formation from seeds
does not necessarily bear a simple relationship to
seed production but is also influenced by loss processes acting on seeds and seedlings and by the seed
dispersal capacity (Orth et al., Chapter 5). In a recent
study (Olesen et al., 2004), the importance of contrasting reproductive strategies to recovery dynamics was studied over 2.5 years on a mixed-species
Philippine seagrass meadow by following patch formation, growth, and mortality in a disturbed gap
area (1200 m
2 ). Different species were involved in
sexual vs. colonization as the large species Thalassia hemprichii and Enhalus acoroides with slow
clonal growth but relatively high production of large,
broadly dispersed seeds were the major contributors to colonization in areas devoid of vegetation.
Although seedling turnover was rapid the high frequency of sexual recruitment (T. hempricii 0.052–
1.31 m
−2 year
−1 and E. acoroides 0.043–0.081 m
−2
year
−1 ) allowed the successful formation and development of new patches and subsequent patch extension through clonal growth. In contrast the small fastgrowing species Cymodocea rotundata and Halodule uninervis with limited seed dispersal ensured
rapid clonal extension (>1.5 m year
−1 ) of surviving patches in areas where disturbances had only
removed part of the existing flora. Hence, where
species of both strategies are present, the scale of
area affected by disturbance and its interaction with
the reproductive strategy of the contrasting species
is fundamental to the recovery dynamics of seagrass
communities.
V. Gap Dynamics
In seagrass species that form extensive meadows,
intense but localized disturbances can cause scars
in the meadow that are akin to canopy light gaps
in forests. Gap dynamics is a key component of
seagrass dynamics (Bell et al., 1999), as gaps are
produced often through physical and biological (e.g.
Nakaoka and Aioi, 1999) disturbances. In such gaps,
the death of later-successional, better competitor
species through many different mechanisms can provide small gaps that allow space for the recruitment of new individuals into the forest. As there
is often an inverse relationship between competitive ability and colonization potential, the first colonizers to these gaps are generally species that will,
through time, be replaced by the original superior
competitor (see Pickett and White, 1985 for a detailed treatment of forest light gaps). In Thalassiadominated seagrass beds of the tropical Western
Atlantic, small scale physical disturbances caused
by wave action or herbivory can remove the dense
Thalassia canopy and provide room for calcareous macroalgae and faster-growing seagrasses like
Halodule wrightii and Syringodium filiforme to become established (e.g. den Hartog, 1971; Patriquin,
1973; van Tussenbroek et al., Chapter 18). These features tend to erode at one end and fill in at the other,
thereby slowly moving through space in a direction
determined by the predominant wave and current
regime. At the trailing edge of these ‘blowouts’, the
rapidly colonizing species are replaced by Thalassia testudinum, as new ground for the early successional species is cleared at the leading edge by
continued erosion. Disturbances like this allow for
the coexistence of competitively inferior species in a