122
R. J. Orth, M. C. Harwell and G. J. Inglis
Biotic agents such as waterfowl, especially migrating waterfowl, manatees, dugongs, or fish also
have the potential to move seeds relatively long distances if they are ingested (Baldwin and Lovvern,
1994; Figuerola and Green, 2002; Figuerola et al.,
2002; 2003). Seagrass species with relatively hard
seed coats (e.g. Halodule) may survive the passage
through the guts of these species better than those
with softer seed coats (e.g. Zostera). Figuerola et al.
(2002) found that germination of Ruppia maritima
seeds was enhanced by passage through the guts of
ducks but was species specific and probably related
to gut structure (i.e. type of gizzard). An important
but overlooked aspect may be the timing of availability of viable seeds and the presence of migrating
waterfowl. For example, in Chesapeake Bay, USA,
Z. marina seeds are produced in May and June and
germinate in late November before wintering waterfowl are present, making it unlikely they could
disperse seeds (Orth and Moore, 1986; Moore et al.,
1993). Knowledge of seed release and germination
periods coupled to an understanding of the feeding strategies of waterfowl that inhabit an area, will
be important in elucidating the role of waterfowl in
seagrass seed dispersal. In more intensively grazed
meadows, the feeding activities of these large herbivores may actually stimulate seed production and
germination (Peterken and Conacher, 1997). The
decline of many of these large herbivore species,
because of over-harvesting or habitat destruction
(Jackson et al., 2001), could have implications for
the natural development of new seagrass beds distant from the original parent and, ultimately, for
the gene flow and genetic diversity of the different
populations.
Disturbance of a seagrass bed by biotic (e.g. turtles, manatees or fish) or abiotic (e.g. storms) can
result in reproductive fragments being ripped and
exported from the bed. Patterson et al. (2001) examined the biomechanical properties of how Z. marina reproductive shoots become fragmented. They
suggested that the characteristics of how a reproductive shoots breaks potentially allows for dispersal of
some reproductive fragments with viable seeds while
allowing some seeds to be retained within the bed for
self-maintenance.
Biotic agents can also limit primary dispersal of
seagrass seeds. Reproductive shoots carrying viable
Z. marina seeds have been found cemented into the
tube cap of the common tube-building polychaete,
Diopatra cuprea (Harwell and Orth, 2001). Drifting plant fragments appear to be entrained into the
tube cap and subsequently incorporated into the construction of the tube. However, this retention may be
important in establishment of beds distant from a
source if Diopatra inhabiting suitable but unvegetated bottom captures fragments that have floated
from afar. In northern Australia, seagrass seeds accumulate within trails excavated in the meadows by
feeding dugongs (Inglis, 2000b).
V. Seed Recruitment and Seedling
Establishment
While studies have shown seagrasses can produce
large numbers of seeds, those few studies that have
followed through have reported a very low rate of
seedling establishment (Table 1). Rarity of seedlings
at a site may be due to recruitment or seed limitation (the inability of propagules to reach a suitable site or too few propagules being produced) or
microsite limitation (the absence of suitable ‘safe’
sites) (Harper, 1977; Eriksson, 1989; Eriksson and
Ehrlen, 1992). Recruitment limitation can also arise
if hydrodynamic conditions, such as currents or
winds, are unable to deliver a propagule to a site,
or if there is a distinct physical barrier to dispersal
such as a land bridge. While Crawley (1990) argued
that microsite limitation is predominant in plant populations, Eriksson and Ehrlen (1992) suggested that
recruitment for clonal plants is the result of both
microsite and seed availability.
The sediment surface may be viewed as a patchwork or ‘lattice’ of sites that may be considered
safe and unsafe for germination (Harper, 1977) with
seeds having to pass through a suite of environmental
obstacles (an ‘environmental sieve’, Harper, 1977),
from the time of release to the time of germination
at a ‘safe’ site. Harper (1977) defined a ‘safe’ site
as a zone where seeds find the required stimuli and
resources to germinate and grow, and where mortality factors are absent. What constitutes a ‘safe
site’ in seagrass systems may, in part, be related
to the topographic complexity of the bottom created by biotic (pits and burrows of animals) or abiotic (sand ripples) factors and how rapidly a seed
can be buried before it is eaten or washed out to
an unsuitable site, leading to the patterns noted in
Fig. 5.
Recruitment behavior of clonal plants has been
classified as species that exhibit either repeated sexual recruitment (RSR) or those species whose seeds
are only responsible for the initial establishment
R. J. Orth, M. C. Harwell and G. J. Inglis
Biotic agents such as waterfowl, especially migrating waterfowl, manatees, dugongs, or fish also
have the potential to move seeds relatively long distances if they are ingested (Baldwin and Lovvern,
1994; Figuerola and Green, 2002; Figuerola et al.,
2002; 2003). Seagrass species with relatively hard
seed coats (e.g. Halodule) may survive the passage
through the guts of these species better than those
with softer seed coats (e.g. Zostera). Figuerola et al.
(2002) found that germination of Ruppia maritima
seeds was enhanced by passage through the guts of
ducks but was species specific and probably related
to gut structure (i.e. type of gizzard). An important
but overlooked aspect may be the timing of availability of viable seeds and the presence of migrating
waterfowl. For example, in Chesapeake Bay, USA,
Z. marina seeds are produced in May and June and
germinate in late November before wintering waterfowl are present, making it unlikely they could
disperse seeds (Orth and Moore, 1986; Moore et al.,
1993). Knowledge of seed release and germination
periods coupled to an understanding of the feeding strategies of waterfowl that inhabit an area, will
be important in elucidating the role of waterfowl in
seagrass seed dispersal. In more intensively grazed
meadows, the feeding activities of these large herbivores may actually stimulate seed production and
germination (Peterken and Conacher, 1997). The
decline of many of these large herbivore species,
because of over-harvesting or habitat destruction
(Jackson et al., 2001), could have implications for
the natural development of new seagrass beds distant from the original parent and, ultimately, for
the gene flow and genetic diversity of the different
populations.
Disturbance of a seagrass bed by biotic (e.g. turtles, manatees or fish) or abiotic (e.g. storms) can
result in reproductive fragments being ripped and
exported from the bed. Patterson et al. (2001) examined the biomechanical properties of how Z. marina reproductive shoots become fragmented. They
suggested that the characteristics of how a reproductive shoots breaks potentially allows for dispersal of
some reproductive fragments with viable seeds while
allowing some seeds to be retained within the bed for
self-maintenance.
Biotic agents can also limit primary dispersal of
seagrass seeds. Reproductive shoots carrying viable
Z. marina seeds have been found cemented into the
tube cap of the common tube-building polychaete,
Diopatra cuprea (Harwell and Orth, 2001). Drifting plant fragments appear to be entrained into the
tube cap and subsequently incorporated into the construction of the tube. However, this retention may be
important in establishment of beds distant from a
source if Diopatra inhabiting suitable but unvegetated bottom captures fragments that have floated
from afar. In northern Australia, seagrass seeds accumulate within trails excavated in the meadows by
feeding dugongs (Inglis, 2000b).
V. Seed Recruitment and Seedling
Establishment
While studies have shown seagrasses can produce
large numbers of seeds, those few studies that have
followed through have reported a very low rate of
seedling establishment (Table 1). Rarity of seedlings
at a site may be due to recruitment or seed limitation (the inability of propagules to reach a suitable site or too few propagules being produced) or
microsite limitation (the absence of suitable ‘safe’
sites) (Harper, 1977; Eriksson, 1989; Eriksson and
Ehrlen, 1992). Recruitment limitation can also arise
if hydrodynamic conditions, such as currents or
winds, are unable to deliver a propagule to a site,
or if there is a distinct physical barrier to dispersal
such as a land bridge. While Crawley (1990) argued
that microsite limitation is predominant in plant populations, Eriksson and Ehrlen (1992) suggested that
recruitment for clonal plants is the result of both
microsite and seed availability.
The sediment surface may be viewed as a patchwork or ‘lattice’ of sites that may be considered
safe and unsafe for germination (Harper, 1977) with
seeds having to pass through a suite of environmental
obstacles (an ‘environmental sieve’, Harper, 1977),
from the time of release to the time of germination
at a ‘safe’ site. Harper (1977) defined a ‘safe’ site
as a zone where seeds find the required stimuli and
resources to germinate and grow, and where mortality factors are absent. What constitutes a ‘safe
site’ in seagrass systems may, in part, be related
to the topographic complexity of the bottom created by biotic (pits and burrows of animals) or abiotic (sand ripples) factors and how rapidly a seed
can be buried before it is eaten or washed out to
an unsuitable site, leading to the patterns noted in
Fig. 5.
Recruitment behavior of clonal plants has been
classified as species that exhibit either repeated sexual recruitment (RSR) or those species whose seeds
are only responsible for the initial establishment