Chapter 5 Seagrass Seeds and Dispersal Strategies
127
within a bed as well as exported, and the fate of each
component. Mechanisms of dispersal need to be considered now that it appears seagrasses are capable
of long-distance dispersal, especially ‘nonstandard’
mechanisms (Higgins et al., 2003). This would be especially true from an historical perspective if some
processes have been lost or minimized, e.g. potential loss from large reductions of turtles and sirenians due to overfishing that could have eaten seeds
and excreted them long distances from feeding sites
(Jackson et al., 2001). Waterfowl may be an underestimated mechanism that could influence local, as
well as regional, population dynamics (Figuerola and
Green, 2002; Figuerola et al., 2002).
Terrestrial seeds have developed various structures to enhance dispersal, e.g. wings, barbs, pappi.
While some species have developed structures to
facilitate entrainment (e.g. barbs of Phyllospadix
seeds, or the hooked bases of Amphibolis), seagrass
seeds appear to be remarkably simple in ornamentation suggesting seeds have not evolved mechanisms
for dispersal. It is entirely possible that the ornamentation seeds do have functions more to retain seeds
where they settle, e.g. the ‘membrane’ surrounding
Posidonia seeds.
Many dispersal studies have emphasized how far
a seed can travel (the tail of the dispersal curve).
However, understanding the shape of the dispersal
curve has important consequences for not only the
rate of spread of a species, but also metapopulation
dynamics (Clark et al., 1998; Williamson, 2002).
Future studies with seagrass dispersal, both empirical and modeling, need to consider both aspects. As
Williamson (2002) concluded “it’s the tail that wags
the dog.”
C. Mating Systems
Greater study of seagrass seeds should also offer some useful insights into the evolution of angiosperm mating systems. Of particular importance
are recent theoretical treatments of the seed-shadow
handicap in dioecious species (Heilbuth et al., 2001).
Because only female plants produce seeds in dioecious populations, the total number of seed bearers is at least half that of an equivalent sized
population of co-sexual species. Maintenance of
dioecious species in the presence of hermaphroditic
competitors, therefore, requires that this disadvantage is overcome by: (1) greater fecundity per plant,
(2) increased division of labor between the sexes,
(3) greater offspring vigor (as a result of reduced inbreeding depression), and/or (4) more broadly dispersed seeds to escape the densely populated seed
shadows around maternal dioecious plants (Heilbuth
et al., 2001). The predominance of dioecy in modern seagrasses points to some unique evolutionary
pressures that have favored the retention of separate
sexes (Waycott et al., Chapter 2). Further comparison of dioecious and co-sexual species may provide
clues as to whether a seed-shadow handicap exists in
these clonal species and, if it does, what traits offset
the disadvantages associated with dioecy.
D. Adult–Seedling Interactions
The importance of adult–seedling interactions in terrestrial plant assemblages argues for greater attention to these issues in seagrass populations (Olesen,
1999; Duarte et al., Chapter 11). Although longdistance dispersal has been demonstrated for seagrass seeds, it is likely that some proportion of seeds
do not travel outside the stands that produce them.
The longevity of some seagrass clones (Reusch et al.,
1999) may mean that seed production and suitable microsites for recruitment occur infrequently
in space and time if clones are sufficiently dense and
continuous to compromise seedling survival. At this
stage, it is unclear if seedling germination, survival
and growth is inhibited by the presence of existing
adults (through more efficient utilization of light and
nutrients by adults) or facilitated by them (through
the provision of shelter, anchorage and the greater
organic content in the sediments of existing stands).
Given the variety in dispersal strategies and adult
size and longevity, we predict that the answers to
these questions will also vary significantly among
species.
E. Recruitment
Understanding the roles of recruitment limitation vs.
microsite limitation in seagrass species is also important for planning appropriate conservation strategies. These questions can only be addressed through
seed sowing experiments, either through seed augmentation where sown species are residents of the
target community, or seed introduction, where sown
species are not part of the community (Turnbull
127
within a bed as well as exported, and the fate of each
component. Mechanisms of dispersal need to be considered now that it appears seagrasses are capable
of long-distance dispersal, especially ‘nonstandard’
mechanisms (Higgins et al., 2003). This would be especially true from an historical perspective if some
processes have been lost or minimized, e.g. potential loss from large reductions of turtles and sirenians due to overfishing that could have eaten seeds
and excreted them long distances from feeding sites
(Jackson et al., 2001). Waterfowl may be an underestimated mechanism that could influence local, as
well as regional, population dynamics (Figuerola and
Green, 2002; Figuerola et al., 2002).
Terrestrial seeds have developed various structures to enhance dispersal, e.g. wings, barbs, pappi.
While some species have developed structures to
facilitate entrainment (e.g. barbs of Phyllospadix
seeds, or the hooked bases of Amphibolis), seagrass
seeds appear to be remarkably simple in ornamentation suggesting seeds have not evolved mechanisms
for dispersal. It is entirely possible that the ornamentation seeds do have functions more to retain seeds
where they settle, e.g. the ‘membrane’ surrounding
Posidonia seeds.
Many dispersal studies have emphasized how far
a seed can travel (the tail of the dispersal curve).
However, understanding the shape of the dispersal
curve has important consequences for not only the
rate of spread of a species, but also metapopulation
dynamics (Clark et al., 1998; Williamson, 2002).
Future studies with seagrass dispersal, both empirical and modeling, need to consider both aspects. As
Williamson (2002) concluded “it’s the tail that wags
the dog.”
C. Mating Systems
Greater study of seagrass seeds should also offer some useful insights into the evolution of angiosperm mating systems. Of particular importance
are recent theoretical treatments of the seed-shadow
handicap in dioecious species (Heilbuth et al., 2001).
Because only female plants produce seeds in dioecious populations, the total number of seed bearers is at least half that of an equivalent sized
population of co-sexual species. Maintenance of
dioecious species in the presence of hermaphroditic
competitors, therefore, requires that this disadvantage is overcome by: (1) greater fecundity per plant,
(2) increased division of labor between the sexes,
(3) greater offspring vigor (as a result of reduced inbreeding depression), and/or (4) more broadly dispersed seeds to escape the densely populated seed
shadows around maternal dioecious plants (Heilbuth
et al., 2001). The predominance of dioecy in modern seagrasses points to some unique evolutionary
pressures that have favored the retention of separate
sexes (Waycott et al., Chapter 2). Further comparison of dioecious and co-sexual species may provide
clues as to whether a seed-shadow handicap exists in
these clonal species and, if it does, what traits offset
the disadvantages associated with dioecy.
D. Adult–Seedling Interactions
The importance of adult–seedling interactions in terrestrial plant assemblages argues for greater attention to these issues in seagrass populations (Olesen,
1999; Duarte et al., Chapter 11). Although longdistance dispersal has been demonstrated for seagrass seeds, it is likely that some proportion of seeds
do not travel outside the stands that produce them.
The longevity of some seagrass clones (Reusch et al.,
1999) may mean that seed production and suitable microsites for recruitment occur infrequently
in space and time if clones are sufficiently dense and
continuous to compromise seedling survival. At this
stage, it is unclear if seedling germination, survival
and growth is inhibited by the presence of existing
adults (through more efficient utilization of light and
nutrients by adults) or facilitated by them (through
the provision of shelter, anchorage and the greater
organic content in the sediments of existing stands).
Given the variety in dispersal strategies and adult
size and longevity, we predict that the answers to
these questions will also vary significantly among
species.
E. Recruitment
Understanding the roles of recruitment limitation vs.
microsite limitation in seagrass species is also important for planning appropriate conservation strategies. These questions can only be addressed through
seed sowing experiments, either through seed augmentation where sown species are residents of the
target community, or seed introduction, where sown
species are not part of the community (Turnbull