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Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
photosynthetic leaves, but also out by the production of new plant modules consisting of a length of
rhizome, associated roots, and a shoot. The branching pattern created by the production of new modules varies from many-branched plants that expand
almost equally in two dimensions (e.g. Posidonia
oceanica) to plants that extend almost exclusively
linearly through space (e.g. Thalassia testudinum).
Eventually, through the action of either senescence
of modules or disturbance, these individuals can become physically separated so that what was once
one plant can become many isolated plants—but
all of these plants are genetically identical—i.e.
they are parts of the same genetic individual (i.e.
genet).
So, when studying the dynamics of seagrass populations, it is important to keep in mind that what
appears above the sediments as a shoot is likely
connected to many more shoots underground. And,
merely because two shoots do not share a common
connection somewhere under the sediments is no indication that these shoots are genetically different.
In fact, there is molecular evidence for genetically
identical shoots of T. testudinum separated by over
3 km in an otherwise genetically diverse, continuous
seagrass bed (Davis et al., 1999). A more thorough
discussion on this topic is provided in Waycott et al.
(Chapter 2).
New genets can enter a population not just through
successful seedlings, but also as adult plant fragments that may drift into a population from some distant source (Setchell, 1929). Seagrasses can float and
survive for extended periods out of the sediment; apparently viable modules of the tropical seagrass Thalassia testudinum can occasionally be found on the
temperate beaches of the North Carolina in the US
(JWF, pers. observ), over 1000 km from the nearest
known T. testudinum populations. Seagrass shoots
can survive for months in the water column, but the
ability of detached shoots to survive when transplanted decreases with time in the water column,
limiting the potential of drifting adult plants to establish new seagrass beds (Ewanchuk and Williams,
1996). Floating seagrass shoots not only have some
potential to become reestablished and expand via
asexual reproduction, but they can also carry viable
seeds (Harwell and Orth, 2002; Orth et al., Chapter
5) and epiphytes (Worcester, 1994) to distant locations. The role of vegetative fragments as vectors
for colonization has likely been underestimated in
seagrass ecology, as these are rare events, that challenge direct observation, although direct evidence
of widespread establishment by fragments has been
recently reported (Campbell, 2003).
Although there are mechanisms to provide genetically unique recruits to seagrass populations,
the importance of these mechanisms in producing
new shoots in seagrass beds is considered low compared to the asexual ramification of plant modules
by clones already extant in populations (Tomlinson, 1974). For most species, observations of successful seedling recruitment are rare (Orth et al.,
Chapter 5). However, the study of sexual recruitment in established populations is complicated by
the difficulty in distinguishing whether shoots are
derived from a single seed or from fragmentation of
a larger clone (cf. Waycott et al., Chapter 2). Moreover, it is possible that the perception that successful seedling recruitment is a rare event may be dependent on insufficient observational effort, as this
process may occur over significant spatial and temporal scales that challenge conventional sampling
strategies.
D. Predicting Population Dynamics Using
Shoot Demography
Most monitoring programs are inefficient at detecting and predicting change in shoot density, because
such change can occur either precipitously (e.g. Robblee et al., 1991) or be too gradual to be detected
within the typically broad error margins of density
and cover estimates used in most monitoring programs (Heidelbaugh and Nelson, 1996). There is,
therefore, a demand for approaches to quantify the
components of seagrass population dynamics with
the aim of allowing an evaluation of their status and
an ecological forecast of possible future trends. Recently, the analysis of age structure data to infer
population growth rate has been applied to seagrass
beds using what has come to be known as the
‘reconstructive technique’ (Duarte et al., 1994),
which has been applied to multiple species since
(e.g. Kenworthy and Schwarzschild, 1998; Marb` a
and Walker, 1999; Guidetti, 2001; Peterson and
Fourqurean, 2001).
Population dynamics reflect the balance between
immigration, emigration, recruitment, and mortality, and the various factors that affect these gains
and losses. For any closed population, the population growth rate per individual (r ) is the difference
Carlos M. Duarte, James W. Fourqurean, Dorte Krause-Jensen, and Birgit Olesen
photosynthetic leaves, but also out by the production of new plant modules consisting of a length of
rhizome, associated roots, and a shoot. The branching pattern created by the production of new modules varies from many-branched plants that expand
almost equally in two dimensions (e.g. Posidonia
oceanica) to plants that extend almost exclusively
linearly through space (e.g. Thalassia testudinum).
Eventually, through the action of either senescence
of modules or disturbance, these individuals can become physically separated so that what was once
one plant can become many isolated plants—but
all of these plants are genetically identical—i.e.
they are parts of the same genetic individual (i.e.
genet).
So, when studying the dynamics of seagrass populations, it is important to keep in mind that what
appears above the sediments as a shoot is likely
connected to many more shoots underground. And,
merely because two shoots do not share a common
connection somewhere under the sediments is no indication that these shoots are genetically different.
In fact, there is molecular evidence for genetically
identical shoots of T. testudinum separated by over
3 km in an otherwise genetically diverse, continuous
seagrass bed (Davis et al., 1999). A more thorough
discussion on this topic is provided in Waycott et al.
(Chapter 2).
New genets can enter a population not just through
successful seedlings, but also as adult plant fragments that may drift into a population from some distant source (Setchell, 1929). Seagrasses can float and
survive for extended periods out of the sediment; apparently viable modules of the tropical seagrass Thalassia testudinum can occasionally be found on the
temperate beaches of the North Carolina in the US
(JWF, pers. observ), over 1000 km from the nearest
known T. testudinum populations. Seagrass shoots
can survive for months in the water column, but the
ability of detached shoots to survive when transplanted decreases with time in the water column,
limiting the potential of drifting adult plants to establish new seagrass beds (Ewanchuk and Williams,
1996). Floating seagrass shoots not only have some
potential to become reestablished and expand via
asexual reproduction, but they can also carry viable
seeds (Harwell and Orth, 2002; Orth et al., Chapter
5) and epiphytes (Worcester, 1994) to distant locations. The role of vegetative fragments as vectors
for colonization has likely been underestimated in
seagrass ecology, as these are rare events, that challenge direct observation, although direct evidence
of widespread establishment by fragments has been
recently reported (Campbell, 2003).
Although there are mechanisms to provide genetically unique recruits to seagrass populations,
the importance of these mechanisms in producing
new shoots in seagrass beds is considered low compared to the asexual ramification of plant modules
by clones already extant in populations (Tomlinson, 1974). For most species, observations of successful seedling recruitment are rare (Orth et al.,
Chapter 5). However, the study of sexual recruitment in established populations is complicated by
the difficulty in distinguishing whether shoots are
derived from a single seed or from fragmentation of
a larger clone (cf. Waycott et al., Chapter 2). Moreover, it is possible that the perception that successful seedling recruitment is a rare event may be dependent on insufficient observational effort, as this
process may occur over significant spatial and temporal scales that challenge conventional sampling
strategies.
D. Predicting Population Dynamics Using
Shoot Demography
Most monitoring programs are inefficient at detecting and predicting change in shoot density, because
such change can occur either precipitously (e.g. Robblee et al., 1991) or be too gradual to be detected
within the typically broad error margins of density
and cover estimates used in most monitoring programs (Heidelbaugh and Nelson, 1996). There is,
therefore, a demand for approaches to quantify the
components of seagrass population dynamics with
the aim of allowing an evaluation of their status and
an ecological forecast of possible future trends. Recently, the analysis of age structure data to infer
population growth rate has been applied to seagrass
beds using what has come to be known as the
‘reconstructive technique’ (Duarte et al., 1994),
which has been applied to multiple species since
(e.g. Kenworthy and Schwarzschild, 1998; Marb` a
and Walker, 1999; Guidetti, 2001; Peterson and
Fourqurean, 2001).
Population dynamics reflect the balance between
immigration, emigration, recruitment, and mortality, and the various factors that affect these gains
and losses. For any closed population, the population growth rate per individual (r ) is the difference