Chapter 25 Seagrass Conservation Biology
603
H. Ecological Implications
of Seagrass Biodiversity
There are two global hotspots of high seagrass
species diversity—the Indo-Pacific and the Red
Sea—where as many as 10–15 species can co-occur.
Even in these two regions there is considerable spatial partitioning among species with depth zonation
and gaps between individual species within contiguous meadows. In these beds, when one species declines the time scale over which it recovers or is
replaced by another seagrass species can be quite
long (Birch and Birch, 1984; Williams, 1990; Preen
et al., 1995; Kenworthy et al., 2002; Peterson et al.,
2002). In some examples of disturbance, no recovery has been observed (Whitfield et al., 2002), or
species may be replaced by macroalgae (Williams,
1990; Hauxwell et al., 2001). Low rates of recovery are due to the fact that many species rely on
vegetative reproduction and clonal growth (Duarte
et al. Chapter 11). Spreading rates by rhizome extension and the re-occupation of space from ramet
expansion is typically <1–3 m year
−1 (Marb` a and
Duarte, 1998; Fonseca et al., 2004). Some species
re-occupy gaps quite rapidly from a sediment seed
bank, clonal growth, and vegetative fragment dispersal (Birch and Birch, 1984; Philbrick and Les, 1996;
Kenworthy, 2000; Inglis, 2000a,b; Peterson et al.,
2002; Hammerstrom and Kenworthy, 2003; Walker
et al., Chapter 23), but this may actually lead to a temporary shift in species dominance (Fourqurean et al.,
1995) or the absence of species during less than optimal growing conditions (Kenworthy, 2000). Recent
studies in the multispecies meadows of the Philippines, where as many as seven seagrasses co-occur,
showed that the presence of one species controlled
the distribution of other species in the same area (Terrados et al., 1998; Duarte, 2000). When one of the
dominant species was experimentally removed from
this community, four others declined, leading to the
loss of biodiversity and formation of gaps within the
meadow.
I. Conservation Biology and Large-Scale
Declines of Seagrasses
Large-scale declines in seagrass populations have
been documented from at least 40 locations worldwide as a result of both natural processes and human
activities (Orth and Moore, 1983; Preen et al., 1995:
Short and Wyllie-Echeverria, 1996; Hemminga and
Duarte, 2000; Duarte, 2002; Walker et al., Chapter 23). The most famous example of natural decline was the ‘wasting disease’ of mono-specific Z.
marina populations on both the American and European coastlines of the north Atlantic during the
1930s (Rasmussen, 1977; Moore and Short, Chapter
16). The precise cause was likely due to a pathogen
and the magnitude of loss associated with this natural catastrophe is still debated today (Muehlstein,
1992). In some areas recovery was either slow or
nonexistent (Cottam and Munro, 1954; Thayer et al.,
1984). Slow recovery of Z. marina populations resulted in large changes to the physical and chemical conditions of the sediments and colonization by
other benthic vegetation, mainly macroalgae (Rasmussen, 1977).
An event such as the wasting disease could have
even more severe and longer-lasting consequences
today than those experienced in the mid-1900s. Recovery of Z. marina populations coincided with the
early stages of the large-scale modern day industrialization and human occupation of coastal environments in the North Atlantic. If the demographic
trends proceed as forecasted, human population in
the coastal zone will continue to grow, causing more
extensive and intensive anthropogenic stress than occurred when the wasting disease prevailed (Duarte
et al., 2002; Green and Short, 2003). These activities include inappropriate fishing methods and overfishing, eutrophication, water diversion, shoreline
armoring and destabilization, freshwater diversion,
agriculture, industrialization, and the anticipated
consequences of global climate change. The combined impact of these stressors and disease has
the potential for eliminating Z. marina from many
locations.
It is clear that while humans lack control over
particular types of natural disturbances that can affect seagrasses such as disease, orogenic uplift, and
volcanic eruption, science and technology exist to
arrest and contain the impacts of industrialization
and human occupation of the coastal zone. Conservation biology makes the direct connection between human activities, habitat loss, and local extinctions (Primack, 2000). Although this approach
has been extended to aquatic ecosystems, only recently has a conservation biology framework been
focused directly on the seagrass biome (Special Issue
of Pacific Conservation Biology Volume 5(4) 2000;
Coles and Fortes 2001; Green and Short, 2003). This
603
H. Ecological Implications
of Seagrass Biodiversity
There are two global hotspots of high seagrass
species diversity—the Indo-Pacific and the Red
Sea—where as many as 10–15 species can co-occur.
Even in these two regions there is considerable spatial partitioning among species with depth zonation
and gaps between individual species within contiguous meadows. In these beds, when one species declines the time scale over which it recovers or is
replaced by another seagrass species can be quite
long (Birch and Birch, 1984; Williams, 1990; Preen
et al., 1995; Kenworthy et al., 2002; Peterson et al.,
2002). In some examples of disturbance, no recovery has been observed (Whitfield et al., 2002), or
species may be replaced by macroalgae (Williams,
1990; Hauxwell et al., 2001). Low rates of recovery are due to the fact that many species rely on
vegetative reproduction and clonal growth (Duarte
et al. Chapter 11). Spreading rates by rhizome extension and the re-occupation of space from ramet
expansion is typically <1–3 m year
−1 (Marb` a and
Duarte, 1998; Fonseca et al., 2004). Some species
re-occupy gaps quite rapidly from a sediment seed
bank, clonal growth, and vegetative fragment dispersal (Birch and Birch, 1984; Philbrick and Les, 1996;
Kenworthy, 2000; Inglis, 2000a,b; Peterson et al.,
2002; Hammerstrom and Kenworthy, 2003; Walker
et al., Chapter 23), but this may actually lead to a temporary shift in species dominance (Fourqurean et al.,
1995) or the absence of species during less than optimal growing conditions (Kenworthy, 2000). Recent
studies in the multispecies meadows of the Philippines, where as many as seven seagrasses co-occur,
showed that the presence of one species controlled
the distribution of other species in the same area (Terrados et al., 1998; Duarte, 2000). When one of the
dominant species was experimentally removed from
this community, four others declined, leading to the
loss of biodiversity and formation of gaps within the
meadow.
I. Conservation Biology and Large-Scale
Declines of Seagrasses
Large-scale declines in seagrass populations have
been documented from at least 40 locations worldwide as a result of both natural processes and human
activities (Orth and Moore, 1983; Preen et al., 1995:
Short and Wyllie-Echeverria, 1996; Hemminga and
Duarte, 2000; Duarte, 2002; Walker et al., Chapter 23). The most famous example of natural decline was the ‘wasting disease’ of mono-specific Z.
marina populations on both the American and European coastlines of the north Atlantic during the
1930s (Rasmussen, 1977; Moore and Short, Chapter
16). The precise cause was likely due to a pathogen
and the magnitude of loss associated with this natural catastrophe is still debated today (Muehlstein,
1992). In some areas recovery was either slow or
nonexistent (Cottam and Munro, 1954; Thayer et al.,
1984). Slow recovery of Z. marina populations resulted in large changes to the physical and chemical conditions of the sediments and colonization by
other benthic vegetation, mainly macroalgae (Rasmussen, 1977).
An event such as the wasting disease could have
even more severe and longer-lasting consequences
today than those experienced in the mid-1900s. Recovery of Z. marina populations coincided with the
early stages of the large-scale modern day industrialization and human occupation of coastal environments in the North Atlantic. If the demographic
trends proceed as forecasted, human population in
the coastal zone will continue to grow, causing more
extensive and intensive anthropogenic stress than occurred when the wasting disease prevailed (Duarte
et al., 2002; Green and Short, 2003). These activities include inappropriate fishing methods and overfishing, eutrophication, water diversion, shoreline
armoring and destabilization, freshwater diversion,
agriculture, industrialization, and the anticipated
consequences of global climate change. The combined impact of these stressors and disease has
the potential for eliminating Z. marina from many
locations.
It is clear that while humans lack control over
particular types of natural disturbances that can affect seagrasses such as disease, orogenic uplift, and
volcanic eruption, science and technology exist to
arrest and contain the impacts of industrialization
and human occupation of the coastal zone. Conservation biology makes the direct connection between human activities, habitat loss, and local extinctions (Primack, 2000). Although this approach
has been extended to aquatic ecosystems, only recently has a conservation biology framework been
focused directly on the seagrass biome (Special Issue
of Pacific Conservation Biology Volume 5(4) 2000;
Coles and Fortes 2001; Green and Short, 2003). This