506
Bronwyn M. Gillanders
1998; but see Rooker and Holt, 1997), although
the structure of assemblages and size of organisms
may vary (Middleton et al., 1984; MacArthur and
Hyndes, 2001). For example, mean species richness
and mean abundance of fishes did not differ between
Posidonia australis and Zostera capricorni at similar
depths in Botany Bay (New South Wales, Australia),
but marked differences were found in the structure
of the assemblages (Middleton et al., 1984). In addition, Zostera tended to be dominated by smaller
fish species and individuals, whereas larger species
and individuals were found in Posidonia (Middleton et al., 1984). For example, smaller species of the
Gobiidae and Syngnathidae were found in Zostera,
whereas larger members of these families were
found in Posidonia. Juveniles of several species were
thought to move from Zostera to Posidonia with increased size. In some comparisons between different
species of seagrass, it is difficult to distinguish the
effects of habitat structure from those of water depth
(e.g. Jenkins et al., 1997b).
Mixed results were found regarding differences
in growth rates of animals among seagrass species.
For example, tiger prawns (Penaeus esculentus and P.
semisulcatus; Penaeidae) grew significantly faster in
seagrass beds where Enhalus acoroides dominated
(Loneragan et al., 1998), whereas no significant difference in growth of red drum (Sciaenops ocellatus; Sciaenidae) between Halodule wrightii and Thalassia testudinum seagrass was found (Rooker and
Holt, 1997). The latter authors suggested that fish
may select habitats where mortality rates are lowest. Although predation between the two types of
seagrass was similar, it was difficult to determine
the significance of this result because shoot density and other morphological features (e.g. blade
widths, blade height) also varied (Rooker et al.,
1998a).
Comparisons of animal abundance also have been
made between natural and artificial seagrass (e.g.
Bell et al., 1985; Sogard, 1989), and between natural
seagrass and transplanted beds (e.g. Sheridan et al.,
2003). No significant differences in number of individuals were found between natural and artificial
seagrass, but there were significantly fewer species
in artificial seagrass, possibly because artificial seagrass was submerged for less time (Bell et al., 1985).
Natural seagrass had significantly higher densities of
fishes, decapods, annelids, and benthic crustaceans
than transplanted shoalgrass (Halodule wrightii) and
the assemblages differed (Sheridan et al., 2003; see
also Section VII.C of this chapter).
B. Seagrass/Non-vegetated Habitats
The majority of comparisons have involved abundances of organisms in seagrass habitats with those
in nearby non-vegetated habitats. Frequently, sampling has been restricted to one seagrass and one nonvegetated habitat at a single site or limited to one bay
or estuary where more than one site was used (Ferrell and Bell, 1991). Comparisons of density between
seagrass and nearby non-vegetated habitat have generally found significantly higher densities of organisms in seagrass compared to non-vegetated habitats
(see Heck et al., 2003 and references therein) (but
see Hanekom and Baird, 1984; Heck and Thoman,
1984; Petrik et al., 1999). In addition, the number
of species is generally greater within seagrass habitats than in non-vegetated habitats (e.g. Gray et al.,
1996; Jenkins et al., 1997b; Jenkins and Wheatley,
1998; but see Jordan et al., 1997). The number of
species was, however, found to decrease in nonvegetated habitat with increasing distance from seagrass (Ferrell and Bell, 1991). Nagelkerken et al.
(2001) also found that fish abundance and species
richness were high in non-vegetated habitats near
seagrass and mangroves. Enhanced diversity and
abundance of prey (e.g. Lubbers et al., 1990; Connolly, 1997; Jenkins and Hamer, 2001), increased
microhabitat availability, reduced risk of predation,
and hydrodynamic effects on larval supply are all
thought to contribute to increased animal abundance
and species richness within vegetated habitats (Orth,
1992; Jordan et al., 1997) (see also ‘Spatial scales—
factors influencing abundance of fish in seagrass
beds’, Section IV).
The composition and relative abundance of mobile invertebrates and fish in seagrass and nonvegetated habitats are often distinctly different (e.g.
Connolly, 1994a; Gray et al., 1996). Differences in
such assemblages occur because many species consistently occur in only one habitat. Seagrass habitats
tend to be dominated by small cryptic species and
juveniles of larger species, whereas non-vegetated
habitats tend to be dominated by species that avoid
predation by schooling or employing camouflage
(Jenkins et al., 1997b). Fish assemblages from seagrass habitats were more similar to those from nonvegetated habitat within 10 m of seagrass than to
those greater than 100 m from seagrass (Ferrell and
Bell, 1991).
Experimental evidence also suggests that individuals inhabiting seagrass experience reduced predation rates compared to organisms in non-vegetated
Bronwyn M. Gillanders
1998; but see Rooker and Holt, 1997), although
the structure of assemblages and size of organisms
may vary (Middleton et al., 1984; MacArthur and
Hyndes, 2001). For example, mean species richness
and mean abundance of fishes did not differ between
Posidonia australis and Zostera capricorni at similar
depths in Botany Bay (New South Wales, Australia),
but marked differences were found in the structure
of the assemblages (Middleton et al., 1984). In addition, Zostera tended to be dominated by smaller
fish species and individuals, whereas larger species
and individuals were found in Posidonia (Middleton et al., 1984). For example, smaller species of the
Gobiidae and Syngnathidae were found in Zostera,
whereas larger members of these families were
found in Posidonia. Juveniles of several species were
thought to move from Zostera to Posidonia with increased size. In some comparisons between different
species of seagrass, it is difficult to distinguish the
effects of habitat structure from those of water depth
(e.g. Jenkins et al., 1997b).
Mixed results were found regarding differences
in growth rates of animals among seagrass species.
For example, tiger prawns (Penaeus esculentus and P.
semisulcatus; Penaeidae) grew significantly faster in
seagrass beds where Enhalus acoroides dominated
(Loneragan et al., 1998), whereas no significant difference in growth of red drum (Sciaenops ocellatus; Sciaenidae) between Halodule wrightii and Thalassia testudinum seagrass was found (Rooker and
Holt, 1997). The latter authors suggested that fish
may select habitats where mortality rates are lowest. Although predation between the two types of
seagrass was similar, it was difficult to determine
the significance of this result because shoot density and other morphological features (e.g. blade
widths, blade height) also varied (Rooker et al.,
1998a).
Comparisons of animal abundance also have been
made between natural and artificial seagrass (e.g.
Bell et al., 1985; Sogard, 1989), and between natural
seagrass and transplanted beds (e.g. Sheridan et al.,
2003). No significant differences in number of individuals were found between natural and artificial
seagrass, but there were significantly fewer species
in artificial seagrass, possibly because artificial seagrass was submerged for less time (Bell et al., 1985).
Natural seagrass had significantly higher densities of
fishes, decapods, annelids, and benthic crustaceans
than transplanted shoalgrass (Halodule wrightii) and
the assemblages differed (Sheridan et al., 2003; see
also Section VII.C of this chapter).
B. Seagrass/Non-vegetated Habitats
The majority of comparisons have involved abundances of organisms in seagrass habitats with those
in nearby non-vegetated habitats. Frequently, sampling has been restricted to one seagrass and one nonvegetated habitat at a single site or limited to one bay
or estuary where more than one site was used (Ferrell and Bell, 1991). Comparisons of density between
seagrass and nearby non-vegetated habitat have generally found significantly higher densities of organisms in seagrass compared to non-vegetated habitats
(see Heck et al., 2003 and references therein) (but
see Hanekom and Baird, 1984; Heck and Thoman,
1984; Petrik et al., 1999). In addition, the number
of species is generally greater within seagrass habitats than in non-vegetated habitats (e.g. Gray et al.,
1996; Jenkins et al., 1997b; Jenkins and Wheatley,
1998; but see Jordan et al., 1997). The number of
species was, however, found to decrease in nonvegetated habitat with increasing distance from seagrass (Ferrell and Bell, 1991). Nagelkerken et al.
(2001) also found that fish abundance and species
richness were high in non-vegetated habitats near
seagrass and mangroves. Enhanced diversity and
abundance of prey (e.g. Lubbers et al., 1990; Connolly, 1997; Jenkins and Hamer, 2001), increased
microhabitat availability, reduced risk of predation,
and hydrodynamic effects on larval supply are all
thought to contribute to increased animal abundance
and species richness within vegetated habitats (Orth,
1992; Jordan et al., 1997) (see also ‘Spatial scales—
factors influencing abundance of fish in seagrass
beds’, Section IV).
The composition and relative abundance of mobile invertebrates and fish in seagrass and nonvegetated habitats are often distinctly different (e.g.
Connolly, 1994a; Gray et al., 1996). Differences in
such assemblages occur because many species consistently occur in only one habitat. Seagrass habitats
tend to be dominated by small cryptic species and
juveniles of larger species, whereas non-vegetated
habitats tend to be dominated by species that avoid
predation by schooling or employing camouflage
(Jenkins et al., 1997b). Fish assemblages from seagrass habitats were more similar to those from nonvegetated habitat within 10 m of seagrass than to
those greater than 100 m from seagrass (Ferrell and
Bell, 1991).
Experimental evidence also suggests that individuals inhabiting seagrass experience reduced predation rates compared to organisms in non-vegetated