Chapter 20 Grazing in Seagrass Ecosystems
491
production, and by inference grazing pressure, is
limited by resources (Edgar 1993; Edgar and Aoki
1993; Edgar and Klumpp 2003), then eutrophication may have little impact on epiphyte accumulation on seagrasses as grazer populations respond
to the enhanced resource availability. Conversely,
if mesograzer grazing pressure is regulated by predation, then grazing is less likely to control epiphytes under elevated nutrient loading. Similarly,
overharvesting of top predators should have little
effect on seagrass biomass if mesograzers are limited by algal production, but could have important cascading impacts on seagrasses if overharvesting releases lower-level consumers from predatory
control.
How does declining biodiversity within seagrass ecosystems affect their structure, functioning,
and stability? Functional redundancy among cooccurring species can enhance stability of ecosystem
structure and processes (Lawton and Brown 1993;
Cottingham et al. 2001), and experiments with synthesized assemblages in several systems have shown
that more diverse communities can be more stable
in the face of perturbations and natural environmental variation (Mulder et al. 1999; Stachowicz et al.
1999, 2002). Even within a species, genotypic diversity can enhance resistance to perturbation, as shown
recently for eelgrass subjected to natural grazing by
geese (Hughes and Stachowicz 2004). Continuing
loss of species and genetic diversity, e.g., through
overharvesting or pollution, might thus render seagrass ecosystems more susceptible to subsequent
disturbances.
How will changes in the structure of seagrass
and coastal food webs influence the resilience of
these systems to excess nutrient loading? This question is related to the previous one. Conservationoriented seagrass research (see Kenworthy et al.,
Chapter 25) has understandably focused primarily
on detrimental effects of nutrient loading and turbidity, as nutrients and light are easily measured
and straightforwardly related to plant growth and
competition. But both theory and a growing number
of experiments show that seagrass systems can be
strongly buffered against effects of nutrient loading
as long as a balanced assemblage of mesograzers
and predators is present. Thus, an important question for seagrass conservation is how grazer assemblages, and food webs generally, are affected by
anthropogenic impacts and how these interact with
nutrient loading.
X. Summary
Seagrass food webs contain two key conduits for the
transfer of primary production to higher order consumers: (1) Seagrass grazing ecosystems are or were
characterized by moderate to intense grazing on living seagrass leaves and rhizomes by abundant large
vertebrate and some invertebrate herbivores, resulting in low seagrass biomass and direct conversion of
much seagrass production into vertebrate biomass;
(2) Seagrass detrital ecosystems are mostly devoid
of large vertebrates, and herbivory is dominated by
small invertebrate mesograzers that feed preferentially on epiphytic algae, indirectly enhancing seagrasses, and resulting in high seagrass biomass and
most seagrass production entering the detrital food
chain. For most of the past 50 My, seagrass food
webs, at least in the tropics, were apparently of the
former type.
This history has implications for understanding
the adaptive significance of characteristic morphological, demographic, and life history traits of seagrasses, many of which—like those of their terrestrial counterparts—may be adaptations to historically chronic grazing. In modern seagrass systems,
direct grazing of seagrasses is relatively low on a
global average (<30% of production), due in part
to overharvesting of vertebrate grazers, but is highly
variable in space and time. Thus, many modern seagrass ecosystems are of the detritus-based type, with
little seagrass production grazed directly.
An important implication is that the seagrass detrital ecosystems on which most research has focused
are an historical anomaly and may have been rare
prior to human impact. Moreover, direct grazing of
seagrasses remains locally intense in certain regions,
both tropical and temperate. Importantly, in both
grazing and detrital seagrass ecosystems, grazing is
an important controlling influence on seagrass community and ecosystem dynamics, whether through
direct consumption of seagrass tissue, or indirectly
as algal grazers mediate competitive interactions between seagrasses and algae.
Seagrasses today remain intimately connected
with herbivores and predators through food-web
linkages. Rigorous understanding of seagrass ecology and responses to anthropogenic change (e.g.,
eutrophication) thus requires a broadening of the historically dominant bottom-up perspective to include
the important structuring roles of consumers. Better
understanding and recognition of the importance of
491
production, and by inference grazing pressure, is
limited by resources (Edgar 1993; Edgar and Aoki
1993; Edgar and Klumpp 2003), then eutrophication may have little impact on epiphyte accumulation on seagrasses as grazer populations respond
to the enhanced resource availability. Conversely,
if mesograzer grazing pressure is regulated by predation, then grazing is less likely to control epiphytes under elevated nutrient loading. Similarly,
overharvesting of top predators should have little
effect on seagrass biomass if mesograzers are limited by algal production, but could have important cascading impacts on seagrasses if overharvesting releases lower-level consumers from predatory
control.
How does declining biodiversity within seagrass ecosystems affect their structure, functioning,
and stability? Functional redundancy among cooccurring species can enhance stability of ecosystem
structure and processes (Lawton and Brown 1993;
Cottingham et al. 2001), and experiments with synthesized assemblages in several systems have shown
that more diverse communities can be more stable
in the face of perturbations and natural environmental variation (Mulder et al. 1999; Stachowicz et al.
1999, 2002). Even within a species, genotypic diversity can enhance resistance to perturbation, as shown
recently for eelgrass subjected to natural grazing by
geese (Hughes and Stachowicz 2004). Continuing
loss of species and genetic diversity, e.g., through
overharvesting or pollution, might thus render seagrass ecosystems more susceptible to subsequent
disturbances.
How will changes in the structure of seagrass
and coastal food webs influence the resilience of
these systems to excess nutrient loading? This question is related to the previous one. Conservationoriented seagrass research (see Kenworthy et al.,
Chapter 25) has understandably focused primarily
on detrimental effects of nutrient loading and turbidity, as nutrients and light are easily measured
and straightforwardly related to plant growth and
competition. But both theory and a growing number
of experiments show that seagrass systems can be
strongly buffered against effects of nutrient loading
as long as a balanced assemblage of mesograzers
and predators is present. Thus, an important question for seagrass conservation is how grazer assemblages, and food webs generally, are affected by
anthropogenic impacts and how these interact with
nutrient loading.
X. Summary
Seagrass food webs contain two key conduits for the
transfer of primary production to higher order consumers: (1) Seagrass grazing ecosystems are or were
characterized by moderate to intense grazing on living seagrass leaves and rhizomes by abundant large
vertebrate and some invertebrate herbivores, resulting in low seagrass biomass and direct conversion of
much seagrass production into vertebrate biomass;
(2) Seagrass detrital ecosystems are mostly devoid
of large vertebrates, and herbivory is dominated by
small invertebrate mesograzers that feed preferentially on epiphytic algae, indirectly enhancing seagrasses, and resulting in high seagrass biomass and
most seagrass production entering the detrital food
chain. For most of the past 50 My, seagrass food
webs, at least in the tropics, were apparently of the
former type.
This history has implications for understanding
the adaptive significance of characteristic morphological, demographic, and life history traits of seagrasses, many of which—like those of their terrestrial counterparts—may be adaptations to historically chronic grazing. In modern seagrass systems,
direct grazing of seagrasses is relatively low on a
global average (<30% of production), due in part
to overharvesting of vertebrate grazers, but is highly
variable in space and time. Thus, many modern seagrass ecosystems are of the detritus-based type, with
little seagrass production grazed directly.
An important implication is that the seagrass detrital ecosystems on which most research has focused
are an historical anomaly and may have been rare
prior to human impact. Moreover, direct grazing of
seagrasses remains locally intense in certain regions,
both tropical and temperate. Importantly, in both
grazing and detrital seagrass ecosystems, grazing is
an important controlling influence on seagrass community and ecosystem dynamics, whether through
direct consumption of seagrass tissue, or indirectly
as algal grazers mediate competitive interactions between seagrasses and algae.
Seagrasses today remain intimately connected
with herbivores and predators through food-web
linkages. Rigorous understanding of seagrass ecology and responses to anthropogenic change (e.g.,
eutrophication) thus requires a broadening of the historically dominant bottom-up perspective to include
the important structuring roles of consumers. Better
understanding and recognition of the importance of