490
John F. Valentine and J. Emmett Duffy
A full understanding of any ecosystem thus
requires a suite of complementary research approaches. A central point of our contribution is that
seagrass ecology would greatly benefit from more
rigorous experimental study of food-web interactions. Despite the limitations of existing mesocosm
experiments, we are struck by their consistency with
one another and with the few field experiments conducted so far. Although they span a wide range of geographic regions, taxa, and designs, most such experiments show that grazers can have strong impacts on
the biomass and community composition of primary
producers in seagrass beds (Tables 1 & 2). Those
grazing impacts are often unrealized in the field, but
this does not mean that they are unimportant. Rather,
it means that perturbations that change abundance or
community composition of grazers, whether large
vertebrates or invertebrate mesograzers, could potentially produce strong changes in structure and
functioning of seagrass ecosystems. To determine
whether and under what circumstances this potential
is realized, seagrass ecology needs creative and ambitious efforts to scale up experimental approaches,
like those that have proven so successful in research
on lake ecosystems (e.g. Carpenter et al. 2001).
In closing, we also wish to emphasize that resolving the role of consumers in seagrass ecosystems is
of far more than simply academic interest. Given the
accelerating pace of seagrass ecosystem degradation
worldwide (Duarte 2002; Kenworthy et al., Chapter
25), understanding the role of community interactions in seagrass systems has increasingly important consequences not only for basic marine ecology
but also for conservation and management of these
ecologically and economically important resources
(Fig. 6). The principal open question for seagrass
ecology remains the fundamental one of bottom-up
vs top-down control: what are the relative importance of resource supply, consumer pressure, and—
most importantly—their interactions, in regulating
the performance of seagrasses, and even the existence of seagrass beds? Despite intensive study of
seagrass light and nutrient relations on the one hand,
and of grazer impacts on seagrasses and epiphytes on
the other, there are surprisingly few data available to
address their interactions rigorously. Effective management and conservation of seagrass ecosystems
will be compromised until we achieve a balanced,
empirically based understanding of seagrass-bed interaction webs that incorporates the important functional roles of consumers. We look with hope to the
rising generation of students, and we believe the following questions will repay close attention.
How important is top-down control in seagrass
ecosystems? The most fundamental question, which
feeds into all the others we consider, involves the
frequency, magnitude, and nature of consumer impacts in seagrass ecosystems. By top-down control
we mean all impacts of consumers on lower trophic
levels, including direct grazing on seagrasses, grazing on algae that indirectly influences seagrasses
through competitive interactions, and cascading impacts of higher-level predators. Whereas numerous studies have documented impacts of consumers
on certain prey populations in seagrass beds, very
few have rigorously tested the community- and
ecosystem-level consequences of grazing and predation in seagrass systems.
What impact does overharvesting of coastal and
estuarine predators have on seagrass systems?
Many estuarine predators, such as the blue crab in
the southeastern USA, are under heavy fishing pressure. Many such predators are also known or suspected to play important structuring roles in benthic
communities, implying that large declines in their
abundance may cascade down to affect other levels
in the food web (Fig. 3). Such processes have been
documented in many other aquatic benthic systems
(e.g., Menge 1995; Shurin et al. 2002) but remain
mostly untested in seagrass beds (but see Heck et al.
2000 and Gillanders, Chapter 21). Does the removal
of large piscivores release herbivores from predator
control, resulting in the high estimates of grazing
reported above? Or do losses of large predators release smaller predators from control, and thus reduce
grazing impact?
To what extent has overharvesting of large vertebrate herbivores and waterfowl reduced the importance of direct grazing on seagrasses in modern food
webs? There is strong indirect evidence that reduction of large marine herbivores has reduced direct
grazing on tropical seagrasses. But why is there so
little direct grazing on temperate seagrasses? Is it
possible that the intense hunting of waterfowl has
led to a similar reduction in direct grazing on seagrasses in temperate zones as well?
What regulates mesograzer populations and communities? Although this may seem a highly specific problem, it is of central importance to understanding most modern seagrass ecosystems, and to
predicting their responses to anthropogenic impacts
(Heck et al. 2000). For example, if total mesograzer
John F. Valentine and J. Emmett Duffy
A full understanding of any ecosystem thus
requires a suite of complementary research approaches. A central point of our contribution is that
seagrass ecology would greatly benefit from more
rigorous experimental study of food-web interactions. Despite the limitations of existing mesocosm
experiments, we are struck by their consistency with
one another and with the few field experiments conducted so far. Although they span a wide range of geographic regions, taxa, and designs, most such experiments show that grazers can have strong impacts on
the biomass and community composition of primary
producers in seagrass beds (Tables 1 & 2). Those
grazing impacts are often unrealized in the field, but
this does not mean that they are unimportant. Rather,
it means that perturbations that change abundance or
community composition of grazers, whether large
vertebrates or invertebrate mesograzers, could potentially produce strong changes in structure and
functioning of seagrass ecosystems. To determine
whether and under what circumstances this potential
is realized, seagrass ecology needs creative and ambitious efforts to scale up experimental approaches,
like those that have proven so successful in research
on lake ecosystems (e.g. Carpenter et al. 2001).
In closing, we also wish to emphasize that resolving the role of consumers in seagrass ecosystems is
of far more than simply academic interest. Given the
accelerating pace of seagrass ecosystem degradation
worldwide (Duarte 2002; Kenworthy et al., Chapter
25), understanding the role of community interactions in seagrass systems has increasingly important consequences not only for basic marine ecology
but also for conservation and management of these
ecologically and economically important resources
(Fig. 6). The principal open question for seagrass
ecology remains the fundamental one of bottom-up
vs top-down control: what are the relative importance of resource supply, consumer pressure, and—
most importantly—their interactions, in regulating
the performance of seagrasses, and even the existence of seagrass beds? Despite intensive study of
seagrass light and nutrient relations on the one hand,
and of grazer impacts on seagrasses and epiphytes on
the other, there are surprisingly few data available to
address their interactions rigorously. Effective management and conservation of seagrass ecosystems
will be compromised until we achieve a balanced,
empirically based understanding of seagrass-bed interaction webs that incorporates the important functional roles of consumers. We look with hope to the
rising generation of students, and we believe the following questions will repay close attention.
How important is top-down control in seagrass
ecosystems? The most fundamental question, which
feeds into all the others we consider, involves the
frequency, magnitude, and nature of consumer impacts in seagrass ecosystems. By top-down control
we mean all impacts of consumers on lower trophic
levels, including direct grazing on seagrasses, grazing on algae that indirectly influences seagrasses
through competitive interactions, and cascading impacts of higher-level predators. Whereas numerous studies have documented impacts of consumers
on certain prey populations in seagrass beds, very
few have rigorously tested the community- and
ecosystem-level consequences of grazing and predation in seagrass systems.
What impact does overharvesting of coastal and
estuarine predators have on seagrass systems?
Many estuarine predators, such as the blue crab in
the southeastern USA, are under heavy fishing pressure. Many such predators are also known or suspected to play important structuring roles in benthic
communities, implying that large declines in their
abundance may cascade down to affect other levels
in the food web (Fig. 3). Such processes have been
documented in many other aquatic benthic systems
(e.g., Menge 1995; Shurin et al. 2002) but remain
mostly untested in seagrass beds (but see Heck et al.
2000 and Gillanders, Chapter 21). Does the removal
of large piscivores release herbivores from predator
control, resulting in the high estimates of grazing
reported above? Or do losses of large predators release smaller predators from control, and thus reduce
grazing impact?
To what extent has overharvesting of large vertebrate herbivores and waterfowl reduced the importance of direct grazing on seagrasses in modern food
webs? There is strong indirect evidence that reduction of large marine herbivores has reduced direct
grazing on tropical seagrasses. But why is there so
little direct grazing on temperate seagrasses? Is it
possible that the intense hunting of waterfowl has
led to a similar reduction in direct grazing on seagrasses in temperate zones as well?
What regulates mesograzer populations and communities? Although this may seem a highly specific problem, it is of central importance to understanding most modern seagrass ecosystems, and to
predicting their responses to anthropogenic impacts
(Heck et al. 2000). For example, if total mesograzer