Chapter 20
The Central Role of Grazing in Seagrass Ecology
John F. Valentine
∗
Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA.
email: jvalentine@disl.org
J. Emmett Duffy
The College of William and Mary, Virginia Institute of Marine Science, Gloucester Point,
VA 23062-1346, USA. email: jeduffy@vims.edu
“[F]or most of the past 50 My, Caribbean seagrass
communities have had to withstand heavy, sustained
grazing pressure from several sympatric lineages of
large mammalian herbivores. This factor is almost
totally absent both from these communities today
(wherein manatees are scarce or absent in most areas) and from the thinking of the aquatic botanists
and marine ecologists who study these communities.
Consequently, the long-established tenet that seagrass ecosystems are largely detritus-based . . . must
be revised to recognize that the modern situation is
anomalous, and that the ‘normal’ pattern throughout most of tropical seagrass history has been that
much (probably most) of the primary productivity
has been channeled through the guts of herbivores,
particularly sirenians.” (Domning, 2001)
I. Introduction
Like kelps, salt marsh grasses, and mangroves, seagrasses are foundation species (sensu Dayton 1975)
of important ecosystems. The physical habitat, production, and predation refuge they supply supports
dense, diverse, and productive faunal assemblages
(Kikuchi 1974; Klumpp et al. 1989; Valentine and
Heck 1993; Williams and Heck 2001). Despite their
dependence on seagrasses; however, animals living
among the blades are hardly passive lawn ornaments.
Both first principles and empirical evidence from
other, better-studied systems strongly imply that sea∗ Author for correspondence; jvalentine@disl.org
grass fauna, dominated numerically by herbivores
(Valentine and Heck 1999; Heck et al. 2000), should
play central roles in mediating the structure and
function of seagrass communities. In this chapter,
we review evidence for such roles, attempt to place
grazing—on both seagrass and attached algae—in
the context of the larger food web and its influence
on seagrass ecosystem structure and function, and
discuss implications for management of seagrassdominated ecosystems. As in all ecosystems, seagrasses and their associated communities are fundamentally influenced by the abiotic environment,
including climate, light and nutrient regimes. There
are several excellent recent reviews of abiotic influences on seagrass ecology (e.g. Hemminga and
Duarte 2000), including chapters in this volume. Our
focus on grazing is not meant to minimize the importance of these influences but to complement them,
and to begin to address how abiotic drivers and consumers interact to shape seagrass ecosystem structure and function.
The relative importance of resource supply
(bottom-up control) versus consumer influence (topdown control) in mediating the structure and functioning of communities and ecosystems is a fundamental issue in ecology, and has been hotly
debated for decades. All ecosystems depend ultimately on primary producers, and thus, in an important sense, all ecosystems are ultimately regulated from the bottom-up (Hunter and Price 1992;
Power 1992). Nevertheless, given a particular level
of resource input, the habitat structure, community structure, biomass, and ecosystem process rates
463–501.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.
The Central Role of Grazing in Seagrass Ecology
John F. Valentine
∗
Dauphin Island Sea Lab, 101 Bienville Boulevard, Dauphin Island, Alabama 36528, USA.
email: jvalentine@disl.org
J. Emmett Duffy
The College of William and Mary, Virginia Institute of Marine Science, Gloucester Point,
VA 23062-1346, USA. email: jeduffy@vims.edu
“[F]or most of the past 50 My, Caribbean seagrass
communities have had to withstand heavy, sustained
grazing pressure from several sympatric lineages of
large mammalian herbivores. This factor is almost
totally absent both from these communities today
(wherein manatees are scarce or absent in most areas) and from the thinking of the aquatic botanists
and marine ecologists who study these communities.
Consequently, the long-established tenet that seagrass ecosystems are largely detritus-based . . . must
be revised to recognize that the modern situation is
anomalous, and that the ‘normal’ pattern throughout most of tropical seagrass history has been that
much (probably most) of the primary productivity
has been channeled through the guts of herbivores,
particularly sirenians.” (Domning, 2001)
I. Introduction
Like kelps, salt marsh grasses, and mangroves, seagrasses are foundation species (sensu Dayton 1975)
of important ecosystems. The physical habitat, production, and predation refuge they supply supports
dense, diverse, and productive faunal assemblages
(Kikuchi 1974; Klumpp et al. 1989; Valentine and
Heck 1993; Williams and Heck 2001). Despite their
dependence on seagrasses; however, animals living
among the blades are hardly passive lawn ornaments.
Both first principles and empirical evidence from
other, better-studied systems strongly imply that sea∗ Author for correspondence; jvalentine@disl.org
grass fauna, dominated numerically by herbivores
(Valentine and Heck 1999; Heck et al. 2000), should
play central roles in mediating the structure and
function of seagrass communities. In this chapter,
we review evidence for such roles, attempt to place
grazing—on both seagrass and attached algae—in
the context of the larger food web and its influence
on seagrass ecosystem structure and function, and
discuss implications for management of seagrassdominated ecosystems. As in all ecosystems, seagrasses and their associated communities are fundamentally influenced by the abiotic environment,
including climate, light and nutrient regimes. There
are several excellent recent reviews of abiotic influences on seagrass ecology (e.g. Hemminga and
Duarte 2000), including chapters in this volume. Our
focus on grazing is not meant to minimize the importance of these influences but to complement them,
and to begin to address how abiotic drivers and consumers interact to shape seagrass ecosystem structure and function.
The relative importance of resource supply
(bottom-up control) versus consumer influence (topdown control) in mediating the structure and functioning of communities and ecosystems is a fundamental issue in ecology, and has been hotly
debated for decades. All ecosystems depend ultimately on primary producers, and thus, in an important sense, all ecosystems are ultimately regulated from the bottom-up (Hunter and Price 1992;
Power 1992). Nevertheless, given a particular level
of resource input, the habitat structure, community structure, biomass, and ecosystem process rates
463–501.
A. W. D. Larkum et al. (eds.), Seagrasses: Biology, Ecology and Conservation, pp.
c
2006 Springer. Printed in the Netherlands.