Chapter 20 Grazing in Seagrass Ecosystems
481
with higher nitrogen content in leaves, to persist in
their foraging grounds (Preen 1995). Similarly, it
has been suggested that green turtles can cultivate
the nitrogen content of their forage by pruning away
older more fibrous turtlegrass leaves, allowing them
access to a greater abundance of younger more nitrogen rich leaves (Bjorndal 1980).
Of course, not all herbivores, in particular smaller
and more sluggish herbivores, have the ability to manipulate the nitrogen content of seagrass leaves. As
a result, such organisms may need to find other ways
to compensate for the low nutritional quality of their
forage, such as compensatory feeding. In the only
experimental study we know of where the possibility of such compensatory feeding on seagrasses has
been considered, Valentine and Heck (2001) manipulated the in situ nitrogen content of seagrass leaves
within cages where urchins (Lytechinus variegatus)
were stocked at densities ranging from 0-20 individuals per square meter of seagrass habitat. The authors found a clear disconnect between the impacts
of these grazers on seagrass density and the proportion of seagrass production they consumed. Because the grazers stimulated above-ground seagrass
production within the unenriched urchin enclosure
cages, the anticipated differences in above-ground
seagrass biomass between nitrogen-enriched and unenriched urchin enclosure treatments were not observed. Relying instead on a mass balance estimate
of urchin consumption in the nitrogen-enriched and
unenriched cages, and on estimates of urchin consumption of nitrogen-enriched and unenriched seagrass leaves embedded in agar, the authors found that
urchins would increase the consumption of leaves
with low nitrogen to meet their nutritional needs.
Because several generalist herbivores increase
their ingestion rates when feeding on marine plants
of low nutritional value (Lares and McClintock
1991), it seems likely that many invertebrate grazers
may be able to compensate for low nutritional quality of their forage as well. Similarly, most herbivorous fishes have high consumption rates and rapid
gut throughput times (Horn 1989). These fishes can
maintain very large populations and high growth
rates on diets low in protein (Russ 1984; Russ and
St. John 1988). Thus, they must process a great deal
of organic material very quickly and could consume
significant quantities of seagrass production when
herbivore biomass is high. Therefore, we believe that
it is premature at this stage to draw firm conclusions
about how important the variability of the nutritional
content of seagrass is in determining herbivore feeding choices. Clearly additional study is needed.
There is new experimental evidence that at least
some herbivores will preferentially feed on nitrogenrich plants in seagrass ecosystems. Goecker et al.
(2005) conducted a series of controlled choice tests
designed to assess the degree to which one abundant specialist seagrass herbivore (Sparisoma radians) would selectively forage on seagrass leaves of
differing nitrogen content. Seagrass leaves of varying leaf nitrogen content were offered on paired tethers placed at multiple locations over two seasons in
the lower Florida Keys. The results of this comparison were surprising in that the bucktooth consumed
virtually all of the enriched leaves while eating minimal amounts of low nitrogen leaves. Because there
was concern that the high nitrogen leaves might represent novel prey, as there were differences in the
color of the high and low nitrogen leaves, a second
set of experiments were conducted in a laboratory
setting. This time the leaves were dried and embedded in agar. The results were similar; these small
parrotfish consumed substantially more of the agar
that was impregnated with the high nitrogen leaves.
These findings suggest an interesting and as yet unconsidered possibility, the perceived poor nutritional
content of seagrass leaves in the field may in some
instances result from preferential removal by herbivores of nitrogen-rich leaves, rather than from limited concentrations of nitrogen in nearshore waters.
There are, of course, many factors other than nutritional or chemical quality of the food that influence
herbivore feeding. Chief among these are interactions between herbivores and other animals, including competitors and especially predators. We return
to the role of such food-web interactions below.
VI. Indirect Effects of Grazing on Seagrass
Communities: A Delicate Balance
Evidence presented in the preceding sections suggests that direct grazing on modern seagrasses may
be more common than has been assumed. Nevertheless, there are large regions of the world, particularly in temperate regions, where such grazing
is now extremely episodic, negligible, or absent.
In such places, the dominant primary consumers
are mesograzers—small grazing and detritivorous
invertebrates, primarily amphipod, and isopod crustaceans, hermit crabs, and gastropod molluscs (Orth
481
with higher nitrogen content in leaves, to persist in
their foraging grounds (Preen 1995). Similarly, it
has been suggested that green turtles can cultivate
the nitrogen content of their forage by pruning away
older more fibrous turtlegrass leaves, allowing them
access to a greater abundance of younger more nitrogen rich leaves (Bjorndal 1980).
Of course, not all herbivores, in particular smaller
and more sluggish herbivores, have the ability to manipulate the nitrogen content of seagrass leaves. As
a result, such organisms may need to find other ways
to compensate for the low nutritional quality of their
forage, such as compensatory feeding. In the only
experimental study we know of where the possibility of such compensatory feeding on seagrasses has
been considered, Valentine and Heck (2001) manipulated the in situ nitrogen content of seagrass leaves
within cages where urchins (Lytechinus variegatus)
were stocked at densities ranging from 0-20 individuals per square meter of seagrass habitat. The authors found a clear disconnect between the impacts
of these grazers on seagrass density and the proportion of seagrass production they consumed. Because the grazers stimulated above-ground seagrass
production within the unenriched urchin enclosure
cages, the anticipated differences in above-ground
seagrass biomass between nitrogen-enriched and unenriched urchin enclosure treatments were not observed. Relying instead on a mass balance estimate
of urchin consumption in the nitrogen-enriched and
unenriched cages, and on estimates of urchin consumption of nitrogen-enriched and unenriched seagrass leaves embedded in agar, the authors found that
urchins would increase the consumption of leaves
with low nitrogen to meet their nutritional needs.
Because several generalist herbivores increase
their ingestion rates when feeding on marine plants
of low nutritional value (Lares and McClintock
1991), it seems likely that many invertebrate grazers
may be able to compensate for low nutritional quality of their forage as well. Similarly, most herbivorous fishes have high consumption rates and rapid
gut throughput times (Horn 1989). These fishes can
maintain very large populations and high growth
rates on diets low in protein (Russ 1984; Russ and
St. John 1988). Thus, they must process a great deal
of organic material very quickly and could consume
significant quantities of seagrass production when
herbivore biomass is high. Therefore, we believe that
it is premature at this stage to draw firm conclusions
about how important the variability of the nutritional
content of seagrass is in determining herbivore feeding choices. Clearly additional study is needed.
There is new experimental evidence that at least
some herbivores will preferentially feed on nitrogenrich plants in seagrass ecosystems. Goecker et al.
(2005) conducted a series of controlled choice tests
designed to assess the degree to which one abundant specialist seagrass herbivore (Sparisoma radians) would selectively forage on seagrass leaves of
differing nitrogen content. Seagrass leaves of varying leaf nitrogen content were offered on paired tethers placed at multiple locations over two seasons in
the lower Florida Keys. The results of this comparison were surprising in that the bucktooth consumed
virtually all of the enriched leaves while eating minimal amounts of low nitrogen leaves. Because there
was concern that the high nitrogen leaves might represent novel prey, as there were differences in the
color of the high and low nitrogen leaves, a second
set of experiments were conducted in a laboratory
setting. This time the leaves were dried and embedded in agar. The results were similar; these small
parrotfish consumed substantially more of the agar
that was impregnated with the high nitrogen leaves.
These findings suggest an interesting and as yet unconsidered possibility, the perceived poor nutritional
content of seagrass leaves in the field may in some
instances result from preferential removal by herbivores of nitrogen-rich leaves, rather than from limited concentrations of nitrogen in nearshore waters.
There are, of course, many factors other than nutritional or chemical quality of the food that influence
herbivore feeding. Chief among these are interactions between herbivores and other animals, including competitors and especially predators. We return
to the role of such food-web interactions below.
VI. Indirect Effects of Grazing on Seagrass
Communities: A Delicate Balance
Evidence presented in the preceding sections suggests that direct grazing on modern seagrasses may
be more common than has been assumed. Nevertheless, there are large regions of the world, particularly in temperate regions, where such grazing
is now extremely episodic, negligible, or absent.
In such places, the dominant primary consumers
are mesograzers—small grazing and detritivorous
invertebrates, primarily amphipod, and isopod crustaceans, hermit crabs, and gastropod molluscs (Orth