638
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
in seagrass (mostly T. testudinum) in Florida, USA
by inserting macroalgal clumps into seagrass areas.
These experimental plots showed increased abundance and evenness of mobile canopy fauna in comparison to unaltered seagrass control plots after 6
months, although the transitory nature of the drift
algae probably made this a temporary effect overall
(Holmquist, 1997). The interaction of macroalgae
and seagrass landscapes may be even more complex,
since faunal colonization of new seagrass patches
was found to be facilitated in Florida, USA by drift
algae (Brooks and Bell, 2001), the very same agent
of disturbance to established beds in the seagrass
landscape studied by Holmquist (1997).
Large-scale disturbance events such as storms or
hurricanes may also have major impacts on seagrass
spatial structure, but few data have been specifically collected to support this suggestion. Fonseca
et al. (2002) reported that disturbance from intense
storms was linked to landscape change of seagrasses
in North Carolina, USA. A decrease in patch size was
detected due to edge erosion during storms in 50 ×
50 m
2 study areas (Fonseca et al., 2002). However,
landscape alteration varied with patch size and physical exposure; greater overall decreases in patch size
were found in higher energy areas (small patches
present) than in less exposed areas (larger patch
sizes) (Fonseca et al., 2002). These findings again
illustrate the interaction between landscape structure
and disturbance. Recent models developed by Fonseca et al. (2002) and discussed by Fonseca and Bell
(1998) provide one methodology for measuring the
physical setting of seagrass beds linked to extreme
events and then predicting landscape structure.
VI. Boundaries and Seagrass Landscapes
Within landscapes, boundaries represent zones
between two contrasting patches, which delimit
heterogeneity (Cadenasso et al., 2003). In terrestrial
applications of landscape principles, boundaries or
edges represent areas of special function (ecotones).
The consistency with which the special functions
seen in terrestrial ecotones apply to seagrass landscape boundaries is not yet clear, but is of particular interest because boundary zones may make up
a significant portion of benthic landscapes (Zajac
et al., 2000). Seagrass beds that have a large number
of patches may have substantial amounts of edge.
In seagrass systems, simple boundaries are typically many easy to observe visually due to the sharp
contrast between dense seagrass vegetation and the
unvegetated sediment matrix (e.g. Kendrick et al.,
1999; see also Fig. 1). However, more complex landscapes with overlapping patches resulting in mixed
boundary regions are common, and whether these
mixed areas function as ecotones or should be treated
as independent patch types is not clear at this time.
Therefore more work delineating boundary properties in seagrass landscapes would be particularly
useful.
Some new approaches to the study of ecotones in
general have recently been proposed. Strayer et al.
(2003) devised a general functional classification
scheme for boundaries that appears useful for seagrass systems. Specifically, they classified boundaries as reflective, transforming, absorptive or semipermeable with respect to fluxes of materials across
patch edges. Accordingly, the fate of nutrients, wave
energy or sediment particles across seagrass patch
boundaries could be studied in this context. A similar approach of following material movement across
ecotones could be used to follow how populations of
plant or animal taxa are influenced by passage across
seagrass patch boundaries. For example, Brooks and
Bell (2001) found that amphipod dispersal from drift
macroalgae was reduced after passing from the interior of a seagrass bed through the seagrass/sediment
boundary, suggesting that for at least some small
mesograzers, such boundaries are readily perceived
and act to modify dispersal rates. Determining the
functional nature of seagrass patch boundaries will
be an important step in understanding the implications of landscape changes (e.g. increasing fragmentation) that increase the proportion of habitat edges.
Much of the existing information on seagrass
edges focuses on faunal usage of edge vs. interior of
seagrass beds. To date no consistent spatial patterns
of faunal habitat utilization have emerged. Bologna
and Heck (2002) found little consistent support for
differential use of seagrass bed edges (defined by
authors as the outer 1 m of a bed) by macrofauna
compared to interior regions of beds, although secondary production of some peracarid crustaceans
was significantly higher in interior areas of seagrass
beds compared to edges. Similarly, Sanchez-Jerez
et al. (1999) found that macroepifaunal abundance
did not differ between edges and interior of seagrass
beds. Other studies have found that some faunal
taxa are more abundant at edges of beds (scallops;
Bologna and Heck, 1999; mysids; Barbera-Cebri´ an
Susan S. Bell, Mark S. Fonseca, and Nathaniel B. Stafford
in seagrass (mostly T. testudinum) in Florida, USA
by inserting macroalgal clumps into seagrass areas.
These experimental plots showed increased abundance and evenness of mobile canopy fauna in comparison to unaltered seagrass control plots after 6
months, although the transitory nature of the drift
algae probably made this a temporary effect overall
(Holmquist, 1997). The interaction of macroalgae
and seagrass landscapes may be even more complex,
since faunal colonization of new seagrass patches
was found to be facilitated in Florida, USA by drift
algae (Brooks and Bell, 2001), the very same agent
of disturbance to established beds in the seagrass
landscape studied by Holmquist (1997).
Large-scale disturbance events such as storms or
hurricanes may also have major impacts on seagrass
spatial structure, but few data have been specifically collected to support this suggestion. Fonseca
et al. (2002) reported that disturbance from intense
storms was linked to landscape change of seagrasses
in North Carolina, USA. A decrease in patch size was
detected due to edge erosion during storms in 50 ×
50 m
2 study areas (Fonseca et al., 2002). However,
landscape alteration varied with patch size and physical exposure; greater overall decreases in patch size
were found in higher energy areas (small patches
present) than in less exposed areas (larger patch
sizes) (Fonseca et al., 2002). These findings again
illustrate the interaction between landscape structure
and disturbance. Recent models developed by Fonseca et al. (2002) and discussed by Fonseca and Bell
(1998) provide one methodology for measuring the
physical setting of seagrass beds linked to extreme
events and then predicting landscape structure.
VI. Boundaries and Seagrass Landscapes
Within landscapes, boundaries represent zones
between two contrasting patches, which delimit
heterogeneity (Cadenasso et al., 2003). In terrestrial
applications of landscape principles, boundaries or
edges represent areas of special function (ecotones).
The consistency with which the special functions
seen in terrestrial ecotones apply to seagrass landscape boundaries is not yet clear, but is of particular interest because boundary zones may make up
a significant portion of benthic landscapes (Zajac
et al., 2000). Seagrass beds that have a large number
of patches may have substantial amounts of edge.
In seagrass systems, simple boundaries are typically many easy to observe visually due to the sharp
contrast between dense seagrass vegetation and the
unvegetated sediment matrix (e.g. Kendrick et al.,
1999; see also Fig. 1). However, more complex landscapes with overlapping patches resulting in mixed
boundary regions are common, and whether these
mixed areas function as ecotones or should be treated
as independent patch types is not clear at this time.
Therefore more work delineating boundary properties in seagrass landscapes would be particularly
useful.
Some new approaches to the study of ecotones in
general have recently been proposed. Strayer et al.
(2003) devised a general functional classification
scheme for boundaries that appears useful for seagrass systems. Specifically, they classified boundaries as reflective, transforming, absorptive or semipermeable with respect to fluxes of materials across
patch edges. Accordingly, the fate of nutrients, wave
energy or sediment particles across seagrass patch
boundaries could be studied in this context. A similar approach of following material movement across
ecotones could be used to follow how populations of
plant or animal taxa are influenced by passage across
seagrass patch boundaries. For example, Brooks and
Bell (2001) found that amphipod dispersal from drift
macroalgae was reduced after passing from the interior of a seagrass bed through the seagrass/sediment
boundary, suggesting that for at least some small
mesograzers, such boundaries are readily perceived
and act to modify dispersal rates. Determining the
functional nature of seagrass patch boundaries will
be an important step in understanding the implications of landscape changes (e.g. increasing fragmentation) that increase the proportion of habitat edges.
Much of the existing information on seagrass
edges focuses on faunal usage of edge vs. interior of
seagrass beds. To date no consistent spatial patterns
of faunal habitat utilization have emerged. Bologna
and Heck (2002) found little consistent support for
differential use of seagrass bed edges (defined by
authors as the outer 1 m of a bed) by macrofauna
compared to interior regions of beds, although secondary production of some peracarid crustaceans
was significantly higher in interior areas of seagrass
beds compared to edges. Similarly, Sanchez-Jerez
et al. (1999) found that macroepifaunal abundance
did not differ between edges and interior of seagrass
beds. Other studies have found that some faunal
taxa are more abundant at edges of beds (scallops;
Bologna and Heck, 1999; mysids; Barbera-Cebri´ an