Chapter 26 A Landscape Perspective
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influence on faunal survival. Irlandi (1994) tested the
effect of ‘patchiness’ (# patches/area) on the survival
of the hard clam Mercenaria mercenaria in mixed H.
wrightii and Z. marina seagrass in North Carolina,
USA. Analysis of clam survival at three different levels of patchiness with the same total seagrass area
showed that survivorship was dramatically greater
where the seagrass was continuous (Irlandi, 1994).
Similarly, Irlandi (1994) showed that siphon nipping
(non-lethal predation) was substantially reduced in
continuous seagrass relative to equivalent areas of
seagrass that were patchy. Using a similar experimental design, Irlandi et al. (1995) also showed a
decrease in survival of bay scallops (Argopecten irradians) in patchy seagrass when compared to continuous seagrass after several weeks. Thus, differences in life history features have been found for
bivalve fauna that utilize seagrass beds with different landscape characteristics.
Irlandi (1996) expanded her earlier work and examined the effect of physical energy regime (high
vs. low), as well as patch size, on two different
size classes of Mercenaria mercenaria. Large clams
grew most quickly in medium-sized patches, and
overall more quickly in seagrass than in unvegetated
sediment, regardless of physical energy; small clams
showed no difference in growth between large and
small patches, but did grow better within seagrass
than without, and better in high vs. low energy seagrass (Irlandi, 1996). However, the exact cause of
these variations in bivalve response to landscape features remains in question. Hovel and Lipcius (2001)
pointed out that predator preference for patchy seagrass or a decreased ability to locate prey in continuous seagrass might also explain the observed
differences in Irlandi’s (1996) study. This series of
experiments further highlighted the need to incorporate seagrass landscape features in studies of marine
fauna and demonstrated the importance of incorporating physical energy into study design.
Studies of fauna and seagrass landscapes have
also focused on blue crabs (Callinectes sapidus), a
dominant member of the nekton in many seagrass
beds on the east coast of USA. Using ASUs to standardize shoot density, Hovel and Lipcius (2001) investigated relationships between seagrass patch size
and blue crab survival. Survivorship of tethered juvenile blue crabs increased with decreasing patch
size, most likely due to the reduced density of cannibalistic adult blue crabs, the primary predator on
juveniles (Hovel and Lipcius, 2001). This inverse relationship between patch size and survivorship also
varied seasonally, and habitat complexity was suggested to be a key factor for juvenile blue crab survivorship in both small and large patch sizes (Hovel
and Lipcius, 2001). Although only pertaining to juvenile survivorship in a short-term tethering experiment, the counterintuitive result, that a small patch
may be more valuable than large ones in some cases,
is an interesting finding and illustrates that plant–
animal interactions may be more complex than previously considered.
3. Complex Interactions
Other examples of the complex nature of seagrass
patch utilization by fauna associated with above
ground vegetation also exist. Eggleston et al. (1998)
used ASUs of different sizes to test for landscapelevel variability in the recruitment of three species
of grass shrimps and the blue crab (C. sapidus) into
patches of different sizes. While all three shrimp
species and the smallest size class of blue crabs were
able to distinguish small-scale patchiness, the results
were not only species-specific, but were also affected
by body size and density of animals (Eggleston
et al., 1998). In an experimental study utilizing
ASUs in Australia, Tanner (2003) reported similarly inconsistent effects of patch shape and orientation on the abundance of different mobile epifaunal
species. Species-specific responses to seagrass habitat heterogeneity in Thailand have also been reported
for mobile epifauna associated with the seagrass,
Enhalus acoroides (Nakaoka and Toyohara, 2000)
and for benthic infauna in H. ovalis and Thalassia
hemprichii beds (Nakaoka, 2001). Total abundance
of epifauna on E. acoroides was significantly related to seagrass patch size (Nakaoka and Toyohara,
2000) while both patch size and within patch location were important for infauna at a species-specific
level (Nakaoka, 2001).
Studies on sediment-dwelling infauna may also be
impacted by seagrass landscape structure. Frost et al.
(1999) compared the infaunal community of a continuous expanse of seagrass (Z. marina) in Britain to
that of a fragmented assemblage of adjacent patches.
The infaunal communities were found to be significantly different in composition, reflecting a large
increase in polychaete abundance in unvegetated areas of the fragmented bed (Frost et al., 1999). The
most plausible explanation for these community patterns was a change in grain size of the sediment as
633
influence on faunal survival. Irlandi (1994) tested the
effect of ‘patchiness’ (# patches/area) on the survival
of the hard clam Mercenaria mercenaria in mixed H.
wrightii and Z. marina seagrass in North Carolina,
USA. Analysis of clam survival at three different levels of patchiness with the same total seagrass area
showed that survivorship was dramatically greater
where the seagrass was continuous (Irlandi, 1994).
Similarly, Irlandi (1994) showed that siphon nipping
(non-lethal predation) was substantially reduced in
continuous seagrass relative to equivalent areas of
seagrass that were patchy. Using a similar experimental design, Irlandi et al. (1995) also showed a
decrease in survival of bay scallops (Argopecten irradians) in patchy seagrass when compared to continuous seagrass after several weeks. Thus, differences in life history features have been found for
bivalve fauna that utilize seagrass beds with different landscape characteristics.
Irlandi (1996) expanded her earlier work and examined the effect of physical energy regime (high
vs. low), as well as patch size, on two different
size classes of Mercenaria mercenaria. Large clams
grew most quickly in medium-sized patches, and
overall more quickly in seagrass than in unvegetated
sediment, regardless of physical energy; small clams
showed no difference in growth between large and
small patches, but did grow better within seagrass
than without, and better in high vs. low energy seagrass (Irlandi, 1996). However, the exact cause of
these variations in bivalve response to landscape features remains in question. Hovel and Lipcius (2001)
pointed out that predator preference for patchy seagrass or a decreased ability to locate prey in continuous seagrass might also explain the observed
differences in Irlandi’s (1996) study. This series of
experiments further highlighted the need to incorporate seagrass landscape features in studies of marine
fauna and demonstrated the importance of incorporating physical energy into study design.
Studies of fauna and seagrass landscapes have
also focused on blue crabs (Callinectes sapidus), a
dominant member of the nekton in many seagrass
beds on the east coast of USA. Using ASUs to standardize shoot density, Hovel and Lipcius (2001) investigated relationships between seagrass patch size
and blue crab survival. Survivorship of tethered juvenile blue crabs increased with decreasing patch
size, most likely due to the reduced density of cannibalistic adult blue crabs, the primary predator on
juveniles (Hovel and Lipcius, 2001). This inverse relationship between patch size and survivorship also
varied seasonally, and habitat complexity was suggested to be a key factor for juvenile blue crab survivorship in both small and large patch sizes (Hovel
and Lipcius, 2001). Although only pertaining to juvenile survivorship in a short-term tethering experiment, the counterintuitive result, that a small patch
may be more valuable than large ones in some cases,
is an interesting finding and illustrates that plant–
animal interactions may be more complex than previously considered.
3. Complex Interactions
Other examples of the complex nature of seagrass
patch utilization by fauna associated with above
ground vegetation also exist. Eggleston et al. (1998)
used ASUs of different sizes to test for landscapelevel variability in the recruitment of three species
of grass shrimps and the blue crab (C. sapidus) into
patches of different sizes. While all three shrimp
species and the smallest size class of blue crabs were
able to distinguish small-scale patchiness, the results
were not only species-specific, but were also affected
by body size and density of animals (Eggleston
et al., 1998). In an experimental study utilizing
ASUs in Australia, Tanner (2003) reported similarly inconsistent effects of patch shape and orientation on the abundance of different mobile epifaunal
species. Species-specific responses to seagrass habitat heterogeneity in Thailand have also been reported
for mobile epifauna associated with the seagrass,
Enhalus acoroides (Nakaoka and Toyohara, 2000)
and for benthic infauna in H. ovalis and Thalassia
hemprichii beds (Nakaoka, 2001). Total abundance
of epifauna on E. acoroides was significantly related to seagrass patch size (Nakaoka and Toyohara,
2000) while both patch size and within patch location were important for infauna at a species-specific
level (Nakaoka, 2001).
Studies on sediment-dwelling infauna may also be
impacted by seagrass landscape structure. Frost et al.
(1999) compared the infaunal community of a continuous expanse of seagrass (Z. marina) in Britain to
that of a fragmented assemblage of adjacent patches.
The infaunal communities were found to be significantly different in composition, reflecting a large
increase in polychaete abundance in unvegetated areas of the fragmented bed (Frost et al., 1999). The
most plausible explanation for these community patterns was a change in grain size of the sediment as