More importantly, on most shores, even within their
distributions, organisms show patchy abundances at
scales of centimeters to tens of meters (Underwood and
Chapman, 1996). This is due to ecological responses of
species: (1) to abiotic factors, e.g., local topography or
wave action; (2) to stochastic variation in processes such
as recruitment or disturbances; and (3) to ecological interactions among individual organisms, e.g., competition,
predation, or grazing. Thus, intertidal shores tend to be
composed of patches of different sizes and ages,
containing a suite of different species occurring in different abundances.
Distributions along estuaries
In the past, it was suggested that estuarine species are
a discrete group, derived from marine and freshwater habitats. More recently, analyses show that, instead, estuarine
assemblages are a continuum of marine and freshwater
species that show gradual changes along the length of an
estuary (Attrill and Rundle, 2002). Freshwater species
become less abundant toward the mouth of an estuary,
and marine species show the opposite pattern. There is
a gradual changeover of species along an estuary, but no
distinct estuarine assemblage with definite and recognizable boundaries.
Mobile species can change their position in an estuary
as conditions change, e.g., with tidal penetration or seasonal changes in freshwater input. Species living on rocky
shores tend to be attached, slow moving, or unable to
migrate as adults from one shore to another. They must tolerate the range of environmental conditions to which any
shore is subjected. Most species on rocky shores in estuaries are marine forms and must be able to survive reduced
and widely fluctuating salinities and little wave action.
They are a subset of the species that can live on open coast
rocky shores, so diversity generally decreases inland;
fewer marine species survive the environmental conditions or do not manage to recruit to sites up an estuary.
Processes affecting distribution and abundance
Wave action
On open coasts, waves are important in determining which
species can survive on a shore. Areas with frequent large
waves are only occupied by species that resist dislodgement. Shores with little wave action can be very stressful
environments during low tide, depending on the local
climate, because of extended periods of emersion without
wetting by waves. Thus, low-shore species can extend
often further up shore on wave-exposed shores, as long
as wave action is not too severe. Because rocky shores in
most estuaries are generally sheltered, except perhaps at
the mouth of the estuary, waves probably have less influence in determining intertidal assemblages than they do
on the open coast. In urbanized estuaries, large waves created by boats and ships can, however, replace the lack of
natural waves (Bishop and Chapman, 2004).
Physical features of the shore
Physiological stress associated with high temperatures and
desiccation during low tide is strongly modified by the
shape and bedrock of the shore. Steep shores may be less
affected by heat during low tide than are near-horizontal
shores because steep shores are likely to be shaded for part
of the day. Animals and plants may therefore be able to
extend farther up shore, but this is countered by increased
vulnerability to dislodgement by waves on steep shores.
Species living on steep shores tend to have strong powers
of adhesion or attachment.
On gently sloping shores, the type of bedrock becomes
important. For example, sandstone does not get as hot as
some other rock types, such as siltstone or basalt. It also
tends to retain water during low tide. So, in estuaries
where wave action is minimal, more species can be found
on sandstone shores than on basalt shores, and they may
be able to occupy a greater area on sandstone shores.
Intertidal distributions are also strongly influenced by
local features of topography (Underwood and Chapman,
2007). Pits, crevices, the undersurfaces of boulders, and
rock pools can provide shaded or damp conditions during
emersion. They are often crowded with animals on estuarine
shores during low tides on hot days, while the rest of the
shore looks quite barren. Although the animals emerge from
these habitats to feed during more benign conditions, many
are slow moving and do not forage over large areas. Thus,
the patchy nature of various microhabitats contributes to
the patchy patterns of distribution of the animals sheltering
in them and those animals and plants on which they feed.
Disturbances
Disturbances affecting intertidal assemblages may be
widespread, e.g., flooding, waterborne pollution, or
storms, or very localized, such as people trampling over
the shore. They tend to damage and remove particular species from the assemblage, often those that are easily
dislodged or broken or those particularly vulnerable to
particular contaminants. Disturbances leave patches of
bare space in reduced assemblages, space that is then
available for other organisms to colonize. Recovery is,
however, not predictable, and a different set of species
may arrive in a disturbed patch to replace those lost. The
new colonists may arrive as propagules from the water
column, or as adults from surrounding areas. What arrives
to colonize a patch will depend on larvae in the water column. It also depends on the variety of species living in the
surrounding area and the rates at which they locate and
encroach on the new patches.
Recruitment
Several processes remove the link between reproduction
of adults and the young that arrive on the shore where they
live. The majority of species release their propagules into
the water column, where they are moved by waves and
currents in the plankton, which may be a long period. During this time, they grow and metamorphose into larvae
508
ROCKY INTERTIDAL SHORES
distributions, organisms show patchy abundances at
scales of centimeters to tens of meters (Underwood and
Chapman, 1996). This is due to ecological responses of
species: (1) to abiotic factors, e.g., local topography or
wave action; (2) to stochastic variation in processes such
as recruitment or disturbances; and (3) to ecological interactions among individual organisms, e.g., competition,
predation, or grazing. Thus, intertidal shores tend to be
composed of patches of different sizes and ages,
containing a suite of different species occurring in different abundances.
Distributions along estuaries
In the past, it was suggested that estuarine species are
a discrete group, derived from marine and freshwater habitats. More recently, analyses show that, instead, estuarine
assemblages are a continuum of marine and freshwater
species that show gradual changes along the length of an
estuary (Attrill and Rundle, 2002). Freshwater species
become less abundant toward the mouth of an estuary,
and marine species show the opposite pattern. There is
a gradual changeover of species along an estuary, but no
distinct estuarine assemblage with definite and recognizable boundaries.
Mobile species can change their position in an estuary
as conditions change, e.g., with tidal penetration or seasonal changes in freshwater input. Species living on rocky
shores tend to be attached, slow moving, or unable to
migrate as adults from one shore to another. They must tolerate the range of environmental conditions to which any
shore is subjected. Most species on rocky shores in estuaries are marine forms and must be able to survive reduced
and widely fluctuating salinities and little wave action.
They are a subset of the species that can live on open coast
rocky shores, so diversity generally decreases inland;
fewer marine species survive the environmental conditions or do not manage to recruit to sites up an estuary.
Processes affecting distribution and abundance
Wave action
On open coasts, waves are important in determining which
species can survive on a shore. Areas with frequent large
waves are only occupied by species that resist dislodgement. Shores with little wave action can be very stressful
environments during low tide, depending on the local
climate, because of extended periods of emersion without
wetting by waves. Thus, low-shore species can extend
often further up shore on wave-exposed shores, as long
as wave action is not too severe. Because rocky shores in
most estuaries are generally sheltered, except perhaps at
the mouth of the estuary, waves probably have less influence in determining intertidal assemblages than they do
on the open coast. In urbanized estuaries, large waves created by boats and ships can, however, replace the lack of
natural waves (Bishop and Chapman, 2004).
Physical features of the shore
Physiological stress associated with high temperatures and
desiccation during low tide is strongly modified by the
shape and bedrock of the shore. Steep shores may be less
affected by heat during low tide than are near-horizontal
shores because steep shores are likely to be shaded for part
of the day. Animals and plants may therefore be able to
extend farther up shore, but this is countered by increased
vulnerability to dislodgement by waves on steep shores.
Species living on steep shores tend to have strong powers
of adhesion or attachment.
On gently sloping shores, the type of bedrock becomes
important. For example, sandstone does not get as hot as
some other rock types, such as siltstone or basalt. It also
tends to retain water during low tide. So, in estuaries
where wave action is minimal, more species can be found
on sandstone shores than on basalt shores, and they may
be able to occupy a greater area on sandstone shores.
Intertidal distributions are also strongly influenced by
local features of topography (Underwood and Chapman,
2007). Pits, crevices, the undersurfaces of boulders, and
rock pools can provide shaded or damp conditions during
emersion. They are often crowded with animals on estuarine
shores during low tides on hot days, while the rest of the
shore looks quite barren. Although the animals emerge from
these habitats to feed during more benign conditions, many
are slow moving and do not forage over large areas. Thus,
the patchy nature of various microhabitats contributes to
the patchy patterns of distribution of the animals sheltering
in them and those animals and plants on which they feed.
Disturbances
Disturbances affecting intertidal assemblages may be
widespread, e.g., flooding, waterborne pollution, or
storms, or very localized, such as people trampling over
the shore. They tend to damage and remove particular species from the assemblage, often those that are easily
dislodged or broken or those particularly vulnerable to
particular contaminants. Disturbances leave patches of
bare space in reduced assemblages, space that is then
available for other organisms to colonize. Recovery is,
however, not predictable, and a different set of species
may arrive in a disturbed patch to replace those lost. The
new colonists may arrive as propagules from the water
column, or as adults from surrounding areas. What arrives
to colonize a patch will depend on larvae in the water column. It also depends on the variety of species living in the
surrounding area and the rates at which they locate and
encroach on the new patches.
Recruitment
Several processes remove the link between reproduction
of adults and the young that arrive on the shore where they
live. The majority of species release their propagules into
the water column, where they are moved by waves and
currents in the plankton, which may be a long period. During this time, they grow and metamorphose into larvae
508
ROCKY INTERTIDAL SHORES