competent to settle, but are subjected to extreme levels of
predation, which is very patchy. Thus, in a body of water
that arrives on a shore, there may be many or few larvae,
but the chances of larvae arriving on their shore of origin,
when they are ready to settle and there is available habitat,
are remote. Apart from species that are direct developers,
e.g., some mollusks or some algae which drop their spores
very close to the parent plant, there is little connection
between breeding adult populations on a shore and new
arrivals.
The importance of this invisible larval stage in determining future intertidal populations has been dubbed
“supply-side ecology” (Underwood and Fairweather,
1989). It can result in very little recruitment on a shore,
sometimes for many years, or the arrival of very large
numbers of individuals, which can dominate a lot of the
space. When there are large numbers of recruits and few
resident predators, the recruits can survive, grow, and
persist for many years. If there are few recruits and/or
predators are very abundant, they will be quickly consumed and the space becomes available once again.
Many settling larvae have complex behaviors, which
allow them to evaluate habitat before settlement. They
may settle in response to features of habitat, such as small
pits, or in response to cues from adults or other larvae.
Together with patchy arrival in the water column, these
behaviors increase local patchiness of species on a shore,
especially for larvae that show gregarious behavior and
settle predominantly where there are already adults.
Grazing and predation
Patterns of distribution and abundance of intertidal organisms are also strongly influenced by ecological interactions among species. Two of the most important of these
are grazing and predation.
Many grazers, such as amphipods and micro-gastropods,
are small and tend to live in cryptic habitats, such as on seaweeds. These species have little impact on the distributions
of species on a shore, but affect the epibiota growing on the
larger plants. The major grazers on rocky shores are mollusks, particularly limpets. These have major influences
on the distribution of seaweeds because they consume the
microalgal stages of development. These grazers are very
competitive, and food is generally in short supply; therefore, they tend to consume most of microscopic stages of
the algae before they have had the chance to grow. So, large
algae and animals and plants, which use them as habitat,
tend to be confined to areas without large numbers of
grazers. This is generally lower on the shore, where algae
can grow fast and occupy a lot of space, excluding large
grazers (Underwood and Chapman, 2007).
The most important predators on rocky shores are starfish, crabs, and whelks, although predatory starfish are
usually not common on sheltered shores. Crabs are common predators and scavengers on rocky shores, particularly low shore, and in boulder fields where they can
shelter under stones. They feed primarily on slow-moving
gastropods, which they crush or “peel” to remove the
shell. The common European crab, Carcinus maenas,
can extend far into estuaries, although it dies in freshwater.
Other common predators on rocky shores are whelks.
These are slow moving and feed primarily on sessile animals, such as barnacles and mussels. They may take many
hours, or even days, to eat a single item of prey, so they
often do not feed far from areas of shelter. This behavior
can cause haloes of bare space around shelters, when prey
near the shelter have been consumed and the survivors are
too far away to be preyed upon. This increases patchiness
created by variable patterns of recruitment or survival in
response to physical stress.
In contrast to open coasts, fish in estuaries can be
important predators on intertidal shores, especially in very
sheltered areas. For example, the common toadfish,
Tetractenos hamiltoni, frequents shallow water in estuaries in New South Wales, Australia. Although generally
eating benthic prey in soft sediments, they can remove
all the small oysters from patches of shore.
Indirect interactions
It is inevitable that within a limited number of pages, one
can only describe some of the major and most important
processes affecting intertidal assemblages. With such
a diverse biota, there are also many indirect interactions
(Wootton, 1993). For example, if a species (A) feeds on
one (B) of two species which compete, large numbers of
the predator may promote the survival of the competitor
which is not consumed (C), by removing its competitor,
even though there is no direct interaction between A and
C. Removal of mussels by whelks might thus promote survival of barnacles, which provide habitat for other species.
Similarly, where large grazers cannot feed in very complex topography or among patches of sessile animals such
as barnacles or mussels, the algae can escape grazing and
grow to form patches outside their general range. Thus, the
more diverse the assemblage, the more possible for its species to interact in direct and indirect ways and the less
likely that the interactions will be similar from place to
place and their outcomes in terms of spatial patterns of
distribution and abundances predictable (Underwood and
Chapman, 2007).
Temporal processes acting on rocky shores
Most of this discussion has focused on spatial patterns of
distributions and abundances. Although it is not possible
to go into detail here, processes alter with time, depending
on rates and types of arrivals onto a shore, regular (e.g., seasonal) and irregular (e.g., storms) disturbances, and changes
in the intertidal assemblage itself. Thus, consumption of
barnacles by whelks may lead to patches being dominated
by mussels until storms remove patches of mussels, when
barnacles, more mussels, or grazers may appear in the
patches. The spatial structure of an intertidal assemblage
is not static, but dynamic, and this, too, interacts at many
spatial and temporal scales (Dayton, 1971).
ROCKY INTERTIDAL SHORES
509
predation, which is very patchy. Thus, in a body of water
that arrives on a shore, there may be many or few larvae,
but the chances of larvae arriving on their shore of origin,
when they are ready to settle and there is available habitat,
are remote. Apart from species that are direct developers,
e.g., some mollusks or some algae which drop their spores
very close to the parent plant, there is little connection
between breeding adult populations on a shore and new
arrivals.
The importance of this invisible larval stage in determining future intertidal populations has been dubbed
“supply-side ecology” (Underwood and Fairweather,
1989). It can result in very little recruitment on a shore,
sometimes for many years, or the arrival of very large
numbers of individuals, which can dominate a lot of the
space. When there are large numbers of recruits and few
resident predators, the recruits can survive, grow, and
persist for many years. If there are few recruits and/or
predators are very abundant, they will be quickly consumed and the space becomes available once again.
Many settling larvae have complex behaviors, which
allow them to evaluate habitat before settlement. They
may settle in response to features of habitat, such as small
pits, or in response to cues from adults or other larvae.
Together with patchy arrival in the water column, these
behaviors increase local patchiness of species on a shore,
especially for larvae that show gregarious behavior and
settle predominantly where there are already adults.
Grazing and predation
Patterns of distribution and abundance of intertidal organisms are also strongly influenced by ecological interactions among species. Two of the most important of these
are grazing and predation.
Many grazers, such as amphipods and micro-gastropods,
are small and tend to live in cryptic habitats, such as on seaweeds. These species have little impact on the distributions
of species on a shore, but affect the epibiota growing on the
larger plants. The major grazers on rocky shores are mollusks, particularly limpets. These have major influences
on the distribution of seaweeds because they consume the
microalgal stages of development. These grazers are very
competitive, and food is generally in short supply; therefore, they tend to consume most of microscopic stages of
the algae before they have had the chance to grow. So, large
algae and animals and plants, which use them as habitat,
tend to be confined to areas without large numbers of
grazers. This is generally lower on the shore, where algae
can grow fast and occupy a lot of space, excluding large
grazers (Underwood and Chapman, 2007).
The most important predators on rocky shores are starfish, crabs, and whelks, although predatory starfish are
usually not common on sheltered shores. Crabs are common predators and scavengers on rocky shores, particularly low shore, and in boulder fields where they can
shelter under stones. They feed primarily on slow-moving
gastropods, which they crush or “peel” to remove the
shell. The common European crab, Carcinus maenas,
can extend far into estuaries, although it dies in freshwater.
Other common predators on rocky shores are whelks.
These are slow moving and feed primarily on sessile animals, such as barnacles and mussels. They may take many
hours, or even days, to eat a single item of prey, so they
often do not feed far from areas of shelter. This behavior
can cause haloes of bare space around shelters, when prey
near the shelter have been consumed and the survivors are
too far away to be preyed upon. This increases patchiness
created by variable patterns of recruitment or survival in
response to physical stress.
In contrast to open coasts, fish in estuaries can be
important predators on intertidal shores, especially in very
sheltered areas. For example, the common toadfish,
Tetractenos hamiltoni, frequents shallow water in estuaries in New South Wales, Australia. Although generally
eating benthic prey in soft sediments, they can remove
all the small oysters from patches of shore.
Indirect interactions
It is inevitable that within a limited number of pages, one
can only describe some of the major and most important
processes affecting intertidal assemblages. With such
a diverse biota, there are also many indirect interactions
(Wootton, 1993). For example, if a species (A) feeds on
one (B) of two species which compete, large numbers of
the predator may promote the survival of the competitor
which is not consumed (C), by removing its competitor,
even though there is no direct interaction between A and
C. Removal of mussels by whelks might thus promote survival of barnacles, which provide habitat for other species.
Similarly, where large grazers cannot feed in very complex topography or among patches of sessile animals such
as barnacles or mussels, the algae can escape grazing and
grow to form patches outside their general range. Thus, the
more diverse the assemblage, the more possible for its species to interact in direct and indirect ways and the less
likely that the interactions will be similar from place to
place and their outcomes in terms of spatial patterns of
distribution and abundances predictable (Underwood and
Chapman, 2007).
Temporal processes acting on rocky shores
Most of this discussion has focused on spatial patterns of
distributions and abundances. Although it is not possible
to go into detail here, processes alter with time, depending
on rates and types of arrivals onto a shore, regular (e.g., seasonal) and irregular (e.g., storms) disturbances, and changes
in the intertidal assemblage itself. Thus, consumption of
barnacles by whelks may lead to patches being dominated
by mussels until storms remove patches of mussels, when
barnacles, more mussels, or grazers may appear in the
patches. The spatial structure of an intertidal assemblage
is not static, but dynamic, and this, too, interacts at many
spatial and temporal scales (Dayton, 1971).
ROCKY INTERTIDAL SHORES
509