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Rocky intertidal shores: Prognosis for the future


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INTRODUCTION What are the current human impacts on rocky shores and what will the ecosystem be like by the year 2025? This assessment builds on the review of Thompson et al. (2002), but provides a more global perspective and outlines actions that are needed to counter threats to rocky shores. The chapter begins by briefly describing the characteristics of rocky shores, the natural driving forces controlling rocky-shore communities and the human and ecosystem services provided by rocky shores. It then provides an overview of past and present human impacts, and projections are made of their intensities to the year 2025. The chapter concludes with recommendations and actions to sustainably manage rocky shores and promote conservation. This assessment includes both anticipated improvements such as more efficient pollution control, and likely future negative impacts such as the irreversible effects of invasive alien species, and considers both local and global scales. An attempt is also made here to identify those societal pressures and consequent impacts that are of an overarching nature (such as global warming) because they apply to all marine and coastal ecosystems. CHARACTERISTICS of ROCKY SHORES AND THEIR CONSEQUENCES Because intertidal rocky shores form a narrow fringe around the coastlines of the world, they are accessible and influenced by events that occur both at sea and on land and extremely vulnerable to human activities compared to many other marine ecosystems (e.g. Chapters 20–22).
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14 !Rocky intertidal shores: prognosis for the future
george m.branch, richard c.thompson,tasman p. crowe,
juan carlos castilla, olivia langmead and stephen j. hawkins
What are the current human impacts on rocky shores and
what will the ecosystem be like by the year 2025? This
assessment builds on the review of Thompson et al. (2002),
but provides a more global perspective and outlines actions
that are needed to counter threats to rocky shores. The
chapter begins by briefly describing the characteristics of
rocky shores, the natural driving forces controlling rocky-
shore communities and the human and ecosystem services
provided by rocky shores. It then provides an overview of
past and present human impacts, and projections are made
of their intensities to the year 2025. The chapter concludes
with recommendations and actions to sustainably manage
rocky shores and promote conservation. This assessment
includes both anticipated improvements such as more effi-
cient pollution control, and likely future negative impacts
such as the irreversible effects of invasive alien species, and
considers both local and global scales. An attempt is also
Aquatic Ecosystems, ed. N. V. C. Polunin. Published by Cambridge University Press. ªFoundation for Environmental Conservation 2008.
In: Aquatic ecosystems Trends and Global Prospects
(Ed: NVC Polunin) Cambridge University Press, 2008
made here to identify those societal pressures and conse-
quent impacts that are of an overarching nature (such as
global warming) because they apply to all marine and coastal
Because intertidal rocky shores form a narrow fringe
around the coastlines of the world, they are accessible and
influenced by events that occur both at sea and on land and
extremely vulnerable to human activities compared to
many other marine ecosystems (e.g. Chapters 20–22). This
vulnerability is tempered, however, by their defining fea-
ture, namely the substratum of hard rock, which is not
altered by most human actions; this is in contrast to coral
reefs (Chapter 16). The relatively two-dimensional nature
of rocky shores compared to many other marine habitats
constrains physical escape from prevailing conditions,
whereas sediment-dwelling organisms can burrow away
from the surface. Conversely, rocky shores do not harbour
pollutants in the same way as do soft sediments.
Rocky shores are also among the most exposed to extreme
sets of physical conditions because of the twice-daily
exposure to air and submersion by the rising tide. Coupled
with their dependence on a rocky substratum, this means
that rocky shore organisms are adapted to variable and
unpredictable conditions. Broad-scale dispersal of propa-
gules and larvae means populations have the capacity to
recover after impacts such as overharvesting or acute pol-
lution over timescales of less than 10 years. Except when
physical conditions are permanently altered, as when sea
defences change the hydrodynamic regime, there is normally
no need for active intervention to enable restoration.
The capacity for recovery of intertidal rocky shores is
exemplified by the flooding of the Orange River, which
caused mass mortalities along 45 km of the west coast of
South Africa (Branch et al. 1990). In the 6 years that fol-
lowed, three patterns emerged. Firstly, all open-coast sites
recovered to resemble (within 10% similarity) the original
community composition. Secondly, the rate of recovery
depended on distance from a source of recruits. Sites
within 5 km of undisturbed communities that could supply
propagules took 2 years to recover; those 45 km away took
4–6 years. Thirdly, physical alteration by the development
of a small harbour at one site transformed it from an open-
coast wave-beaten site into one protected from wave action.
At this site, the community never returned to its original
condition; instead it came to resemble that of natural
sheltered coves (Fig. 14.1).
Compared with lakes, estuaries and lagoons (e.g.
Chapters 5, 7 and 13), rocky shores tend to be relatively
open systems (Gaines & Roughgarden 1985; Underwood
& Fairweather 1989; Menge 1991; Small & Gosling 2001),
although this is not the case universally (Todd 1998;
Todd et al. 1998; Cowen et al. 2000). The larvae and
adults of most rocky-shore organisms disperse and
migrate over distances spanning centimetres to thousands
of kilometres, and they are influenced by inputs of
materials such as nutrients and particulate matter from
afar. Both factors mean that rocky shores can vary con-
siderably in time and space (Lewis 1976; Bowman &
Lewis 1977; Underwood et al. 1983; Hartnoll & Hawkins
1985; Johnson et al. 1998; Underwood 1999; Jenkins et al.
2001), often making it difficult to distinguish human-
induced from natural changes.
These features have major consequences for manage-
ment, notably vulnerability to human activities, tolerance
1993 1992 1991
Stress = 0.14
45 km from source, on open coast
5 km from source, on open coast
45 km from source, in harbour
Fig. 14.1. Recovery of rocky shores on the west coast of South
Africa, following flood-induced mass mortalities in 1988. The
conditions on shores that were 5 km versus 45 km from any
source of recruits was assessed over a 5-year period by multi-
dimensional scaling analyses of entire communities between low
and high spring tide levels, based on log-transformed data of
percentage cover. (G. M. Branch, unpublished data.)
to varying physical conditions, resilience in the face of
change and considerable spatial and temporal variability
(Table 14.1).
Rocky shores provide human society with many goods and
services. Shellfish, finfish and seaweeds are collected and
caught on rocky coastlines and have provided subsistence
foods since prehistory. Living and dead marine organisms
plus fossils have generated curios and ornaments, consti-
tuting currency in some societies. Pressure has increased
because of global tourism. Bait for line fishing and traps is
also collected from rocky shores. These goods are crucial to
sustaining the lives of many people in less-wealthy coun-
tries of Latin America, Africa and Asia. Even in richer
countries, foods previously used for subsistence are
increasingly becoming luxury items (for example limpets
in the Azores: Hawkins et al. 2000), putting further pres-
sure on diminishing resources as supplies fail to match
demand. Commercialization, the development of global
markets and rapid international transportation have all
intensified demands for food.
Rocky coastlines are natural sea defences and head-
lands can provide sheltered anchorages. Soft-rock coast-
lines, especially of chalk or sandstone, are often
reinforced artificially to retard erosive processes in
heavily populated areas where land is expensive. Except
when at the base of cliffs, rocky shores are generally
accessible to people from land; they are used extensively
for recreation, although probably not to the same extent
as sandy shores (Chapter 17). Activities include bird-
watching, fishing and food collection for personal use, and
appreciation of nature, although rising labour costs mean
that even the most well-heeled amateur naturalists no
longer have access to the ‘stout-backed quarrymen’ who
helped find cryptic fauna and were eulogized by Kingsley
(1890). Early recreational use of shores often had a strong
element of self-improvement by appreciation of nature,
and their high biological diversity makes rocky shores
an invaluable educational resource that is accessible to
people of all ages and levels of education. Shores are also
superb areas for engaging public interest in both the
natural world and science as a whole.
Research on rocky shores has included ground-breaking
field experimental ecology (for example Hatton 1938; Jones
1946; Connell 1961; Paine 1969), much of it richly used in
teaching. There is concern, however, that heavy use of
sites for research and education can be damaging (Hawkins
The most important ecosystem service of rocky shores is
the redistribution of algal detrital material from both the
intertidal and subtidal zones (Chapter 15) which fuels many
inshore and coastal systems. As highly productive and
structurally complex systems, rocky shores are important
feeding areas for birds and fish, and nursery grounds for fish
and mobile invertebrates. Materials and energy are thus
imported and exported by waves and currents, and by both
tidal and seasonal movements of animals.
Enclosed bays, rias and fjordic coastlines can be
bounded by extensive rocky substrate supporting large
numbers of filter-feeders that can influence water quality
(Hily 1991). Similar effects can occur on the artificial rocky
shores of former dock basins redeveloped for amenity and
recreational usage (Conlan et al. 1992; Hawkins et al. 1999,
Table 14.1. Natural characteristics of rocky shores and their consequences for management
Natural characteristics Consequences for management
Small in extent Vulnerable to human activities
Impacted by events at sea and on land
Stable physical rock matrix Tolerant and resilient to change
Physical gradient steep, variable
System open owing to larval dispersal and
migration plus inputs of materials Impacts and outcomes of management difficult to predict
Variable in time and space
Rocky intertidal shores 211
Rocky shores are bounded to the landward by terrestrial
systems, and to their seaward side they are part of a con-
tinuum with subtidal reefs or sedimentary shores (see
reviews by Lewis 1964; Stephenson & Stephenson 1972;
Little & Kitching 1996; Raffaelli & Hawkins 1996). Thus
they have steep environmental gradients between land and
sea and between exposed headlands and sheltered bays and
inlets (for review see Raffaelli & Hawkins 1996). The
vertical gradient is essentially unidirectional with increas-
ing stress owing to greater exposure to air at higher shore
levels. There are also horizontal gradients associated with
exposure to different intensities of wave action. This stress
gradient is not unidirectional; some organisms such as
suspension feeders function better in wave-swept condi-
tions, whilst others such as some of the large algae thrive in
shelter. Salinity is another major environmental gradient
that is evident in rock pools. Climatic influences operate
over much larger biogeographical scales, associated with
latitude and modified by ocean currents and upwelling
regimes. Therefore, biota found at any given location are
determined ultimately by their ability to colonize sites and
tolerate conditions, and proximately by smaller-scale
physical influences, propagule supply and interactions with
other organisms (see reviews by Lewis 1964; Connell 1972;
Stephenson & Stephenson 1972; Little & Kitching 1996;
Raffaelli & Hawkins 1996).
Offshore hydrographic conditions, including upwell-
ing, strongly influence most rocky shores (Menge et al.
1997, 2003; Menge 2000). Waves, tides and currents
transport material and hence energy onto, away from and
within rocky shores. Mobile marine animals, including
fish and crustaceans, as well as birds, reptiles and mam-
mals, extensively exploit rocky shores as feeding, resting,
spawning and nursery areas (Rangeley & Kramer 1995;
Bradshaw et al. 1999; Burrows et al. 1999; Coleman et al.
1999; Thompson et al. 2000a). There are also major
functional links between rocky shores and other inshore
habitats and the land itself.
Thus the interaction of many physical and biological
factors influences the structure and functioning of rocky-
shore communities, operating at scales ranging from global
to local effects (Table 14.2). Superimposed on these natural
factors is a set of human impacts, including overarch-
ing sociopolitical issues that impinge on all ecosystems.
Interactions among the natural factors complicate predic-
tions about their combined effects (see Chapter 13). For
instance, around the 2600-km coastline of southern Africa,
differences in biomass are primarily attributable to local
productivity interacting with wave action. In the case of
filter-feeders, wave action plays an overwhelming role,
leading to biomass values that differ more over short dis-
tances of tens of metres between sites with differing levels of
wave action than over a gradient of productivity spanning
hundreds to thousands of kilometres (Bustamante & Branch
1996a,b). Conversely, grazers respond more to differences in
productivity and the input of subsidies from other systems
than they do to wave action (Bustamante et al. 1995a,b).
Thus, simple predictions about the role of biotic interactions
are often not easy to make because of complex interactions
between physical and biological processes.
Pollution can have impacts at all levels of biological
organization from molecules to ecosystems: impairment of
molecular immune responses (Harvell et al. 1999); endo-
crine disrupters exerting multi-level influences on organ-
isms (from cells up to populations); oil spills and toxic algal
blooms directly and indirectly affecting populations and
communities; eutrophication directly influencing commu-
nities and ecosystems; and mining causing severe and
lasting community disruption (reviewed in Thompson
et al. 2002).
Marine organisms can develop tolerance to pollution
via phenotypic responses such as metallothionein produc-
tion (Bebianno & Langston 1991, 1992, 1995; Bebianno
et al. 1993) and sequestration in inert structures (Brough &
White 1990). In species with limited dispersal, tolerant
strains can develop locally, for example in algae (Fielding
& Russell 1976) and in polychaetes (Grant et al. 1989;
Hateley et al. 1992). In the case of the periwinkle Littorina
saxatilis (which has direct development), specimens from
sites historically contaminated with heavy metals from
mining at the Isle of Man (UK) (Daka et al. 2003) have
proven tolerant when subjected to toxicity testing (Daka &
Hawkins 2004), and this tolerance is suspected to be
genetically determined.
Pollution has been combated and continues to be abated
in mature industrialized economies. Many traditionally dirty
industries are in decline in richer countries but are being
relocated or otherwise developed in poorer countries that
have cheaper labour costs and often less stringent environ-
mental legislation and enforcement. As standards of living
rise, awareness of pollution and the financial ability to
impose controls can follow, but with some lag. There can
therefore be optimism about pollution effects to the 2025
time horizon, providing there is no major economic collapse
or widespread warfare.
The ability of anthropogenic substances to disrupt normal
endocrine function in animals has raised concern. One of
the best-publicized examples is from the worldwide effects
of tributyl tin (TBT) pollution, which originates from the
leachates of antifouling paints. Whelks have proven par-
ticularly susceptible to TBT because they respond to much
lower concentrations than other marine organisms (Ellis &
Pattisina 1990; Matthiessen & Gibbs 1998), developing
imposex (superimposition of male sex characteristics on
females), leading in badly affected cases to female sterility.
This has devastated populations and led to local extinc-
tions in sheltered bays and estuaries (Bryan et al. 1986;
Gibbs & Bryan 1987; Spence et al. 1990; Hawkins et al.
1994) and sublethal effects on open coasts.
Dogwhelks are long-lived, and because imposex is
irreversible it persists in populations for extended periods.
Recovery in the UK, following legislation in 1987 to ban
TBT paints on small boats, has taken at least 10–15 years
(Thompson et al. 2002). Other substances have also been
recognized as having endocrine disruptive effects for a
variety of marine organisms including other molluscs,
crustaceans, echinoderms and polychaetes (Depledge &
Billinghurst 1998), but here also enactment of legislation is
likely to lag behind detection of impact.
The effects of oil spills are some of the best-recorded
community-level impacts of anthropogenic stress (Clark
et al. 1997; Hawkins et al. 2002b). Some species such as
barnacles (Southward & Southward 1978) and mussels
(Southward & Southward 1978; Newey & Seed 1995) can
be remarkably tolerant of oiling. Many are affected more
during the clean-up than by the oil itself (see reviews
of Southward & Southward 1978; Foster et al. 1990).
Grazing molluscs seem to be particularly susceptible to
both oil and chemical dispersants (Smith 1968; Hawkins
& Southward 1992; Newey & Seed 1995). Considerable
disturbance can also be caused by physical cleaning,
especially if high water pressures or temperatures are
Table 14.2. Summary of natural factors, human impacts and societal context influencing rocky shore ecosystems
Scale of operation Natural factors Human impacts Overarching socioeconomic effects
Global Climate Global change Population growth
Latitudinal gradients Macroeconomics
Biogeographical effects Globalization
Regional Productivity Pollution War and famine
Currents Modification of coastal processes
Tidal regime
Riverine input
Substrate type
Local Waves Power generation
Sand inundation Mining
Regional/local Recruitment and dispersal Genetic modification Societal and demographic shifts
Local Adult–recruit interactions Pollution Wealth and poverty
Competition Harvesting Export and transport
Predation Introduction of alien species
Disease and parasites
Rocky intertidal shores 213
used (for example after the Exxon Valdez spill: Shaw
1992; Peterson et al. 2000a).
In the well-studied Torrey Canyon oil spill in Cornwall
(UK), the major damage was not caused by the estimated
14 000 tonnes of oil that came ashore, but by excessive
treatment with over 10 000 tonnes of dispersants. These
killed the principal gastropod grazers, Patella spp. (mainly
P. vulgata), Osilinus (Monodonta)lineata and Littorina spp.,
leading to dense growths of algae on many shores
(Smith 1968; Southward & Southward 1978; Hawkins &
Southward 1992; Hawkins et al. 1994). This macroalgal
cover subsequently provided a favourable environment for
early survival of P. vulgata. Treated shores recovered over
the next 10–15 years through a series of damped oscilla-
tions, while an untreated shore recovered within 2–3 years
(Southward & Southward 1978; Hawkins & Southward
1992). The Exxon Valdez spill had similar patterns of
impact and recovery. Here the clean-up did not involve
dispersants, though methods including manual wipe-up,
removal of oiled rocks and seaweed, bioremediation and
pressurized hot-water washing were used. The last method
kills animals as effectively as dispersants, and recovery of
rocky shore assemblages treated this way was still slow.
The brown alga Fucus gardneri had still not recovered to its
original levels some 7 years after the spill (see reviews of
Paine et al. 1996; Peterson et al. 2000a).
Oil spills are one of the most visible and newsworthy
forms of pollution and will no doubt continue to occur on
shores adjacent to major shipping routes and oil refineries.
The frequency of spills is likely to decline with increases in
the size of vessels and improvements in design and legis-
lation, but the potential for large-scale devastation from
incidents with supertankers will probably increase to the
2025 time horizon.
Eutrophication is a significant problem in enclosed seas in
many parts of the world (for example the Baltic: Bonsdorff
et al. 1997; the Irish Sea: Allen et al. 1998; Canada:
Meeuwig et al. 1998; and the USA: Cloern 2001), driven
largely by intensive agricultural runoff (Iversen et al. 1998)
contaminated by high fertilizer loads and, to a lesser
extent, sewage discharges (Nixon 1995). Nitrogen depos-
ition from the atmosphere is also important (Paerl &
Whitall 1999). In the Baltic, the main effect of eutrophi-
cation has been a decline in the perennial macroalgae
(Fucus vesiculosus), including reduction in depth range
(Kautsky et al. 1986) and increases in the abundance of
ephemeral algae (Schramm 1996; Worm et al. 1999).
Eutrophication can also directly affect microbial commu-
nities on rocky substrata (Meyer-Reil & Koster 2000), with
potential indirect effects on grazers.
Management of sewage and fertilizers may reduce
eutrophication in some developed regions of the world by
2025, and some dramatic improvements have already been
observed (Cloern 2001). Developing countries, however,
are expanding their use of inorganic fertilizers and
increasing sewage discharges into the sea. Levels of
eutrophication are therefore expected to follow a trajectory
similar to that observed in developed countries (Nixon
1995). However, oligotrophic tropical systems are expected
to exhibit stronger responses to eutrophication than those
in the temperate zone (Corredor et al. 1999). Atmospheric
inputs of nitrogen are increasing (Paerl & Whitall 1999)
and forecasted increases in rainfall may also exacerbate
eutrophication by leaching terrestrial organic nitrogen
(Hessen et al. 1997).
In addition to direct effects, eutrophication has been
linked to increases in the incidence of harmful algal
blooms (Smayda 1997; Paerl & Whitall 1999; Wu
1999; Cognetti 2001). Warm water associated with El
˜o–Southern Oscillation (ENSO) events was also
implicated in initiating blooms on the coasts of China in
1997–8 (Yin et al. 1999), and the cysts of harmful algae
can be spread in ballast water (Hallegraeff 1998). When
toxic blooms occur, substantial mortality of rocky-shore
filter-feeders, grazers (Southgate et al. 1984) and preda-
tors (Robertson 1991) has been observed. For example,
when a bloom of Chrysochromulina polylepsis was washed
ashore over large areas of the Scandinavian coast in 1988,
it decimated populations of dogwhelks and other marine
invertebrates in some areas (Bokn et al. 1990; Robertson
1991; Wu 1999). The impacts on the whole community
can resemble that of a badly treated oil spill, with a
proliferation of algae produced by decreases in the
abundance of grazers (Southgate et al. 1984).
Both the direct effects of mining and the side effects of
dumping unwanted mine tailings can radically alter the
nature of rocky-shore communities. Much of the west
coast of South Africa and Namibia is mined for diamonds,
affecting rocky shores by two different processes. Firstly,
shallow-water divers work from the shore operating
suction pipes that suck up diamond-bearing gravel which
is screened on the shore and then fed back into the sea.
In the process, the shore is abraded, grazers and filter-
feeders are reduced in numbers and algal growth
enhanced, converting shores into seaweed-dominated
ecosystems (Pulfrich et al. 2003a). Secondly, land-based
mining produces large quantities of waste (tailings), and
in certain areas this is deposited into the sea, building up
beaches and smothering or scouring adjacent rocky
shores. This increases the abundance of filter-feeders but
depletes grazers and again leads to algal domination
(Pulfrich et al. 2003b). The effects are acute, but merci-
fully local.
In northern Chile, several copper mines are located in
the Andes, but the waste tailings from these are diverted to
the coast. Between 1976 and 1989, one mine alone
deposited 130 million tonnes of solid waste on the coast,
with a copper content of about 6000–7000 lg per litre. This
transformed a previously diverse intertidal community to
one dominated by the ‘sentinel’ green algal species Enter-
omorpha compressa (Castilla 1996; Correa et al. 1999; Farina
& Castilla 2001). From 1990, waste was purified by settling
and only the ‘clear water’ was then released at the coast.
Despite this, the depauperate community has persisted
(Fig. 14.2).
These mining activities tend to be local in their effects,
but their impacts are severe. Moreover, although effects of
mining on rocky shores are the focus here, mining takes
place offshore at far larger scales (Chapter 22). The present
forecast is that mining will increase in intensity, particu-
larly offshore where lucrative deposits await the develop-
ment of technology that will allow economically viable
extraction, and where the effects will be less visible and less
easy to monitor. By its very nature, mining disturbs entire
communities and therefore should not be permitted in
marine protected areas. On rocky shores, the alteration of
biotic communities is usually obvious, and it is predicted
here that increasingly stringent environmental legislation
will mitigate against the worst effects, particularly in cases
where human health is threatened.
Human harvesting
In some areas rocky shores are used intensively for food.
Although it is difficult to separate fisheries statistics into
species collected from the intertidal versus subtidal and
pelagic, both gastropods and bivalves, which constitute 2%
and 29% respectively of world marine mollusc catches, are
harvested from the intertidal zone. The world gastropod
fishery shows clear variations among regions with collec-
tion in South America, Asia and Oceania far exceeding that
in Europe and Africa (Leiva & Castilla 2001). The prin-
cipal species harvested and the techniques used vary con-
siderably between regions. In the Americas, the principal
species are Concholepas concholepas in Chile, strombid
species of conch in the Mexican Caribbean, and Haliotis
spp. in Baja California (Mexico). Japan, the Republic of
Korea and Australia accounted for 95% of the total catches
in Oceania and Asia between 1979 and 1996, the main
species here being Haliotis spp. and Turbo truncatus the
horned turban shell. France, the UK and Ireland account
for over 90% of the gastropod fishery in Europe, the main
species being the common periwinkle Littorina littorea and
the subtidal whelk Buccinum undatum. The harvest in
100 Control site
Impact site
Other sessile
Percentage cover
01975 1980 1985 1990 1975 1980 1985
1990 199
Fig. 14.2. Condition of intertidal assemblages on the coast of Chile (1975–95) at control sites and at sites where copper tailings were
disposed of. (After Castilla 1996.)
Rocky intertidal shores 215
Africa appears relatively low, although only three African
countries are registered in world gastropod fishery records.
In addition, species collected for their ornamental value as
marine curios (Newton et al. 1993) have not been included
in these statistics. Methods of harvesting vary considerably
from artisanal subsistence gathering to large-scale com-
mercial collection (Kingsford et al. 1991; Kyle et al. 1997;
Crowe et al. 2000). However, even small-scale fisheries can
exert considerable pressure on stocks where there are large
numbers of collectors, as for example in Chile, where there
are some 45 000 registered artisanal fishers engaged in
small-scale fisheries.
Harvesting has direct effects on target species and
can also have distinct indirect effects on other intertidal
organisms. In some regions, the effects are so extensive that
whole shoreline landscapes may be modified (Paine 1994).
Direct effects include reduced abundance and average size
of individuals (Hockey & Bosman 1986; Underwood 1993).
When intensive collection occurs at a large spatial scale,
recruitment overfishing may result. There are several
examples of this on isolated islands, such as overexploitation
of intertidal limpets in Hawaii (Cellana spp.), the Azores,
Madeira and Canaries (Patella spp.) (for review see Hawkins
et al. 2000).
In Chile, exclusion of humans from a Marine Protected
Area at Las Cruces led to an increase in the body size and
abundance of the predatory muricid gastropod Concholepas
concholepas (Fig. 14.3a), which had previously been
exploited (Moreno et al. 1984; Duran & Castilla 1989;
Castilla 1999, 2000, 2001). This was followed by a reduc-
tion in the abundance of mussels, which are its main prey.
Keyhole limpets also increased in abundance. Loss of the
mussels created areas of open space, which became col-
onized by barnacles (Castilla & Duran 1985). Subsequently
there was a decline in the abundance of Concholepas and
Fissurella, which was attributed to food shortages. Hence,
reducing collection by humans led to predictable direct and
less predictable indirect changes in community structure
(Hawkins et al. 2002a).
In South Africa, intense harvesting of the brown mussel
Perna perna by subsistence and recreational fishers
(Tomalin & Kyle 1998) has depleted the natural stocks and
affected the community structure as a whole. Where
mussels are depleted, foliose algae proliferate. In particu-
lar, beds of articulated coralline algae develop, and have the
ability to exclude mussels for prolonged periods of time
(Lambert & Steinke 1986). As erect algae outcompete
encrusting corallines, the latter also decline in abundance
at localities where mussels are depleted. The net effect of
intense subsistence harvesting is that communities become
less diverse, converging on a common homogenized state
(Fig. 14.3b). From the point of view of the mussel stocks,
there are two additional adverse effects. Firstly, removal of
adult mussels inevitably removes a by-catch of unwanted
juvenile mussels. Secondly, mussel beds are the preferred
settlement site for recruits. Thus, depletion of mussels not
only diminishes the adult stocks, but also diminishes future
recruitment into the fishable stock and transforms the
community structure into one that retards recovery of
mussels (Harris et al. 2003).
Harvesting of rocky-shore organisms is likely to
intensify to the 2025 time horizon as a consequence of the
increasing demand for food resources. In the Third World,
this is likely to be dominated by subsistence and artisanal
fishing, linked to poverty and the need to obtain food.
However, use of non-traditional harvesting methods and
improved transportation and storage will all increase the
potential for export markets to be developed for further
commercialization of harvesting in these regions. In eco-
nomically more affluent countries, harvesting on rocky
shores may well decline in intensity overall. However,
exploitation of more lucrative species is likely to increase,
and has already led to substantial illegal activities and
depletions of stocks. In both cases, the immobility and
accessibility of the resources makes them particularly
vulnerable and also makes it difficult to enforce control
Alien species
There are many examples of non-native (‘alien’) species
that have invaded biogeographical provinces from which
they were previously absent. The barnacle Elminius mod-
estus has colonized much of the European coastline from
Australasia. The alga Sargassum has spread from the Pacific
coast of Asia to both the north American Pacific coast and
to Europe. Many species have been exported from Europe,
including the common shore crab (Carcinus maenas), which
has found its way to both coasts of the Americas, South
Africa and Australia. This is a multi-way traffic, the fre-
quency of which seems to be increasing. Invasions occur
frequently and are more likely in some areas: Europe,
North America and south-western Australia seem to be
particular hotspots. Proximity to major harbours, volume
of trade and intensity of aquaculture operations increase
the likelihood of such events (e.g. Chapters 10, 11 and 13).
= Areas protected from harvesting
= Areas where harvesting occurs
In harvested areas In Marine Protected Area
Mussels Mussels
Siphonaria Siphonaria
+ +
?+ ?+
Fig. 14.3. Effects of human harvesting on rocky shore
communities. (a) Comparisons between community structure
and negative (") and positive (þ) biological interactions, in
areas where harvesting takes place versus those inside a
Marine Protected Area on the Central Chilean coast. The sizes
of the ellipses indicate the relative biomass of different
organisms, and the thickness of the arrows shows the strength
of the interactions (modified from Branch & Moreno 1994).
(b) Correspondence analysis of lower balanoid communities at
protected and harvested areas in Transkei (South Africa)
(modified from Hockey & Bosman 1986). Harvested
communities converge to a common state, whereas areas
protected from harvesting are represented as ‘satellite’
Rocky intertidal shores 217
Major canals such as Panama and Suez can provide
short-cuts enabling transfer directly among tropical
regions without extensive sea journeys through cold
temperate areas. There is also some evidence that low-
diversity or disturbed ecosystems are more susceptible to
invasion than those less disturbed and more diverse.
Many of the species that invade are eurythermic and
euryhaline, and occupy a wide range of habitats such as
both soft and hard substrata, and are thus ideally adapted
for low-diversity disturbed ecosystems (Carlton 1999;
Grosholz et al. 2000). Littorina littorea and Carcinus
maenas are examples. In cases such as L. littorea in the
USA, there is dispute about whether invasion or range
extension has occurred, although molecular genetic tools
can now be used to unravel past species’ distributions
(Wares et al. 2002). The consequences of range expansion
and invasions are similar, though impacts are likely to be
greater if the immigrant comes from a different biogeo-
graphical province with a different suite of competitors
and consumers (as is the case for C. maenas in Australia:
Grosholz 2002; Thresher et al. 2003).
The Mediterranean mussel Mytilus galloprovincialis was
first recorded on the west coast of South Africa during the
1970s, but has now spread to occupy almost 2000 km of
coastline. In the process, it has transformed communities on
exposed shores, displacing the endemic mussel Aulacomya
ater (Hockey & van Erkom Schurink 1992). In addition, it
competes with the limpets Scutellastra granularis and
S. argenvillei for primary space. Being a relatively small
species, S. granularis can survive on the alternative sub-
stratum provided by the mussels themselves. Indeed, its
densities and reproductive output at population level have
increased in areas where mussels have invaded, as settlement
on mussels is preferable to bare rock. However, large S.
granularis cannot survive on mussels, so maximum size and
reproductive output per individual are reduced (Griffiths
et al. 1992). The situation for the much larger S. argenvillei
is more perilous. Mytilus galloprovincialis displaces all indi-
viduals of this limpet that are large enough to reproduce.
Taking such interactions a step further, physical factors may
moderate biotic effects. For example the competitive dom-
inance of M. galloprovincialis over S. argenvillei is moderated
by wave action (Fig. 14.4). At low intensities of wave action
both species are absent, while on extremely exposed shores
both are present but are under stress, and neither achieves
domination. Between these extremes, however, the inter-
action is powerfully influenced by wave action. On semi-
exposed shores, M. galloprovincialis is recruited at low levels
and grows slowly, so it has almost no impact on the abun-
dance, size structure and attainment of sexual maturity by
S. argenvillei. On more exposed shores, however, M. gal-
loprovincialis becomes dominant because of its faster growth
and higher settlement, and occupies 80–95% of available
space (Steffani & Branch 2003a). Although the limpet can
inhibit mussel settlement on bare rock, it is incapable of
preventing lateral encroachment, which leads to the exclu-
sion of most of the limpets; those that do co-occur with the
mussel are small and virtually none attain sexual maturity
(Branch & Steffani 2004).
In the future, there is likely to be an increase in the
frequency of introductions, particularly in areas that
receive heavy shipping traffic and on coasts that have
sheltered waters, because organisms that survive trans-
portation in ballast are likely to be adapted to calm waters.
Harbours provide such a haven, but further spread of
aliens is most probable where there are natural calm-water
bays that provide stepping stones. The amount of ship-
ping, the size of vessels and the speed of ships are all
increasing. The faster the travel, the greater the likelihood
that organisms attached to hulls or carried in ballast water
will survive the journey. Stricter international laws will be
required to prevent substantial increases in the rate of
introduction of alien species.
Species introductions also occur as a consequence of
mariculture. For example, attempts to introduce the South
African abalone Haliotis midae into mariculture ventures in
California resulted in the disastrous accidental introduction
of a sabellid epizoite that burrows into the shell and stunts
growth. Its effects on its normal host H. midae are usually
minimal, but it causes severe stunting of Californian species
of Haliotis. The sabellids also escaped from mariculture and
infected other molluscs in the wild, necessitating the closure
of most abalone mariculture sites in California and strenu-
ous efforts to locate and eliminate molluscs that had become
infested in the wild (Leighton 1998; Ruck & Cook 1998).
Thus the impacts of introductions via mariculture are not
confined to the target species; accidental introduction of
associated species, parasites and diseases can wreak havoc on
native species.
Mariculture is rapidly expanding, and genetic modifi-
cations associated with mariculture are also likely to
multiply. Undesirable side effects of mariculture are likely
to increase, and some may prove extremely difficult to
contain, partly because they are often unpredictable.
One of the greatest potential risks to rocky shores is from
the introduction of alien species that can lead to permanent
changes in community structure. This is especially so for
invasive pathogens, which may find hosts lacking natural
resistance (see Harvell et al. 1999), and high-ranking com-
petitors or predators such as M. galloprovincialis in South
Africa (Griffiths et al. 1992) and C. maenas in California
(Grosholz et al. 2000).
Some non-native species, however, can also have
positive effects, especially if the species is an ecosystem
engineer providing new or additional habitat. For example,
Pyura praeputialis, a non-native species on the Chilean
coast, is an ecosystem engineer and provides additional
habitat in the interstices amongst individual animals, thus
increasing biodiversity (Cerda & Castilla 2001). Similarly,
higher diversity of invertebrates has been found amongst
the alien Sargassum muticum than amongst native algae
(Norton & Benson 1983). In some cases, the alien species
provides additional food resources for local people.
Examples include M. galloprovincialis in South Africa and
P. praeputialis in Chile.
Alteration of coastal geomorphological
Many sedimentary coastlines worldwide adjoin low-lying
land or are threatened by flooding or coastal erosion. This
has led to construction of sea defences, for example in the
Netherlands, on the Adriatic Sea and in Japan (Koike
1993). Virtually all of what were originally sedimentary
coastlines in these areas have now been ‘hardened’ by sea
defences. Most artificial rocky shores support communities
similar to, but less diverse than, natural shores (Southward
& Orton 1954; Hawkins et al. 1983; Chapman & Bulleri
2003). These artificial shores can provide stepping stones
between populations presently isolated by distance. This
has consequences for population genetics (see Kimura &
Weiss 1964; Keenan 1994) and may lead to range exten-
sions of species with restricted dispersal. For example,
littorinids have extended their range along the Belgian
coastline, which lacks natural rocky shores, via breakwaters
( Johannesson & Warmoes 1990).
The severity of flooding and coastal erosion is likely
to increase during the next few decades because of sea-
level rise and the increasing frequency of storms
(Rodwell et al. 1999; Grevemeyer et al. 2000). This will
lead to increasingly ‘hardened’ coastlines over the 2025
time horizon as more sea defences such as offshore
breakwaters and sea walls are constructed. The
(a) Relative abundance
Wave exposure
S. argenvillei
S. argenvillei
by mussels
Sheltered Moderate Exposed Extreme
Mussel recruitment
(numbers per 100 cm2)
Limpet mean
size (mm)
Sexual maturity
of limpets (%)
–M –M
Mussel growth
rate (mm per yr)
Fig. 14.4. Synopsis of interactions between an alien mussel
(Mytilus galloprovincialis) and an endemic limpet (Scutellastra
argenvillei) on sheltered, moderate, exposed and extremely
exposed shores on the west coast of South Africa. (a) Relative
abundances of the two species; (b) intensities of mussel
settlement in the presence (þL) or absence ("L) of the lim-
pet; (c) growth rates of mussels; (d) mean sizes of limpets; and
(e) percentage of the limpet population reaching sexual
maturity in the presence (þM) or absence ("M) of mussels.
Data are derived from Steffani & Branch (2003a,b,c) and
Branch & Steffani (2004).
Rocky intertidal shores 219
connectivity between rocky shores will increase as a result,
as will the extent of habitat available for rocky-shore
organisms. However, changes in hydrodynamics will have
consequences for sediment-dwelling organisms and
will lead to increased fragmentation of sedimentary
habitats (M. Frost, P. S. Moschella, R. C. Thompson &
S. J. Hawkins, unpublished data).
The amount of suspended sediment in the water column
is expected to increase by 2025 as a consequence of
increased coastal erosion, riverine inputs, artificial
replenishment of coastlines and dumping of sediment
overburden from mining. When deposited, this material
may smother rocky habitats, particularly in sheltered
locations. This will lead to an increase in the abundance
of sand-dwelling organisms such as polychaetes and sand-
tolerant species such as the anemone Anthopleura ele-
gantissima and the red alga Chondrus crispus (Daly &
Mathieson 1977; Taylor & Littler 1982). Organisms with
opportunistic life histories, such as ephemeral and some
turf-forming algae, will also be favoured on rocks that are
subject to intermittent inundation (Airoldi 1998). Hence
diversity at the ‘shore’ scale of resolution may increase as
a consequence of an increase in the variety and frag-
mentation of habitats (Littler et al. 1983; McQuaid &
Dower 1990).
The La Rance tidal barrage scheme in France, where tidal
amplitude has been reduced, hydrographic exchange
altered and the distribution of sheltered rocky shore
organisms changed (see Little & Mettam 1994; Retie
1994), emphasizes the view that if physical conditions are
changed, rocky-shore communities will be irreparably
altered. There has been a recent resurgence of interest in
tidal barrages. Other renewable energy sources such as
offshore wind turbines and wave-energy machines are
likely to expand rapidly. These devices will inevitably lead
to more artificial hard structures and, in the case of wave-
energy machines, will certainly reduce wave action to the
landward side. There will also be the risk of occasional
wreckage of structures on the shore causing localized
Recreational gatherers collect a wide variety of organisms,
mainly invertebrates, for use as fishing bait, for their
ornamental value or for food. Although the daily take may
be small, the cumulative effects can be substantial
(Underwood 1993). Additionally, habitats can also be
damaged by human trampling and disturbance associated
with collection (Newton et al. 1993), and by other forms of
recreation often associated with tourism, such as access to
the sea for scuba diving (Hawkins & Roberts 1992), surfing
and swimming as well as educational visits (Fletcher &
Frid 1996; Bellan & Bellan-Santini 2001).
Disposable income, leisure time and availability of
personal transport have all increased dramatically over
the last 50 years in the developed world. This has been
associated with a decline of subsistence gathering of food
and collection of algae, but an increase in recreationally
related impacts (Fletcher & Frid 1996), including those in
areas of the Third World that are being developed for
international tourism (Hawkins & Roberts 1992). Para-
doxically, these impacts can be particularly heavy in
conservation areas where public access is encouraged to
promote awareness of marine wildlife (Fletcher & Frid
1996). Increases in the amount of time available for leis-
ure are likely to lead to an increase in this form of dis-
turbance by 2025. Despite some localized negative
impacts of research and educational activities (Hawkins
1999), these activities will be beneficial in influencing
perceptions and attitudes to coastal environments and in
providing information for management.
Global change and large-scale phenomena
Global changes in many climatic variables (temperature,
insolation, ultraviolet radiation (UV), sea level and wave
action) anticipated by 2025 (IPCC [Intergovernmental
Panel on Climate Change] 2001a; Hulme et al. 2002) will
no doubt lead to shifts in the geographical distribution of
some intertidal organisms. Most intertidal organisms have
morphological or physiological adaptations to cope with
changes in environmental stresses greater than those
anticipated by 2025. Consequently, although there may be
changes in species abundance at the fringes of their dis-
tribution (see for example Herbert et al. 2003), these are
only likely to affect organisms at local to regional scales and
will depend on the gradient concerned. For instance, shifts
might be expected to occur along the vertical emersion
gradient at a scale of centimetres to metres, along hori-
zontal gradients of wave exposure at a scale of tens to
hundreds of metres, and along geographical climatic
gradients at a scale of tens to hundreds of kilometres.
Changes in the distributions of species are likely to have
significant effects where they are associated with shifts in
the relative proportions of functional groups or in the
abundance of organisms that have a key role in structuring
communities. Similarly, considerable shifts in species
distributions would occur if small changes in climate led to
large-scale changes in ocean circulation.
Climate can directly influence the distribution of intertidal
species. This operates through shifts in competitive abilities
at the northern and southern distributional boundaries, and
at the upper vertical distributional boundary (see Denny &
Paine 1998), where temperatures are either too low or too
high for growth and reproduction. For example, in response
to elevated temperature, species will become increasingly
stressed at the equatorial margins of their distribution,
leading to local extinctions. At higher latitudes, if low
temperatures are also a major factor limiting their distri-
bution, species with broad dispersal will be able to colonize
polewards quite rapidly (see review of Clarke 1996), while
those with limited dispersal will face a bottleneck, their
distribution being squeezed by increasing temperature from
the equator and their restricted ability to disperse towards
the poles.
Organisms may respond to an environmental challenge
by (1) moving somewhere else, (2) staying and adapting to
the changes or (3) going extinct (Clarke 1996). Although
historical examples of all of these responses can be dem-
onstrated for climate change (for example range expansion
and morphological evolution: Hellberg et al. 2001), there is
no real understanding of the balance between them. Given
the gradual nature of changes in the biogeographical axis,
the steepness of vertical gradients and the ability of shore
organisms to tolerate environmental fluctuations, the
likelihood that rocky-shore animals and algae will survive
and eventually move seems high.
Different species will, however, move in distinct ways
depending on their dispersal characteristics (Hiscock et al.
2004). The influence of climate on the distribution and
abundance of invertebrates is also mediated through
reproductive output (Southward & Crisp 1954; Southward
1967; Kendall et al. 1985; Lewis 1996). For example,
southern species of barnacles reaching their northern
geographical limits in the British Isles have fewer broods
than further south in Europe, and the success of early
broods released into the plankton is very low (Burrows
et al. 1992).
Because of the connectivity of rocky-shore habitats,
rocky-shore organisms will probably respond to climate
change by shifts in distribution and abundance along envir-
onmental gradients over biogeographical scales. Hence local
extinctions will generally be matched by colonization of new
areas. For example, abundances of the boreal cold–temperate
barnacle Semibalanus balanoides and the subtropical Chtha-
malus spp. fluctuate markedly over time, with numbers being
strongly positively correlated with sea-surface temperature
with a time lag of 2 years (Southward 1991; Thompson et al.
2002, Fig. 6). Similar fluctuations have been seen in northern
(Patella vulgata) and southern (P. depressa) limpet species
(see reviews of Southward et al. 1995, 2005). Consequently,
rocky-shore plants and animals may provide valuable indi-
cators of more extensive change offshore (Southward 1980,
1991; Southward et al. 1995).
The effects of changes in temperature may be amplified
or reduced by interactions with other physical factors. For
example, based on thermal tolerances alone, the ranges of
some tropical and warm–temperate macroalgal species are
expected to extend to higher latitudes as a consequence of
increases in global temperature. Apart from temperature,
however, the distribution of many algae is also regulated by
day length. Hence expansions in algal distribution from
lower latitudes may be limited by their inability to adapt
to prevailing photoperiods rather than by temperature
(Beardall et al. 1998).
Interactions with other organisms will also modify the
direct effects of temperature, further restricting prediction
of the consequences of climatic changes. For example, small
changes of about 2 $C in air temperature across the Cape
Cod peninsula appear to fundamentally alter the influence of
Ascophyllum nodosum canopy on survival of S. balanoides. In
the south, macroalgal canopy has been shown to provide a
refuge from thermal stress and to enhance barnacle survival
(Leonard 2000; Thompson et al. 2002). Further north,
where shading is less important, the algae provide refuge for
a predatory whelk that feed on the barnacles and reduce
barnacle survival relative to areas higher on the shore above
the Ascophyllum zone (Leonard 2000).
Climate change will enhance the success of some non-
native species invasions and they will be able to spread
further. For example, Crepidula fornicata, an immigrant
to the UK, was initially found only on the south and west
coasts of England and Wales, but it has now been found
as far north as Scotland ( J. Davenport, personal com-
munication 2004). It is likely that climate change will lead
to greater variability of temperature as well as rising
Rocky intertidal shores 221
averages. This may favour species transfer from climate
regions strongly influenced by the continent (American
East Coast, Eastern Pacific) to more equitable regions
ameliorated by a strong oceanic influence including
warming currents such as the Gulf Stream (USA Pacific
and European coasts). Invasion seems to be less likely in
high-diversity communities and so human activities that
reduce diversity (eutrophication, chronic pollution or
harvesting of living resources) will probably increase the
risk of further invasions.
Based on predictions of a 1.4–5.8 $C rise in temperature
by 2100 (IPCC 2001a), typical horizontal shifts in species
distributions in the region of tens to hundreds of
kilometres and small changes in vertical distribution
(depending on the tidal range at a given location) are
predicted by 2025. Different species will respond in dif-
ferent ways depending on life-history traits and habitat
requirements (Helmuth et al. 2006), with recent range
extensions of southern species being recorded (Herbert
et al. 2003; Mieszkowska et al. 2005; Lima et al. 2006).
Assuming the current pattern of gradients in sea and air
temperature remains, the speed of horizontal shifts in
distribution will tend to be more rapid in regions where
isotherms are widely separated. However, there will be
exceptions, and there is potential for local extinctions and
major shifts in community structure and ecosystem pro-
cesses in some regions (see Harley et al. 2006, Helmuth
et al. 2006 for reviews).
Ultraviolet radiation, principally UV-B, can have inhibi-
tory effects on photosynthetic performance, growth and
nutrient uptake, and can also damage DNA in algae
(Beardall et al. 1998). It can also alter behaviour (for
example in the frequency with which sea urchins cover
themselves with debris in response to elevated light levels:
Adams 2001), shift sex ratios and reduce survival in
invertebrates (Chalker-Scott 1995). Although UV-B
radiation is maximal in the tropics, the greatest ecological
effects are likely to occur at higher latitudes, where
organisms may lack adaptive mechanisms such as screening
compounds, methods of repair or behavioural strategies
that help reduce the deleterious effects of UV-B (Beardall
et al. 1998). Gaps in the ozone layer also occur at higher
latitudes. To date, most work has focused on individual
responses; more research is required on potential effects
at the level of communities and ecosystems (see review of
El-Sayed et al. 1996).
Only small changes in sea level are anticipated by 2025
(IPCC 2001a), the ‘best estimates’ for 2020 being 6–7 cm.
Direct consequences for rocky-shore organisms are likely
to be minimal compared to the effects of changes in climate
or storminess, and relative to natural fluctuations in tidal
rhythms associated with short-term (18.6-year) changes in
the astronomical cycle (Denny & Paine 1998). An excep-
tion to this might occur in regions where a small change in
tidal level leads to a substantial shift in the extent of
horizontal versus vertical rocky shoreline with associated
consequences for the area and aspect of intertidal substrata
(Graham et al. 2003).
Also predicted are greater frequency of extreme events
such as storms and increased rainfall in winter or hot
spells in summer (IPCC 2001a). An increase in wave
action associated with storms will lead to an increase in
the relative abundance of filter-feeders, which do well in
these conditions, and a reduction in the abundance of
grazers. These changes are likely to be associated with an
increase in biomass and a reduction in species diversity
(see Bustamante & Branch 1996b; Ricciardi & Bourget
1999). Extreme thermal events are likely to result in kills
at the upper limits of distribution of many intertidal
species during both hot (Schonbeck & Norton 1978;
Hawkins & Hartnoll 1985) and cold weather (Todd &
Lewis 1984). The abundance of intertidal epilithic
microalgae is also strongly influenced by weather condi-
tions, declining during summer. Hence climatic extremes
will also affect primary productivity leading to alternating
conditions of feast or famine for the molluscs that graze
these microbial films (reviewed by R. C. Thompson et al.
2000). There will be consequent changes in the balance
between bottom–up regulation versus top–down control
of intertidal communities (Thompson et al. 2004). These
features would all lead to increased temporal variability in
the structure of rocky-shore communities, with extreme
conditions leading to periodic mortality of some species
and an associated increase in the frequency with which
space is made available for recolonization. The frequency
and magnitude of disturbance are likely to increase.
As natural shifts in atmospheric and oceanic circulation
across entire ocean basins, the ENSO and North Atlantic
Oscillation (NAO) ( Jaksic 1998) may be linked to climate
change driven by increased anthropogenic carbon dioxide
in the atmosphere (see Urban et al. 2000). On rocky shores,
the most significant impact of ENSOs is to reduce the
frequency and extent of upwelling of cold nutrient-rich
water along the west coast of the Americas and other
upwelling coastlines. ENSO events lead to warmer nutri-
ent-poor water and changed currents and storm regimes
(Allison et al. 1998), and have been associated with changes
in species distributions and abundances, prevalences of
marine diseases and marine primary production levels
(see Fields et al. 1993; Harvell et al. 1999). Recorded
changes have been particularly striking at localities such as
the Galapagos (Ecuador) that lie astride complex and
contrasting currents (Vinueza et al. 2006).
Impacts of the ENSO similar to those on pelagic
fisheries and subtidal kelp forests (Dayton et al. 1992,
1998) are apparent in intertidal communities, with both
positive and negative effects depending on the species
concerned. For example, in Chile, the 1982/3 ENSO was
thought to have caused massive die-offs of brown algae
(Soto 1985) and of littoral invertebrates (Tomicic 1985).
Settlement of a keystone predator (Concholepas con-
cholepas) was low in years associated with either El Nin
or La Nin
˜a (negatively related to the ENSO Index:
Moreno et al. 1984), with potential consequences for both
intertidal communities and food gathering (Castilla &
Camus 1992). In California, recruitment of intertidal
barnacles (Connolly & Roughgarden 1998) and tide-pool
fish (Davis 2000) was affected during the 1997 ENSO.
ENSO events have been a persistent feature of late
Quaternary climate variation and have been occurring since
at least the Pleistocene (Keefer et al. 1998; Bull et al. 2000).
There has been an apparent trend towards more severe El
˜os at the end of the twentieth century ( Jaksic 1998;
Stone et al. 1999). If this forms part of a longer-term trend,
impacts on affected rocky shores are likely to become more
frequent and intense over the next few decades.
Impacts on rocky shores will range from sublethal effects
on individuals through to populations and community-
level responses. Many of these impacts will be localized
and stem from point pollution sources and local human
usage. Collection of living resources can affect entire
coastlines, and both commercial and recreational usage
may have widespread effects. Creation of new hard sub-
strata as part of sea-defence schemes will also occur on a
large scale in many parts of the world. These changes will
have direct and indirect effects on individuals, popula-
tions, communities and ecosystems, leading to changes in
primary and secondary productivity, with consequences
for both commercially exploited and unexploited species,
together with ecosystem goods and services.
In trying to anticipate the future it is important to view
the past. Many problems already existed in the 1960s (e.g.
Thompson et al. 2002; Table 14.1), but perhaps were not
appreciated at the time. Some, such as oil spills, sprang
into public consciousness after major events such as the
Torrey Canyon wreck off the coast of Cornwall (UK) in the
1960s. Scientific detective work in the 1980s unravelled the
effects of TBT on gastropods and oysters in the face of
scepticism by industry (Ludgate 1987). Declining yields
and concerns about the need for and scope of marine
protected areas prompted work on intertidal resources in
the 1970s and 1980s, which showed the extent and scale of
human predation on rocky shores in southern Africa
(Branch & Odendaal 2003), Chile (Castilla & Duran 1985)
and New Zealand (Towns & Ballantine 1993).
Looking forward, what if any will be the surprises?
Maybe a new wonder pesticide (as TBT was hailed in the
1970s) will be seen to have a host of unforeseen side effects.
An area of current concern is the side effects of various
preparations used to treat ectoparasites in aquaculture (e.g.
Collier & Pinn 1998). Given the pace of biotechnology,
genetically modified organisms could be developed for
algal and animal mariculture. Fast-growing strains of sea-
weeds, mussels, oysters, crabs and shrimps could all be
developed and escape from culture. These would influence
biodiversity at the within-species level in terms of genetic
variation but could also influence population and com-
munity processes (e.g. Kapuscinski & Hallerman 1991;
Hallerman & Kapuscinski 1995).
The traditional historical human afflictions of war,
pestilence and famine will no doubt feature over the next
few decades. The consequences of even quite small
regional wars have been illustrated in the Arabian Gulf
where oil pollution destroyed many marine communities
( Jones et al. 1994). Even small-scale insurrections lead to
breakdown of law and order, and an early casualty is
enforcement of conservation and anti-pollution legislation.
The ability to predict the consequences of changes in a
single impact vary from reasonable certainty, in the case of
some pollutants on single species, to considerable uncer-
tainty, for example in terms of ecosystem responses to
changes in global climate or the introduction of non-native
Rocky intertidal shores 223
species (Thompson et al. 2002; Table 14.1). Unfortunately,
the ability to forecast the combined interactive effects of
several environmental factors is at best fairly modest.
Hence, unpleasant surprises can be expected to 2025, but
their nature remains largely unpredictable. The greatest
ecological surprises are likely to occur where (1) environ-
mental change induces shifts between alternate states
(Barkai & McQuaid 1988; Paine et al. 1998; but see Dayton
et al. 1998, for an example of stability in algal communities
despite loss of considerable biomass); (2) an organism is
particularly susceptible to a pollutant (for example dog-
whelks and TBT); or (3) an exotic species has a much more
prominent role in an invaded community than at home (as
is the case for Sargassum muticum).
Gaps in scientific knowledge about rocky shores are
outlined in Thompson et al. (2002). Here, the focus is on
policy and management priorities. Rocky shores must not
be studied in isolation. Table 14.3 summarizes the four
major effects of human activities on rocky shores, and
the actions necessary to mitigate them. However, part of
the danger lies in developing such lists without also
ensuring that appropriate actions are then taken to
resolve them.
It is worth focusing on one of these activities here,
namely harvesting of rocky shores by subsistence com-
munities, a ubiquitous activity in most developing coun-
tries. There are many examples where this has led to
unsustainable depletion of stocks. Solutions are not easily
found in conventional fisheries management, but there
have been some success stories for co-management, namely
the involvement of users and central authorities in joint
responsibility and decision-making for the management of
renewable resources. One illustrative case has been mussel
harvesting on the eastern coast of South Africa (Harris
et al. 2003). Under previous regulation, subsistence fishers
were given no recognition and were entitled to harvest only
amounts allowable to recreational fishers, which were never
enough to provide a regular source of food, and this forced
subsistence fishers into illegal harvesting. Confrontation
with authorities became violent, and to reduce detection
the fishers turned to nocturnal and quick but destructive
methods of stripping the mussels. The impasse was only
broken when co-management was initiated. This allowed
Table 14.3. Actions available to help preserve rocky intertidal habitats together with a qualitative assessment of their relative
importance and the regional scale at which action would be required in order to be effective
Scale Actions
Major threats
Species invasions Harvesting
Changes to coastal
geomorphology Global change
Local Regulations **
Co-management ***
Marine Protected Areas *** *
Education/awareness ** *** * **
National Equitable access ***
Policy and legislation *** *** ***
Eliminate subsidies ***
International Research and monitoring *** *** *** ***
Regulation required:
Aquaculture and
aquarium trade
Shipping operations ***
Greenhouse gases ***
*, useful; **, important; ***, essential.
the fishing community equal participation in making
decisions about regulations, involved them in research and
improved enforcement. Benefits were mutual, although
considerable effort and time were necessary to achieve
them. Determination of a sustainable total allowable catch
was based on scientific experiments that actively involved
the community and led to joint agreement on desired levels
of harvesting. Enforcement remained the responsibility of
the governmental authority, but was based on cooperation
by the community and consequent improvements in
compliance. The community gained knowledge, training,
legal and exclusive access to the local resources and an
equitable way of sharing them. There were gains for
conservation, decision-making and the development of
policies and regulations.
Co-management (Fig. 14.5) should not be regarded as a
panacea, but it clearly has huge advantages over previous
top–down regulations imposed by edict (Berkes et al.
2001), and the principles are particularly relevant to rocky
shores for five reasons. Firstly, harvesting is one of the
major factors affecting rocky shores. Secondly, they are
extremely accessible, and can be harvested by low-tech
means by almost anyone. Thirdly, rocky shores are diffi-
cult to police and manage. Fourthly, it is relatively easy
to assign ‘ownership’ to local communities because the
physical habitat lies close to user communities, and most of
the resources are sessile or slow-moving so that local users
can benefit from the control measures to which they agree.
Finally, most of the resources that are harvested have
relatively low commercial value, and are better used as a
source of food than for commercial sale, thus reducing the
incentive to break rules and cheat for short-term gains at
the expense of long-term sustainability. Perhaps more so
on rocky shores than anywhere else, biological, social and
economic factors conspire to make co-management a good
bet. Winning local communities around to agreeing on and
setting controls is essential if management is ever to
achieve sustainability.
Monitoring Compliance
Fig. 14.5. Summary of the roles, responsibilities and benefits
that can be jointly attained among scientists, managers and
users in a system of co-management.
Rocky intertidal shores 225
... Due to the exposed location of rocky shore species at the landsea interface and the number of potential drivers affecting them, environmental impacts on rocky shore species are often more severe than in other coastal ecosystems (Thompson et al., 2002;Branch et al., 2008). This results in a pronounced disturbance history of rocky shore communities and demands their ability to frequently adapt to changes in the environment (Mieszkowska, 2016). ...
... With the increasing intensity of anthropogenically-driven changes, rocky shore species are experiencing more intense stress at local, regional, and global scales. Impacts of various drivers such as pollution, exploitation by humans, introduction of alien species, man-made alterations to coastal geomorphological processes, and large-scale phenomena on rocky shores have been well reviewed in the past (Thompson et al., 2002;Branch et al., 2008;Mieszkowska, 2016). As global warming is an "umbrella" driver prevalent across systems (Jackson et al., 2021), we will focus specifically on climate driver impacts on rocky shore intertidal systems in this section. ...
... Such increases in temperature have already led to changes in the abundance and distribution of intertidal species: it has been shown that warm water species are becoming more abundant, while those species adapted to cold water conditions are either moving poleward Pitt et al., 2010;Wernberg et al., 2011) or contracting their ranges (Smale et al., 2017). The latitudinal shift in species distributions can vary from a few kilometers to a few hundred kilometers (Branch et al., 2008). Range shifts will lead to alterations in community composition and, therefore, may affect ecosystem structure and functioning Mieszkowska, 2016). ...
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Most intertidal rocky systems are exposed to severe tidal, diurnal, and seasonal changes in environmental parameters. In addition, they show extreme vulnerability to anthropogenic impacts. Research on multiple drivers is therefore crucial to understand the complexity of their potential interactions. Here, we first give an overview of the natural environment and impacts of climate change on rocky shore intertidal systems, and then focus on the impacts of multiple drivers. We further provide a summary of existing multiple driver studies in the literature with the aim for a better understanding of multiple driver interactions. As multiple drivers can affect rocky shore intertidal systems at different spatial and temporal scales, and the outcome of their effects are still more of an “ecological surprise,” we recommend a more widespread assessment of the environmental and biological context. We propose a new, integrated approach based on existing literature: this complements previous frameworks but with an improved understanding of co-occurring multiple driver systems of the rocky intertidal, in order to find management solutions based on accurate and informed predictions in these times of global change.
... Intertidal rocky shores are important ecosystems, forming a narrow fringe between land and sea and providing many goods and ecosystem services (Branch et al. 2008). Rocky intertidal communities have been extensively studied worldwide and have proved to be tractable systems for experimental ecology, contributing much to our general understanding of population and community ecology (Martins et al. 2008). ...
... Rocky intertidal communities have been extensively studied worldwide and have proved to be tractable systems for experimental ecology, contributing much to our general understanding of population and community ecology (Martins et al. 2008). They are influenced by events that occur both at sea and on land and extremely vulnerable to human activities compared to many other marine ecosystems (Thompson et al. 2002;Branch et al. 2008). Many naturally occurring factors have been recognised as important in influencing marine benthic hard-bottom intertidal communities (see, e.g. ...
... Many naturally occurring factors have been recognised as important in influencing marine benthic hard-bottom intertidal communities (see, e.g. Bustamante et al. 1997;Wallenstein and Neto 2006;Branch et al. 2008). Physical environmental factors, including substratum type, are usually considered strong elements in structuring intertidal populations and composition of assemblages (Little and Kitching 1996;Griffin et al. 2009;Firth et al. 2013; Communicated by I. Kjersti Sjøtun Electronic supplementary material The online version of this article ( ...
Rocky intertidal communities have proved to be tractable systems for experimental ecology, contributing much to our general understanding of population and community ecology. Physical environmental factors are usually considered strong structuring elements for these assemblages. In this study, we adopted a mixed model sampling design to study the effects of substratum type and shore orientation (i.e. different wave exposure) on intertidal assemblages of Madeira Island (NE Atlantic) across time. We included both macrofauna and macroalgae and compare their abundance and distribution in boulders and rocky platforms on north and south coasts of the island. Generally, assemblages moderately differed between boulders and rocky platforms whereas orientation had little influence on the distribution of most taxa. A high variability was observed across a range of spatial and temporal scales, suggesting that interactions of both physical variables and biological parameters may be influencing distribution of intertidal organisms. The results obtained provide pioneer quantitative data on intertidal assemblages of Madeira.
... Some authors considered that artificial structures built with locally quarried materials are likely to have minimal effects on inter-tidal assemblages, and therefore predicted that the epibiotic communities on such structures would be similar to those colonizing nearby natural rocky habitats (Thompson et al. 2002;Branch et al. 2008). However, there is evidence that this is not always true and differences have been found between assemblages on natural habitats and artificial structures made of locally quarried rock (see Bulleri & Chapman 2010 for review). ...
... Some studies have suggested that artificial structures provide habitats for epibiotic communities that are qualitatively similar to those of nearby natural reefs (e.g. Branch et al. 2008) but quantitative studies (e.g. Moschella et al. 2005;Firth et al. 2014), have shown that artificial structures often have significantly lower abundances of gastropods, barnacles and coarsely branched algae, indicating some lack of similarity to natural rocky shores. ...
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On a global scale, urbanization has resulted in substantial proportions of coasts being replaced by artificial structures such as marinas, breakwaters and seawalls. There is broad consensus that coastal defense structures are poor surrogates of the natural habitats that they replace. Here we investigated the effects of the type and roughness of materials used for the construction of artificial structures on the surrounding biota by comparing abundances and distribution of key inter-tidal taxa between natural shores and coastal defenses. Lower abundances of gastropods and barnacles were found on artificial coastal defense structures (regardless of the material type). At small spatial scales, abundances of key taxa increased with increasing roughness. Our results suggest that the choice of materials used for the construction of coastal defense structures has little effect on community structure per se, but that enhanced roughness could make coastal defenses better surrogates of natural habitats by supporting assemblages that are more similar to those found on natural shores.
... What can, however, be managed is the interaction between global climate change and other impacts such as non-native species. One of the most pressing issues is the interactions between invasive species from other biogeographic realms (global homogenization) and climate change (Thompson et al., 2002; Branch et al., 2008); there is good evidence that climate change favours non-native species (Stachowicz et al., 2002; Sax et al., 2007). This can only be tackled by greater bio-security and vigilance against both deliberate and accidental introductions. ...
The Earth and its oceans are going through a period of unprecedented change driven by increasing human population and economic development. Increasing greenhouse gases are influencing climate; temperatures are rising (IPCC, 2007; Burrows et al., 2011); sea levels are rising and conditions are getting stormier (Lowe and Gregory, 2005); stratification of shelf seas may intensify (Richardson and Schoeman, 2004); ocean circulation patterns may change (Bryden et al., 2005). While not strictly a climate effect, increased carbon dioxide in the atmosphere is leading to a reduction of the pH of the oceans (‘Ocean Acidification’) (Caldeira and Wickett, 2003; Orr et al., 2005). Superimposed on these physical and chemical changes are biological impacts at global scales such as homogenization of floras and faunas by species invading from other biogeographic realms (Maggs et al., 2010) and overfishing of large pelagic species (Myers and Worm, 2005). Marine plastic litter is a global problem (Thompson et al., 2004). On a regional scale all seas are showing signs of overfishing for demersal species and to some extent smaller pelagic species. Some enclosed seas (e.g. Baltic, N. Adriatic) are showing the impacts of eutrophication which can interact with fishing to reshape ecosystems (Österblom et al., 2007). Local scale impacts are myriad from point source pollution, recreational use of coastal areas, marine noise and most pervasive of all, coastal development leading to habitat degradation and total loss (Airoldi and Beck, 2007). Such local degradation can scale up to whole regions (i.e. coastal defences in the Northern Adriatic and the southern North-Sea, Airoldi et al., 2005). In this brief article I will consider the major issues facing marine conservation in a rapidly changing world. I will outline an approach based on managing the interactions between global climate change and other global, regional and local impacts. This builds on work published elsewhere (Hiscock et al., 2004; Hawkins et al., 2008, 2009, 2010a,2010b; Firth and Hawkins, 2011). I will then give some brief pointers for the way forward for conservation including marine protected areas (MPAs). The crucial need for understanding connectivity better is emphasized. As this article has a 10–25 year time horizon, I will not consider the longer-term threat of reducing pH of the ocean as much has been written about ‘Ocean Acidification’ elsewhere, although some recent work has suggested that effects may already be occurring (Wootton et al., 2008).
Intertidal ecosystems are key habitats that are being replaced by artificial hard substrates due to the increment of human activities in coastal areas. These new substrates host generally less biodiversity mainly due to differences in complexity and composition. It is a global phenomenon and has led to the development of strategies in the framework of eco-engineering. However, mitigating measures, such as new eco-designs, must cope with the high spatial variability of the region where they are applied. Therefore, in order to assess if differences in biodiversity detected at local scales in previous studies could be scaled up to predict patterns at a wider scale, we studied taxa richness and taxonomic structure of intertidal communities across the Alboran Sea (western Mediterranean Sea). We compared four different types of artificial substrates (cubes, rip-raps, seawalls and tetrapods) to assess which produces less impact. Overall, artificial substrates host low benthic biodiversity, specially on seawalls, whereas boulder-like artificial structures such as rip-raps were more similar to natural ones. Nevertheless, the effect of a particular type of artificial structure at a regional scale seems unpredictable, highlighting the challenge that eco-engineering measures face in order to establish global protocols for biodiversity enhancement and the importance of local scale in management programmes.
The world’s 40% of the population lives in coastal areas (<150 km from the sea), and this is set to increase in upcoming years. This urban sprawl leads to the proliferation of artificial coastal defence structures along the coasts to save the populace from coastal erosion, storms, and hurricanes. Deployment of artificial coastal defence structures has direct or indirect impacts on the local and global scenario, but the ecology of artificial habitats was studied poorly. Therefore, the current study aimed to focus on the role of artificial coastal defence structures in enhancing coastal biodiversity. A total of 228 epibiotic species associated with the artificial coastal defence structures were identified. The study recorded high species richness and diversity of epibenthos in artificial habitats compared to natural habitats. Among various types of artificial habitats, the assemblage pattern of epibiotic species in sandstone surfaces differs from non-sandstone surfaces. Apart from the structure surface, local epibenthic biodiversity also plays a significant role in determining the artificial structure assemblages. The length, vertical height, and age of the structures are the major deciding factors in the species composition of the structures. The overall study concluded that the artificial coastal defence structures could act as a surrogate surface for epibiotic assemblages. The input of coastal biodiversity component while designing the artificial coastal defence structures can be an added advantage.
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Rocky shore ecology has been studied for a long time, starting with qualitative descriptions and becoming more quantitative and experimental over time. Some of the earliest manipulative experimental ecological studies were undertaken on rocky shores. Many, over time, have made considerable contributions to ecological theory, especially highlighting the importance of biological interactions at the community level. The suitability of rocky shores as convenient test systems for ecological experimentation is outlined. Here we consider contributions from rocky shores to the emerging concepts of supply-side ecology, the roles of competition, predation and grazing, disturbance and succession and positive interactions in structuring communities along environmental gradients. We then address alternative stable states, relationships between biodiversity and ecosystem functioning, and bottom-up and top-down control of ecosystems. We briefly consider the feedback and synergies between ecological concepts and experimental work on rocky shores, whilst still emphasizing the traditional values of marine natural history upheld in JMBA since its first publication. The importance of rigorous experimental designs championed by Underwood and co-workers is emphasized. Recent progress taking advantage of new technologies and emerging approaches is considered. We illustrate how experimental studies have shown the importance of biological interactions in modulating species and assemblage-level responses to climate change and informed conservation and management of coastal ecosystems.
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Sustainable ecotourism development (SED) in areas with a low potential for development through other sectors, can be a sustainable income-generating activity for locals and a place for tourists to spend their free time. In this regard, planners and policymakers must focus on SED in areas with a meanwhile, if we want to develop ecotourism, we need to evaluate the existing ecotourism areas in each region. Indeed, a study is required that can follow this goal using sustainability indicators. This study was conducted to develop a model to recognize the indicators of evaluating the Ecotourism Sustainability (EES) and to fill the gap of research for SED in the west of Iran. Accordingly, we identified 30 indicators in three dimensions (economic, social, and environmental) of sustainability for EES in six eco-touristic areas in the west of Iran. The best area (Gahar Lake) was selected using the Composite Index (CI). Then, we formulated and ranked the strategies in the SWOT-AHP-TOWS analysis. The results of the CI section showed that the weight of economic indicators in EES was greater than that of other dimensions. AHP analysis revealed that the most important criteria for the SED in Gahar Lake were the opportunities in this area. Also, according to the ranking of strategies extracted in the matrix of TOWS, the most important strategy was “Advertising and introducing the beautiful nature of the lake nationally and internationally”. Our main suggestion is for Iran’s Tourism Organization to pursue more serious negotiations with international organizations for the global registration of the lake as an ecotourism zone.
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This is the first study of the ecological significance of rocky breakwaters as habitat for intertidal biota in marine environments that freeze in winter. Percent cover of intertidal seaweeds and invertebrates was quantified on exposed (high wave action and winter ice scour) and sheltered sides of 18 breakwaters (>5 yr old) and compared with 18 natural rocky intertidal areas along 430 km of the southern Gulf of St. Lawrence coast (Atlantic Canada) in the summer of 2010. Sheltered areas of breakwaters differed from natural rocky shores in having lower biotic richness and total abundance. However, these indices were not significantly different between habitat types for exposed areas. Multivariate analysis revealed significant differences in community composition between breakwaters and natural rocky shores in both sheltered and exposed areas. Ulva spp. (U. intestinalis and U. lactuca), Hildenbrandia rubra, and Mytilus edulis (exposed areas only) were more abundant on breakwaters than on natural rocky shores, while Semibalanus balanoides, Calothrix spp., Fucus spp., Chordaria flagelliformis (exposed areas only), and Ascophyllum nodosum (sheltered areas only) were less abundant on breakwaters. Our study shows that breakwaters from marine shores affected by winter sea ice support substantially different biotic communities than natural rocky intertidal areas. Thus, the findings of this study provide vital information for management decisions related to habitat loss and compensation when the coastal landscape is altered through the construction of breakwaters.
Exploitation of key consumers can have major consequences for community and ecosystem functioning. Limpets are key grazers exploited in regions such as Macaronesia, southern Africa, Chile and California. Here we describe a field experiment designed to simulate human exploitation of British limpets that are unexploited and used as model populations. Our aim was to evaluate the effects of size-selective harvesting on the composition of the rocky shore community of non-target species. Limpet populations were subjected to simulated exploitation of large size classes for 18 mo at 2 locations in the southwest of England, by systematic removal at 2 different intensities: low and high exploitation compared with unexploited plots. The exploitation of limpets led to establishment of Fucus spp. to differing degrees at each location, but while variation in percentage cover of Fucus spp. decreased over the course of the experiment in unmanipulated control plots, it increased in plots with either low or high exploitation. Multivariate analyses showed that communities at the 2 locations responded differently to the same intensity of exploitation: un - manipulated controls were similar to low-exploitation treatments at Constantine, while at Trevone low-exploitation treatments were similar to high-exploitation treatments. This was mainly due to increases in percentage cover of F. vesiculosus var. evesiculosus with exploitation, indicating that site-specific differences in assemblage structure and the size structure of the harvested populations will determine its assemblage-level responses. Therefore, reductions in density of grazers may have divergent consequences for different rocky shore communities.
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