There is strong evidence from the MarClim project that recent rapid climate change has resulted in significant increases in abundance and extensions in the northern limits of southern, warm-water species that reach their northern biogeographical limits on rocky shores in the UK since the mid-1980s. Between 2001 and 2005 a number of these species increased their range around N. Scotland and along the English Channel. These range extensions were not trimmed back by the recent cold winter of 2005/2006 (the coldest since 1995/1996 for England and Wales, since 2000/2001 for Scotland and Northern Ireland). Decreases in the abundance of northern, cold-water species have been observed, however, there is less evidence on rocky shores of northern cold-water species retreating northwards.

These observed shifts reflect predictions by climate models based on increased sea surface temperatures, and are occurring faster than most recorded changes in the terrestrial environment, but they are highly species-specific. This is likely to have consequences for biodiversity as the rate and extent of changes will not be synchronous, and biological interactions will be affected.  Sea-level rise and storms may have an important indirect impact as sea defences create artificial habitats in areas between natural rocky shores, acting as "stepping stones" by allowing intertidal species to extend their range.

Key points:

• The warmwater macroalga (seaweed) Bifurcaria bifurcata has established a new range boundary at Portland Headland in the last 5 years, 150km east of previous records.

• Some warm water invertebrates and algae show continued increases in abundance and have extended their ranges around northern Scotland and along the English channel over the last 20 years.
• Coldwater species have shown a continued decrease in abundance throughout the period 2001-2007. (e.g. Semibalanus balanoides, the acorn barnacle, and Alaria esculenta, dabberlocks algae).
• Rates of change are mostly faster than in terrestrial systems and highly species-specific.

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What is happening now - Medium

Overall, we have 'moderate confidence' that climate is already affecting distributions of species

This is based on the existence of a large archive of high quality time-series data with wide geographical coverage that has been collected over long time periods prior to and during the current period of climate warming (see supporting evidence). Modelling undertaken to date has accurately replicated past and present observed species' distributions using changes in marine climate variables.

What could happen in the future - Medium

We have 'moderate confidence' in our predictions for the future due to the current limitations in climate scenario forecasts and limited knowledge of the impacts at the community and ecosystem levels.  The confidence level can only be increased by continuing monitoring surveys to validate model forecasts and increase the amount of baseline data for areas such as the east coasts of Scotland and England, and by investigating the bioprocesses governing species responses to climate change.

Continued annual observations are critical to prevent anomalous environmental or anthropogenicA change within part of the climate system that can be attributed to human action, rather than natural causes. events from being misinterpreted as climate-induced effects on the ecosystem.  The rate of temperature increase is accelerating and ecological observations therefore need to be made on a regular basis in order to accurately map and predict future responses to climatic drivers. An annual sampling periodicity has been employed throughout MarClim and subsequent surveys from 2001-2007 and is strongly recommended for future climate impact projects. 

Further investigation is also required into the biological mechanisms by which species are responding to rapid climatic fluctuations, causing the resultant observed changes in distribution and abundance.  The impacts of indirect effects of climate change including species' interactions require more in-depth investigation to tease apart the various driving factors causing the observed changes in biogeography and population level changes.  The influence of factors other than temperature including ocean acidification, sea level rise, stormier seas and the impact of artificial coastal defence structures must also be taken into consideration in forecasts of future states of rocky intertidal ecosystems.  The data shown in Table 1 was used to a) develop and test hypotheses on the impacts of climatic change on marine biodiversity in Britain and Ireland, b) forecast future marine community changes on the basis of the Met Office's Hadley Centre climate change models and the United Kingdom Climate Impacts Partnership's climate change scenarios (the broad range of species known or thought likely to be temperature sensitive were covered).  In parallel, models were constructed on interactions between northern and southern indicator species, focussing on barnacles and validated by comparisons with long-term time series. Models were also constructed to predict past and current distributions of indicator species and validated against MarClim archived and contemporary data.

Specifically predictions on the future responses of intertidal rocky shore fauna and flora to changes in environmental temperature regimes in Britain are:

0-5 years:

• Northern species will continue to retreat northwards and their abundance will decline; such changes are likely be driven by a reduction in reproductive output and/or decreased juvenile or adult survival during hotter summer periods.
• Southern species will continue to expand their ranges northwards and abundances will continue to increase. The mechanisms underlying these responses are likely to be recruitment driven via some combination of an increase in reproductive output and/or larval and juvenile survival during warmer spring and summer periods and milder winters.
• The  extent  to which  range extensions  can occur  will also  be dependent  on  whether larval production  and  dispersal  increases  sufficiently  in  response  to  a  warming  climate  to enable Pertaining to the physical properties of the oceans and seas- including its depth, temperature and salinity.hydrographic barriers to be breached.
• The construction of artificial sea defences beyond existing range edges will allow range extensions in areas of unsuitable natural habitat via 'stepping stones'.
• Biological interactions including competition, facilitation and predation will modulate the responses of southern and northern species, with implications for community structure and ecosystem functioning. For example, in barnacles the northern species (Semibalanus balanoides) is competitively superior and the southern species (Chthamalus montagui, Chthamalus stellatus) are released from competition by recruitment failure in S. balanoides associated with warmer spring temperatures.  Such mechanisms may occur in other species.
• Biodiveristy will increase in the short term due to the increases in range and abundance of southern, warm-water species occurring faster than the decrease in abundance and retractions of southern range limits for cold-water species. 

20-50 years:

• Changes will be greater during the 21st century than in the last warm period in the 1950s.
• Biodiversity  will  begin  to  decrease as northern, cold-water species are lost from shores, initially in  the  south  west  of  England  that  are  located  close  to  the  boundary  between boreal and lusitanianDescription of the bio-geographic area broadly applicable from the western Iberia through to the south-wetern part of the British Isles. waters.  Major step changes may occur.

In summary, we have medium confidence in future predictions due to limitations with future climatic data predictions and incomplete knowledge of how ecosystems will be affected as changes accelerate.

• Investigations of the biological mechanisms determining species responses to climate change.
• Further investigations into indirect effects of climate change via species interactions.

• Potential for greater invasion of non-native species.

Nova Mieszkowska, Pippa Moore, Steve Hawkins
Marine Biological Association of the UK, The Laboratory, Citadel Hill, Plymouth, PL1 2PB

Mike Burrows
Scottish Association of Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyle