This report is published as a BTO Research Report and was funded by Defra. It comprises a review of the primary literature on climatic change and its effect on migratory species. It arises from collaboration between the BTO, Aberdeen University and CEH. Extra support and information were provided by participants at the conference “Climatic Change and Migratory Species”, held at Madingley Hall, Cambridge on 16 and 17 March 2005.

Our climate is changing, both naturally and due to man’s actions. There is already compelling evidence that animals and plants have been affected in both their distribution and behaviour. Migratory species are likely to be affected by climate change as their behaviour usually involves seasonal movement between different geographic areas and across national boundaries. The primary framework for migratory species conservation is the Convention on the Conservation of Migratory Species of Wild Animals (CMS). Several other international policies cover some migratory species, but only the Ramsar Convention (migratory waterbirds) currently provides for climate change.

To be able to successfully predict the impacts of climate change we need a better understanding of how populations of animals and plants will respond. Effects on breeding performance and survival are crucial, yet are poorly understood. Our understanding of the likely impacts of future climate change also varies greatly between taxonomic groups, with the best information being available for birds. Of the bird species listed on the CMS, 84% face some threat from climate change. Almost half of these are because of changes in water usage; this is equivalent to all other man-made causes put together. Although it is thought that no species has yet become extinct solely because of climate change (Golden Toad is a possible exception), many extinctions (of both migratory and non-migratory species) are predicted in the near future.

The act of migration itself may become more difficult due to climate change. For example, many migratory birds use the Sahel region of Africa to refuel before making the northerly crossing of the Sahara Desert. Decreased rainfall and over-grazing is causing increased desertification in this area, leading to declines in a number of species such as the Whitethroat. Further declines in trans-Saharan migrants might be expected with climate change.

Interactions between climate change and human exploitation are widespread, though poorly understood. Examples include:

changes in migratory journeys of Wildebeest in Africa are hampered by the presence of park fences.
changes in rainfall patterns in Southern America are leading to the construction of dams that are proving a major barrier to the migration of the Tucuxi (a river dolphin).
many waterbirds are reliant on a network of a few, widely separated wetland sites for migration, which are at risk from rising sea levels.
many sites also face development and increasing water removal (due to climate change), making direct climate-driven threats even worse.

A major conservation concern is for arctic and montane species (most of which are migratory), the distributions of which cannot shift further north. Many migratory waders, such as the Red Knot, face large population declines and some, such as the endangered Spoon-billed Sandpiper, face extinction. Among mammals, Polar Bear and northern seals are of key concern, due to the loss of Arctic sea ice.

Sea-level rise has the potential to affect a range of species. Examples include:

turtles could lose their nesting beaches - 32% of beaches used by nesting turtles in the Caribbean could be lost with a 50cm sea-level rise
seals need beaches upon which to raise their pups - e.g. the endangered Mediterranean Monk Seal
there could be a net loss of shallow coastal areas used by whales, dolphins, dugongs and manatees - e.g. White-beaked Dolphins require cold water less than 200 m deep

A major effect of climate on migratory (and other) species will be changes in prey distribution, some of which are already well documented. Such changes are a major threat in marine ecosystems. Changes in sea surface temperatures have already been linked to large shifts in distribution (as much as 10° latitude) and abundance of plankton communities (with declines to as little as one thousandth of former values). These changes have affected the distribution and abundance of many marine species, such as Cod, Salmon, Long-finned Pilot Whale and a number of penguin species.

Changes in the timing of parts of the life cycle are already well documented. For example, migratory British birds are arriving in their breeding areas two to three weeks earlier than thirty years ago. Laying dates have also advanced for both birds and turtles. However, changes in laying date of migratory birds (typically 2 days earlier for a rise of 1oC) appear to be less than those in the life cycles of vegetation and invertebrates (typically 6d/1oC). This may lead to a mismatch between the birds and their prey. There is also evidence to suggest that long-distance migrant birds, such as the Swallow, may be less able to adapt their timing than short-distance migrants, such as the Chiffchaff. Warmer winters are also encouraging the earlier emergence of bats from hibernation but the population impacts of this are unknown.

Survival of individuals is also strongly related to climatic conditions. For birds, warmer winter temperatures are likely to increase survival for those that winter in northern latitudes, while those that migrate south in the winter are likely to suffer from reduced rainfall.

Changes in population size are a combination of changes in survival and breeding performance and the impact of climate change will depend on the relative balance of these two factors. For example, in one colony, increased sea surface temperatures meant that Emperor Penguins had to forage further from the breeding colony (reducing survival), but the penguins benefited from increased hatching success. The effects on survival were greater and colony size declined. In general, changes in survival and breeding success will interact with population density (through a process known as density-dependence), and thus scenarios where there are changes in population size require further study.

On land, changes in water availability (e.g. due to increased water abstraction and drought frequency) and loss of vulnerable habitat (particularly Arctic tundra) are likely to affect the greatest number of migratory species. While adaptation (through habitat management) to climate change may bring benefits in terrestrial ecosystems, a reduction in greenhouse gas emissions will be required to achieve significant benefits in marine systems where habitat management is less feasible. In many cases, a reduction in human impacts (such as over-exploitation or habitat loss) will help animal and plants to adapt. In more general terms, the maintenance of large population sizes, in order to maximise genetic variation, will allow populations the greatest chance of adapting to changes.

Some migratory species require a coherent network of discrete sites, so site management will need to be flexible in response to changing conditions. Other species will require continuous habitat corridors and broad-scale land-use planning. Changing patterns of human exploitation in response to climate change are a major threat and conservation measures need to take these into account, both as threats and as opportunities for providing benefits through the management of habitats to benefit both people and wildlife (multi-functional ecosystem management).

It is critical that there is a commitment to long-term monitoring schemes to detect the impacts of long-term climate change and to assess the abilities of plants and animals to adapt to it. There is also a need to gather information on migratory stopover sites so as best to target conservation action. Targeted implementation and enforcement of existing measures should provide much of the protection needed, as would the broader use of existing guidance codes. However, much more detailed research is still needed to be able to make accurate predictions of the effect of climate change on plants and animals.
The full text of the report is available on the Defra web site

Robert A. Robinson, Jennifer A. Learmonth, Anthony M. Hutson, Colin D. Macleod, Tim H. Sparks, David I. Leech, Graham J. Pierce, Mark M. Rehfisch & Humphrey Q.P. Crick.

Published in August 2005 by the British Trust for Ornithology, The Nunnery, Thetford, Norfolk, IP24 2PU, UK.

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