Written November 2015.
Climate change is the topic of the week with COP21 currently taking place in France, with countries hoping to reach an agreement for climate change action. There is no doubt the world’s climate is altering due to the increased presence of greenhouse gases, with anthropogenic actives causing large amounts of these gases to be released. The IPCC suggests that a temperature rise of above 2oC would cause severe changes, which would impact species across the world. Previous, current and future climate change is something that threatens Eurasian bitterns in the UK.
The Bittern, Botaurus stellaris, is the much loved reedbed star which always has birders on the edge of their seats, especially beneficial when the seats are uncomfortable wooden planks. This bird is a secretive heron, with a detailed brown plumage, making it perfectly camouflaged amongst reeds – much to the birdwatchers’ frustration. The male’s deep boom can reach 5 km, and sends shivers through listeners’ spines. Given that this species now makes regular appearances on TV programmes such as BBC Springwatch, it is hard to believe that in 1886 in the UK they were extinct, although thankfully recolonised areas during the 20th century (Wotton et al, 2009). This species is however red listed due to its low population size. According to the RSPB the population consists of around 80 booming males and approximately 600 overwintering birds. They can mainly be found in the south of the UK, including sites like RSPB Minsmere in Suffolk.
Many other species also have low populations and specific habitat requirements so what makes Bitterns so vulnerable to climate change? Due to the ecology of this species they are suited to reedbed habitats, as they feed on the freshwater fish in shallow water amongst the reeds and they also make their nest in reed stands surrounded by water, this aids protection from predators (Gilbert et al, 2005). The main threat to bitterns due to climate change is the rising sea level as this will flood the reedbed habitat, destroying nesting areas as well as causing saline incursions, creating brackish conditions, meaning the freshwater fish the bitterns feed on will perish. Wotton et al (2009), found that 26/29 nesting sites for bitterns were at risk from saltwater incursions, this included sites at Minsmere. Extreme weather events will also worsen this threat for bitterns as storms near coastal areas could cause largescale saline incursion into freshwater reedbeds, rapidly making them unsuitable for bitterns, which is what happened at Benacre Broad in Suffolk (Wotton et al, 2009).
Temperature increases will also impact bitterns as water tables could be lowered, reducing freshwater fish survival, as well as making the conditions less suitable for the common reed, Phragmites australis, which makes up the reedbed. The density of fish will in turn affect whether male bitterns establish a territory and therefore future breeding success of the species (Gilbert et al, 2007). Furthermore the changing vegetation community will influence whether the habitat is even suitable in the future. However warmer temperatures would mean that during the winter the water bodies that bitterns rely upon to fish in may not freeze, making the prey accessible. In 1997, due to the harsh winter, the UK population declined by half to just 11 (RSPB, 2014).
Coastal flooding still remains a large issue as many bittern strongholds are at coastal locations, therefore trying to increase connectivity of these habitats is important so birds will be able to migrate when conditions change. The populations on reserves near the coast are monitored as well as protected via designations, as five SPAs for breeding bitterns were created by the EU Birds Directive. However in the face of climate change, rising temperatures and sea levels, trying to increase connectivity is vital which is why habitat creation and restoration is occurring at various sites. For example the Great Fen project aims to restore and connect fen habitat between Peterborough and Huntingdon, and at Woodwalton Fen, which forms part of the project, in 2013 a booming male was heard, the first for 20 years. Further schemes have included 3000 ha of reedbed being created as part of the EU Life Project, for example in Cambridgeshire at Kingfishers Bridge where, in 2007, bittern nests were recorded (Wotton et al, 2009). There are however feasibility issues with these schemes as they need to be in areas which bitterns can colonise therefore near other sites, and comprise of areas not at risk from flooding. Furthermore it is thought to take approximately ten years for a ‘new’ reedbed to function adequately for it to be in the right condition for nesting bitterns to be sustained (Wotton et al, 2009).
Overall the rising temperature and increasing sea level puts bitterns at risk as the suitability of their habitat for nesting and their prey is affected. It is clear that in the long-term habitat availability and connectivity is a vital component for the future survival of this iconic reedbed species.
Gilbert, G., Tyler, G., Dunn, C. and Smith, K. (2005) ‘Nesting habitat selection by bitternsBotaurus stellaris in Britain and the implications for wetland management’, Biological Conservation, 124, 547-553.
Gilbert, G., Tyler, G., Dunn, C., Ratcliffe, N. and Smith, K. (2007) ‘The influence of habitat management on the breeding success of the Great Bittern Botaurus stellaris in Britain’,Ibis, 149, 53-66. [Abstract].
RSPB. (2014) Bringing reedbeds to life [www document]. http://www.rspb.org.uk/whatwedo/projects/details.aspx?id=tcm:9-210865 (Accessed 30 October 2015).
Wotton, S., Brown, A., Burn, A., Dodd, A., Droy, N., Gilbert, G., Hardiman, N., Rees, S., White, G. and Gregory, R. (2009) ‘Boom or bust – a sustainable future for reedbeds and Bitterns?’, British Wildlife, 20, 305-315.