Download Enhancing Habitat Diversity

Enhancing Habitat Diversity: turning the
tide on natterjack population decline at
Caerlaverock National Nature Reserve.
The saltmarsh or ‘merse’ at Caerlaverock National Nature Reserve on the
Solway Firth is a soft low-lying section of coastline vulnerable to rising
sea level. Home to the only population of natterjack toads in Scotland,
there is some concern amongst conservationists about the effects of
sea level rise on the long-term survival of the toad colonies. Natterjacks
thrive in naturally dynamic coastal habitats, but also have some very
specific habitat requirements. Their coastal marsh habitat has already
come under pressure from changes in coastal land use, and across
Britain numbers have dwindled as a result. Sea level rise and the human
response to the threat of coastal flooding are likely to be further drivers
of habitat change. Caerlaverock National Nature Reserve team have been
working with others to better understand the habitat requirements of
natterjacks so they can enhance and create optimum habitat, giving them
the best possible chance to adapt to the threat of rising sea levels.
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1. The climate risk: sea level rise
The latest projections of global sea level rise suggest that even under a highly optimistic emissions
scenario we might see over half a meter of sea-level rise by 2100 (figure 1), with serious impacts on
many coastal areas, including coastal erosion and an increased risk of flooding.
Figure 1. Past and future global sea-level rise. For the past, proxy data are shown in light purple and tide gauge
data in blue. For the future, the IPCC projections for very high emissions (red scenario) and very low emissions
(blue scenario) are shown. Source: Church et al, (2013) p1204
Sea level rise affects all parts of Scotland’s coast and, crucially, now outpaces land uplift on most of
the Scottish coast (Rennie and Hansom, 2011). Long term rates of sea level rise vary between 0.5 and
2 mm/yr; however, recent short term rates are higher (up to 6 mm/yr). There is no way of knowing if
the recent increase is temporary or likely to continue at this rate into the future.
The impact of coastal flooding is already worsening in Scotland (Ball et al, 2008), with low-lying
‘soft’ coasts such as the Uists, parts of the Orkney Isles and some inner firths such as the Solway,
particularly vulnerable (figure 2).
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Figure 2. Map showing the distribution of low-lying coastal areas in Scotland. Caerlaverock NNR lies in a
vulnerable part of the Solway Firth.
Caerlaverock National Nature Reserve on the Solway Firth (figure 2) is a soft low-lying coastline
dominated by saltmarsh. Uncertainty over the response of some coastal habitats to sea level rise
makes it tricky to predict how species that live there will fare. Coastal erosion, increased salt water
inundation and flooding associated with sea level rise at Caerlaverock could threaten one of our rarest
species – the natterjack toad Bufo calamita (figure 3).
Figure 3. The natterjack toad Bufo calamita
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Natterjack toads thrive in habitats where things change constantly, and as a result their population
dynamics are typically ‘boom and bust.’ In Britain they occur almost exclusively on coastal dunes, upper
saltmarshes (or ‘merses’ as they are known locally at Caerlaverock) and inland heaths. Between the
late 1800s and 1970, they vanished from 70–80% of their British range, primarily as a consequence of
habitat change (Beebee, 1976; 1977). In many coastal areas land claim for agriculture was the main
driver of habitat change.
Caerlaverock is home to the most northerly population of natterjack toads in Britain, occurring as
isolated populations along an 8 km stretch of low-lying coastal marsh. Research conducted in 2012
by Amphibian and Reptile Conservation (ARC) suggested that natterjack toads in Scotland number
only around 250 individuals, making them rarer than the Scottish wildcat (P. Minting pers. comm).
The net effect of climate change on natterjacks is unclear – in the south of England, a 30 year study
of the toads found that warmer May’s have a positive effect on the species numbers (Beebee, 2011).
Natterjacks at Caerlaverock favour the upper saltmarshes where fresh and salt water interact, since
they are dependent on ephemeral ‘brackish’ pools in which they breed. The occasional salt water
inundation keeps the spawn safe from predators and competitors who cannot tolerate salty conditions.
At Caerlaverock, any benefits of warmer temperatures may be offset by the threat of rising sea level or
storm surges causing existing breeding pools to become too saline for spawn to survive.
Some saltmarshes appear to be keeping pace with sea level rise by accreting or ‘growing’ vertically
(e.g. Teasdale et al, 2011; Mossman et al, 2013), but there is uncertainty about how widespread this
response is. Although increased storm frequency has not yet been identified (Comhairle nan eilean
Sar, 2012) it is expected to occur (Lowe et al, 2009). Where the habitat can’t adapt quickly enough to
the anticipated rising sea levels over the coming decades, the same storms could flood further inland
as they over-ride a higher mean sea level (Dixon and Tawn, 1997). Without the sediment supply to
grow vertically, or space to shift inland due to farmland or sea defences, there could be a reduction in
the freshwater dominated areas of marshes crucial to natterjack survival.
We must also consider the human response to sea level rise, particularly the effects of changes in land
management on the Solway’s natterjack toad population (figure 4). The combination of these ‘human’
pressures has undoubtedly been a far greater cause of natterjack habitat degradation to date than
climate change, but this may change in the future.
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‘Human’ Threats
Implications For Natterjacks
Habitat
fragmentation
Natterjacks are unlikely to cross extensive areas of unsuitable habitat to
move between their summer/winter habitat and breeding ponds
Abandonment
of traditional
grazing practice
Scrub encroachment and development of rank vegetation following
abandonment can stabilise the habitat too much.
Coastal
Squeeze –
loss of habitat
Land claim of upper merses for more intensive agriculture, coastal
development or hard coastal defences can all impede natural coastal
processes of erosion and deposition of sands, muds, and interaction of
fresh and salt water. Natterjacks depend on these dynamic processes.
Recreational
disturbance
Dogs swimming in pools during breeding season can disturb breeding
natterjacks.
Modification of
merse drainage
Changes to tidal inundation regime through construction of
embankments, and drainage channels can alter the balance of fresh and
saltwater, rendering some areas unsuitable for natterjacks, or increasing
suitability for predators.
Figure 4. Summary of the ‘human’ threats to natterjacks in Scotland
2. Adaptation measure: enhancing habitat diversity
The first step towards ‘future-proofing’ the vulnerable natterjack population at Caerlaverock is to
understand what their optimum habitat is:
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open, un-shaded areas with extensive areas of un-vegetated or sparsely vegetated ground;
vegetation with short sward to ensure presence of invertebrate prey;
some shelter (often beneath rocks, or drystone walls); and
un-shaded, ephemeral ponds with shallow, gradually shelving margins with few predators or
competitors in which to breed. A small amount of salt water inundation also helps to kill off
predators such as fish and larger aquatic invertebrates – which would otherwise eat the toads’
eggs and tadpoles.
We can use the above information to help the vulnerable natterjack populations adapt to increasing
human and climatic pressures facing their habitat, by creating or enhancing areas especially for them.
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Habitat creation on farmland
Aquatic habitat creation on farmlands immediately adjacent to the saltmarsh has been a key part of the
management to date. By excavating shallow pools on farmland inland of the Reserve, the natterjacks
can venture further inland to breed – something they may be forced to do in future in response to
higher sea levels. Breeding pool management has been carried out on all the Solway colonies, with
mixed results. Some natural pools have been slightly deepened, or ‘topped up’ so that they are less
likely to dry out in warm periods, but care must be taken not to over-deepen and allow predators or
competitors to colonise.
Figure 5. Breeding pool creation and an existing pool. Breeding pool management is challenging for site mangers
to get just right for the toads to breed in.
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Ensuring the pools are maintained in the long-term is difficult, but Scottish Natural Heritage are
continuing with their rolling programme to maintain the toad breeding pools on the Reserve, together
with provision of grants to local landowners to create suitable pools for toads on land beyond. This
attempts to revert the historical development (i.e. habitat loss) but also safeguards space for the
natural processes, habitats and species to adapt (Brazier et al, 2012).
Improved Terrestrial Habitat
Natterjack toads need a short sward adjacent to their breeding pools for hunting. Terrestrial habitat
management has included scrub clearance and some re-instatement of grazing. In late summer, once
the toads have finished breeding, cattle are let into the areas around their pools. This keeps the grass
short so the toads can forage for food, and it stops plant growth from smothering the shallow pools.
Artificial refuges for the toads are also provided in strategic positions.
Figure 6. Carefully managed cattle grazing at Caerlaverock NNR helps to keep a short sward for natterjack toads
3. Results: hope for long-term natterjack survival on the Solway
Large species populations are likely to be more resilient than small ones, so boosting breeding
success by focusing on the aquatic habitat seems logical. However, creation of more breeding pools
alone is not enough to boost natterjack numbers in the long term. Doing so successfully requires
close monitoring of pool conditions which is both highly interventionist and time-consuming. Many
natterjack conservationists feel that we are running very fast to stand still! After many years of trial and
error in creation and management of the toad’s aquatic habitat, plans are underway to create some
new pools to replace those which have become over-deepened and unsuitable.
On-going natterjack conservation involves working with others to improve terrestrial land management
on the upper marshes. In the absence of grazing, marsh vegetation becomes too dense for natterjacks
to hunt in, and also causes the marshes to stabilise. Natterjacks fare better where natural coastal
processes are unhindered by artificial drainage, coastal defences and dense vegetation. Reserve staff
are working hard with partner organisations and land owners to secure suitable long-term grazing for
natterjack toads at Caerlaverock and beyond.
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Darwin told us that variation is the raw material of adaptive change; and adaptive change is what
enables an organism to thrive in unfamiliar conditions (Quammen, 2014). With this in mind, the
Reserve’s long term goal to ensure a diverse, dynamic and well-connected mosaic of the toads
favoured terrestrial and aquatic habitat is likely to be the best way to encourage their continued survival
here.
4. Wider benefits
Fortunately, the level of grazing on the Caerlaverock reserve required for natterjack toad conservation
also suits the Barnacle geese who flock here in their thousands during the winter. Keeping the sward
fit for grazing geese also encourages them to stay on the Reserve where visitors can see them, and
helps relieve pressure from the surrounding farmland, where the geese might otherwise feed on
developing winter cereal crops.
Despite years of highly specialised species-focused conservation for natterjacks, it has become
increasingly clear that alongside these specific measures, the species thrives best against a
backdrop of natural coastal processes. Management at Caerlaverock aims to halt or reverse the
loss of coastal habitats and features from coastal squeeze and to sustain and revitalise the natural
coastal sedimentary processes – something we have shown is important for natterjack conservation
but also for us! Soft coastal habitats such as saltmarsh, sand dunes and mudflats are our natural
coastal defences; absorbing and dissipating the force of the sea and reducing the risk to people and
development. Trying to stabilise inherently dynamic natural systems can do more harm than good.
Ensuring that the natural processes continue to operate as sea levels rise, together with well managed
grazing on the marshes should help to change the fortunes of one of our most vulnerable species,
whilst at the same time helping to protect us from coastal flooding.
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REFERENCES
Ball T., A. Werrity R.W. Duck, A. Edwards, L. Booth & A.R. Black (2008) Coastal flooding in Scotland:
A Scoping study - Technical Summary. SNIFFER. Final Report. FRM10),
Beebee, T.J.C. 1976. The natterjack toad in the British Isles, a study of past and present
status. British Journal of Herpetology 5 p 515-521.
Beebee, T.J.C. 1977. Environmental change as a cause of natterjack toad (Bufo calamita)
declines in Britain. Biological Conservation 11 p 87-102.
Beebee, T.J.C (2011) Modelling factors affecting population trends in an endangered amphibian.
Journal of Zoology 284, Issue 2 p 97-104.
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Climate: The Role of Geodiversity in Biodiversity Conservation. Scottish Geographical Journal 128:
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Horsburgh, T. Reeder, G. Jenkins, J. Ridley, S. Dye, and S. Bradley (2009) UK Climate Projections
science report: Marine and coastal projections. Met Office Hadley Centre, Exeter, UK.
Comhairle nan Eilean Siar (2012) South Ford Hydrodynamic Study - Final Report. September 2012.
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Rennie, A.F., and J.D Hansom (2011) Sea level trend reversal: land uplift outpaced by sea level rise
on Scotland’s coast. Geomorphology 125 (1) p 193-202.
Teasdale, P.A., P.F Collins, C.R Firth & A.B Cundy (2011) Recent estuarine sedimentation rates from
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For more information on Caerlaverock National Nature Reserve and its management, please visit
http://www.nnr-scotland.org.uk/caerlaverock/
For more information on how SNH is helping nature adapt to climate change, please contact
Christina Wood [email protected]
or visit www.snh.gov.uk/climate-change/what-snh-is-doing/helping-nature-adapt/
www.snh.org.uk
www.nnr-scotland.org.uk
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