Guest blog by Dr Matthew Carroll, Conservation Scientist at RSPB Centre for Conservation Science
Craneflies as a food source for upland breeding birds
The British uplands are home to internationally-important breeding bird populations. And for some of these species, a key part of their diet during the breeding season is something you might not have expected – craneflies, or daddy longlegs as they’re often better known. For birds that breed on blanket bogs – deep peat soils that occur on rolling hills in the wettest areas of the uplands – craneflies can make up a big part of the diet during this critical period.
Photo of a cranefly by Matthew Carroll
Both chicks and adults can rely on cranefly larvae and adults for food during the breeding season. For species like golden plover and red grouse, it’s been shown that chick growth and survival rates are higher when there are more craneflies available. So these unassuming flies are a very important part of the food chain. But research we’ve just published in Nature Communications suggests that climate change might threaten the whole food web.
Craneflies need wet peat to survive and thrive
The key link in the chain is soil moisture. Cranefly larvae live in the top few centimetres of peat, and they die when the peat becomes too dry. Some of our previous research published in Global Change Biology, Maintaining northern peatland ecosystems in a changing climate: effects of soil moisture, drainage and drain blocking on craneflies has shown that higher cranefly abundances are associated with wetter peat, so things like inappropriate drainage can cause their populations to fall.
Climate warming could make peat too dry
But it’s not just drainage that could make the peat become drier. Climate change could lead to warmer, drier summers, and this could threaten cranefly populations even on bogs that have never been drained. So our work examines this system, to see what the impacts of climate change might be.
We used a model that predicts how wet peat bogs are, based on weather (temperature and rainfall) and landscape characteristics (altitude, steepness of slopes). We then combined these predicted water-table depths with observations of cranefly abundance so that, for a given set of weather and landscape conditions, we could estimate cranefly abundances. And then we used our cranefly predictions to see how bird populations responded to varying food supplies, and how things might change in the future.
First, we looked at the Peak District, where big bird surveys were carried out in 1990 by English Nature and 2004 by Moors for the Future Partnership. We found that where our model predicted the highest cranefly abundances to be, the largest breeding bird populations had been seen. This pattern was found for three iconic upland species that eat craneflies during the breeding season – golden plover, dunlin and red grouse. So, where there was more food, there were more birds!
Cranefly declines = golden plover and dunlin declines
Then, we used climate change projections to see what might happen in the future. We found that as summer temperatures rise and rainfall decreases, blanket bogs are indeed predicted to become drier. This would mean that cranefly populations could fall by up to 80% by the late 21st Century. In some places, high cranefly abundances would be restricted to the wettest bits of the landscape. Relatively dry areas like the North York Moors may end up with very few craneflies left at all.
Photo of golden plover by Andy Hay (rspb-images.com)
And what would these declines in cranefly abundance mean for the birds? Well in the Peak District, we could see big abundance declines. Dunlin abundance was projected to decline by about 50%, whilst golden plover abundance was projected to decline by nearly 30% - these are clearly big losses. By the late 21st Century, both dunlin and golden plover might only be found in the wettest areas that still retain enough food for them.
Conservation action for craneflies
So, it might seem a bit bleak – our model suggests that climate change could threaten some of our important upland bird populations. But there is hope. Our previous research has shown that blocking drainage ditches, which increases soil moisture levels, can increase cranefly populations. Drain blocking, along with re-vegetating bare peat, are key parts of blanket bog restoration and conservation strategies that the RSPB are involved with, in places such as Dove Stone in the Peak District and Lake Vyrnwy in mid Wales. These actions should help to keep bogs wet and provide a better future for the whole ecosystem as the climate changes. Knowing how climate affects hydrology, how hydrology affects invertebrates, and how prey availability affects birds, we can start to identify conservation actions. To maintain our upland bird populations in the face of a changing climate, we need to ensure that blanket bogs stay wet and craneflies remain abundant.
The paper Hydrologically driven ecosystem processes determine the distribution and persistence of ecosystem-specialist predators under climate change was published in the journal Nature Communications 31 July 2015.