New Scientist: Life aloft: The unexplored ecosystem above your head
Earlier this year, as a first step to protecting the biodiversity of airspace, Davy, along with Kevin Fraser at the University of Manitoba and Adam Ford at the University of British Columbia, put forward the idea that we should think about aerial habitats as layers, similar to the way that marine habitats are characterised by depth. They propose three subdivisions of the troposphere, the lowest zone of the atmosphere rising to roughly 15 kilometres up. The basoaerial habitat extends from the ground up to 1 kilometre. Here human threats range from tall buildings to wind turbines and moving vehicles (see diagram). The mesoaerial habitat, between 1 and 8 kilometres in altitude, is characterised by steadily decreasing temperatures and oxygen levels; the main threats here are light pollution and aircraft. In the epiaerial habitat, between 8 and 13 kilometres up, temperatures plunge towards -56°C at mid-latitudes; its inhabitants, mainly microorganisms, require special adaptations to survive.
A better definition of habitats is only part of what’s needed if “aeroconservation” is to take off, however. For a start, we’re not even really sure how big the problem is we’re trying to solve. A meta-study published in 2014 put the number of birds killed in building collisions at between 365 million and 968 million a year in the US. It is estimated that 140,000 to 328,000 birds are killed annually by wind turbines and thousands by civilian aircraft. In the UK, the British Trust for Ornithology estimates that 100 million birds crash into windows annually, and in Canada, more than 50 million adult birds are thought to die each year from collisions with buildings, wind farms, communication towers and other human structures that invade the skies.
On their own, though, such numbers only say so much. “What we have are mortality counts,” says Davy. “We don’t have the data that we need to be able to say whether [such counts equate to] 1 per cent or 100 per cent of the population.” That’s because we just don’t know how many creatures call the sky home.
For birds, efforts to estimate populations are well under way, aided by decades of counts, ringing schemes and newer methods such as tracking with telemetry and GPS. But for other airborne creatures, we are further in the dark. Population estimates for bats are often murky or non-existent. Some early attempts to quantify insects, meanwhile, have produced staggering numbers: more than a trillion are thought to migrate over the southern UK each year, for example.