How ecology crosses marine-terrestrial boundaries

I’ve been off the grid for a while, first at the British Ecological Society’s annual meeting last week, then in grant writing mode this week, and tomorrow I head off to Aberdeen for the World Conference on Marine Biodiversity, hoping to have the two talks I will be giving in some kind of reasonable shape. But for a break from all the everyday business of science, here’s a little bit of cool natural history. Let’s start with a question: How do forests in Alaska end up getting fertilised with nitrogen which comes from the sea?

The answer involves an icon of wildlife photography, and a famous rhetorical question. The image first: think of Alaskan streams and one picture more than any other is likely to spring to mind: a brown bear, stood in a waterfall, plucking a leaping salmon from mid-air. Of course, those samlon are returning upstream to spawn, having spent years feeding at sea (and thus assimilating nitrogen, as well as various marine-derived minerals). As for how the nitrogen then gets into the forests, well, is the Pope a Catholic? Or, hang on, do I mean the other one….?

This kind of transfer of nutrients from one ecosystem to another is known as an ecological subsidy, and is actually pretty common. Think about the billions of insects which emerge every year from their aquatic larval homes, and how many of them will end up, somehow or another, fertilising the banks or shores of their nursery river or lake with matter originally derived from the aquatic system. An individual mayfly might not provide much of an N input, but multiply by any realistic number of individuals and you start to see what an impact such subsidies might have.

I’ve been fascinated by this phenomenon for a while, partly I guess because I spend quite a bit of time comparing and contrasting marine and terrestrial ecological communities. But it came to mind this week because of a new paper I happened across in the Journal of Avian Biology.

In this paper, Therrien et al. have tracked snowy owls overwintering in the Canadian Arctic. Because these magnificent birds specialise in the summer months on hunting small mammals, and this is likely to be rather tricky in the winter, it had previously been thought that most owls winter further south where little furry things were still active.

But this new study shows owls spending several weeks over the sea ice, around 40km (and up to 210km) from shore. It seems that this small mammal specialist turns generalist in the winter, targeting small areas of open water and the seabirds that gather there.

The authors focus their discussion on the likely consequences for owls of reduced sea ice in the future, and I agree that the effects of a changing sea ice regime on the structure and functioning terrestrial ecosystems has been overlooked. But I also think this is another neat example of the slow, ecologically-catalysed cycling of elements between the land and the seas.