Two recent trips to the coast, one for pleasure and one for work, have led me to ruminate on some of the peculiar issues that make marine ecology both especially intriguing and particularly tricky to study. One of the striking features of the marine habitat is the unignorable third dimension. Studying terrestrial organisms, especially at the kinds of biogeographic scales that interest me, we can confidently talk in terms of areas – the range size of any given species will typically be quoted as a geographical extent, in km2. Even the most aerial of birds – swifts, say, or the great albatrosses – are tied to terra firma for breeding purposes. In the sea, it is different. For some marine species, especially those closely associated with the sea bed, we can perhaps take the same approach. But for others, that multitude of pelagic organisms inhabiting the water column – well, many of them undertake extensive vertical migrations every day, without ever in their lives experiencing the hard certainty of rock, sand or silt. Their environment extends in all directions, and their ranges should surely be quantified as volumes.

Conceptually, that can be difficult to grasp, particularly if you have grown accustomed to seeing species ranges, and other patterns of biodiversity, represented on flat maps. But for me, visualising an ecological problem is very often a great help, if not an essential prerequiste, to solving it. So in Wales, gazing out to sea, I practised the following thought experiment: imagine if you could see through the sea, as if it were air.

View of Ramsey Island, Pembrokeshire. Imagine if, instead of blue, the sea were see through; imagine seeing the sandy plain and its inhabitants, rather than the mirrored sky.

Extending from the base of the cliffs, then, would be a vast sandy expanse, fringed in places with dense patches of large algae, but otherwise devoid of visible plantlife. But this plain would not be lifeless. Closer inspection would reveal plentiful signs of busy animal existences. It is riddled with the burrows of countless worms and molluscs. More obvious, through your binoculars you will see the larger epibenthic predators and scavengers – the crabs, urchins and sea stars. From time to time, a flatfish may reveal itself, lifting from the sea bed and rippling this way or that.

But drawing most attention will be the swimmers. Darting around the base of the rocks are wrasse and other reef fish; further out, pollock, or perhaps haddock or cod, would be hunting on the plain; and above them, the more flittish flocks of sandeels and mackerel, shimmering like starlings coming in to roost.

And you would see in their entirity the chases of air-breathing predators. The diving gannets, usually curtailed at the sea surface, could be followed right into the shoals of fish several metres below; you could see their misses and, when they succeed, watch them swallowing their grabbed fish on their ascent. Auks would swim deeper among the sandeels like fierce sheepdogs, puffins stuffing their comic bills with fish after fish.

You would be hoping, of course, for something larger: to see the ballet of grey seals at play, or possibly even porpoise or basking sharks, lording it over this shallow plain like the great beasts of the savannah.

People would feature too, of course. In shallow coastal Pembrokeshire, you’d see more evidence of leisure than of industry, although the occasional lobster pot would announce itself with its bright balloon hanging high above. But visit the more productive seas and you would witness carnage of a kind unthinkable on land. Imagine ploughing a field, with a vast net attached to the plough such that everything in your path is collected: the grubs and worms in the soil, the attendant gulls, the fleeing hares and pheasants. That’s beam trawling.

Or imagine casting an immense mist net to enclose those roosting starlings; or suspending kilometers of baited hooks across the Straits of Gibraltar during the autumn raptor migration; or harpooning buffalo; or dynamiting rabbit warrens; or poisoning tropical forest canopies and catching whatever falls out; or… Well, any means you can conceive of to catch and kill animals in commercial quantities has probably been attempted in the sea. If it is effective, it probably still goes on.

Of course, seeing a problem does not automatically mean it would be fixed, but I wonder what marine conservation would be like if every human impact on marine ecosystems were as visible as the Gulf oil spill. Certainly, we would be better placed to understand the state of the marine environment, and the effects of conservation measures, if we had a photographic record of historical habitats and the superabundance of large animals in the recent past. Perhaps it’s my job, as a marine ecologist, to try to assemble such pictures, using the scaps of evidence that we have been able to obtain by fumbling in the dark.