Conservation contemplates the pre-emptive strike
Among those who work on and care about the preservation of nature's plants and animals, the word "endangered" is an absolute touchstone.
Endangered species will in general have more conservation resources devoted to them. There may be political or community or scientific action aimed at saving them, the most extreme examples being captive breeding programmes or seed banks.
This approach hasn't been drawn up as some master plan - it's just evolved, a natural tendency to devote resources in situations where some kind of danger is clearly evident.
But is it sensible? Why wait until species become endangered? Why not intervene at an earlier stage, devoting resources to keeping ecosystems intact and keeping common species common rather than allowing them to become rare - to give a medical analogy, favouring prevention rather than cure?
It's a particularly pertinent time to ask the question given that we've just entered the UN-designated International Year of Biodiversity, when various groups of people are going to be thinking more than usual about nature preservation; and it's cropped up in this week's edition of the journal Science, in an article by Kevin Gaston from the UK's University of Sheffield.
(Science is a subscription publication so you may not be able to see the article from the link above - apologies, but not much I can do about it.)
One of his main arguments is that common species, by dint of being common, contribute most to the ecosystem that contains them - providing the majority of services such as photosynthesis and nutrient flow.
Given that human societies depend to a greater or lesser extent on these same services, it's not just an altruistic point.
The main driver of nature loss through human history (and the main thing pushing species currently on the Red List of Threatened Species towards extinction) has been removal or degradation of habitat.
National parks and protected areas are aimed, of course, at combating this, at preserving some areas where habitat is more or less unchanged.
About 13% of the world's land surface is now under some kind of protection. And while that's a figure and an achievement that many conservationists are proud of, it does mean that 87% of the land is without protection.
And here's the catch in Professor Gaston's argument. As he puts it: "To a first approximation, common species are habitat loss" - in other words, when you change the habitat, you change or remove the common species.
It's sometimes assumed that you can reduce pieces of forest or coral reef or whatever it is down and down in size, and whatever lives there will continue to live there, albeit in proportionately smaller numbers.
Ecologists know this isn't true; and Kevin Gaston gives two cases where modification of habitat during modern settlement of North America had catastrophic impacts on very abundant species, the rocky mountain locust (or grasshopper) and the passenger pigeon.
The Red List entry for the pigeon reads:
"Over the 19th Century, the species crashed from being one of the most abundant birds in the world to extinction... It was a nomadic species, breeding and foraging in vast flocks millions of birds strong... The precise cause of its extinction is difficult to determine, but the widespread clearance of the hardwood trees which provided its mast food, and the proliferation of the rail network and telegraph system which enabled efficient location of nesting colonies and the transport of young birds to market are probably the two single most important factors."
And here's an obituary for the grasshopper; a fascinating piece of social as well as ecological history.
Both abundant species; both sliding through rarity to extinction in less than a century - and neither, it appears, capable of enduring in lower numbers in small protected pockets.
Preserve the habitat, preserve the species.
And there are other reasons why focusing on common as well as rare species might be a good idea.
One fairly obvious point is that as Darwin noted, common species become less common before entering the threatened list; so even if preventing extinctions is your only aim, you can achieve that by intervening earlier.
Another is a point made by Jean-Christophe Vie from the International Union for the Conservation of Nature (IUCN) in an article in our Green Room series: what do we want to preserve species for?
"Do we want nature to be confined in zoos and botanic gardens or isolated pockets where rich tourists could go and watch what once covered most of our planet?"
Do we want species to exist in such small numbers that they cannot play a significant role in the ecosystem they inhabit?
An example: two years ago, researchers showed that heavy depletion of cod in the Baltic Sea had impacts down the food chain that resulted in increasing volumes of sometimes toxic algal blooms. The cod are still there, but in numbers too small to play their former role in the ecosystem.
To highlight the International Year of Biodiversity, IUCN has launched a "species of the day" initiative allowing you a "daily download" of something rather beautiful from around the natural world. They've also provided a rather pleasing gallery of nature pictures (I've scattered a couple of them around this post).
Not all of them are rare; not all of them are endangered. I guess their recommendation and Kevin Gaston's might be: let's keep it that way.
I'm Richard Black, environment correspondent for the BBC News website. This is my take on what's happening to our shared environment as the human population grows and our use of nature's resources increases.
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