Many of us keep a list of the bird species that show up in our back gardens, or the types of fungi in our local wood. The species is perhaps our most simple, fundamental, and useful tool for describing the living world. But it's a more slippery concept than it might first appear — and the harder one looks, the more slippery it becomes.
Take, for example, the hoverfly Microdon mutabilis. The flies' larva infiltrates an ant's nest — probably by chemically mimicking its host — and spends two years eating ant eggs and larva before pupating. In 2002, entomologist Karsten Schönrogge of the Centre for Ecology and Hydrology in Dorset (soon to be closed by the UK government) and his colleagues showed that M. mutabilis was in fact two species, each specialized to parasitize a different species of ant.
Now, Schönrogge's team have found that the fly's diversity is still finer-grained. Working on the Scottish island of Mull, they moved eggs of M. mutabilis from the ant nests in which they were laid to different nests of the same species.
Almost no eggs survived a move of more than three kilometres. Ants from a different neighbourhood were able to recognize and destroy hoverfly eggs before the larvae could gain entry.
So the species that we have called Microdon mutabilis seems, in fact, to be many populations closely adapted to their local ant hosts, and unable to survive elsewhere. As the fly is found from Ireland to Japan, the one name may conceal a mind-boggling number of cryptic fly species.
This kind of extreme specialisation, and the diversity that comes with it, is especially likely to evolve in parasites, which must trick their hosts into accepting their malign embrace. Faced with overcoming the host's powers of detection, natural selection will hone the parasite's powers of deception, until it may become so expert at exploiting one host that it closes off all its other options, and paints itself into an evolutionary corner.
Microdon, however, might have a trick up its sleeve. It prefers to eat small larvae or eggs. This causes the ant's brood to become biased towards large larvae. These are more likely to be potential queens and males — the sexual forms that found new colonies. And so the hoverfly might be able to promote the reproduction of the colonies that it infests, and ensure a new supply of susceptible hosts.
In general, the smaller you get, the vaguer our notions of biodiversity become. We have a pretty good idea of how many species of bird and mammal there are, and what a species of bird is. But we have almost no idea how many different types of insect, fungi or microbe there are, or how we should classify them. DNA sequencing is revealing yet another world of diversity, that biologists are unsure how to reconcile with their more traditional categories.
Our understanding of what diversity is, and how much of it there is, also has practical implications. Microdon is listed in the Red Data Book as being of conservation concern. But what should we be trying to conserve? The species as a whole? Individual populations? And how should we conserve it? It certainly doesn't look as if one could move hoverflies from one place to another.
The hoverfly is a small example of how important it is to keep the fabric of life intact. It's not even that we won't know what we've lost until it's gone. Even then, we still won't know.