The difficulty in deciphering what bioindicators are telling us is exemplified in the decline of the house sparrow in Britain. The national population has declined by a staggering two-thirds in little more than 25 years. Counts in London's Hyde Park show a population of 2,604 individuals there in 1925, dropping decade by decade until they became extinct in the park in 2002.
Clearly, something is amiss. But what? Here we have a bioindicator whose gauge is on "red alert", and the alarm bells are ringing. But why? What does this indicate about the world that the sparrow lives in, a world which largely corresponds to our own? No one yet knows.
In this case, it is tempting to leap to the conclusion that it must be the increased number of predators such as sparrowhawks, magpies, or even cats.
And yet other common garden birds such as great tits and blue tits, which also feature in the predators' diets, have actually shown big population increases.
So are the fumes from unleaded petrol killing the insects needed by young sparrows? Or do sparrows no longer have a place to nest any more with the closing up of nesting holes under the eaves? Could it be the removal of privet hedges, or the decline in allotment use, or disease? Or are mobile phone masts to blame? All these factors have been put forward as serious theories, and a combination of some or all of them may be to blame. Only lengthy and detailed research, currently under way, might be able to unravel the mystery.