In Chapter 1, I proposed a definition of extreme organisms by reference to the life boxes of organisms: the range of physical conditions under which they exist (the non-biological features of their ecological niche). I also proposed that we could recognise two groups of extreme organisms. Extremophiles have life boxes which are beyond the limits of those of the majority of organisms. Cryptobiotes have life boxes which are the same as, or substantially overlap, those of the majority of organisms, but, when the conditions of their environment deteriorate beyond those under which they can maintain their life processes (metabolism), rather than dying (as do most organisms), they cease metabolism and await the return of favourable conditions. We could thus recognise extreme environments as being those which have rather few species of organism (and those which are present are extrem-
ophiles) and/or those in which there are, at least periodically, species in a state of cryptobiosis.
I have mentioned a number of extreme environments in this book (places that are hot, cold, dry, salty, acidic etc.). I chose them, to be honest, not because they fitted the life box definition but because they seemed extreme to a human like me. In spite of this, most do in fact seem to fit the definition. There are fewer species that live in extreme environments (such as deserts and polar regions) than live in environments which are not extreme (such as tropical rainforests and coral reefs). There is, however, at least one extreme environment for which this is not true. We think of the deep sea as being an extreme environment because of the high pressures faced by the organisms that live there. Now that the problems of sampling organisms from this environment have been overcome, we have realised that, rather than being a biological desert, as had been assumed, it is populated by a very diverse range of species. The diversity of deep-sea organisms may be even greater than that found on land (see Chapter 2, The cold deep sea). The deep sea thus does not fit the definition of extreme that I proposed in Chapter 1. This definition focusses on the range of conditions that the majority of species could survive. Extreme environments would thus be expected to have a low diversity (rather few species would tolerate the physical conditions found there), whereas non-extreme environments would have a high (or, at least, a higher) diversity. According to this criterion, either the deep sea should not be considered to be an extreme environment or there is something wrong with the definition. I will return to this problem later.
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