The problem of biodiversity

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The diversity of nature, exemplified by Darwin's entangled bank, is so familiar from everyday experience that it is easy to accept it without much critical thought. We are so used to a multitude of species, and higher taxa, that it is difficult to appreciate the fundamental theoretical problems this diversity presents.

Consider these data from a 15 X 15 cm quadrat I recorded at an altitude of 1,890 m in the Dolomites (northern Italy). Within this very small area of ground in a coniferous forest I found mosses, sedges, grasses, and broad-leaved plants from three different families (Asteraceae, Ranunculaceae, and Ericaceae) along with a small pine seedling. With the exception of the seedling this quadrat was quite typical of the forest floor at this location. However, green plants can be considered as just ways for life to access solar energy, so why does it take so many different types of 'solar panels' to do this job on 225 cm2 of ground in a forest on an Italian mountainside? An even smaller scale example comes from the bark and wood chip mulch used on some of the soils on the science campus of my University in Liverpool. Using a microscope I counted 100 individual testate amoebae (Fig. 4.1) from a small sample of this mulch and found they belonged to at least 14 species from 10 different genera (since I made no attempt to split several difficult taxa which appear nearly identical under conventional light microscopy, the real species richness was probably slightly greater). Why is more than one species of protozoan needed to do the job of eating bacteria and other small organic particles in this apparently simple habitat?

These two examples illustrate biodiversity at the small scale; plants on 225 cm2 of ground or protozoa in less than a gram of bark chippings. However, the problem is also apparent at the planetary scale. Robert May (1988), in a famous paper in Science, drew attention to the fact that we have very little idea as to how many species currently inhabit the Earth. One result that most authors agree on is that insects appear to be especially rich in species, for example in the

Fig. 4.1: Shell of the testate amoebae Nebela collaris sensu lato one of the species found In the wood chip mulch on my University campus In Liverpool, UK. This species Is typically between 100 and 180 ^m long and feeds mainly on immobile microorganisms (such as micro algae and fungal mycelia) along with senescent and dead microbes (Gilbert etal., 2003). The encysted organism can be seen within the shell, when active its pseudopodia emerge from the 'mouth' of the shell to collect food particles.

Fig. 4.1: Shell of the testate amoebae Nebela collaris sensu lato one of the species found In the wood chip mulch on my University campus In Liverpool, UK. This species Is typically between 100 and 180 ^m long and feeds mainly on immobile microorganisms (such as micro algae and fungal mycelia) along with senescent and dead microbes (Gilbert etal., 2003). The encysted organism can be seen within the shell, when active its pseudopodia emerge from the 'mouth' of the shell to collect food particles.

early 1980s Erwin (1982) estimated that there were at least 30 million insect species, a conclusion he reached by extrapolation from the ratio of described to undescribed species in a study of beetle species richness in trees in Panama. Using a similar extrapolation approach, but based on data on hemipteran bugs in Indonesia, Hodkinson and Casson (1991) estimated a lower figure of approximately 2 million, although some other estimates have reached 80 million (reviewed by Stork and Gaston, 1990). The actual numbers produced by these estimates are best treated with a high degree of scepticism, however, they do make two important points.

1. The range of estimated numbers of insect species (from 2 to 80 million) strikingly illustrates our current lack of quantitative knowledge of biodiversity. Since the 1980s there has been a general consensus that the highest values are probably overestimates (Purvis and Hector, 2000; Novotny and Basset, 2005), although it is still impossible to answer the question 'how many insect species?' to the nearest 10 million with any great confidence!

2. Even the lowest figures, such as Hodkinson's and Casson's (1991) estimate of 1.84-2.57 million species are still surprisingly large if one considers the obvious functional questions, 'what do all these species do?, and why are there so many?'

One of the reasons for concentrating on insects in the above analyses is that they form a significant proportion of all described species (Wilson, 1992) so if we had a good estimate for insects we could make an educated guess at the total species richness of all organisms on Earth. However, this assumes that the relatively low numbers of described species of microorganisms are a real measure of their diversity and not an artefact of them being small and difficult to study; a crucial point appreciated by May (1988) in his Science paper. Currently, the extent of geographical isolation, and hence presumably species richness, in microorganisms is very controversial, this is not helped by the difficulty in defining 'a species' for many microbial groups. Some high-profile studies have been claiming that almost all free living protozoa show a complete lack of geographical isolation (see Finlay 2002 or Finlay et al., 2004 for reviews), however, other authors suggest that although protozoa are more likely to be cosmopolitan than larger organisms there is still evidence that many species have limited ranges (e.g. Foissner, 1999; Hillebrand et al., 2001; Wilkinson, 2001a). By analogy what applies to free living protozoa may also apply to other groups of microbes such as bacteria (Finlay and Clarke, 1999), so the outcome of this debate could greatly affect the estimated number of species on Earth; although there is always the possibility that different microbial groups may differ in their behaviour. If free living microbes are very species-rich then calculations that assume insects make up approximately half of all species will be spectacularly wrong (this assumption would also be wrong if it turns out that the average insect has several species-specific parasites, although in this case insect species richness could still be used as a guide to total species richness). This debate also has implications for conservation biology, if some microbes have limited ranges then our actions could cause their extinction much more easily than if their ranges are global with the species occurring anywhere in the world where the correct habitat is found (Finlay et al., 1997).

The previous chapter discussed reasons why any planetary ecology should consist of two or more principal guilds, such as photosynthetic autotrophs or decomposers, and argued that multiple guilds are so fundamental that they would be found on any planet with life. However, those arguments give us no reason to expect that these guilds should be subdivided into large numbers of different types (species), which is what we observe on Earth. In this chapter, I consider one of the fundamental processes which can cause such biodiversity—the idea of tradeoffs. Without tradeoffs the role of species-rich guilds could be filled by

Tradeoffs illustrated by human sporting performance 43

single taxa, such as a 'perfect' photosynthetic plant. Such 'Darwinian Demons' would dominate a planet leaving no room for speciation. I argue that these tradeoffs would tend to cause within-guild biodiversity on any planet with life. I also consider the Gaian effect of this resulting biodiversity, a topic of more than academic interest as we are currently in the process of greatly reducing biodiversity on Earth through our actions.

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