Seeing Further (32 page)

Recent historical accidents may also have a large influence upon ecological trends. In the Pyrenees altitudinal changes in floral richness are much confused by the fact that farmers have modified lowland habitats more than they have those far above them. One surprise has been to find that what seem to be pristine habitats have long been modified by man: itself a complication when trying to establish natural patterns of variability. Even Darwin’s Atlantic forest of Brazil, together with the vast biological storehouse of the Amazon jungle, are partly human constructs, for their structure has been much disturbed by the large indigenous population that lived there in pre-Columbian times and turned parts of it into parkland (Heckenberger
et al.
2008).

A century and a half of research has improved our knowledge of ecosystems, but many regions of the globe and – more important – many habitats remain relatively unexplored. Sometimes, detailed sampling reveals astonishing patterns of diversity: a single bay on the island of Flores, in the East Indies, has more species of fish than does the entire tropical Atlantic (Briggs 2005). As a result, the geography of diversity has begun to look more complicated than it did. The Conservation International organisation names 34 patches of land as ‘hotspots’ that contain almost half the world’s known plant species and a third of its vertebrates. Together, they represent less than 2 per cent of the terrestrial world. The hottest spots of all are indeed in the tropics – Sundaland, Madagascar, Brazil’s Atlantic forest and the Caribbean. Together, in one two-hundredth of the total land surface,
they boast a fifth of known plants and a sixth of vertebrates (Sodhi 2008). For mammals, in contrast, the high points of variation include the Andes and the Hengduan Mountains of south-western China (Schipper 2008).

Mediterranean ecosystems such as those of South Africa, of Western Australia, or of the Mediterranean itself, also contain large numbers of creatures although they are well away from the equator. Hotspots are important in conservation, but (Grenyer 2006) there is often little congruence in the distribution of threatened species, particularly when the rarest creatures with the smallest ranges are considered – indeed, if anything they tend to be found in different places. A study of large-scale spatial change in amphibia, birds and mammals across the Western hemisphere suggests that there is some congruence of pattern for birds and amphibians (but much less for all three groups considered together) when areas of high local differentiation are considered. However, the opposite is not true – the three have large regions of relative homogeneity in quite different places (McKnight 2007). In the deep sea, too, there are few consistent associations of biological diversity with depth, latitude, sediment type, or water quality.

Many rules of diversity have been proposed. Food, predators, climate, efficiency of energy transfer and complexity of the habitat have all been appealed to as agents underlying community structure. Some cases are convincing, but a closer look reveals a disappointing lack of consistency from one ecosystem to another. Some are under top-down control through the action of predators, while others respond to forces that well upwards from the primary producers and yet more depend on an interaction between the two. It might seem obvious that a complicated place like a rainforest is more productive and more diverse than a peat bog, and a survey of dozens of habitats suggests that the most connected communities
may be more efficient. Even that evidence is not always persuasive, for experiments in which particular species have been removed one by one from grassland or pond to see how well the remainder survive give results that are ambiguous at best. A search for order behind local or global patterns of ecological change has not always been a success.

The most productive parts of the world are, it is often said, the most blessed with unique forms of life, perhaps because they have more energy input from sunshine. Whales and dolphins also tend to be most diverse and most abundant in middle latitudes such as the southern Indian Ocean; and although these are not close to the equator they are regions of high productivity (Schipper
et al.
2008). Metabolic rate may be the main driver, with small or relatively warm-blooded creatures living more speedy lives and generating more species than do their opposites (although the shared geographic patterns of change in warm- and cold-blooded creatures does not fit this notion).

An alternative view emphasises the importance of predators in maintaining community structure and a whole science of food webs attempts to analyse the patterns of eating and being eaten among species in a search for regularity. The re-introduction of wolves to Yellowstone National Park led to more corpses being scattered across the landscape and to increased opportunities for a variety of scavengers, while browsers increase the plant diversity of the pastures upon which they feed. Conservation biologists often believe that large predators help maintain the structure of a community and much effort is devoted to ensuring the survival of such creatures (Sergio
et al.
2008). Once again a wider look at the dozens of claims made for the importance of predators reveals a depressing lack of consistency. Although the trophic effects of a large predator may be important in some places they do not seem to be important general agents as many creatures seem to live lives rather detached from those of most other species around them. A meta-analysis of twenty food webs (Vermaat
et al.
2009) hints that they might fall into two classes, highly interconnected or more linear, with
fewer links; but the tie between predation, energy flow and community structure is not clear, and may involve further attributes of each species such as how easy they are to eat.

In the past few years, a new notion has emerged: that community structure can best be explained with a radical and at first sight absurd assumption that, in effect, all the species involved are equivalent and that their abundance turns on random fluctuations in survival and in reproduction (reviewed in Leigh 2007). This ‘neutral model’ of ecology has parallels with its equivalent in genetics, in which levels of inherited variation emerge from a balance between random mutation and the accidents of genetic drift. That model has been tested against the real world, and although it sometimes fails, at the level of DNA sequence it retains considerable explanatory power (Clark 2009). In ecology, too, a random model of communities may carry more general conviction than does a series of special cases that explain some patterns in some places but have little predictive power overall.

Darwin accepted random change when he noted that islands contain fewer species than do nearby tracts of mainland and the claim that island life is driven by the accidents of migration and extinction has held up well. The same is true in other populations, on a variety of timescales. Thus, when cataclysms strike, as in the five great extinctions of the past five hundred million years (most associated with comets or great geological upheavals), huge numbers of species of many kinds disappear through mere ill luck, and rules that might help predict their ability to withstand everyday pressures do not much apply (Jablonski 2004). Other geological events quite unrelated to the biological universe such as continental drift also have persistent effects on the diversity of communities. In the same way, the last ice age stripped Northern Europe of most life and the glaciated
regions are still depauperate as the result of an ancient historical accident rather than as a response to modern conditions.

The peak of coastal marine species variety is in Indonesia and on the northern coasts of Australia (Renema
et al.
2008). There, coral reefs flourish. Such places are often appealed to as an epitome of undisturbed and productive nature, in which new kinds of creature can evolve to add to the treasury of life. A closer look at the fossils and the genes shows that in fact the occupants of the reefs have moved across the globe as conditions changed. During the Eocene, marine diversity found its peak in south-west Europe and North Africa, along the Arabian Peninsula and in what is now Pakistan. As these lands were raised from the sea when Arabia crashed into Asia, many of their inhabitants migrated to more congenial places, the present Indo-Australian region included. Most of the animals supposed to have originated there have in fact an ancient and dispersed history. Global disasters of fifty million years ago have done more to shape the geography of today’s teeming reefs than have climate, food or sunlight. Evolution, that reminds us, works on a far longer timescale than does ecology.

As in genetics, there are many non-linear interactions in ecology (Andersen
et al.
2009) and, as in the weather and the stock market, a small disturbance can lead to a sudden and unpredictable change in state. An attempt to shoot foxes to increase the numbers of red grouse prey backfired, for the predators normally caught only the birds most filled with parasites and once they were removed disease spread and killed many more birds than before. In a related case, an attack by one insect herbivore on a leaf often alters its attractiveness to other grazers, while plants that activate a pathway that fights fungal disease may reduce their own ability to combat insect attack with a different biochemical strategy. All these and many more multiple interactions (Strauss & Irwin 2004) emphasise that – as in genetics – many of the connections among species within a community are far from simple.

The importance of randomness first came to attention with the ‘paradox of the plankton’, the discovery that the apparently homogeneous environment of the sea was host to a vast diversity of drifting creatures all apparently in competition for the same resources, in contradiction to the supposedly fundamental principle of exclusion of species with similar demands (Scheffer
et al.
2003). The plankton have become even more paradoxical with the discovery of vast numbers of new marine bacteria. The same is true of the world beneath the soil, whose organisms differ wildly from place to place, but generate roughly the same mix of nutrients. Perhaps each of those habitats really is filled with a chance assemblage of ecologically equivalent creatures, each arriving more or less by accident.

That radical notion may have a wider validity, for it seems to apply to some very different terrestrial and freshwater habitats. Fish species diversity across eight hundred tributaries in the entire Missouri–Mississippi river system can be explained by the random loss of species of varying dispersal in a pattern that diffuses from a centre of abundance into streams of smaller and smaller size (Muneepeerakul 2008) with no need for any consideration of the nutrient status of streams, of other species, or of climate. The same is true of patterns of diversity in mature forests.

Temporal shifts, too, hint at an underlying lack of order. In a somewhat heroic experiment (Beninca
et al.
2008) a series of laboratory containers containing samples of plankton from the Black Sea was cultivated for seven years, in – as far as they could be attained – constant conditions. The abundance of the various species varied dramatically with time, and the relative numbers of each type could not be predicted with any confidence over any period longer than a month (which is, incidentally, the longest period for which the British weather forecast is even slightly dependable). The system was driven by something close to chaos – but, even so, most species persisted at high or low frequency within the containers.

Natural ecosystems can also remain stable until a threshold is reached and then collapse. The effect is familiar to fisheries managers, for a trophic cascade may be set off by overfishing, with unpredictable results. In the Black Sea itself, there was, from the 1970s on, a shift from large (and valuable) fish to anchovies that feed on plankton, and then to gelatinous creatures such as jellyfish and ctenophores, which now teem in huge numbers and have, within a few decades, replaced what seemed a stable ecosystem. A similar shift in the Pacific from sardines to anchovies and back twice in the second half of the twentieth century may also have turned on small changes in climate in a regime poised on the edge of stability that moves unpredictably from one to another. There have been dozens of climate shifts from cool to warm and back again every few hundred or thousand years, in the past hundred thousand years, each of which was no doubt accompanied by sudden upheavals in what might have seemed like stable ecosystems. Even on a much shorter timescale, the numbers of birds and mammals in a particular place when studied for long enough swing wildly for no obvious reason (as in the collapse of the British house-sparrow). Unexpected outbreaks can also destroy whole ecosystems (as in Dutch Elm disease, which appeared from almost nowhere and killed millions of trees). Such fluctuation might maintain a complex community with no external driver, in which case the paradox of the plankton (and, by extension, of land-based ecosystems too) could be explained in terms of random change.

A recent review claims that ‘ecological surprises’ of this kind have proved to be almost universal (Doak
et al.
2008). Not only do they reveal our ignorance of the laws behind biodiversity, but they hint that chaos and complexity may be the rule rather than the exception. Darwin himself was well aware of the difficulties of disentangling the patterns of nature. The term ‘complexity’ appears in
The Origin
almost fifty times, and ‘innumerable’ and ‘endless’ almost as often (although ‘inextricable web of infinities’ makes it just once). The tension between order and disorder remains unresolved and more than a century and a half since that remarkable work we may understand rather less (although we know considerably more) about the patterns of nature than we imagined just a decade ago.