Generations of ecologists have puzzled over the causes of biodiversity and its relationship with productivity. Dr. Mae-Wan Ho investigates.
This was the question asked by distinguished ecologist Evelyn Hutchinson  in 1959, the centenary of Darwin's Origin of Species, a question that has remained as enigmatic today as it was then .
There are about a million described species of animals, three-quarters of them being insects, of which disproportionately large numbers belong to the order Coleoptera, or beetles. In contrast to land animals, there are far fewer species in the sea.
Hutchinson considered a number of possible explanations. Could food chains or feeding relationships suffice? If one supposes an energy conversion efficiency of 20% at every link of the chain, and each predator being twice as big as its prey, the fifth animal link will have a population of one ten thousandth (10-4) of the first, which is about as long as it would get. Food chains could hardly generate a great deal of biodiversity.
Natural selection isn't going to help; an overly efficient predator will simply eat itself out of prey, thus breaking the link and making itself extinct in the process. While lengthening the chain is difficult, shortening the chain is not, the most dramatic example is the whale-bone whale, which can feed largely on plankton.
What about the diversity of terrestrial plants which provide a variety of different structures - bark, leaves, flowers and fruits - for different animals to feed on. A major source of biodiversity of land animals was indeed introduced by the evolution of almost 200 000 species of flowering plants, and the three-quarters of a million species of insects are a product of that diversity. But then, why are there so many different kinds of plants?
Part of the answer is that instead of linear food-chains, nature is replete with food-webs. Most predators eat more than one species of prey, which reduces the danger that it will eat its prey and itself extinct. So, at least part of the answer to why there are so many kinds of animals and plants is that biodiverse communities are better able to persist than less diverse communities. And that was the origin of the idea that complex ecosystems are more stable, which has been hotly debated to this day. While it may be intuitively obvious that the more flexible the links in the foodweb, the less likely they will break; mathematicians find it extraordinarily difficult to represent such flexibility, and more so, to agree what constitutes stability, let alone complexity .
Going back to biodiversity, ecologists have long noticed that while a hectare of tropical rainforest contains an estimated 200 to 300 species of trees, the same area of temperate forest contains only 20-30 species. One hypothesis is that diversity is ultimately determined by the amount of energy available to an ecosystem. Support for this idea came from measures of productivity and biodiversity in different ecological communities. Productivity is the rate of production of biomass by an ecosystem, and is in general determined by the rate of energy supply.
High proportions of land and freshwater species on earth do occur in the tropics, which receive the highest amount of the sun's radiant energy. Average species richness increases from high to low latitudes and this has been documented for a wide spectrum of taxonomic groups, including protists (single-celled organisms), trees, ants, woodpeckers and primates, and for data across a range of spatial resolutions . Species richness also appears to increase with energy, measured as mean annual temperature, and evapotranspiration.
But that doesn't seem to be the whole story. Relationship between diversity and productivity was found to vary at different spatial scales . At large geographical scale, such as across continents in the same latitude, diversity generally increases with productivity. At smaller local scales (metres to kilometers), several different patterns emerge.
Early studies found biodiversity peaking at intermediate levels of productivity in a unimodal curve (a curve with a single hump). More recent reviews came up with a variety of relationships, with diversity increasing, decreasing or remaining unchanged as productivity increases. Although some of these patterns suggest that energy is causally involved, other factors may also be important, such as environmental heterogeneity: spatial or temporal variation in the physical, chemical or biological features of the environment.
In a simple lab experiment , the bacterium Pseudomonas fluorescens was used to test the relationship between environmental heterogeneity and diversity. This bacterium is known to rapidly differentiate into distinct 'morphs' in different microhabitats in unmixed culture vessels. One major morph flourishes at the interface between air and the liquid growth medium, another does best in the center of the culture vessel and a third occupies the bottom of the vessel. The researchers found that there are further variations within each major morph, so that a total of ten types can be distinguished. Shaking the vessel eliminated environmental heterogeneity and, with it, diversity. With a gradient of productivity, a unimodal diversity curve was obtained. In other words, diversity increased with energy available up to a point, and then decreased as available energy increased further.
Ecosystems typically consist of plants and animal species of vastly different sizes, from big mammals to birds, insects and microbes in the soil, which would use resource that matches their size. Thus, the more finely the species can divide up space and resources, the more species can coexist in the same habitat. But how best to represent this environmental heterogeneity?
Mark Ritchie from the University of Utah, Logan, in the United States, and Han Olff in Wageningen Agricultural University, in the Netherlands, reasoned that the distributions of habitat, food and resources often appear to be statistically self-similar over three to four orders of magnitude. If so, their volume or area can be described with fractal geometry .
A fractal is a structure that has dimensions in between the usual 1, 2 or 3; and 'self-similar' refers to the property that the structure appears the same over many scales. Typical examples are fern leaves, branching blood vessels and the coastline.
In a fractal environment, body size determines the abundance of food and resources that a species perceives, and it sets limits to the similarity in body size between any two species. Ritchie and Olff derived a body size ratio between species of adjacent sizes that declines with increasing organism size. That in turn predicts how diverse the community can be.
Thus, energy, productivity and environmental heterogeneity all appear to play a role in creating biodiversity.
In the next article ("Why are organisms so complex?" this series), I shall show how biodiversity and productivity are intimately linked through energy capture and storage in a sustainable system.
Article first published 23/01/04
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