Though it is not a very picturesque simile, ecosystems are like wholesalers for goods and services. When we fill our baskets with the weekly shopping at our local store, all the food we buy has come from the ecosystems that supply that shop in wholesale quantities. Only the fact that most of us do not grow our own food, but instead buy it neatly packaged and branded, obscures this obvious reality. If you have to grow a crop for yourself, knowledge of what role different organisms play in the ecosystem is invaluable.
For example, introducing earthworms to the previously depauperate soils of a tea plantation in India increased yields by over 200 per cent, thereby adding thousands of dollars per hectare to the profit obtained from the crop. Inoculating soils with the best-performing types of mycorrhizal fungi can dramatically improve the growth of many crops. The bacterial nitrogen-fixers attached to the roots of clover and related plants provide a source of nitrogen in pastures. In rice paddies, a floating aquatic fern called Azolla harbours cyanobacteria that can contribute 76kg nitrogen per hectare per year (68lb per acre per year) to the ecosystem. Such natural sources of usable nitrogen obviate the need to add nitrogen fertiliser, which is expensive to buy and energy-costly to make and to transport. All of these useful organisms, plus the insects that pollinate crops and many more, can be found in the ecosystem supermarket next to the food section, in a well-stocked aisle labelled Farmers' Friends.
At the far end of the Farmers' Friends aisle, hidden discreetly from view to spare the blushes of customers who have abused their ecosystems and are suffering the ugly consequences, is the Ecosystem Reclamation section. For example, a remedy for ecosystem degradation in the Sahel can be found in this section. In this semi-desert region of Africa live some of the poorest people on Earth, most of them engaged in subsistence agriculture. Continuous cultivation and grazing and trampling by livestock, combined with drought, have caused soils in the Sahel to become covered in a crust that rain cannot penetrate and that is bare of vegetation. A simple remedy is to cover the surface in a mulch of straw and twigs, which is eaten and incorporated into the soil by termites that are already present in the area. Termite activity breaks the soil crust and improves soil structure, allowing rain to percolate downwards and enabling plants to grow. After 3 years of such a treatment an experiment in Burkina Faso found that treated plots produced over 3 tonnes per hectare (1.2 ton per acre) of plant dry matter, while untreated plots produced nothing at all. Plots with mulch but no termites produced only one-third the plant yield that mulch plus termites produced.
Resistance is useful
‘I am Locutus of Borg. Resistance is futile. Your life as it has been is over. From this time forward, you will service us.'
With these words, uttered in an episode of Star Trek, the captain of the Starship Enterprise learns that he is going to join the ranks of the exploited, providing services for another species. Our relationship to nature is not unlike that which the Borg intended for the captain and crew of the Enterprise. Given our dependence upon ecosystems and the species in them, how can we be sure that nature can carry this burden? How resistant to human impact are ecosystems? Contrary to the words of the Borg, when it comes to ecosystems, resistance is useful, not futile. A resistant ecosystem is one that does not change under the impact of exploitation. If it does change, we can ask how resilient is it? Resilience is the ability to bounce back after an impact. Resilience is useful too.
Resistance and resilience are properties of ecosystems, but their efficiency depends upon how living organisms respond to being exploited. Very moderate felling of trees in a forest should permit most of its ecosystem functions and services, such as regulation of water supply, to continue, and hence the ecosystem could be said to be resistant to this level of exploitation. Clear-felling of patches of forest has a larger impact, but in resilient ecosystems the patches are eventually recolonised by trees, and recover normal ecological functions.
In the 19th century, chestnut was an abundant and economically very important tree in the Appalachian forests of eastern USA, providing edible nuts, rot-resistant timber and bark for tanning leather. Chestnut blight, caused by an introduced fungus, wiped out the species in the first half of the 20th century. The valuable products once obtained from chestnut were irretrievably lost, but other tree species, such as oaks and maples, took the chestnut's place in the forest. Because the forest ecosystem as a whole recovered, it could be regarded as resilient in this respect, but the farmers living on the edge of subsistence, for whom chestnut trees provided a valuable source of income, might not agree.
How many species do we need?
Research on several types of food web suggests that having many species makes them more resistant and/or resilient. For example, the majority of marine fisheries are under severe pressure from overfishing and many fish stocks have collapsed to below 10 per cent of their peak. Such collapses have been more frequent in areas where there were few fish species than in places where there were many. The causes are difficult to pin down, but where many species are available, fishers can more easily switch away from a species that is becoming rare to a more common one, allowing overfished stocks to recover.
In grasslands, the presence of more plant species can increase the total productivity of the ecosystem and decrease its variation from year to year. Different species of plants have different periods of maximum growth during the year, have different good and bad years, or exploit different sources or kinds of nutrients. As a consequence, a species-rich community of plants can be better at making use of all available resources under all weather conditions than a species-poor one.
Higher plant productivity produced by increasing species diversity does not mean that adding species is the best way to achieve high yields (if that is the aim), because adding fertiliser or improving irrigation generally has a much greater effect. While adding resources (nutrients or water) increases total plant production, it may also reduce the number of plant species present, because the strongest competitors for the added resources out-compete the weaker ones. For this reason the most productive natural ecosystems, such as estuarine marshes, where nutrients and water are plentiful, are dominated by just one or two plant species. The types of natural vegetation with the highest biodiversity, such as calcareous grassland or ‘fynbos' in the Cape [South Africa], all occur on the most nutrient-poor soils. These associations show that it is impossible to achieve maximum plant productivity and high plant biodiversity simultaneously from the same ecosystem. From a practical standpoint, the relevant question becomes not so much ‘How many species do I need?' as ‘How many species do I want?'. A choice, or at least a compromise, must be made between productivity and biodiversity.
Jonathan Silvertown is professor of ecology at the Open University and editor and co-author of Fragile Web: What next for nature? (Open University/Natural History Museum, 2010, £14.99), from which this article is an extract.
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