Читать книгу Mutual Aid - Pablo Servigne - Страница 19

In this last example, the relation between animals (predators) and plants is hardly peaceful. But this is far from a general rule. Plants and animals collaborate very well. Agriculture, which is an example of the domestication of plants by animals, is the typical case of ‘win-win’ interactions between kingdoms.¹²

People are not the only ones to practise agriculture. At the origin of the apple, for example, there is the bear. On the borders of China and Kazakhstan, bears have carried out a long and valuable process of selection based on a kind of wild apples, small and sour, prized by some birds, gradually giving rise to a sweeter, much larger apple. By some miracle, this part of the world has not been completely deforested, and can today be celebrated as the true cradle of one of the commonest fruits on earth.¹³ The animal beneficiary of the pact has simply changed en route …

In addition to having been the first breeders (of aphids), social insects were also the pioneers of agriculture. This has been extensively described for ants, but less is known about termites, of which 330 species out of the more than 2,600 described¹⁴ make up a compost from plant debris and the droppings of workers to cultivate a fungus which they feed on. The ecological advantage of this feeding method is considerable: we see these species thrive in drier habitats and consume fifteen times more dry matter per hectare than species that do not grow fungi.¹⁵ Another group of insects also cultivates fungi, this time on the walls of the galleries they dig in wood. After 60 million years of this practice,¹⁶ more than half of the 7,500 species of bark beetles, small beetles greatly feared by foresters, have opted for this symbiosis.

Plants, not at first glance the most mobile of beings, have found animals to be a great way to get around – or, more precisely, to transport their gametes (pollen) and their embryos (seeds). They generally attract animals by offering an attractive food reward: fleshy, sweet or nutritious. Birds, bats, lizards, mammals and of course insects participate in this exchange of services, one that takes place across a wide range of behaviours and involves generalist species (honey bees that forage on many species of flowers) as well as superspecialists (a flower that can only be pollinated by one species of butterfly).

This last case is beautifully illustrated by the endemic Madagascan orchid, Angreacum sesquipedale, which differs from neighbouring species by its spur, up to 30 centimetres long, at the end of which lies the coveted nectar. Looking at this plant, Darwin, a connoisseur of the relationship between insects and orchids, had wagered as early as 1877 that there surely existed a sphinx (butterfly) with a proboscis of equivalent length.¹⁷ Xanthopan morgani praedicta was duly described in 1903.¹⁸ This type of mutualist, exclusive, specialized and audacious relationship is risky, however, because each species then depends entirely on the survival of the other.

A little less volatile, but just as efficient, the African elephant is responsible for the spread of thirty-seven tree species from Côte d’Ivoire, including thirty for which it is the only known disperser.¹⁹ On the other side of the Atlantic, in Central America, another elephant, now extinct, surrounded the seeds of the ancestors of courgettes and pumpkins with nutritious dung that conferred an evolutionary advantage. The bitterness of the flesh of these vegetables initially repelled the appetites of smaller animals, before humans replaced the exterminated mastodons by selecting varieties with a more palatable taste – those that we know today.

There are also small examples of original relationships. In Asia, for example, a carnivorous plant from Borneo, the Nepenthes rajah, grows on trees, far from the ground, and so far away from nitrogen sources. But, rather than trap insects in order to obtain protein, like its cousins the carnivorous plants, it has developed a fragrant nectar that treeshrews drink. These small arboreal mammals then use the plant as a latrine, thus providing it in return with the nitrogen it needs.²⁰

Finally, there are the great fusional relationships. One of the most successful symbioses is undoubtedly the one which led to lichens – an extraordinary chimaera, halfway between fungi and algae. Algae provide the fungus with the sugars they produce by photosynthesis, in exchange for which the fungus give them water, mineral salts and, above all, a shelter, thus allowing them to live in what, for algae, are amazing places.²¹ Thanks to this association, lichens are found on surfaces as diverse as bark, leaves, walls or on the ground, on all continents (they are the dominant vegetation on 6% of the planet’s landmass)²² and in all climates, including the most inhospitable, such as deserts, mountain rocks and the dry valleys of Antarctica.²³ This symbiosis takes its time: the annual growth of some species is limited to half a millimetre per year, with lifespans of up to 5,000 years.²⁴ Here is yet another confirmation of the power of mutual aid, since 20% of the 75,000 species of fungi described to date are engaged in this type of symbiosis.²⁵

However, the prize goes to an old association dating back 220 million years,²⁶ namely corals, which constitute a particularly rich example of a complex interweaving of beneficial associations. They are a union between colonial polyps (animals close to jellyfish) and zooxanthellae (unicellular algae). The latter provide their animal partners with sugars, in exchange for shelter, nutrients (droppings) and carbon dioxide.²⁷ This circular economy allows the polyp to reduce its energy bill by 90%. Very recently, researchers have found that this symbiosis was even a ménage à trois (at the very least), involving several groups of fungi, some of which directly provide algae with nitrogen.²⁸