Читать книгу Mutual Aid - Pablo Servigne - Страница 23

Not all interactions between living creatures are beneficial. Like humans involved in corporations, governments and property management, the species of the six kingdoms sometimes scam, ignore, avoid, attack and manipulate each other – not to mention the fact that these relationships may change in the course of a day, a season, a life or a geological era. Tensions ease, friendships fester, pacts are loosened, friends betrayed. Some links are very stable while others are very volatile.

How are we to classify all these interactions between species? One simple way to do this (one which doesn’t account for complexity, but allows us to put our ideas into order) is to draw up a small table with two columns (species A and species B), and consider the benefits (+) and the costs (−) of the interactions between the two.⁴⁴

On the first line there are the mutually beneficial relations between two species (+/+ relations), those that we have just been discussing. They are called mutualisms in general, and symbioses in the particular case where these two species cannot live without each other.

When species tend to politely avoid each other, we speak of coexistence (0/0 relations). This is the case in the Amazon rainforest, for example, among arboreal ants, whose hunting territory is an entire tree (and woe betide any other insects that land on it!). In the evening, the diurnal species returns to its nest and leaves room for the nocturnal species until the early hours of the morning. Our intuition as biologists is nevertheless that the box ‘coexistence’ in our table is a grabbag in which we temporarily store the interactions whose advantages and disadvantages are as yet unclear to ecologists.

Species A	Species B	Relationship
+	+	Mutualism/symbiosis
0	0	Coexistence
+	0	Commensalism
−	0	Amensalism
+	−	Predation/parasitism
−	−	Competition

Because there may indeed be disadvantages. This is the case with (more or less) asymmetric relationships. For example, between the fungus Penicillium, which naturally produces an antibiotic, and certain closely related bacteria, the relationship will be neutral for the former, but frankly negative for the latter. We then speak of amensalism (0/– relations). On the contrary, a small epiphytic orchid that lives perched on the branch of a giant of the rainforest will find great benefit in being 20 metres up in the air, but will not offer any advantage to the tree. We then speak of commensalism (relations +/0).

As for frankly asymmetric relations, these are the best known: they are called predation and parasitism (relations +/−). In the case of predation, the predator is larger than or similar in size to its prey (an owl and its mouse-dinner in the first case, a wolf and an elk in the second), which it will usually kill before ingesting it; parasites, however, are mostly much smaller in size than their hosts, and they do not necessarily kill them.⁴⁵

You will have noticed that there is one category left: mutually negative relationships (−/− relationships). In fact, these are just a matter of competition. Behaviours expressing struggle, mutual aggression, combat, threat or tension are generally observed in connection with questions of territory (setting limits) or reproduction. But these are expensive relationships for both species, because each of them risks losing a few feathers or tufts of hair, or even their lives. To avoid this risk and minimize energy loss or spilled blood, many species have developed ways of avoiding combat by simulating it with large gestures or threats. These are the so-called ‘ritualized’ behaviours: they are used to signal their intention, their determination or strength to others, but without coming to blows with claws/beaks/feet/teeth/poison. Think about the fights between deer, roosters, beetles or felines. Competition is by definition a source of stress; so in general it is only occasional (in animals), because it is exhausting and dangerous.

But why, if competition is so bad, does it lie at the heart of our Western culture? Why is it so hard for us to accept the pervasiveness of beneficial relationships in nature? To understand this, let’s look at the story of two naturalists who left their mark on their age.