Ecology: The Biology of Interactions. 4.07. Mutualism
From the definition of mutualism as mutually beneficial relationships between populations that occur together in nature, one might think this is some exotic phenomenon. This is far from the case: for example, developed terrestrial life exists only thanks to mutualistic relationships...
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4.06. Classification of Population Interactions
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Chapter 4. Population Ecology
April 8. Protocooperation
4.07. Mutualism We noted that mutualism is an obligate mutually beneficial relationship between populations. How can we establish that a particular relationship is obligate? If organisms in a mutually beneficial relationship are found together only in natural conditions, this indicates the inseparable nature of their connection. Mutualism is a common phenomenon. For example, developed terrestrial life exists only thanks to mutualistic relationships. The colonization of land by higher plants occurred precisely because of their close interaction with fungi. Traces of mycorrhiza (the etymology of this word means something like "fungus-root") have been found on the remains of Devonian plants. Mineral nutrition of terrestrial plants is an extremely complex process, and in many cases, it is impossible without fungi. In mycorrhiza, fungal hyphae envelop plant roots, penetrate these roots, or even individual cells. Fungi have a significantly more efficient mechanism for obtaining mineral substances than plants, but they are limited by a lack of organic matter. Plants transfer part of the organic matter they synthesize to the fungal hyphae, receiving the necessary salts from them. Endosymbiotic nitrogen fixers, similar to Rhizobium bacteria, also participate in mineral nutrition. On the roots of a number of plants (legumes, alder, etc.), special porous outgrowths form, which are inhabited by bacteria capable of fixing atmospheric nitrogen. This process benefits not only the plant itself, which develops nitrogen-fixing nodules, but also other plants, as it leads to soil enrichment with nitrogen. Thus, the developed terrestrial plant cover is the result of mutualism between plants and fungi and bacteria. However, the consumption of plant biomass is also largely related to processes in this category. All herbivorous mammals, to varying degrees, use the endosymbiotic microflora of their intestines. In some cases, bacteria capable of breaking down cellulose and synthesizing essential amino acids settle in the posterior part of the intestine. To benefit from their biochemical activity, rodents and lagomorphs have to eat their own excrement. However, endosymbiotic complexes reach their greatest development in ruminant artiodactyls. The stomach of these animals consists of several compartments, one of which – the rumen – is essentially a microbiological fermenter, a reservoir for growing microorganisms. The microflora and microfauna of the rumen of ruminants are quite complex and not fully understood. Their essential components include both protozoa (e.g., flagellates and ciliates) and bacteria. The result of the work of this multi-species complex is the deep processing of cellulose. Termites are an important component of tropical forests, ensuring the rapid decomposition of dead organic matter and the return of its content to the cycle. A complex of bacteria and protozoa also inhabits the intestines of termites, and for its effective functioning, repeated consumption of each other's excrement by termites is necessary. Naturally, mutualism is not limited to land. For aquatic ecosystems, mutualism between animals and various endosymbiotic algae is extremely characteristic. For example, reef-building corals and the largest mollusks, tridacnas, exist thanks to endosymbionts. Finally, lichens – lichenized fungi – are a classic example of mutualism. These organisms can exist only under conditions of close contact between fungal hyphae and the cells of cyanobacteria or algae. Lichenized fungi have evolved from parasitizing photosynthetic organisms to true mutualism. What organisms can become mutualistic? They must differ significantly. If the ecological niches of two populations are similar, meaning these populations require the same resources, competition will arise between them. The condition under which mutualism and protocooperation can develop is a difference in niches. Mutualism most often develops when each member of a pair of species gives the partner a resource that is less valuable to itself and receives a more valuable one. The optimal situation is when the waste products of one organism are a resource for another, and vice versa.
4.06. Classification of relationships between populations
D. Shabanov, M. Kravchenko. Ecology: Biology of Interaction. Chapter 4. Population Ecology
April 8. Protocooperation