Mutualism is a biological interaction that contributes to the survival and growth of both partner species. In other words, this is a form of symbiosis. Lichens are a classic example illustrating what mutualism is.
Another example is the relationship between legumes and nitrogen-fixing bacteria in nodules on their roots. Mutualism is also some close interrelations of pollinators with pollinated plants, for example, Lepidoptera Tegeticula yucasella and Yucca.
Optional and obligate mutualism
It is necessary to distinguish between these two concepts. Both obligate and optional mutualism are mutually beneficial cooperation. The interaction in this case is useful for one and for another species. However, in the second case, each species is able to exist in isolation. Obligatory mutualism is obligatory. This means that organisms cannot exist separately.
Mycorrhiza
One of the most interesting and important from an environmental point of view examples of the phenomenon we are interested in is the interactions that exist between fungi and vascular plants. The roots of most of these plants form complex structures with fungi. These structures are called mycorrhiza. Without it, normal plant growth would not have been possible. Mycorrhiza, apparently, played a key role in their settlement of land. Mutualism (symbiosis) has been widespread since ancient times.
Mycorrhizal connections
The more we learn about them, the clearer their importance for vascular plants becomes. In many species, non-mycorrhizal individuals are rare in nature, even if their growth is possible without fungi with careful selection of growth conditions. Most vascular plants are “double” organisms in the same sense as lichens, although this duality is usually invisible in their aerial parts. According to S. Wilde, a soil scientist at the University of Wisconsin, a tree extracted from the soil is just a part of a whole plant that is surgically separated from its absorbing and digestive organ. In most plants, fungi play a vital role in the uptake of phosphorus and other essential nutrients.
Fungi that form mycorrhiza with most plants are zygomycetes. This type of it is called endomycorrhiza. It is characteristic of most grasses, shrubs and trees. Some groups of conifers and dicotyledons - mainly trees - form mycorrhiza with basidiomycetes, as well as with some ascomics. In this case, we are talking about ectomycorrhiza. Sometimes it is very specific: one type of fungus interacts only with a certain type of vascular plant or with a group of related species. It is known, for example, that Basletiomycete Boletus elegans is associated only with coniferous larch (Larix). Other fungi form mycorrhiza with forest species of more than a dozen genera. Ectomycorrhiza is especially characteristic of relatively tree-poor communities of trees living in high latitudes of the Northern Hemisphere or in high mountains.
Acacia and ants
The most complex examples of mutualism are found in the tropics, where the diversity of organisms is much higher than in temperate regions. So, in the tropics and subtropics, acacias (trees and shrubs of the genus Acacia) are widespread. The relationships between certain species of these plants on the plains of Mexico and Central America and the ants living in their spikes are a wonderful example of the complex interactions between animals and plants. They are especially evident for ants of the genus Pseudomyrmex.
These acacias have a pair of bloated thorns at the base of each leaf, the length of which is more than 2 cm. Nectaries are located on the petioles, and small nutrient organs called Belt bodies are located at the ends of leaflets. Ants live inside hollow spikes, eating sugars from nectaries and Belt bodies, which contain fats and proteins. Acacias grow extremely fast and are especially characteristic of disturbed areas, where competition between fast-growing colonizing plants is often very intense. Thomas Belt first described the relationship between Pseudornyrmex and these trees in his book Naturalist in Nicaragua, published in 1874.
Other types of mutualistic relationships
There are many other types of organism-binding relationships in which mutualism is manifested. These are, for example, trees in the forest (as well as grass), which are often spliced by their roots. As a result, nutrients are transferred from one plant to another in a complex and completely unexpected way, and the survival of a species in a particular area literally depends on the presence of another with which it forms such a connection. Tree stumps can live indefinitely, although they are devoid of photosynthetic organs, because they are spliced by roots with other individuals and can receive nutrients from them. Some diseases, such as oak wilt in the Midwest and Eastern United States, can also be transmitted through such root “vaccinations.”
As you can see, mutualism is a common occurrence in nature. It is a special form of symbiosis.