The structure of a community is the ratio of groups of organisms according to any characteristic or according to the nature of their distribution in the environment.
One of the most important indicators of a community is the species structure.
The species structure of a community includes the species composition of its organisms and the quantitative ratio of species populations.
A community is judged primarily by its species diversity and species richness.
Species richness is a general set of species in a community, which is expressed by lists of representatives of different groups of organisms.
Species diversity is an indicator that reflects not only the qualitative composition of the biocenosis, but also the quantitative relationships of species.
Species diversity is a sign of ecological diversity: the more species, the more ecological niches, that is, the higher the richness of the environment.
Species diversity is also related to the sustainability of a community: the greater the diversity, the wider is the possibility for the community to adapt to changed conditions, be it climate change or other factors.
The number of species in a community depends on many factors, such as its geographic location. It increases markedly as it moves from north to south.
Example: in a tropical forest on one hectare, you can meet a hundred species of birds, while in a temperate forest in the same area, their number does not exceed a dozen. But in both cases, the number of individuals is approximately the same. The fauna on the islands is usually poorer than on the continents, and it is the poorer, the smaller the island, and the farther it is from the mainland.
The variety of living organisms is determined by both climatic and historical factors.
Example: in areas with a mild, stable climate, with abundant and regular rainfall, without severe frosts and seasonal temperature fluctuations, the species richness is higher than in areas located in areas of severe climates, such as tundra or highlands.
The species richness increases with the evolutionary development of the community. The more time has passed since the formation of a community, the higher its species richness.
Example: in such an ancient lake as Baikal, for example, there are 300 species of amphipods alone.
Agricultural communities have the shortest history, they are created artificially, their lifetime is measured in several months. But if a peasant field remains uncultivated and uncultivated for two or three years, it acquires a completely different look: forbs grow, new species of insects, birds, and rodents appear.
In any community, as a rule, there are relatively few species represented by a large number of individuals or a large biomass, and relatively many species that are rare. Species with a high abundance have a significant impact on the community. Species-environment-formers play a special role.
Example: in bog ecosystems, sphagnum is often the habitat; it forms a special type of soil – sphagnum peat and largely determines the conditions for the survival of other species of plants and animals.
At the same time, rare species often turn out to be the best indicators of the state of a community, since certain sets of environmental conditions are required for the existence of such species. Under the normal state of the community, such conditions are maintained by its functioning. Therefore, the disappearance of rare species allows us to conclude that the balance in the ecosystem has been disturbed.
Species diversity is a sign of community resilience.
Species diversity can be viewed as an indicator of the well-being of a community or ecosystem as a whole. Its decrease often indicates trouble much earlier than a change in the total number of living organisms. In communities where there are many species, some of them find themselves in a similar position, since they occupy the same territory and perform similar functions. Such communities are less affected by various factors. So, any change in conditions may cause the death of one of the species, but this will have little effect on the state of the entire community, since the loss will be compensated for by species that are close in function.