The species diversity of a community depends on the following factors. Species richness depends on community structure

Population by tiers in a forest ecosystem?

The composition of the community is judged primarily by species diversity. Diversity refers to the species richness of a community.

The number of species in a community depends on many factors, such as its geographic location. It increases markedly when moving from north to south. In one hectare of tropical forest one can meet hundreds of species of birds, while in the temperate forest in the same area the number does not exceed ten. But in both cases, the number of individuals is approximately the same. On the islands, the fauna is usually poorer than on the continents, and the poorer it is, the smaller the island and the further it is from the mainland.

Variety of living organisms determined by both climatic and historical factors. 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 severe climate zones, such as, for example, the tundra or highlands.

Species richness grows with the evolutionary development of the community. The more time has passed since the formation of the community, the higher its species richness. Agricultural communities have the shortest history, they are created artificially, the time of their existence is measured in several months. But if a peasant field remains unsown and uncultivated for two or three years, it takes on a completely different appearance: forbs increase, new species of insects, birds, and rodents appear. The longer the development ecosystem, biocenoses and populations, the richer its species composition. In such an ancient lake as Baikal, for example, only 300 species of amphipods live.

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 (Fig. 60). Species with a high abundance play a huge role in the life of the community, especially the so-called habitat-forming species. In forest ecosystems, for example, these include the species of predominant woody plants: they determine the conditions necessary for the survival of other plant and animal species - grasses, insects, birds. animals, small invertebrates of the forest litter, etc.

At the same time, rare species are often the best indicators of the health of a community. This is due to the fact that strictly defined combinations of various factors (for example, temperature, humidity, soil composition, certain types of food) are required to maintain the life of rare species. resources and etc.). Maintaining the necessary conditions largely depends on the normal functioning of ecosystems, so the disappearance of rare species allows us to conclude that the functioning of ecosystems has been disrupted.

Species diversity can be considered as an indicator of the well-being of a community or an ecosystem as a whole. Its decrease often indicates trouble much earlier than a change in the total number of living organisms, moreover, species diversity is a sign of community stability. In communities with high diversity, many species occupy a similar position, inhabiting the same area of ​​space, performing similar functions in the system of matter-energy metabolism. In such a community, a change in living conditions under the influence of, for example, climate change or other factors can lead to the extinction of one species, but this loss will be compensated by other species close to the retired one in their specialization. Thus, the greater the diversity, the more resilient the community is to sudden changes in physical factors or climate.

Morphological and spatial structure of communities.

Any communities, regardless of the location or composition of the species present in it, have some features that facilitate their analysis and comparison with each other. These features include the ratio of organisms with certain types of external structure and the spatial organization of the community.

Certain types of external structure of organisms that have arisen as adaptations to habitat conditions are called life forms.

The life forms of plants and animals are very diverse.

They are distinguished by a combination of signs of structure and lifestyle. So, the most common life forms of plants are trees, shrubs, herbs.

The characteristic features of a plant community, for example, can be judged by the ratio of life forms present here. After all, the number of life forms, as a rule, is significantly less than the number of species forming the community, and the predominance of certain forms characterizes the general living conditions of organisms. In arid climates, succulents with fleshy leaves or stems predominate, with a lack of light in the tropical forest - vines, in the tundra, highlands with low temperatures, dryness and strong winds - postlans and pillow plants. The species composition of deciduous and coniferous forests is different, and these communities are similar in terms of the ratio of life forms.

The set of life forms and their ratio determine the morphological (from the Greek morphe - form) structure of the community, which is used to judge its belonging to a particular type, for example, forest, meadow, shrub.
The life forms of animals for different systematic groups are distinguished according to different characteristics. In animals, one of the main signs for distinguishing life forms is the means of movement (walking, running, jumping, swimming, crawling, flying). Characteristic features of the external structure of ground jumpers, for example, are long hind limbs with strongly developed thigh muscles, a long tail, and a short neck. These usually include inhabitants of open spaces: Asian jerboas, Australian kangaroos, African jumpers and other jumping mammals living on different continents.

Life forms of aquatic organisms are distinguished by the type of their habitats. The inhabitants of the water column are united by a special life form plankton (from the Greek planktos - wandering) - a set of organisms that live in suspension and are not able to resist currents. Plankton contains both plant (algae) and animal (small crustaceans) organisms. The inhabitants of the bottom "form benthos (from the Greek benthos - depth).

Various life forms are spatially isolated from each other in a certain way. This isolation characterizes the spatial structure of the community. Any plant community, for example, is divided into tiers - horizontal layers in which the ground or underground parts of plants of certain life forms are located. Layering is especially pronounced in forest phytocenoses, where there are usually 5-6 layers (Fig. 61). But even in meadow or steppe communities, at least two or three tiers can also be distinguished.

The animal population of the community, "attached" to plants, is also distributed over tiers. For example, the microfauna of soil animals is the richest in the litter. Different types of birds build nests and feed in different tiers - on the ground (wagtail), in bushes (robin, nightingale), in tree crowns (rooks, magpies).
Horizontally, the community is also divided into separate elements - micro-groups, the location of which reflects the heterogeneity of living conditions. This is especially evident in the structure of the ground (ground) cover - in the presence of a "mosaic" of various microgroups (for example, tussocks or patches of grasses, light-loving grasses in the "windows" of forest glades, shade-tolerant grasses under trees, patches of mosses or bare ground).

The morphological and spatial structure of the community is an indicator of the diversity of the living conditions of organisms, the richness and completeness of their use of environmental resources. To a certain extent, they also characterize the stability of communities, that is, their ability to withstand external influences.

trophic structure.

Maintaining the integrity of the community is provided by a variety of relationships between organisms. Animals can use plant organisms as sources of food, shelter, building material. Plants, in turn, use the "fruits of activity" of animals that spread their seeds, participate in the processing of organic matter, the products of which, returning to the soil, are again used by plants.
Different types of organisms in the community are closely related to each other, interdependent on each other. Of greatest importance in nature are food ties, due to which a continuous material-energy exchange is carried out between living and inanimate matter of nature.

For any community, it is possible to draw up a diagram of all the nutritional relationships of organisms. This diagram is in the form of a network. A food web (its interlacings are very complex, usually consists of several food chains, each of which is, as it were, a separate channel through which matter and energy are transferred (Fig. 62). A simple example of a food chain is given by the following sequence: herbivorous insect - predatory insect - insectivorous bird of prey.

In this chain, a unidirectional flow of matter and energy from one group of organisms to another is carried out. On the figure 62 arrows show the flow of matter in the food web.

Different species occupy different positions in the food chain.

Only green plants are capable of fixing light energy and using simple inorganic substances in their nutrition. Such organisms are divided into a group and called autotrophs (self-feeding, from the Greek autos - self and trophe - food), or producers - producers of biological matter. They are the most important part of any community, because almost all other organisms directly or indirectly depend on the supply of matter and energy stored by plants. On land, autotrophs are usually large plants with roots, while in water bodies microscopic algae floating in the water column (phytoplankton) take on their role. Such organisms are isolated as independent organisms. All other organisms are classified as heterotrophs (from Greek, heteros - different), feeding on ready-made organic substances.

Heterotrophs decompose, rebuild and assimilate complex organic substances created by primary producers. All animals are heterotrophs, and many microorganisms also belong to them. In turn, heterotrophic organisms are divided into consumers (consumers) and decomposers (decomposers).

Consumers are mainly animals that eat other organisms (vegetable or animal) or pulverized organic matter. The decomposers are represented mainly by fungi and bacteria, which decompose the false components of the dead cytoplasm, reducing them to simple organic compounds, which can later be used by producers. Intensive heterotrophic activity is concentrated in those places where organic matter accumulates in soil and silt.

The position of an organism in the food chain is characterized by its remoteness from the main source of energy entering the community. Different organisms occupy different positions: in these cases they are said to be located at different trophic levels. Autotrophs occupy the first trophic level, and heterotrophs occupy all subsequent trophic levels: herbivorous organisms - [the second, carnivores - the third, predators that feed on frugivorous animals - the fourth, etc.)

Figure 63 simplistically conveys the structure of two types of communities related to terrestrial and aquatic ecosystems. These communities radically differ in the composition of organisms, with the exception of some bacteria that can exist in both environments. However, they are similar in trophic structure: both here and there the main ecological components are present: autotrophs, heterotrophs, nonsuments and decomposers (explanations in the text under the figure).

species diversity. species composition. Autotrophs. Heterotrophs. Producers. Consumers. Reducers. Layered. Rare species. Environment-forming species. Food chain. food web. life forms. trophic level.

1. What factors increase the species richness of a community?
2. What is the importance of rare species?
3. What properties of the community characterize the diversity of species?
4. What is a food chain and food web? What is their significance?

Kamensky A. A., Kriksunov E. V., Pasechnik V. V. Biology Grade 9
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The species richness of the communities of trees and insectivorous birds in the Western Caucasus is determined both by the sequence in which species occupy parts of the ecological niche and by the number of species of the surrounding area potentially able to exist in these communities. The relative role of these factors varies depending on the ratio of the number of species (the rank structure of abundance) of these communities.

In the article by V.V. Akatova and A.G. Perevozova (Maikop State Technological University, Caucasian State Natural Biosphere Reserve) considers the reasons that affect the species richness in the communities of trees and birds of the Western Caucasus. The higher the level of dominance, i.e. the proportion of individuals of the most numerous species of the total number of individuals in the community, the less resources remain for other species of the community, the lower their numbers and the higher the probability of extinction as a result of random processes. Accordingly, the lower the species richness.

The authors give a description of the main models of the ratio of species abundance in a community (for a comparison of models that characterize the species structure of communities, see: In search of a universal law for the structure of biological communities, or Why did ecologists fail? "Elements", 12.02.08).

Particular attention is paid to the geometric series model (Y. Motomura, 1932) or “predominant niche capture”, which was used in this work. The geometric series model assumes that species of a community, ranked in descending order of abundance, consume the same proportion of the remaining total resource of the community. For example, if the most numerous species takes 1/2 of the resource, then the next most important species consumes half of what is left (i.e. 1/4 of the original), the third species again consumes half of the remaining (1/8 of the original), and so on. . The model implies a hierarchical principle of resource sharing. The larger the share of the resource is intercepted by the dominant species, the more of the remaining resources are used by the subdominant species, and the less resources are transferred to the smaller species. Communities with such a distribution are characterized not only by a smaller amount of resources available to non-dominant companion species, but also by their more “rigid” distribution. The number of species is proportional to the share of resources they have inherited, and represents a geometric progression. Such a geometric model describes the capture of the lion's share of the resource by a small number of species with a strongly pronounced dominance. It applies to simple animal or plant communities in the early stages of succession or existing in harsh environmental conditions, or to individual parts of communities.

The hyperbolic model (A.P. Levich, 1977) is close to the geometric one, but reflects an even less uniform distribution of resources: the abundance of the first species decreases more sharply, while the abundance of rare species, on the contrary, is smoother. Compared to the Motomura model, the hyperbolic model better describes complex communities and large samples.

The lognormal model (Preston, 1948) is typical for more evenly distributed resources and species abundances, here the number of species with an average abundance increases.

In the distribution described by the “broken rod” model (R. MacArthur, 1957), species abundances are distributed with the highest possible uniformity in nature. The limiting resource is modeled by a rod randomly broken in different places. The abundance of each species is proportional to the length of the piece he got. This model is suitable for communities living in a homogeneous biotope, one trophic level with a simple structure, where the abundance of species is limited by the action of some one factor or accidentally divides an important resource.

In addition to dominant species, the species richness of a local community is influenced by the species fund (pool) - the totality of species living in a given area and potentially able to exist in this community. By local species richness is meant, for example, the average number of plant species on the site, and by the species fund - the total number of tree species recorded in the forest areas of the entire region. The size of the species fund is determined by regional environmental conditions, including climate. Under extreme conditions, only a modest set of species can exist, which automatically limits the number of possible dominants. Under favorable conditions, both the total number of species and the number of candidates for the role of dominants increase. The more favorable the conditions, the greater the number of species able to achieve high abundance, and the lower the level of dominance of each of them in specific areas. The size of the species pool also depends on the rate of speciation and the history of the region: for example, the biomes of the regions close to the poles that experienced the Pleistocene glaciation may be relatively poorer in species compared to those located to the south, including because of their youth.

V.V. Akatov and A.G. Perevozov studied trees in 58 areas of lowland and mountain forests and communities of insectivorous birds in 9 biotopes of the Western Caucasus. In relation to the entire data set, the maximum impact (50–60%) on local species richness was exerted by the number of individuals of associated species. In all studied communities, a high correlation was found between the level of dominance and species richness. The level of dominance of the strongest competitor determined about 15–20% of the variation in the number of species in the community. Apparently, this means that the relationship between the level of dominance and species richness is largely a consequence of a simple redistribution of resources from companion to dominant species. In turn, the size of the species fund influenced both the level of dominance and species richness.

To assess the ratio of the roles of the level of dominance, the number of associated species, and the species fund, the studied communities were divided into two groups - with high and low correspondence of the species structure to the geometric model (GM).

In areas with high GM correspondence, species richness depended more strongly on local conditions, namely, on the number of individuals of associated species and on the level of dominance, which reflects the nature of the niche space distribution.

On the contrary, in areas with low correspondence of the species structure to the geometric model, the role of the species fund increased, while the role of local factors decreased. In such communities, species richness turned out to be relatively independent of the abundance of dominants.

Thus, the authors obtained the expected result: the relative contribution of various mechanisms to local species richness depends on the rank structure of species abundance in communities, including the correspondence of this structure to the geometric model.

Question 1. What factors increase the species richness of the community?
The species diversity of a community depends on the following factors:
one). geographical location (when moving from north to south in the Northern Hemisphere of the Earth, and vice versa, in the Southern island fauna is usually poorer than on the mainland, and it is poorer, the smaller the island and the further it is from the mainland);
2).climatic conditions(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 severe climate zones);
3).duration of development(The more time has passed since the formation of the community, the higher its species richness.

Question 2. What is the importance of rare species?
To maintain the life of rare species, strictly defined combinations of various environmental factors (temperature, humidity, soil composition, certain types of food resources, etc.) are required, which largely depends on the normal functioning of the ecosystem. Rare species provide a high level of species diversity and are the best indicators (indicators) of the state of the community as a whole. For example, if crayfish live in a reservoir, then this may be an indicator that the ecosystem is developing normally in this reservoir. If the reservoir is “overgrown” with algae, then this is a signal that the balance of the ecosystem is disturbed in this reservoir.

Question 4. What is a food chain and food web? What is their significance?
The transfer of energy from its original source - plants - through a series of organisms, each of which eats the previous one and serves as food for the next, is called power circuits. For any community, you can map out all the food relationships - food web. food web consists of several food chains. The simplest example of a food chain: a plant - a herbivorous insect - an insectivorous bird - a bird of prey.
For each of the food chains that form the food web, matter and energy are transferred, i.e., a material-energy exchange is carried out. The implementation of all ties in the community, including food, helps to maintain its integrity.
In the biocenosis, all components are sequentially distributed according to the trophic levels of food chains and their interacting combinations - food webs. As a result, a single functional system of metabolism and energy conversion is formed within the biocenosis (Fig. 4.).

1. What is the layering of a plant community?

The layering of a plant community is the division of a community into horizontal layers, in which the above-ground or underground parts of plants of certain life forms are located.

2. How is the animal population distributed over the tiers in the forest ecosystem?

Plants of each tier and the microclimate caused by them create a certain environment for specific animals:

In the soil layer of the forest, which is filled with plant roots, soil animals (various microorganisms, bacteria, insects, worms) live;

Insects, mites, spiders, numerous microorganisms live in the forest bedding;

Higher tiers are occupied by herbivorous insects, birds, mammals and other animals;

Different types of birds build nests and feed in different tiers - on the ground (pheasant, black grouse, wagtails, skates, buntings), in bushes (thrushes, warblers, bullfinches), in tree crowns (finches, goldfinches, kinglets, large predators).

Questions

1. What factors increase the species richness of a community?

The diversity of living organisms is determined by both climatic and historical factors. 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 severe climate zones, such as, for example, tundra or highlands.

Species richness grows with the evolutionary development of the community. The longer the development of an ecosystem, the richer its species composition. In such an ancient lake as Baikal, for example, only 300 species of amphipods live.

2. What is the importance of rare species?

Rare species are often the best indicators of the health of a community. This is due to the fact that strictly defined combinations of various factors (for example, temperature, humidity, soil composition, certain types of food resources, etc.) are required to maintain the life of rare species. Maintaining the necessary conditions largely depends on the normal functioning of ecosystems, so the disappearance of rare species allows us to conclude that the functioning of ecosystems has been disrupted.

In communities with high diversity, many species occupy a similar position, inhabiting the same area of ​​space. In such a community, a change in living conditions under the influence of, for example, climate change or other factors can lead to the extinction of one species, but this loss will be compensated by other species close to the retired one in their specialization.

3. What properties of the community characterize the diversity of species?

Species diversity makes it possible to determine how resilient a community is to sudden changes in physical factors or climate.

4. What is a food chain and food web? What is their meaning?

A food web usually consists of several food chains, each of which is, as it were, a separate channel through which matter and energy are transferred.

A simple example of a food chain is the sequence: plant - herbivorous insect - predatory insect - insectivorous bird - bird of prey.

In this chain, a unidirectional flow of matter and energy from one group of organisms to another is carried out.

Thanks to food connections, a continuous material-energy exchange is carried out between living and inanimate matter of nature, which helps to maintain the integrity of the community.

Tasks

Figure 85 simplistically conveys the structure of two types of communities related to terrestrial and aquatic ecosystems. Analyze the structure of these ecosystems. Compare the features characteristic of them. Draw a conclusion about how these communities are fundamentally different and how they are similar.

For terrestrial ecosystems, the main abiotic factors that determine the composition and primary biological production are water and soil richness in mineral nutrition elements. In ecosystems with a dense canopy of plants - broad-leaved forests, tall reedbeds or canaries on the river bank - light can be a limiting factor. There is no shortage of water in aquatic ecosystems, it is always in excess: if a reservoir dries up, then its aquatic ecosystem collapses and is replaced by another, terrestrial one. The main factors in them are the content of oxygen and nutrients in the water (primarily phosphorus and nitrogen). In addition, as in terrestrial ecosystems, it can be the availability of light.

In food chains in terrestrial ecosystems - usually no more than three links (for example, clover - hare - fox). In aquatic ecosystems, there may be four, five or even six such links.

Aquatic ecosystems are very dynamic. They change throughout the day and seasons of the year. In the second half of summer, eutrophic lakes "bloom" - microscopic unicellular algae and cyanobacteria massively develop in them. By autumn, the biological productivity of phytoplankton decreases, and macrophytes sink to the bottom.

The biological production of aquatic ecosystems is greater than the stock of biomass. Due to the fact that the main "workers" of the autotrophic and heterotrophic workshops of the aquatic ecosystem do not live long (bacteria - a few hours, algae - a few days, small crustaceans - a few weeks), at any given time, the stock of organic matter in the water (biomass) may be less than the biological production of the reservoir for the entire growing season. In terrestrial ecosystems, on the contrary, the stock of biomass is higher than the production (in the forest - 50 times, in the meadow and in the steppe - 2–5 times);

Animal biomass in aquatic communities may be greater than plant biomass. This is because zooplankton organisms live longer than algae and cyanobacteria. This does not happen in terrestrial ecosystems, and the biomass of plants is always greater than the biomass of phytophages, and the biomass of zoophages is less than the biomass of phytophages.

Similarities: in the communities under consideration, the following organisms are present without fail: producers (vegetation on land and phytoplankton in water), consumers, decomposers.

The primary source of energy in reservoir and forest communities, as in most ecosystems, is sunlight.

Productivity. For plants, the productivity of an environment can depend on whichever resource or condition is most restrictive of growth. In general, there has been an increase in primary production from the poles to the tropics as light, mean temperatures and growing seasons increase. In terrestrial communities, a decrease in temperature and a decrease in the duration of the growing season with height lead in general to a decrease in production. In reservoirs, production, as a rule, falls with depth in parallel with temperature and illumination.

There is often a sharp decrease in production in arid conditions, where growth can be limited by a lack of moisture, and its increase occurs almost always when the influx of key nutrients such as nitrogen, phosphorus and potassium increases. In the broadest sense, environmental productivity for animals follows the same patterns, as it depends on the amount of resources at the base of the food chain, temperature, and other conditions.

If the increase in production leads to an expansion of the range of available resources, it probably contributes to an increase in species richness. However, environments with different productivity can differ only in the amount (intensity of supply) of the same resources with the same range of resources. This means that the difference between them will not be in the number of species, but only in the size of the populations of each of them. On the other hand, it is possible that even with the same general range of resources, some rarely encountered categories (or unproductive parts of their spectrum), insufficient to provide species in an unproductive environment, will become so abundant in a productive environment that additional species can be included in the community. . Arguing in a similar way, we can conclude that if competition prevails in a community, then an increase in the amount of resources will contribute to a narrowing of specialization. ; in this case, the density of populations of individual specialized species will not necessarily decrease significantly.

Thus, in general, one can expect an increase in species richness as productivity increases. This has been clearly shown by Brown and Davidson, who found very good correlations between the number of species and the level of precipitation in both seed-eating ants and seed-eating rodents in the deserts of the southwestern United States. In these arid areas, the average annual rainfall is closely related to primary production and hence to the amount of seed stock available. It is especially noteworthy that in species-rich areas, there are more very large (consuming large seeds) and very small (feeding small seeds) species among ants. There are also more species of very small rodents. Apparently, in more productive communities, either the size range of seeds is wider, or there are so many of them that additional types of consumers can feed.

It is not easy to point out any other unambiguous relationships between species richness and productivity, because although both of these parameters often change in parallel (for example, with latitude or height above sea level), other factors usually change along with them, i.e. the correlation that is found may be due to them.

However, a direct relationship has been described between the number of lizard species in the deserts of the southwestern United States and the length of the growing season, an important factor in the productivity of arid environments.

Brown and Gibson, using data from the work of Whiteside and Harmsworth, showed a diversity of planktonic cladocerans in 14 unpolluted lakes in the state. Indiana is positively correlated with the total production of these bodies of water, expressed in grams of carbon per year. .

On the other hand, the growth of diversity with an increase in productivity cannot be considered a general pattern. This is demonstrated, for example, by a unique "lawn" experiment conducted from 1856 to the present day at the Rothamsted Experimental Station (England). The pasture with an area of ​​about 2 hectares was divided into 20 plots; two of them served as control, the rest were fertilized once a year. From 1856 to 1949, the species diversity of the herbaceous plant community changed on the control plots and those that received a full set of fertilizers. While the former remained practically unchanged, the latter showed a gradual reduction in species diversity. This decline in diversity (called "enrichment paradox" of the environment) has also been identified in some other geobotanical studies.

Similarly, anthropogenic eutrophication of lakes, rivers, estuaries and coastal areas leads to a decrease in phytoplankton diversity (parallel to an increase in primary production). It should also be mentioned that the two types of communities that are among the most species rich in the world develop on soils that are extremely poor in nutrients (we are talking about the South African and Australian communities of thickets of thickets of thickets in a climate close to the Mediterranean), while in the them, on more fertile soils, the diversity of vegetation is much less.

It is logical to assume that when increasing productivity means expanding the range of resources, an increase in species richness should be expected (at least some observations support this). In particular, a more productive and diverse plant community is likely to have a richer phytophage fauna, and so on until the end of the food chain. On the other hand, when increased productivity is due to an increased supply of resources, and not to an expansion of their range, the theory allows for the possibility of both an increase and a decrease in species richness. Evidence, especially from the field of geobotany, suggests that more often than not, an increase in the availability of resources leads to a decrease in the number of species.

In connection with all this, it is not superfluous to dwell on properties of light as a resource for plants. In highly productive systems (such as tropical forests), where it enters very intensively, it is reflected and scattered over a thick layer of vegetation. Therefore, there is not only a high initial illumination, but also a long smooth gradient of its decrease, and also, possibly, a wide range of light frequency spectra. Thus, an increase in the intensity of solar radiation, apparently, is necessarily associated with a large variety of light regimes, due to which the possibility of specialization and, consequently, the growth of species richness increases. Another implication from this is that the tallest forms must be able to function throughout the range of light as they grow from soil level to the top of the canopy.

Spatial inhomogeneity. The patchy nature of the environment, with an aggregated distribution of organisms, can ensure the coexistence of competing species. In addition, in environments with more spatial heterogeneity, higher species richness can be expected due to the fact that they have more diverse microhabitats, a wider range of microclimates, more types of shelter from predators, etc. In short, the spectrum of resources is expanding.

In some cases, it was possible to show the relationship between species richness and the spatial heterogeneity of the abiotic environment. Thus, a plant community occupying a number of soils and landforms will almost certainly (ceteris paribus) be richer floristically than a phytocenosis on a flat area with homogeneous soil.

Climatic fluctuations. The impact of climatic fluctuations on species diversity depends on whether they are predictable or unpredictable (on time scales relevant to particular organisms). In a predictable environment with regular seasons, different species can be adapted to live at different times of the year. Therefore, it should be expected that more species can coexist in a seasonal climate than in unchanging environmental conditions. For example, different annuals in temperate regions sprout, grow, flower, and produce seeds at different points in the seasonal cycle; here, in large lakes, there is a seasonal succession of phyto- and zooplankton with alternate dominance of one or another species, as changing conditions and resources become most suitable for them.

On the other hand, in non-seasonal habitats, there are opportunities for specialization that are absent in an environment with pronounced seasonality. For example, it would be difficult for a long-lived obligate frugivorous organism to survive in a climate where fruits are available only at certain very short times of the year. But in a non-seasonal tropical environment, where the fruits of one or another plant are constantly present, such specialization is very common.

Unpredictable climatic fluctuations can have various effects on species richness. On the one hand, under stable conditions, there may be specialized species that are unlikely to survive where conditions or resources are subject to sudden fluctuations. ; species saturation is more likely in a stable environment, and it follows from theoretical considerations that niche overlap will be greater in more constant environments . All this can increase species richness.

On the other hand, it is in a stable environment that the probability is higher that populations will reach their limiting densities, competition will intensify in communities and, consequently, competitive exclusion will occur. Therefore, it would be logical to consider unpredictable climatic fluctuations as one of the forms of disturbance, and species richness, apparently, will be maximum at its “intermediate” levels, i.e. it can both increase and decrease with increasing climate instability.

Anecdotal studies seem to support the notion that the number of species will increase as climate fluctuations decrease. For example, MacArthur, studying birds, mammals and gastropods of the western coast of North America (from Panama to Alaska), found a significant negative correlation between species richness and the range of average monthly temperatures. However, many other parameters also change at this distance, so that such a dependence can only be indirect. Other studies of climatic fluctuations also did not lead to unambiguous conclusions.

The harshness of the environment. An environment dominated by some extreme abiotic factor (often referred to as harsh) is not as easy to recognize as it seems at first glance. From a purely human point of view, "extreme" will be both very cold and very hot habitats, and unusually salty lakes, and heavily polluted rivers. However, species living in such habitats have arisen, and what seems very cold and extreme to us must seem appropriate and quite ordinary to the penguin.

A more objective definition can be given by highlighting for each factor on a continuous scale of its values ​​extreme - maximum and minimum. However, would relative humidity close to 100% (air saturated with water vapor) be as "extreme" as zero? Can the minimum concentration of a pollutant be called extreme? Of course not.

You can completely bypass the problem, leaving the body to "solve it for itself." In this case, we will call this or that environment "extreme" if the organisms are not able to live in it. But as soon as it is required to prove that species richness is low under extreme conditions, such a definition leads to a tautology.

Perhaps the most reasonable definition of extreme conditions implies the presence in any organism capable of enduring them, special morphological structures or biochemical mechanisms that are absent in the nearest species and require certain costs - either energy or in the form of compensatory changes in the biology of the organism required to adapt to such environment. For example, plants living in highly acidic soils may suffer either directly from exposure to hydrogen ions or from low pH-mediated deficiencies in available nutrients such as phosphorus, magnesium and calcium. In addition, the solubility of aluminium, manganese and heavy metals can increase to toxic levels, disrupting mycorrhizal activity and nitrogen fixation. Plants are able to tolerate low pH values ​​only if they have special structures or mechanisms that allow them to avoid or resist these effects.

In the uncultivated meadows of northern England, the average number of plant species per square meter is the lowest at low soil pH. Similarly, the diversity of benthic invertebrates in the streams of the Ashdown forest (southern England) in more acidic waters was markedly reduced.

Extreme habitats with low species diversity include hot springs, caves and very salty waters (eg the Dead Sea). However, the difficulty lies in the fact that they have other features associated with low species richness. Many such systems are unproductive and (probably as a consequence) spatially relatively uniform. They are often short-lived (caves, hot springs) or at least rare compared to other types of environments (only a small proportion of the flowing waters in southern England are acidic). Thus, often "extreme" habitats can be considered as small and isolated islands. Although it is logical to assume that only a few species will survive in an environment with extreme properties, it is extremely difficult to confirm this.

Community age: evolutionary time. It is known that the relatively low species richness of the community may be due to the lack of time for the settlement of the territory or evolution on it. In addition, the unbalanced structure of many communities in disturbed habitats is the result of their incomplete recolonization. However, it has often been suggested that individual species may also be absent in communities occupying vast territories and rarely disturbed, precisely because they have not yet reached ecological or evolutionary equilibrium [for example, Stanley, 1979]. It follows from this that communities can differ in species richness due to the fact that some are closer to a state of equilibrium than others, and, therefore, are more fully saturated with species. .

This idea was most often put forward in connection with the restoration of ecosystems after the Pleistocene glaciations. For example, the low diversity of forest species in Europe compared to North America was explained by the fact that the most important mountain ranges in the first case extend in the latitudinal direction (Alps and Pyrenees), and in the second - in the longitudinal direction (Appalachians, Rocky Mountains, Sierra Nevada) . Therefore, in Europe, the trees were squeezed between glaciers and mountains and, having fallen into a kind of trap, died out, and in America they simply retreated to the south. The time that has passed since then is evolutionarily insufficient for European trees to achieve equilibrium diversity. Apparently, even in North America, during the interglacial epochs, the balance did not have time to be restored; post-glacial settlement of rocks displaced by the glacier is too slow.

More broadly, it has often been assumed that the tropics are richer than the temperate regions, at least in part because of their long continuous evolution, while the regions closer to the poles have not yet recovered from the Pleistocene (or even older) glaciations. However, it is possible that environmentalists in the past have greatly exaggerated the long-term stability of the tropics.

When the climatic and natural zones of the temperate regions shifted towards the equator during the glaciation, the tropical forest, apparently, was reduced to a few small refugia surrounded by grass formations. Therefore, it is impossible to simplistically contrast the unchanging tropics with disturbed and recovering temperate zones. If we want to at least partially attribute the poverty of the circumpolar biota to a state far from evolutionary equilibrium, we will have to resort to a complex and unproven argument. It is possible that the shift of temperate zones to completely different latitudes led to the extinction of a much larger number of forms than the reduction in the area of ​​tropical systems without changing their latitudinal distribution. A detailed geological record would help solve the problem, showing that the tropics have always been characterized by about the same species richness, and in the temperate regions either there were many more species in the past, or now their number is noticeably increasing here. Unfortunately, we have no such evidence. So, in all likelihood, some communities are indeed farther from equilibrium than others, but it is not possible to speak precisely or at least with certainty about relative proximity to it with current knowledge.

Species richness gradients. Latitude. Perhaps the most famous pattern of species diversity is its increase from the poles to the tropics. This can be seen in a wide variety of groups of organisms - trees, marine bivalves, ants, lizards and birds. In addition, this pattern is observed in terrestrial, marine, and freshwater habitats. For example, it has been found that 30-60 species of insects usually live in small rivers of tropical America, and 10-30 species in similar water bodies in the temperate zone of the USA. Such an increase in diversity is noticeable when comparing not only large geographic regions, but also small areas. Thus, 40–100 different tree species can grow on one hectare of tropical rainforest, usually 10–30 in the deciduous forests of eastern North America, and only 1–5 in the taiga in northern Canada. Of course, there are exceptions. Separate groups, such as penguins or seals, are most diverse just in the polar regions, and conifers - in temperate latitudes. However, for each such group there are many others that live only in the tropics, such as the New World fruit-eating bats and the giant bivalves of the Indian and Pacific Oceans.

A number of explanations for this general pattern have been proposed, but none of them can be accepted without reservation. First of all, the wealth of tropical communities was attributed to intense grazing. It has been suggested that natural enemies may be a major factor in maintaining the high diversity of tree species in tropical forests: disproportionately high mortality of undergrowth of the same species should be observed near mature trees, since the parent tree is a rich source of species-specific phytophages. If the probability of renewal of the same species is low next to a mature tree, the chances of other species settling there increase, and, consequently, an increase in the diversity of the community. Let us note, however, that if eating specialized in a certain type of food favors the diversity of tropical ecosystems, it still will not be its main cause, for it is itself a property of them.

In addition, diversity has been associated with increased productivity from the poles to the tropics. In the case of the heterotrophic components of the community, this seems to be true: a decrease in latitude means a wider range of resources; a larger selection of their types, presented in sufficient quantities for operation. But is this explanation true for plants?

If increased productivity in tropical areas means "more of the same" (light, for example), then one would expect a reduction rather than an increase in species richness. At the same time, more light can also mean an increase in the range of light regimes, and due to this an increase in diversity, but this is just an assumption. On the other hand, plant production is not determined by light alone. Soils in the tropics tend to be poorer in nutrients than in the temperate zone, so tropical species richness could be considered the result of low environment productivity. Tropical soil is depleted in nutrients, since most of them are contained in a huge biomass, and the decomposition and release of nutrients proceeds relatively quickly here. So the argument related to "productivity" should be formulated as follows. Illumination, temperature and water regime of the tropics determine the presence of a large (but not necessarily diverse) plant biomass. This leads to the formation of poor soils and possibly a wide range of light regimes, which in turn leads to a wide variety of flora. Of course, this is no longer simply an explanation of latitudinal trends in diversity by "productivity".

Some ecologists have attributed the high species diversity in the tropics to climate. Of course, in the equatorial regions there is not as pronounced seasonality as in the temperate zone (although in the tropics, in general, rainfall may follow a strict seasonal cycle), and for many organisms conditions are probably more predictable (although this assumption is extremely difficult to test, since the “predictability” of the environment for it largely depends on the size of the body and the time of generation of each species). The assertion that a climate with less seasonal fluctuations promotes a narrower specialization of organisms has been repeatedly tested in recent times.

Carr, for example, compared the state's bird communities. Illinois (temperate climate) and tropical Panama. Both the shrub formations and the forests of the tropics host many more breeding species than comparable temperate ecosystems, with 25 to 50% of the increase in species richness coming from specialized frugivorous forms, and the other part from birds feeding on large insects. , which are only available in the tropics throughout the year. Thus, the presence of some food sources creates additional opportunities for the specialization of tropical avifauna. In contrast to birds, two groups of beetles, namely bark beetles and wood beetles (families Scolytidae and Platypodidae) in the tropics are not as narrowly specialized in forage plants as in temperate regions, despite the fact that the number of their species in the tropics is much greater.

Finally, as a reason for the high species richness of tropical communities, their greater evolutionary age was put forward. As mentioned above, this theory is quite plausible, but its validity still needs to be proven.

In general, it has not yet been possible to clearly and unambiguously explain the presence of a latitudinal gradient in species richness. This is hardly surprising. The elements of a possible explanation - trends associated with productivity, climate stability, etc. - are by themselves still far from completely clear to us, and yet at different latitudes they interact in different ways with each other and with others, sometimes oppositely directed. forces. However, the explanation may turn out to be very simple - and here's why. Imagine that there is some external factor that contributes to the establishment of a latitudinal gradient in species richness, for example, among plants. Then an increase in the volume, diversity and heterogeneity of resource distribution will stimulate the growth of species richness of phytophages. Consequently, their influence on plants will increase (causing a further increase in the diversity of the latter) and the diversity of resources for carnivorous forms will increase, which, in turn, will increase the pressure of predation on phytophages, etc. In short, a small external force can create a cascading effect, eventually leading to a well-defined diversity gradient. However, we do not yet have conclusive data on what could trigger such a reaction.

Part of the problem lies in the many exceptions to the general pattern. It is clear that explaining their presence is just as important as the general trend. One of the large categories of such evasive communities is the islands. In addition, deserts are very species-poor even near the tropics, possibly due to their extremely low productivity (associated with lack of moisture) and extreme climatic conditions. The salt marshes and hot springs are relatively poor in species, although the production of these communities is high; apparently, the point here is the severity of the abiotic environment (and in the case of sources, also the "island" nature of these small habitats). It has been shown that the species richness of neighboring communities may differ simply because they are subjected to physical disturbances with different intensity.

Height. In terrestrial habitats, the decrease in species richness with height is as common a phenomenon as its decrease with distance from the equator. A person climbing a mountain near the equator will first pass the tropical habitats at the foot, then alternately pass through climatic and biotic zones, strongly reminiscent of the nature of the Mediterranean, temperate and Arctic regions. If the climber also happens to be an ecologist, he will most likely notice how the number of species decreases as he climbs. This has been described in the birds of New Guinea and the higher vascular plants of the Nepalese Himalayas.

Therefore, at least some of the factors that determine the latitudinal gradient of diversity should also play a certain role in the formation of the dependence of diversity on altitude (this, apparently, does not apply to evolutionary age and is less likely for climate stability). Of course, the problems that arise in explaining the latitudinal trend remain here, and one more circumstance is added to them. The point is that high mountain communities almost always occupy a smaller area than the corresponding lowland biomes and, as a rule, are more isolated from similar ecosystems, without forming extended continuous zones. Naturally, the limited surface and isolation cannot but contribute to the reduction of species richness with height.

Using the example of landscapes with a slight difference in heights, it has been established that the number of species can differ quite sharply in depressions and on mounds of highly rugged terrain (meadow). It is worth paying attention to what serious fluctuations in the composition and diversity of the biota can be observed in a very small area, i.e., apparently, within one community.

Depth. In the aquatic environment, changes in species diversity with depth occur in much the same way as on land with height. Naturally, there are fewer species in the cold, dark, and oxygen-poor depths of large lakes than in the thin surface layer of water. Similarly, in the seas, plants are found only in the euphotic zone (where photosynthesis is possible), rarely going deeper than 30 m. Therefore, in the open ocean, there is a rapid decline in diversity with depth, disturbed only by some, often bizarrely shaped animals that live on the bottom. It is interesting, however, that the change in species richness of benthic invertebrates with depth does not follow a smooth gradient: at a depth of about 2000 m, a diversity peak is observed, approximately corresponding to the continental slope boundary. It is thought to reflect an increase in the predictability of the environment from 0 to 2000 m depth. Deeper, beyond the continental slope, the species richness decreases again, probably due to the extremely scarce food resources of the abyssal zone.

Succession. cascading effect. Some geobotanical works indicate a gradual increase in species richness in the course of succession, up to the climax or to a certain stage, after which the depletion of the flora follows as some late successional species disappear.

To a certain extent, the successional gradient in species richness is a natural consequence of the gradual colonization of the site by species from the surrounding communities that are at the later stages of succession, i.e. increase in saturation with species . However, this is far from a complete explanation, since the essence of succession is not in the simple addition of species, but in their change.

As with other gradients, succession inevitably has a cascading effect. In fact, you can imagine that she is this cascade effect in action. The first species will be those that are better than others able to populate free spaces and compete for them. They immediately represent previously absent resources and provide a heterogeneous environment. Thus, pioneer plants create areas of soil depleted in nutrients, increasing the spatial heterogeneity of the concentration of plant nutrients. The plants themselves expand the set of microhabitats and the food spectrum for phytophagous animals. Increased grazing and then predation through feedback can contribute to a further increase in species richness, providing an ever greater choice of food resources, increased environmental heterogeneity, etc. In addition, temperature, humidity and wind speed in the forest are much less variable than in early successional communities, and increasing environmental constancy can provide stable conditions and resources that allow specialized species to settle and establish themselves. Indeed, a number of data confirm this concept, for example.

As with other gradients, it is difficult to separate cause from effect. And yet, in the formation of a successional gradient of diversity, the close interweaving of causes and effects seems to be the very essence of the problem.