Influence of competition on community structure
Competition has an important influence on the formation of community structure. The most direct evidence of the role of interspecific competition in the formation of community structure may be the introduction or removal of species in natural communities. Schoner and Cornell made statistics on the literature about interspecific competition (184 and 72 cases respectively), with an average of 90% of the cases explaining interspecific competition, indicating that competition is quite common in natural communities. Their results also show that the interspecific competition ratio of marine organisms is greater than that of terrestrial organisms; The height between large organisms is higher than that between small organisms; The competition among herbivorous insects is low, because there are green plants everywhere, which are abundant and rarely eaten up, so the possibility of competing for food resources is relatively small.
Existing evidence shows that competition is an important driving factor for community formation. However, the importance of competition is obviously different among multiple communities, and it is often only a small part of the interaction between species.
So why do many survey results show that competitiveness is often not strong?
It is generally believed that: ① natural selection may have effectively avoided competition through niche division (or erased the traces of past competition); (2) In an environmental patch, competitive species exist because they don't use the same resources; (3) When resources are scarce, species may only compete in population outbreaks.
Effects of Predation on Community Structure
The influence of predation on the formation of community structure varies with whether the predator is a generalized species or a specialized species. For generalized species, predation eases interspecific competition and promotes diversity. However, when the feeding intensity is too high, the number of species will also decrease. For specialized species, it varies depending on whether the species chosen to eat is superior or inferior. If you choose dominant species, predation can improve diversity, and if predators like to eat inferior species with weak competitiveness, diversity will decrease with presentation.
Influence of disturbance on community structure
Interference is a common phenomenon in nature. Disturbances, unlike disasters, do not cause great damage, but they often occur repeatedly, leaving species with insufficient time to evolve. Most modern ecologists believe that disturbance is a beneficial ecological phenomenon, which causes the unbalanced characteristics of the community and emphasizes the role of disturbance in the formation and dynamics of the community structure.
(1) Interference and the gap between communities
It is a very common phenomenon that gaps appear in continuous communities, and gaps are often composed of interference. Gaps in the forest may be caused by strong winds, lightning, logging, fire, etc. The disturbance of grassland community includes grazing, animal digging and trampling. After the disturbance causes the gap in the community, some will gradually recover without continuing the disturbance, but the gap may also be occupied by any species in the surrounding community and develop into a dominant species. Who is the winner depends entirely on random factors, which can be called the lottery competition for the gap.
However, the gaps formed by some communities have regular species replacement. The newly opened gap is often invaded by one or several pioneer species with strong diffusion ability. Because of their activities, the conditions were changed, which promoted the invasion of middle succession species and was eventually replaced by top species. In this case, the diversity began to decrease and increase in the middle of succession, but it often decreased slightly at the top.
② Moderate interference hypothesis
The frequency of gap formation affects species diversity, and accordingly, T.W.Connell and others put forward the hypothesis of moderate interference, that is, moderate interference level can maintain a high degree of diversity. The reason is:
First, after a disturbance, some pioneer species invaded this gap. If the disturbance is frequent, the pioneer species cannot develop to the middle stage of succession, so the diversity is low;
Second, if the interference interval is long, the succession process can develop to the top stage, and the diversity is not very high;
Third, only moderate interference can maintain the highest level of diversity, thus allowing more species to invade and settle down. Examples of similar modes of action Predators inhibit competitors' oceans: sea otters, sea urchins and land: herbivores that selectively eat seeds according to their size inhibit competitors, elephants, rabbits, pathogens and parasites inhibit predators, herbivores, slime molds and tsetse flies, and competitors inhibit species replacement in the succession of competitors. For example, the dominant tree species and weed species in the forest have been effectively propagated. Pollinators and disseminators of plant resources that depend on key species physically interfere with the nutrient transfer rate of rabbits, gophers, termites, beavers and hippos. The environment of nitrogen-fixing bacteria and mycorrhizal fungi decomposers is uneven. The higher the degree of spatial heterogeneity, the more diverse niches there are, so more species can be allowed to exist.
A large number of data in plant community research show that there will be more species in communities with rich soil and terrain changes; The community diversity of flat homogeneous soil is low. Harman studied the correlation between species number of molluscs in freshwater system and spatial heterogeneity, and obtained a positive correlation result that the more habitat types, the more species of freshwater molluscs.
By studying the relationship between bird diversity and plant species diversity and feeding diversity, MacArthur found that the correlation between bird diversity and plant species number is not as obvious as that between bird diversity and feeding-high diversity. Therefore, for bird life, the layered structure of vegetation is more important than species composition. In grassland and shrub communities, the vertical structure is not as obvious as the forest community, while the horizontal structure, namely mosaic and patch, may play a decisive role.
As can be seen from the table, each plant community consists of several life forms, but one life form is dominant, which is closely related to the environment. The dominance of high-bud plants is the characteristic of communities in warm and humid climates, such as tropical rain forest communities; The dominant community of ground bud plants reflects the long cold season in this area, such as temperate coniferous forest and deciduous forest community; Underground bud plants are dominant, reflecting that the environment in this area is relatively wet and cold, such as the dark coniferous forest in the cold temperate zone of Changbai Mountain; The dominance of annual plants is the characteristic of desert grassland community in arid climate (such as temperate grassland in Northeast China). Community type ph.ch.h.cr.t Xishuangbanna tropical rain forest 94.7 5.3 0 0 0 Dinghushan subtropical evergreen broad-leaved forest 84.55.444.1.4.1.00 Zhejiang subtropical evergreen broad-leaved forest 76.71.03. 38+0 7.8 2 Temperate deciduous broad-leaved forest on the northern slope of Qinling Mountains 52.0 5.0 38.0 3.7 1.3 Cold temperate dark coniferous forest in Changbai Mountain 25.4 4.4 39.8 26.4 3.2 Northeast temperate grassland 3.62.041.19.033.
Because the island is isolated from the mainland, the intensity of biological species entering and leaving is lower than that of the surrounding continuous continent. Many studies have confirmed that the number of species on the island is closely related to the area of the island. The larger the island, the more species there are. The relationship between the island area and the number of species on the island can be described by a simple equation:
S = cAz
Or expressed logarithmically: lgS = lgc+z (lgA)
Where: s-number of species;
Area a;
Z and c- two constants.
3.4.2 MacArthur's equilibrium theory.
Although the number of species on the island depends on the area of the island, it is the result of the balance of species migration, emigration and extinction, which is a dynamic balance. Species are constantly dying, and species death is constantly compensated by the migration of the same species or other species.
The species balance on the island can be illustrated in Figure 3-2. Taking the mobility curve as an example, when there are no resident species on the island, any individual who moves in is new, so the mobility is high. As the number of resident species increases, the mobility of species decreases. When all species in the provenance bank (species on the mainland) are on the island, the mobility is zero. The extinction rate is the opposite. The more species remain, the higher the extinction rate. Mobility depends on the distance between the island and the mainland and the size of the island. Large islands close to the mainland have high mobility. Far and small island, low immigration rate. Similarly, the extinction rate is also affected by the size of the island.
The number of species at the intersection of mobility curve and extinction rate curve is the predicted number of species on the island. According to the equilibrium theory, the following four points can be explained:
① The number of species on the island does not change with time;
This is a dynamic balance, that is, extinct species are constantly replaced by newly migrated species;
③ The big island can "feed" more species than the small island;
(4) With the distance from the island to the mainland from near to far, the species at the balance point gradually decrease.
Equilibrium theory and non-equilibrium theory
There are two opposing views on the general theory of community structure, namely, equilibrium theory and non-equilibrium theory.
According to the equilibrium theory, species living in the same community are in a stable state. Its central idea is:
(1) * * * Symbiotic species contain each other through interspecific interactions such as competition, predation and mutual benefit;
(2) The biological community has the characteristics of global stability, and interspecific interaction leads to the stability of the community. In the stable state, the species composition of the community and the number of various communities have little change;
(3) The actual changes in the community are caused by changes in the environment, which is called interference, and the interference is gradually decreasing. Therefore, the equilibrium theory regards biological community as a stable entity existing in the ever-changing physical environment.
The main basis of non-equilibrium theory is moderate interference theory. According to this theory, the species that make up the community are always changing, and it is impossible for the community to reach an equilibrium state. Natural communities have no global stability, only resistance (the ability of the community to resist external interference) and resilience (the ability of the community to recover after interference).
Comparing the equilibrium theory with the non-equilibrium theory, there is a basic difference except the emphasis on interference: the equilibrium theory pays attention to the nature of the system in equilibrium, but not enough attention to timeliness and variability; Non-equilibrium theory focuses on the behavior change process of the system near the equilibrium point, with special emphasis on timeliness and variability. Another important difference between these two theories is that communities are regarded as closed systems or open systems.