12.3Coexistence of Biocenoses and Diverse Species
Trophic Levels and Food Chains
In the natural world, a multitude of species coexist in one habitat. The whole of individual groups of species linked to each other by interspecific interaction is referred to as a biocenosis. A biocenosis comprises plants photosynthesizing organic compounds from inorganic compounds, some animals—namely herbivores—feeding on the plants, and, on top of them, carnivores feeding on the herbivores. Such classification is referred to as trophic levels, and these relationships linked by predator-prey processes are called food chains. In addition to such predator-prey relationships, interspecific competition arises between organism species that require similar food and habitats. Consequently, connecting these interactions between concerned species in a biocenosis with lines creates an intricate mesh-like structure, that is to say, many interaction networks constitute a biocenosis.
In researching biocenoses, it is impossible to target all the species inhabiting the same areas. It is therefore often the case to focus on a group of organism species utilizing similar food sources (i.e., a guild) along with prey and predators associated closely with the group as the representative of a biocenosis. Figure 12-6 depicts where 3 species of Pieridae utilize Brassicaceae plants, as well as by which predators (parasitoid wasps or parasitoid flies) exploit them.
Coexistence of Diverse Species Constituting Biocenoses
There are two major theories to explain the reason for the coexistence of diverse species in a biocenosis. One is called the biocenotic theory based on the aforementioned niche differentiation, which attributes the main factor behind the formation of biocenoses in the natural world to interspecific competition. This theory contends that since the natural world is in equilibrium between the supply and demand of resources, various species are able to coexist by diversifying the manner in which they utilize food and habitats, i.e. niches, to avoid competitive exclusion. The typical examples include Figures 12-3, 12-4, and 12-6 shown above.
The opposing viewpoint to this theory argues that the population density of each individual group is suppressed to a much lower level by fluctuations in climate and predation action by predators than by the strong influence of interspecific competition. The ratio of supply to demand of resources is much lower than 1, and it is rare for it to reach as high a level as causing exclusive competition. The abundance of food and habitats therefore enables a multitude of competing species to coexist without the need for distinct niche differentiation. This hypothesis is referred to as the non-equilibrium coexistence theory.
An Example in Support of Nonequilibrium Coexistence Theory
A research on an intertidal community provided an example of predators impeding competitive exclusion, thereby helping multiple species coexist without niche differentiation. Normally, this community is a biocenosis that consists of diverse species with starfish being the top predator (Fig. 12-7). As a consequence of artificially removing the starfish from the research zone, however, barnacles occupied most of the rocky area after three months, and then mussels dominated the rock surface rapidly after one year, leaving a sparse population of predatory conches, Thais clavigera. Having been expelled from the rock surface, the population of algae plummeted sharply, thus prompting chitons and limpets that feed on them to disappear. In this intertidal zone, competition was so fierce that the starfish was feeding on the highly competitive mussels and barnacles to prevent these species from dominating the surface of the rocky area. A hypothesis in which predators facilitate the coexistence of multiple species by impeding the dominance of competitive species is called the predatory theory.
Fig. 12-8. Disturbance by Waves in a Coral Reef and the Number of Coexisting Coral Species
Revised from Cornell, J. H.: Science, 199: 1302, 1979
Disturbance by climatic changes can sometimes promote coexistence of multiple species depending on its degree. Figure 12-8 illustrates a study comparing the number of coral species in an Australian coral reef between the northern slope, which is prone to coral damage due to cyclone waves and the southern slope, which is free from the damage. The cover degree of the living coral was in inverse proportion to the extent of the damage incurred by the waves. This figure demonstrates a tendency that the places with about 30% cover degree of the living coral had the most diverse coral species with a greater or lesser extent of influence of the waves decreasing the number of coexisting species. This therefore suggested that mesoscale disturbance could promote the coexistence of multiple species by preventing biocenoses from reaching equilibrium in order to give rise to competitive exclusion (the mesoscale disturbance theory)
Plants play the roles of supplying organic material to animals as their food and of providing them with habitats. The presence of plants therefore is crucial in natural ecology. A group of plants is referred to as a plant community. A plant community consists of a number of species, of which species that cover the wide expanses of the ground surface or that have many individuals are called dominant species.
As mentioned in the beginning of this chapter, organisms living in an environment spontaneously exert an environment-forming effect on the environmental conditions. Consequently, the environmental conditions in a biocenosis constantly change with the presence of its constituent species, thereby promoting the entry of new species. The transition of constituent species in a biocenosis in such a manner is referred to as community succession.
There are two types of ecological succession of land plant communities: primary succession starting from the state completely devoid of any soil that would permit growth of plants, e.g., a stratum of a lava plateau formed after volcanic eruption, containing neither plant seeds nor stalks, and secondary succession proceeding from lands such as logged areas of forest and fallow lands. Let us explain primary succession by referring to the case in Miyakejima that has had volcanic eruptions in 20-year cycles for the past 70 years (Fig. 12-9). As weathering of the country rock progresses, lichens and mosses that are tolerant to dryness infiltrate the lava plateau. Taking advantage of the depressions in the ground where the mixture of the remains of these plants and weathered soil accumulates slightly, perennial plants such as Japanese knotweed and Japanese pampas grass start taking root and spreading sporadically. When herbaceous plants make inroads, organic compounds and nutrient salts increase in the soil as a result of decomposition of dry grass, thus steadily forming nutrient-rich soil. At around this stage, fast-growing, shade-intolerant trees such as Mallotus japonicus and Alnus sieboldiana start invading the woods, which all soon change into the sun tree woods.
However, the formation of woods darkens its floor gradually. This triggers the invasion of shade-tolerant trees such as Castanopsis sieboldii and Machilus thunbergii, which are capable of growing even under dark conditions, albeit with slower speed. They replace the shade-tolerant trees by degrees, eventually establishing the shade tree woods. Once that occurs, the lower layer of woods becomes so dark that shade-intolerant trees are unable to grow any longer, thus permitting only shade-intolerant trees to constitute the woods in a stable manner. This state is referred to as climax. The type of climax differs depending on the climatic conditions of a particular region. In Japan, evergreen broad-leaved forests are present in the warm-temperate zone to the west of the Mainland Japan, summer-green forest in the temperate zone to the north of the eastern Mainland, and coniferous forest in the subfrigid zone.
The biomes of the world (cohesiveness of the landscape of a large area with regard to the vegetation) are also sorted out by air temperature and precipitation rate (Fig. 12-10). While temperate regions like Japan are positioned in the center of Figure 12-10, across the globe there are various biomes which extend from arid regions to the far north.
Soil Animals as Decomposers
With succession of a plant community, the state of the soil undergoes great changes, which are manifested most noticeably among soil animal communities (Column Fig. 12-1). Only a small number of species such as orbited mites and springtails (which decompose fallen leaves) appear at the earliest stages of succession. When the succession progresses a little, bacteria and fungi start decomposing fallen leaves and dead branches to form soil along with weathered country rock, which become humus. This stage further increases both the number of the species and the population density of these mites and springtails, thus promoting further decomposition of humus. When the further progress of succession expands the plant community, the amount of fallen leaves increases drastically, thereby producing damp humic soil. By this stage, a soil animal community consisting of copious species such as Campodeidae, sow bugs, Gammaridae, and larvae of predatory insects in addition to mites and springtails is formed.
Humus used in organic farming methods is composed of fallen leaves and branches decomposed by soil animals, fungi (molds and mushrooms), microorganisms, etc. in woods and meadows.
Conservation of Tropical Forests
In the torrid zone centering around the equator, a variety of tropical forests such as tropical rainforests, tropical seasonal forests, and tropical savannah forests, as well as mangrove forests are distributed. They account for nearly half of the forest area on the ground (approximately 17 million km2). An enormous array of species is estimated to inhabit these tropical forests, especially, rainforests. In recent years, tropical forests have been disappearing without cease; e.g., the area of tropical forests vanished during the 15 years from 1980 amounted to 1.214 million km2 (Column Table 12-1). This is equivalent to about 5 times the land area of Japan (approximately 370 thousand km2). The main causes of the disappearing rainforests are attributable to haphazard swidden cultivation, mass utilization as fuel sources, commercial logging, etc. In traditional swidden cultivation, burnt fields would be laid fallow for around 20 years. According to the current practice, on the other hand, no fallow periods are being observed, leading to forests being reduced to wasteland without any chances of recovery. Since tropical soil contains vigorously active microorganisms, organic matter such as fallen leaves is promptly decomposed and absorbed into trees, thus leaving only a subtle amount of nutrient salts in the soil. Consequently, once torrential rains erode away the surface soil, only the substrata remain in the burnt fields. This turns the fields into bare grounds where plants can scarcely grow, rendering it difficult for the tropical forests to recover. Furthermore, heavy precipitation on the bare grounds after logging and consequent soil erosion will have a major impact on marine ecosystems such as coral reefs.