Awb

Other conifers

a 49i Vaccinio-Piceion

Alnion glutinoso-incanae ! 38 • 37

Alno-Ulmion are able to occupy the same habitat. Relations between ecological potential of species and the resource potential of their habitat were considered. Spatial dimensions are just as important as nutrition.

In Elton's (1927) niche concept the coexistence of species in a niche is possible as soon as these species differ significantly in their demands, but are otherwise linked, e.g. via a nutrition chain. Niches are a micro-habitat linked to functional aspects; there are similar niches in every ecosystem which may be occupied by different species with the same function. Elton speaks of the "profession" of individual species, now called site equivalence. Finally, he not only considers the environmental demands of organisms in a niche, but the influence they have on their environment, particularly regarding the use of resources.

Hutchinson (1978) introduced another niche concept, separate from the ambiguous connections with spatial dimensions, defining a niche as the sum of all environmental factors influencing an organism. In an n-dimensional coordinate system where each axis represents an environmental factor, a virtual habitat may be defined in which an organism is able to exist and function in relation to its requirements (Cody 1991). This ecological niche offers the required abiotic and biotic factors. A niche is thus a system in which the organism occupies a permanent position ('job', role, status, address) and should not be confused with an actual space (e.g. ecotope) nor with the conditions in a site. The term niche shows that each organism has its place in an ecosystem and plays a certain role there with particular functions. In such a system new species may only settle when the niche is empty.

Competition arises in such an ecological niche when several organisms occur in the same niche at the same time, making demands on the same resources. It is thus important to differentiate between fundamental and attained niches. The former characterises a niche where an organism has unrestricted access to all available resources which are used to achieve particular functions. The attained niche is the area actually occupied by the organism, with sharing of resources and achieving certain functions in supplementing ways. This is shown in Fig. 4.3.26 where the niche of a plant is outlined three-di-mensionally, corresponding to three selected resources. Each point in the space, resulting from

Moisture

Moisture

":ig. 4.3.26. Schematic representation of the niche of a plant species in relation to the resources of moisture, light and size of the pollinator. (After Howe and Westley 1993)

the variables light, moisture and size of pollinating bees, is shown in the virtual space and represents the conditions under which the plant is able to survive and propagate. In reality this space is reduced as bees also pollinate other plants so this resource has to be shared.

In interspecific competition several species have the potential to use the same niche, but are restricted to an attainable niche; consequently, with several competing species, the attainable niche becomes smaller for each. In contrast, intraspecific competition, and the resulting strengthening of the affected species, causes consolidation of the dominant species or expansion of the attainable niche. It is, however, also assumed that niches may overlap. According to Schoener (1985), this overlap is only slight if the competing species have 'arranged' themselves previously. If, however, there is a strong competition in small habitats (e.g. for nutrients), then niche overlap is possible. In large areas of resource-rich habitats, overlap will be small even if competition is strong. The actual niche is the product of intraspecific and interspecific competition. The niche structure of a plant community is the result of a long evolutionary adaptation in the sense of synevolution (Dierschke 1994). Competition may have been important in the past, even without differentiation of niches, which decreases interspecific competition but increases intraspecific competition.

To make the multidimensional, theoretical term 'niche' more practical, Grubb (1977) modified the concept distinguishing habitat niches, life form niches, regeneration niches and pheno-logical niches. Habitat niches are characterised physically and chemically, life form niches have close relations to production, the regeneration niche comprises all conditions for propagation, and the phenological niche is determined by temporal development. Temporal niches occur for species flowering at different times but at the same site and having the same pollinator; this also applies to spring geophytes in nemoral forests regarding the use of light.

Terms such as spatial niche (e.g. stones in a forest occupied by lichens and mosses) or structural niches (e.g. the habitat in dead wood) are also used, and there are nutritional niches and breeding site niches, together with niches for species and individual plants. This inflation of terms contributed to criticism of the niche concept. However, it should be remembered that, even though the concept of the ecological niche was questioned, it enriched discussions on basic questions of competition and coexistence, in connection with invaders (neophytes) and the question of 'open positions'. In this context, the principle of competitive exclusion (Gause 1934) is very important; this states that two species with identical demands cannot coexist in the same living space. The counter argument is, however, provided by niche overlap of species. Similar to the question of niches, the key phrase 'competitive exclusion' also leads to basic concepts of coexistence.

Competitive Exclusion or Coexistence?

The consequence of the principle of competitive exclusion enunciated by Gause (1934) is that species with identical ecological demands are not able to exist side by side. Thus, sympatric species are ecologically distinguished and do not compete.

Lotka (1932) and Volterra (1926) developed models supporting this principle mathematically. They assumed that each further individual arriving in an existing population (or community) worsens the existence and growth conditions as a whole. In this model of competition, growth rates of two species are related to each other in an equation. Gause established his ideas with Paramecium, but they are also applicable to plants arriving in an existing plant community (Law and Watkinson 1989).

Aarssen (1983), in particular, considered this principle of competitive exclusion postulating that one condition required for competition exclusion was a long period with constant conditions. However, these do not exist, because of continuous environmental changes. Endogenous developments and exogenous interference lead continuously to the formation of new niches, enabling competing species to coexist and avoiding competitive exclusion. Permanent niche differentiation is an important process. Positive interactions which exceed the negative influences of competition lead to a (dynamic) contact between individuals and species, and to coexistence.

Aarssen (1983, 1989) developed three hypotheses which would lead to avoidance of competition and thus guarantee coexistence. In the first, the balance between supply and demand is decisive for the occurrence of species. Exclusion is only possible when supply is smaller than demand. The second relates to the limited demands of several species for the same resource: If the demands of species are focused on a particular resource, exclusion is the consequence, but not if the resources are different. The third hypothesis relates to the strength of competition: If this is the same or similar, coexistence is possible. These three hypotheses are the building blocks of Aarssen's general evolution theory of coexistence. His idea of ecological combining ability (niche differentiation) and competitive combining ability (competitive strength) aims at the evolutionary aspects of conditions at the site and the general conditions important for the coexistence of organisms. The classical theory of competitive exclusion is contrasted against a modern theory in which spatial and temporal scales of coexistence are incorporated with the same characteristics of competition. Differences in demographic processes have been little noted, nor the distribution of niches under aspects of patch dynamics, or succession and regeneration.

Hulme (1996) listed other theoretical approaches to explain coexistence (Table 4.3.5). They may be grouped into those where competition is regulated by resources, and those where competition is linked to the concept of ecological niches and niche differentiation ("peaceful niche sharing"). These results from the 1970s and 1980s are still discussed and show that there is still no unanimous theory or final clarification of the interconnections between competition and coexistence.

Growth of plants in close proximity cannot occur without mutual interference. Competition alone is not the decisive factor, as many species obviously do not compete with each other, but coexist. Competition and coexistence should not be regarded as opposites. Coexistence may be

Table 4.3.5. Summary of mechanisms often advanced for the coexistence of plants in natural communities. (After Hulme 1996)

Shorthand term for

General description

Most important assumptions

Author

theory

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