Competitive exclusion & Niche concept

Transcription

1 Competitive exclusion & Niche concept [Academic Script] Subject: Course: Paper No. & Title: Zoology B.Sc. 3 rd Year Z-301B Ecology Topic Title: Topic - 5 Competition in nature intraspecific and interspecific. Niche concept. Competitive exclusion Lecture No. & Title: Lecture 2 Competitive exclusion & Niche concept

2 Academic Script 1.Introduction During this lecture, we will learn about competitive exclusion principle and niche concept in detail. Now, let us get the detailed knowledge about Competitive exclusion principle. In laboratory experiments of Gause, made on Paramecium species (i.e. P. caudatum and P. aurelia), as given in the figure, it has been found out that both the species approximate to logistic growth with sigmoidal growth curve when cultured independently. But in mixed cultures, P. aurelia survives and P. caudatum dies out. It has been proved by the experiments of Crombie (1947) that habitat diversification can reduce competition and allow coexistence instead of exclusion. He had carried out the experiment on two different species of flour beetle Tribolium and Oryzaephilus. He observed that in finely ground wheat flour, Tribolium always displaces Oryzaephilus but when cracked whole grains are used, both the species coexist indefinitely because Oryzaephilus larvae pupate within the grains, thereby escaping from Tribolium. This effect of physical protection for the pupae was demonstrated further by using ground flour to which capillary tubes were added. The Oryzaephilus pupated inside the tubes and both populations persisted. Thus, when a simple one-

3 niche environment is changed to two-niche environment, the competition is reduced sufficiently for the support of two species. Based on such results and experiments made on paramecium species, Gause suggested the hypothesis which has been restated by Hardin (1960) as the competitive exclusion principle. Competitive exclusion principle in its simplest form states that complete competitors can not coexist. This concept is considered as the cornerstone of ecology which suggests that ecological segregation can lead to reduction in competition and permit coexistence. The competitive exclusion principle explains that two species that use the same resource in the same way in the same space and time can not coexist and must diverge from each other over time in order for the two species to coexist. One species will often exhibit an advantage in resource use. The superior competitor will out-compete the other with more efficient use of the limiting resource. As a result, the inferior competitor will suffer a decline in population over time. Ultimately, it will be excluded from the area and replaced by the superior competitor. Thus, one of the two competitors will always overcome the other, leading to either the extinction of this competitor or an evolutionary or behavioral shift towards a different ecological niche. 2. Competition & Ecological niche

4 Let us get an overview of the relation between competition and ecological niche. Competition amongst the organisms is also found in the ecological niche. A useful extension of the ecological niche concept is the distinction between fundamental and realized niches as per the figure. The fundamental niche of a species includes the total range of environmental conditions that are suitable for existence of an organism without interspecific competition or predation from other species. In this case, an organism is free of interference from other species and can utilize full range of available resources to survive and reproduce. But in actuality, as a result of interspecific competition, species are usually forced to occupy a niche that is narrower than this, and to which they are highly adapted. This is termed as the realized niche. Thus, realized niche describes a part of the fundamental niche actually occupied by the species. Closely related species of competitors will have similar requirements along the niche dimensions so that their niches overlap one another. If the fundamental niche of one species completely overlaps that of the other, then one of the species will be eliminated according to the competitive exclusion principle. If the niches overlap partially, coexistence is possible in two ways: Either one species fully occupies its own fundamental niche, excluding the second species from part of its fundamental niche and leaving it to occupy a smaller realized niche or both species have restricted realized niches, each utilizing a smaller range of particular niche dimension than they would in the

5 absence of other species. Niche theory leads to ecological segregation and can be regarded as an extension of Gause s hypothesis. 3. Niche Concept Now, let s move on to the study of niche concept. The word niche is derived from the French word nicher, meaning to nest. The term niche was for the first time coined by Joseph Grinnel (1971) to explain microhabitats. The general meaning of ecological niche is a place which not only involves the physical space occupied by an organism but also its functional role in the community and its position in the environmental gradients including other conditions of existence. According to Grinnel, niche is the ultimate distributional unit, within which each species is held by its structural and instinctive limitations... no two species in the same general territory can occupy for long identically the same ecological niche. Odum has considered niche as the profession of an organism. According to Charles Elton, Niche is the functional status of an organism in its community. Ecological niche of any organism concerns not only the space where it lives but also what it does (i.e. how it transforms energy, responds and modifies its physical and biotic environment) and how it is constrained by other species. 4. Ecological Niche

6 There are three different aspects of ecological niche: (i) Spatial or habitat niche which refers to the physical space occupied by an organism (ii) Trophic niche which suggests the functional role of an organism at particular trophic position and (iii) Multifactor or hypervolume niche which designates the position of an organism in the environmental gradients. Let s understand all the three aspects one by one. (i) Spatial or Habitat Niche This type of niche is concerned with the physical space occupied by an organism. O Neill in the year 1967 has provided a good example of spatial niche in the distribution of seven species of millipede in forest floor of a maple-oak forest. All the seven species occur in the same general habitat i.e. forest floor and all belong to the same basic trophic level (i.e. all are decomposers in their role as detritivores). But for searching a particular millipede species, one needs to examine different layers of the forest floor. Each species of millipedes indeed predominates in its own specific different microhabitat. There exist several gradients in decomposition stage from the centre of the log to the position underneath the leaf litter, and these gradients constitute distinct microhabitats in the same general habitat, the forest floor. Thus, Euryurus erythropygus predominates in the heartwood at centre of logs, Pseudopolydesmus serratus in superficial wood of logs, Narceus americanus in the outer surface of logs beneath the bark, Scytonotus granulatus under log but on log surface, Fontaria virginiensis under log but on ground surface, Cleidognia caesioannularis within leaves of litter and Abacion lacterium beneath litter on ground surface.

7 (ii) Trophic Niche It is concerned with the trophic position of an organism. In this case, two species live in the same habitat but they occupy different trophic niches, because of differences in food habits. Finches of Galapagos Islands in America provide a very good example of trophic niche. These birds belong to three genera, viz., Geospiza (ground finche), Camarhynchus (tree finche) and Certhidia (warbler finch). These all birds live in the same general habitat, but differ in terms of their trophic position. For example, in tree finches, one of the species, Camarhynchus crassirostris has a parrot like beak and is basically vegetarian, feeding on buds and fruits. Rest of the tree finches are insect eaters, three of which feed on insects of different sizes. Similarly, the occurrence of weaver birds in vicinity of Lake Mweru of Africa is another good example of trophic niche. All of three weaver birds differ in their choice for food, thus occupying different trophic niches. Of them, two, Ploceus melanocephalus and Ploceus collaris, living in same nest, one is insect eater and the other feeds on seeds. Another example is of two aquatic bugs, Notonecta and Corixa which live in the same pond but occupy different trophic niches. Notonecta is an active predator, whereas Corixa feeds largely on decaying vegetation. (iii) Multifactor or Hypervolume Niche

8 Zoologist of Yale University G. Evelyn Hutchinson in the year 1965 developed the concept of an abstract n-dimensional hypervolume niche. He suggested that the niche could be visualized as a multidimensional space or hypervolume within which the environment permits an individual or species to survive indefinitely. For example, the niche of a plant might include the temperature range that it can tolerate, the intensity of light required for photosynthesis, specific humidity regimes, and minimum quantities of essential soil nutrients for uptake. Suppose that we measure the range of some environmental variable like temperature over which a particular species can live and reproduce and we put this on a graph. Then we do the same for another environmental variable humidity and put this on the second axis of the graph. The space that is enclosed will represent the niche of the species. If the effects of two variables (i.e. temperature and humidity) are independent, the space would be a two dimensional box. As temperature and relative humidity are not independent in their biological effects, under such condition of interacting variables, the niche would be an ellipse in shape instead of box like structure. Suppose now we consider available phosphorus as the third variable, then niche representing three variables would be three-dimensional. If we now have a fourth variable, the space enclosed would be a hypervolume with four dimensions. Since there are a large number of environmental factors, both biotic and abiotic affecting the population, the niche is n-dimensional hypervolume. This is considered as the fundamental niche of the species. If the fundamental niche of two species overlaps then the two species are considered as competing. Competitive exclusion will be

9 observed in case of the complete overlapping of n-dimension niches. 5. Advantages of Niche Segregation Let s quickly get an overview of advantages of niche segregation. The major advantage which organisms gain by occupying different niches is the escape from continuous intense competition. It provides suitable substratum and microclimate to species for its survival. Niche segregation avoids confusion of activities between organisms in community which permits more orderly and efficient life cycle on part of each species. Segregation into different niches also permits the occupancy of the area by a large number of species, since they will better divide the available resources between them. This also helps the species in avoiding the conflict with its neighbours and leading a life that is orderly, productive and quite efficient. Thus, we can say that competitive exclusion principle and niche concept are two important aspects of ecology very closely associated with each other. 6.Summary In this particular part of lecture, we have studied two important aspects of ecology. In the starting, we have understood competitive exclusion principle in brief. Then we have got an

10 overview of the relation between competition and ecological niche. We have also differentiated between fundamental niche and realized niche of an organism. During this lecture, we have also covered the niche concept in detail. We have understood the general meaning of niche along with three different aspects of ecological niche as spatial niche, trophic niche and hypervolume niche with appropriate examples.