Principles of Ecology Interspecific population Interactions: Competition

Transcription

1 Paper No. : 12 Module : 13 Development Team Principal Investigator: Co-Principal Investigator: Paper Coordinator: Content Writer: Content Reviewer: Prof. Neeta Sehgal Head, Department of Zoology, University of Delhi Prof. D.K. Singh Department of Zoology, University of Delhi Prof. D.K. Singh Department of Zoology, University of Delhi Dr. Laxmi Narula S G T B Khalsa College, University of Delhi Prof. K.S. Rao Department of Botany, University of Delhi 1

2 Description of Module Subject Name Paper Name Module Name/Title Module Id Keywords Zool 12 Interspecific population Interactions M13 Competition Interference competition, allelopathy, Exploitation competition, Gause s principle of Competitive exclusion, Lotka Volterra equation, Isocline, ecological niche, resource partitioning, character displacement Contents 1. Learning Outcomes 2. Introduction 3. Types of Competition 3.1. Direct or Interference Type 3.2. Indirect or Exploitation 4. Gause s Principle 4.1. Laboratory Studies 4.2. Field Studies 5. Lotka-Volterra Equation 5.1. Isocline Graphical Presentation of Competition 5.2. The Graphical Solutions/Strategies of the Lotka-Volterra Competition Model 6. Field studies - Testing the Model in Natural Settings 7. Competition and Evolution of New Species 8. Summary 2

3 1. Learning Objectives After studying this module, you shall be able to know about: Types of competition Gause s Principle of competitive exclusion Experimental evidences to prove Gause s principle Field examples Lotka Volterra equation for competition Strategies of competition Evolutionary perspectives of competition leading to the origin of new species. 2. Introduction Requirement for food, shelter and protection for the survivor of all living organisms is common in all biologically diverse forms of life in the ecosphere. All organisms depend on the available resources for these requirements. Competition refers to the interaction between the two organisms of the same or different species striving for the same resource. Intraspecific competition is between the organisms of the same species and interspecifc between the individuals of two different species. Intraspecific competition is more severe because of the common requirements of the similar individuals than interspecific competition.. Competition can also be defined as the act of striving against another force for the purpose of achieving dominance or attaining a reward or goal for the better survival. It results in adverse effect on the growth of a population. Ecological significance of competition is to bring about separation of two closely related or similar organisms by the process of dispersal or adaptations Competition is an important step in the process of evolution for the origin of new species 3. Types of Competition There are two types of competitions namely A. Direct or Interference Type B. Indirect or Exploitation Type Both the types are observed in intraspecific as well as interspecific competition. Figure 1: Male-male competition in red deer during rut is an example of intraspecific interference competition. 3

4 3.1. A. Direct or Interference Competition In interference competition, one organism prevents other organisms from using the common resource. Interference competition can occur, particularly where the resource is "patchy" only (occurring in discrete patches) and thus can be defended. An elephant might be able to prevent other animals from using a water hole, but would be unlikely to be able to chase them away from a river with its long banks. It occurs directly between individuals by aggression when the individuals interfere with foraging, survival, reproduction of others or by directly preventing their physical establishment in a portion of the habitat. Interference competition is not restricted to cases where one mobile organism physically chases off another. Even the sessile organisms and plants can practice interference competition. One common method is allelopathy, in which toxic chemicals are produced by one organism - often a plant and directed at potential competitors. For example walnut tree produces toxic chemicals both at the leaves and from the roots that kill competitors in its vicinity. Some sponges and corals likewise engage in "chemical warfare" by putting chemicals into the soil. Similarly the desert plants space themselves by protecting an area from which their roots will absorb infrequent rain water 3.2. B. Indirect or Exploitation Competition Exploitation competition occurs indirectly through a common limiting resource which acts as an intermediate. The excess use of resources depletes the amount available to others, that may be food or space. In exploitation competition one species uses resources directly and makes it unavailable for other species to use. A good example of exploitative competition is found in aphid species competing over the sap in plant phloem. Each aphid species that feeds on host plant sap uses the resource more than required leaving behind less for the competing individual / species. Parrots feed on the fruits of the trees and waste a major part of it not available to others. The resource is misused. 4. Gause s Principle Gause s principle of competitive exclusion or just Gause's principle or the competitive exclusion principle is a proposition which states that two species competing for the same resource cannot coexist, if other ecological factors are constant. When one species has even the slightest advantage or edge over another it will dominate in the long term. One of the two competitors will always overcome the other, leading to either the extinction of its competitor or an evolutionary or behavioral shift towards a different ecological niche. The principle has been paraphrased into "complete competitors cannot coexist". Based on field observations Joseph Grinell, formulated the principle of competitive exclusion in 1904: "Two species of approximately the same food habits are not likely to remain long evenly balanced in numbers in the same region. One will crowd out the other. It occurs indirectly through common limiting resources which act as an intermediate. The over use of resources by one species depletes the amount available to others, that may be food or space. Organisms use up resources directly in exploitation competition. Once used, the resource is no longer available for other species Laboratory Experiments that Support Principle of Competitive Exclusion Russian microbiologist Georgy Gause formulated the law of competitive exclusion based on laboratory experiments using two species of Paramecium, P. aurelia and P. caudatum. He maintained the culture by adding fresh water and a constant input of food every day. Although P. caudatum initially dominated P. aurelia recovered and subsequently drove P. caudatum extinct via exploitative resource competition. P. aurelia has a higher reproductive rate and can also tolerate higher population densities. When a limited amount of bacterial food was provided to the mixed culture P. caudatum died out completely. In another culture he reared P. caudatum with P. bursaria. These two species could co-exist 4

5 because P. caudatum remained suspended in the solution and P. bursaria confined itself to the bottom of the culture tube for feeding on the bacteria. The two species had separate special niche in the same culture. However, Gause was also able to let the P. caudatum survive by differing the other environmental parameters such as food and water. Thus Gause s principle is applicable only if the ecological factors remain constant. Figure 2. Paramecium aurelia and P caudatum growth patterns in independent and mixed cultures. Russian microbiologist G. F Gause studied competition between two species of yeast, Saccharomy cescerevisiae. and Schizosaccharomyces kefir ( also Kephir) He cultured these two species in isolation and found that both of them have a sigmoid growth pattern as described by Verhulst Pearl equation By performing various experiments he concluded that accumulation of alcohol which is its own metabolite kills the new buds separated from the mother and thus limits the population growth. When two species of yeast were grown separately the values of r and K were observed as given in the table 1 ( Ref Charles J. Kreb - page 240) Table 1 Species K r Saccharomyces Schizosaccharomyces Figure 3. Showing population growth of schizosaccharomyces and saccharomyces in separate cultures. 5

6 Figure 4 (a). Growth of Saccharomyces in pure and mixed culture. Figure 4 (b). Population growth of Schizosaccharomyces in pure and mixed culture. Figure 5. Amount of alcohol produced by yeast as a factor of its population growth. 6

7 With respect to the carrying capacity (culture) K 1 ; One volume of Schizosaccharomyces is equal to 3.15 volumes of Saccharomyces. The amount of ethanol produced by the two species of yeast is also different. Schizosaccharomyces is more in size and produces more alcohol. Table 2. Amount of ethanol produced by the two species of yeast. Species % ethanol / cc of yeast ( alcohol production) Saccharomyces Schyzosaccharomyces The value of Alpha and beta can also be calculated from the ratio of the alcohol produced by the yeast species since it is one of the limiting factor α = / = 2.18 β = / = 0.46 Thus Schizosaccharomyces has more negative effect on the population growth of Saccharomyces than vice- versa To study the effect of growth of one species on the growth of the other species, he prepared a mixed culture of the two species of yeast. There was a reduced rate of population growth of each species in the mixed culture as compared to the pure culture. The competition coefficient changes with age of the culture (value of α 21 and α 12 or α and β) as shown in the table 3 Table 3. Change in the competition coefficient with age. Age of the culture (hr) α ( Schizosaccharomyces) β (Saccharomyces) Mean value He found that Schizosaccharomyces kefir consistently outcompeted Saccharomyces cerevisiae by producing a higher concentration of ethyl alcohol. Environmental factors and coexistence of predators and parasites can also change the strategies of competition. For example when the grain beetles Calendraoryzae and Rhizopertha dominica were reared at lower temperature (29.3 degree C) Calendra won over Rhizopertha and at higher temperature (32.3 o C ) Rhizopertha species was the winner (Birch 1953). 7

8 Temperature = 29.1 degrees Temperature = 32.3 degrees Figure 6 (a) & (b). Population growth of two species of grain beetle namely Calendra and Rhizopertha at 29.1 and 32.3 degree celcius Crombie performed another experiment with grain beetle using Rhizopertha and Oryzaephilus. He showed that under overcrowding conditions the two species can coexist by changing their ecological niche. He reared beetle Rhizopertha and Oryzaephilus on wheat and observed that the larvae of Rhizopertha feed inside the grain and that of Oryzaephilus outside. The adults of both the species feed outside the grain, these two beetle populations can coexist indefinitely Field Studies of Competition in Natural Populations In natural populations competition for space is one of the strongest competitions, particularly in sessile organisms. Once a plant or a barnacle or a mussel attaches itself to a place it is very unlikely to move; thus the space is out of play for others until the organism occupying the space dies. In the figure below the open space on the rock has been settled by a number of young (small) barnacles. These shrimp relatives encase themselves in moveable rocky shells that are permanently fixed to the rock. Some larger barnacles were already there, and some of the small barnacles have settled on them. This is not good for them, since the bigger barnacles get all the food (barnacles reach out from their shells to filter food from the water). But the younger ones get very little left over food, once in a while, 8

9 not fulfilling their requirement. The barnacles, try to get an open place before anyone else does settle there. Figure 7. Competition in open space on the rock settled by a number of young (small) barnacles, large barnacles and mussels. Competition for space is so severe that both mussels and barnacles will grow on rocks exposed at low tides, even though this means that they will not be able to feed for part of the day and that they will have to survive the hot sun and the dangers of desiccation. Often, the barnacles or the mussels will outcompete and eliminate the other in certain microhabitats on the rock. Over the surface of the rock the barnacles dominate, but in the crevices (inset) the mussels have the edge. Perhaps the barnacles are more able to withstand the rigors of the open rock but are unable to compete effectively in the moister, protected areas of the crevices. Robert MacArthur s work with wood warblers is a well known example. He found five species of wood warbler living and raising offspring in the same woods. All feed on insects gleaned from surfaces or caught in flight and in general use resources in ways that seemed very similar, an apparent violation of the competitive exclusion principle. MacArthur watched the warblers carefully and monitored their foraging habits. He found that each species had a unique pattern of foraging in a tree. At the top of the trees, Near the ground, In the middle of the trees, Toward the trunk, At the tip of the branch. He found that their foraging behaviors differed in other ways as well. In other words, the species shared resources, and avoided competitive exclusion, by foraging in different microhabitats and employing different foraging behaviors. Another example is observed from the eastern deciduous forests of North America by the nuthatch and the brown creeper. These two birds forage on the same trees for insects. However, the creeper works its way UP the trunk, while the nuthatch forages down the trunk. In this way they see (and eat) an entirely different set of insects. These examples prove the competitive exclusion principle that two species cannot share the same ecological niche. This will be discussed later as coexistence by niche separation. 9

10 5. Lotka-Volterra Equations Theoretically, the competitive exclusion principle is based on a mathematical model developed independently by Vito Volterra and Alfred Lotka.Their construction of the competition model was based on an earlier mathematical logistic model of population growth. One simple version of the logistic model is expressed in the equation: dn/dt = rn((k - N)/K). For two different populations using the same resource the equations can be given as follows dn 1 /dt = r 1 N 1 ((K 1 - N 1 ) /K 1 ) dn 2 /dt = r 2 N 2 ((K 2 - N 2 ) /K 2 ) N is the number of individuals in a population or the population density, K is the number of individuals or population density of a species that the environment can support. K is called the carrying capacity of the environment for that species. The r in the equation is the intrinsic rate of increase of the population (the rate at which the population can increase when there is plenty of food and other conditions are ideal for its growth). dn/dt is the calculus notation for the change in N (population size or density) with time. The d s can be thought of as symbols indicating change: hence dn/dt is the change in population size with change in time. (K - N)/K is the environmental resistance to growth of the population. It includes food shortage, predation, disease, competition, etc. (K - N)/K becomes zero and so does dn/dt, when N = K, that is, when the population size or density reaches the environment s carrying capacity the population stops growing. Without environmental resistance ((K - N)/K), the population would grow exponentially at a rate equal to rn, an unrealistic expectation for any extended period of time, although it could occur for a short time when the population (N) is very small. Under those conditions (K N)/K approaches one and dn/dt approaches rn.( subscript 1 and 2 can be used to indicate population one and two. Starting with the logistic model for population growth, Lotka and Volterra independently set out model for interspecific competition, which is competition between members of different species, as opposed to competition between members of the same species. The latter is called intraspecific competition. In either case, competition occurs when two or more individuals use a resource and that resource is in short supply. Consider the simplest version of the model which treats competition between two species, rather than the more realistic situation of several species competing with one another. Figure 8. This indicates the resource utilization by two species rat and rabbit. Rabbit requires requires double the space than rat α is 2 β (or α 12 = 2 α 21 ) 10

11 Two mathematical equations make up the model based on the logistic equation for population growth for each of the two competitor species. The two species can be designated as species 1 and species 2, and each would have a population size or density (N 1 and N 2 ), an intrinsic rate of increase (r 1 and r 2 ), and the environment would have a carrying capacity for each (K 1 and K 2 ). The two logistic equations, with competition present as an unidentified part of the environmental resistance ((K 1 - N 1 )/K 1 ) and ((K 2 - N 2 )/K 2 ) would be are also affected by the interspecific competition. The impacts of interspecific competition on populations have been formalized in a mathematical model called the competitive Lotka Volterra equation, which gives a theoretical prediction of interactions. It combines the effects of each species on the other. These effects are calculated separately for the first and second population respectively: or = = In these formulae, N 1 and N 2 is the population size, t is time, K 1 and K 2 is the carrying capacity, r 1 and r 2 is the intrinsic rate of increase and α (α 12 and, α 21 or α and β ) is the competition coefficient. The results show the effect that the other species has on the species being calculated. The results can be graphed to show a trend and possible prediction for the future of the species population. Assumptions of Lotka Volterra model are lack of migration constancy of the carrying capacities competition coefficients of both species. The complex nature of ecology determines that these assumptions are rarely true in the field but the model provides a basis for improved understanding of these important concepts. The two equations can be solved graphically, and the solutions give rise to the mathematical-theoretical version of the competitive exclusion principle. Before solving the combined equations, it is important to consider the individual graphs for each equation which are represented as isoclines for each population Graphical Presentation of the Strategies of Competition Isocline: Isocline represents graphically zero population growth for a species. For the species 1, the graph consists of a horizontal (X) axis containing values for the population size of species 1 (N 1 ) and a vertical (Y) axis with the population size of species 2 (N 2 ). The equation is a linear equation, therefore the graph is a straight line with constant slope. The model s solution is based on the 11

12 zero growth lines for each species on this graph. Consider species 1 alone, and the conditions that result in dn 1 / dt = 0. Two points on the line are readily attainable from the equation for population 1 s growth. These are the intercepts of the zero growth line on the X axis where N 2 = 0 and N 1 = K 1 ; and on the Y axis where N 1 = 0, N 2 = K 1 /α (Figure 1). Figure 9: The graph of species 1's zero growth line in competition with species 2. (α 12 is also written as α and α 21 as β). When N 1 = K 1 and N 2 = 0; dn 1 /dt = 0 As well when N 2 = K 1 /α and N 1 = 0; dn 1 / dt = 0 The X intercept zero growth point [N 1 = K 1 :N 2 = 0] is somewhat intuitive in the context of the equation. If species 1 is at carrying capacity, it will be using up all the resources and there will be none left over to support species 2 (which will therefore be absent). According to the logistic model for population growth, species 1 can not grow beyond its carrying capacity therefore [N 1 = K 1 :N 2 = 0] is an intuitively logical zero growth point as well as one generated by the model. These substitutions into the equation are shown below: dn 1 /dt = r 1 N 1 [(K 1 N 1 β(0) ] /K 1 dn 1 /dt = r 1 N 1 (0-0)/K 1 = r 1 N 1 (0)/K 1 = 0 The Y intercept zero growth point is not so intuitive, but it is clearly true within the context of the equation, and the model. Substitutions of these Y axis values into the equation for growth of species 1 are shown below: dn 1 /dt = r 1 N 1 {(K 1 0 α (K 1 /α)}/k 1 dn 1 /dt = r 1 N 1 (K 1 K 1 )/K 1 = 0 The graph in Figure 1 shows the zero growth line for species 1 when competing with species 2 (dn 1 /dt = 0). Because we have two points and the equation represents a linear relationship, all points on the line drawn between those two points will result in stopping the growth of species 1 s population. All combinations of population sizes (N 1 and N 2 ) below the line allow growth of species 1 s population, as indicated by the right- 12

13 directed arrow. All combinations of population sizes above the line result in a decline of species 1 s population, indicated by the left-directed arrow. This line is called isoclines for species 1. K 2 Decreasing N 2 Increasing N 1 Figure 10: Isocline for Species 2. Thus the Isocline for Species 2 will indicate its zero population growth (dn 2 /dt = 0 ) and a similar graph can be generated for species 2 s growth in competition with species 1. Leaving the axes the same (N 2 on the Y axis, N 1 on the X axis), the graph in Figure 2 is plotted. The up and down arrows indicate graphical regions of growth and decline, respectively of population of species 2. An important point for interpretation of the graphs that follow is that any point (represented by a value for N 1 and a value for N 2, and written [N 1, N 2 ]) on the graph which falls above the zero growth line for a species results in a decline in that population toward the zero growth line. Any point [N 1, N 2 ] on the graph which falls below a species' zero growth line results in an increase in that population toward the zero growth line The Graphical Solutions of the Lotka-Volterra Competition Model To solve the two equations and explore competition in the context of the model they represent, both zero growth lines (isocline) must be placed on the same axes and the interpretation described above must be employed. There are four possible outcomes. The model conditions that give rise to the four graphs as follows: 1. Species 1 wins and excludes species 2 2. Species 2 wins and excludes species 1 3. Unstable equilibrium between the two populations 4. Stable equilibrium between the two populations 1. Species 1 wins and predicts competitive exclusion of species 2 Species 1 wins K 2 N 2 STABLE EQUILIBRIUM POINT N 1 K 1 Figure 11. Species I wins and Species 2 is completely excluded. Horizontal red arrows indicate growth or decline of population1 and vertical red arrows indicate growth or decline of population 2 in each region of the graph. The black arrows associated with each pair of red arrows indicate the movement of the two population sizes in that region of the graph. 13

14 This solution is generated if K 1 > K 2 / β on the X axis; and K 1 / α > K 2 on the Y axis. It indicates that both species populations grow in the triangular zone below both zero growth lines (Isocline lines). Both species populations decline in the zone above both zero growth lines. In the space between the zero growth lines, (above species 2 s and below species 1 s,) species 2 must decrease and species 1 will increase, as indicated by the arrows in that region. Whatever population sizes (N 1 and N 2 at point [N 1 :N 2 ]) this two-species community starts with, growth and decline of the two species will continue until the point [N 1 = K 1 :N 2 = 0] is reached. At that point species 1 s population is at its carrying capacity and species 2 s population is at zero (species 2 is absent). [N 1 = K 1 :N 2 = 0] is the only stable equilibrium point on the graph. The model concludes that under these conditions. Species 1 will competitively exclude species 2. To summarize -the environment s carrying capacity for species 1 is relatively larger than it is for species 2, and species 1 interferes with the growth of species 2 more intensely than species 2 interferes with the growth of species 1. This intuitive conclusion follows because: For K 1 > K 2 /β, K 2 must be relatively small and α (in the denominator, and an indicator of the strength of the competition of species 1 on species 2) must be relatively large. The same logic applied to the Y axis gives a relatively small K 2 and small β The environment has a greater carrying capacity for species 1 than for species 2, and species 1 interferes more with species 2 s growth than species 2 does with species 1 s. Both relationships favour species 1. Species 1 is the superior competitor under these conditions, and species 1 competitively excludes species 2 at equilibrium 2. Species 2 wins and species 1 is completely excluded K 2 STABLE EQUILIBRIUM POINT Species 2 wins N 2 N 1 K 1 Figure 12. Species 2 wins and species 1 is completely excluded. If the axial relationship is reversed so that on the X axis K 1 < K 2 /β and on the Y axis K 1 /α <K 2 Parallel logic to that given above gives [N 1 = 0:N 2 = K 2 ] as the only stable equilibrium point, and at equilibrium species 2 s population will be at the environment s carrying capacity for species 2, while species 1 s population is at zero. Species 1 will be competitively excluded and the species 2 will win. Which species is competitively excluded is often determined by the relative sizes of the initial populations. If there is a large population of species 2 (N 2 is large) and a small population of species 1 (N 1 is small), the equilibrium result will usually be arrested at [N 1 = 0:N 2 = K 2 ]. If the initial population sizes are reversed, the point [N 1 = K 1 :N 2 = 0] is likely to be achieved at equilibrium. 14

15 3. Unstable Equilibrium Third Graphical Solution Predicts Competitive Exclusion of Species 1 or 2. Consider the situation with each species having a larger carrying capacity value on its axis, or On the X axis K 1 > K 2 /β and on the Y axis K 2 > K 1 /α There are three equilibrium points on this graph but only two are stable equilibrium points, [N1 = K1:N2 = 0]; and [N1=0:N2 = K2]. The point at which the two zero growth lines cross is an unstable equilibrium point. Much of the graph s area directs the populations to that point, but if perturbed from that point into either adjacent triangle (above and to the left or below and to the right) population sizes will be driven to one of the stable equilibrium points where one species is at its carrying capacity and the other has been completely excluded. Above the line K 2, K 2 / β species 2 is unable to increase and above the line K1, K 1 /α Species 1 is unable to increase. In the triangle K 2, E, K 1 /α only species 2 can increase towards carrying capacity ( E is the point where two isoclines are intersecting ) and in the triangle K 1,E, K 2 / β only the species 1 can increase. Both the populations tend to move towards their own carrying capacity making E unstable as the arrows are directed away from E towards K 1 or K 2. K 2 STABLE EQUILIBRIUM POINT Competition can go either way UNSTABLE EQUILIBRIUM POINT N 2 STABLE EQUILIBRIUM POINT N 1 K 1 Figure 13. Unstable Equilibrium 4. Stable Equilibrium It predicts the requirements for the coexistence Finally, consider the fourth possible solution, both carrying capacities represent the smaller values on their axes, r On the X axis, K 2 / β > K 1 and on the Y axis, K 1 /α> K 2 Whatever the original sizes of the two populations in this graphical space they will end up at the point where the two zero growth lines intersect. E is the only stable equilibrium point on with both species present (neither N 2 nor N 1 is zero). In other words, this is the only one of the graphical solutions that results in coexistence of the two species over time. What is special about this solution? For the terms K 1 /α and K 2 /β to be large (larger than K 2 and K 1 respectively), the two competition coefficients must be small. They are in the denominator of a large fraction, if denominators are large, fractions will be small. The intersect (E) of two isoclines has converging arrows indicating stability or coexistence of the two populations Using the same intuitive logic, in each of the other three solutions (graphs), one or both of the competition coefficient (α and β) are large. And in each of those cases, one species competitively excludes the other. 15

16 Thus the Lotka-Volterra model concludes that species coexist only when the competition between the species is minimal. In such populations intraspecific competition may play more important role in population increase or decline. Coexistence K 2 N 2 STABLE EQUILIBRIUM POINT N 1 K 1 Figure 14. Stable equilibrium Species 1 and species 2 can co-exist. 6. Field Studies Testing the Model in Natural Settings So, laboratory experiments supported the competitive exclusion principle, but the results might just be artifacts of highly oversimplified systems. Could the principle be tested in the more natural and complex context of ecological communities? A number of experiments and observations have been carried out attempting to answer this challenge. Differently stated, each species may have been competitively excluding the others from its foraging microhabitat, thus allowing coexistence in the larger habitat; with the use of different foraging techniques enhancing resource sharing. A large number of similar studies have supported the competitive exclusion principle, though some have refused it. More elaborate models have been generated to study competition. Many of them start with the Lotka-Volterra model and build additional complexity into it. Probably all of them trace their roots to Lotka and Volterra, even if they do not build directly on their equations. 7. Competition and Evolution of New Species Competition can influence natural selection and resource partitioning reduces the direct competition by specific physiological, morphological and behavioral adaptations. The chapter on competition is incomplete without discussing Darwin s theory of natural selection and origin of new species which is completely based on competition. Following Phenomenal evidences are used to deduce that competition has occurred in the past and has resulted in the origin of new species? 1. Niche separation or Resource Partitioning 2. Habitat shift 3. Character displacement 4. Competitive exclusion 5. Competitive release 16

17 1. Niche separation: This process allows two species to partition certain resources so that one species does not out-compete the other as dictated by the competitive exclusion principle; thus, coexistence is obtained through the differentiation of their ecological niche. Niche partitioning may not occur if there is sufficient geographic and ecological space for organisms to expand into. Niche differentiation is a process which occurs through several different modes and on multiple temporal and spatial scales. Resource partitioning occurs when two species coexist in spite of apparent competition for the same resources. Close study would reveal that they actually occupy slightly different niches. By pursuing slightly different resources or obtaining resources in slightly different ways, individuals minimize competition and maximize success. Dividing up resources in this manner is called resource partitioning. Five species of warblers coexist in spruce trees by feeding on insects in different regions of the tree and by using different feeding behaviors to obtain insects. Figure 15. Resource partitioning in 5 species of warblers. ( 2. Habitat fragmentation or habitat shift describes the emergence of discontinuities (fragmentation) in an organism's preferred habitat. In other words it results in small suitable islands of habitat surrounded by sea of unsuitable habitat causing population fragmentation. Habitat fragmentation can be caused largely by geological processes that slowly alter the layout of the physical environment or by human activity such as land conversion, which can alter the environment much faster and causes extinction of many species. Two species bluegill sunfish (Lepomis macrochirus) and green sunfish (Lepomis cyanellus) is good example of niche fragmentation. Bluegill sunfish in small ponds feeds on large prey associated with the vegetation. However in the presence of green sunfish it shifts to feeding on smaller and less preferred prey in the open water. Thus the two species do not change their feeding habit. Blue gill takes a refuse on truncated food in open waters the green sun fish is a stronger competitor utilizes the wider food. 17

18 Figure 16. Bluegill and green sunfish. 3. Character displacement: The term character displacement is generally described as the morphological difference/s due to competition. This pattern results from evolutionary change driven by competition among species for a limited resource (e.g. food). It refers to the phenomenon where differences among similar species whose distributions overlap geographically are accentuated in regions where the species co-occur, but are minimized or lost where the species distributions do not overlap. Galapagos finches are good example of character displacement. Each closely-related species differs in beak size and beak length, allowing them to coexist in the same region since each species eats a different type of seed: the seed best fit for its unique beak. The finches with the deeper, stronger beaks consume large, tough seeds, while the finches with smaller beaks consume the smaller, softer seeds. The rationale for character displacement stems from the competitive exclusion principle. (Gause's principle). Figure 17. Darwin s finches on Galapagos Island (Copy). (Ref 4. Competitive exclusion: Which contends that to coexist in a stable environment two competing species must differ in their respective ecological niche or without differentiation, one species will eliminate or exclude the other through competition. For example two birds found in American forests: the nuthatch and the brown creeper. They both seek food from the same trees, but the brown creeper travels up the trunk, while the nuthatch goes down for insects. 18

19 Figure 18. Gray squirrel replaced the red squirrel in Britain. (Ref: Gray squirrel is stronger competitor and better adapted to the environmental conditions has completely replaced the red squirrel from the forests of Britain. 5. Competitive release (Grant 1972), It is defined as the expansion of an ecological niche in the absence of a competitor or relief from competition. In the presence of competitor a population occupies its realized niche whereas in its absence it occupies its ecological or fundamental niche. Competitive release occurs when one of two species competing for the same resource disappear, thereby allowing the remaining competitor to utilize the resource more fully than it could in the presence of the first species. Joseph Connell (1961) demonstrated the competitive release in two species of barnacles which are sessile organisms. He observed that in the Scottish coastal region. Balanus (blue Barnacles) normally occupy the intertidal zone on rocky coast while Chthamalus (brown Barnacles ) normally occupy the coast above the high tide. When Connell experimentally removed the Balanus he found that the Chthamalus colonized the intertidal zone and overgrew any new Balanus that settled there and thus excluded Balanus from successfully establishing in the intertidal zone. Thus he showed that the experimental removal of Balanus from the fundamental niche (above high tide and the intertidal zone) of Chthamalus allowed it to undergo competitive release(relief from competition) and expand its ecological niche where as in the presence of competitor it occupied only its realized niche ( above the high tide). Figure 19. Balanus species excludes Chthamalus from low tide to high tide. 19

20 End results of competition How does competition influence natural selection and the evolution of species? Darwin s finches are a good example to prove the end results of competition: Three situations with respect to the resource utilization can be: a. No overlap b. Partial overlap c. Significant overlap If the curves are separate (a), natural selection and evolution indicates that a species that can capture the unused portion will have more fitness therefore, you should see a shift of both species towards the middle. Indicating no competition When there is slight overlap both competitors can survive whereas, if the curves are overlapping largely there is strong competition (c) may occur Arrows indicate the direction of evolution pattern. In (a) curves are completely separate some food resources are not used by species A and B (b) If the curves overlap only slightly, each species has a set of food sizes for itself and will be able to survive (c) if the curves overlap greatly both the species eat the same food and the competition is severe (Ref ; C.J. Kreb fourth edition) Figure 20. Hypothetical resource utilization curves for two species. Food size for which the competition. 8. Summary Competition refers to the interaction between the two organisms of the same or different species striving for the same resource. Interspecific competition is an extrinsic mechanism of population regulation at a given place and given time. Competition as a regulatory mechanisms in plant and animal populations is spectacular in population dynamics and plays an important understanding of the ecological and evolutionary processes. It can be exploitative or interference type. In the exploitative type the two competitors may have an equal access to the resource but one may be more efficient in utilizing it than the other one. In interference competition the access to the resource is denied by the competitor species. This can be simply an interference in the access of other species to the resource or a direct confrontation between the two competitors. In both the cases growth of the weaker species is affected. Outcome of competition change with time and other environmental conditions, such as the presence of predators, parasites and changes in physical conditions like temperature, light and humidity etc. 20

21 Strategies of Competition can be: i) complete exclusion of one species either 1 or 2; or may result in ii) stable or iii) unstable equilibrium between the two species. The stable equilibrium may be achieved by - Niche separation, resource portioning or character displacement. Darwin s theory of natural selection and origin of new species which is completely based on competition. Competition can influence natural selection. It results in resource partitioning, habitat fragmentation, niche separation and character displacement to reduces the direct competition by specific physiological, morphological and behavioral adaptations for the existence of a species. 21