Class 9Populations 3Species Interactions – Competition: Class 9 Populations 3 Species Interactions – Competition Readings Krebs – Chapter 12


Slide2: Competition 1. Species Interactions Mechanisms Effects 2. Types and characteristics of competition 3. Models of competition Lotka-Voltera Tilman 4. Competition in natural populations Fundamental versus realized niches Character displacement Coexistence-the paradox of competition Does competition ever occur? 5. Adaptive strategies r and K Grime’s plant competition strategies


Mechanisms for Interactions: Mechanisms for Interactions Competition – 2 species use or seek the same resource to the detriment of both Predation – one animal species eats all or parts of another animal species (cannibalism – same species predation) Herbivory – one animal species eats all or parts of a plant species Parasitism – 2 species live in a close, obligatory association where the parasite depends metabolically on the host Disease – an association between a pathogenic microorganism and a host species, where the host suffers physiologically Mutualism – 2 species live in close association with one another to the benefit of both


Direct competition –PredationTypes of predators:: Direct competition –Predation Types of predators: Herbivores Animals that prey on plants or their seeds or fruit (often does not kill plant) Carnivores Animals that eat herbivores and other carnivores Parasites Plants or animals that live on or in their host and depend on the host for their nutrition Parasitoids Insects that lay there eggs on or near their host which is subsequently killed and eaten Cannibals Special case where the predators and prey are the same species


Slide5: Competition 1. Species Interactions Mechanisms Effects 2. Types and characteristics of competition 3. Models of competition Lotka-Voltera Tilman 4. Competition in natural populations Fundamental versus realized niches Character displacement Coexistence-the paradox of competition Does competition ever occur? 5. Adaptive strategies r and K Grime’s plant competition strategies


Types of Competition: Types of Competition Interspecific (different species) vs. Intraspecific (same species) Resource competition (scramble or exploitive competition) – a number of organisms utilize common resources that are in short supply All individuals equally affected – no winners/losers Interference competition (contest competition) – organisms seeking a resource harm one another in the process, even when resources are not in short supply All individuals not equally affected – winners and losers


Diffuse Competition: Diffuse Competition The combined effects of many species on a specific species In many cases, a species is in multiple competitions These competitions can be the result of direct and indirect factors


Characteristics of Competition: Characteristics of Competition Competition occurs for a variety of resources that influence the fitness of the organism – Very different resources for plants and animals For competition to exist, a resource must be scarce and organisms share a common requirement for it


Important Aspects of Competition: Important Aspects of Competition Animals may not see or hear their competitors – may deplete a resource at different times Most animals occupying the same habitat are not competitors Plants mainly compete for space, because they are rooted Space provides access to sunlight, water, nutrients


Slide11: Competition 1. Species Interactions Mechanisms Effects 2. Types and characteristics of competition 3. Models of competition Lotka-Voltera Tilman 4. Competition in natural populations Fundamental versus realized niches Character displacement Coexistence-the paradox of competition Does competition ever occur? 5. Adaptive strategies r and K Grime’s plant competition strategies


Lotka-Voltera Competition Model: Lotka-Voltera Competition Model Uses the basic logistic (sigmoid) growth model dN / dt = r N (K – N) / K where N is the population size t is time r is the intrinsic capacity for increase K is the carrying capacity


Lotka-Voltera Competition Model: Lotka-Voltera Competition Model Model assumes you have two populations (denoted by the subscripts 1 and 2) that are in competition for the same resource dN1 / dt = r1 N1 (K 1– N1) / K1 dN2 / dt = r2 N2 (K2 – N2) / K2


Slide14: 1 2 2 2 = 2  is the relative resource use by species 2 with respect to the requirements of species 1. So  N2 is the equivalent number of species 1 individuals that would use the same amount of resource as 4 individuals of sp 2.


Lotka-Voltera Competition Model: Lotka-Voltera Competition Model If we have N2 individuals present, then they will reduce the resources required for increases in the N1 population dN1 / dt = r1 N1 (K 1– N1 -  N2 ) / K1


Slide16: Change of species 1 in the face of competition of species 2 Important numbers – K1 is the carrying capacity for species 1, in the absence of species 2, there will be K1 of species 1 K1/ is the equivalent carrying capacity for species . Not necessarily equal to K2 The blue line represents the equilibrium point towards which the population will move. Called the zero isocline Model assumes that relationship is linear Blue arrows show what happens to the population of species 1 When the population of species 1 is below the curve, it will increase If it is above the curve, it will decrease Note that change in population is dependent on both populations


Slide17: Remember that K1 is dependent on environmental conditions. If the conditions change, the carrying capacity is going to change also The changes in conditions are likely to effect the two species differently Thus, the slope of the curve is going change as well


Slide18: 1 2 2 2 = 2  is relative resource use by species 1 with respect to the requirements of species 2  N1 is the equivalent number of species 2 individuals that use the same amount as 1 sp 1 individual


Lotka-Voltera Competition Model: Lotka-Voltera Competition Model If we have N1 individuals present, then they will reduce the resources required for increases in the N2 population dN2 / dt = r2 N2 (K 2– N2 -  N1 ) / K2


Slide20: dN2 / dt = 0 In this case, the carrying capacity for species one is defined in terms of species 2 Blue arrows show the direction of change in the population of species 2


Possible Outcomes of Competition: Possible Outcomes of Competition Species 1 out competes species 2  species 2 becomes extinct Species 2 out competes species 1  species 1 becomes extinct Neither has an advantage  species co-exist


Slide23: Competition 1. Species Interactions Mechanisms Effects 2. Types and characteristics of competition 3. Models of competition Lotka-Voltera Tilman 4. Competition in natural populations Fundamental versus realized niches Character displacement Coexistence-the paradox of competition Does competition ever occur? 5. Adaptive strategies r and K Grime’s plant competition strategies


Tilman’s Competition Model: Tilman’s Competition Model Tilman developed a multiple-factor (resource) competition model More complex than that developed by Lotka-Volterra Models how the availability of resources influence population growth and competition


Slide25: Population growth in laboratory experiments carried out by the Russian scientist Gause on growth rates in two different yeast species Each of the species has the same food – e.g., sugar The plots in this curve were developed for populations growing separately Both populations exhibited the classic sigmoid growth curve


Slide26: Growth decreased prior to the point where food or energy were exhausted in each case. Concentration of alcohol limits growth - a side-product when yeast consumes sugar under anaerobic conditions Alcohol has a toxic effect on the yeast populations. High alcohol concentrations kill young yeast buds as they break off from their mothers


Slide27: Gause conducted experiments on the growth of the two yeast species together, competing for the same food source. Each grow less when in competition. Presence of alcohol limits growth. When one species produces alcohol, it limits the growth of both.Thus the carrying capacity of both are reduced Alpha and beta for competition between the species estimated and the Lotka-Volterra model described the results well


Slide28: T = 29.1 degrees T = 32.3 degrees Research by Birch on grain beetles represent a situation where competition leads to extinction of one species or the other In his study, Birch showed that one species went extinct in the face of competition when the temp was 29 degrees, but by changing the temperature by 3 degrees, the other species then went extinct Temperature influences the metabolic pathways of the species differently, so variations in temperature influence the relative fitness of the individuals and so competition between species is dependent on ambient environmental conditions Example of cases 1 and 2 of the Lotka-Volterra model


Slide29: The 29 degree case equals Case 1, e.g., species 1 out competes species 2 The 32 degree case equals Case 2, e.g., species 2 out competes species 1


Slide30: Competition 1. Species Interactions Mechanisms Effects 2. Types and characteristics of competition 3. Models of competition Lotka-Voltera Tilman 4. Competition in natural populations Fundamental versus realized niches (see Class 2) Character displacement (see Class 2) Coexistence-the paradox of competition Does competition ever occur? 5. Adaptive strategies r and K Grime’s plant competition strategies


Competition in Natural Populations: Competition in Natural Populations Gause’s hypothesis – Two species with a similar ecology cannot live together in the same place Competitive exclusion principle -Complete competitors cannot co-exist


Coexistence-The Paradox of Competition: Coexistence-The Paradox of Competition How does the theory of competitive exclusion and the observation of extinction of closely related species in laboratories reconcile itself with the observation that a large number of similar species can actually co-exist in nature


Explanation 1- situations exist where competition is not expected: Explanation 1- situations exist where competition is not expected Resources are not limiting, ample for all Unstable, fluctuating environments exist, that reverse direction of competition before extinction is possible


Slide34: Explanation 2 - Since in many cases, resources are not limiting, competition is actually rare – there really is no competition Explanation 3 – Competition is not rare, but is common – has resulted in adaptations and natural selection that serve to limit competition – so it only appears to be rare


Slide35: Where competition was thought to exist, in fact the species under study were occupying different niches Feeding positions of warblers found in coniferous forests occupy different parts of the canopy, and therefore exploit different resources. Hence no competition.


Slide36: How does competition influence natural selection and the evolution of species? Three situations: a. No – overlap c. Significant overlap b. Partial over-lap Case A - If the curves are separate, 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


Slide37: Example of food size for terns living in the same area (Christmas Island in the Pacific Ocean) that illustrates how selection and evolution work to create separate food niches. Each species has a unique region of fish sizes that it feeds on, therefore, they are avoiding competition With respect to the sooty and brown noddy, even though their food sizes are similar, they actually feed in different regions, so they are not in competition


Criteria for establishing that competition exists: Criteria for establishing that competition exists Species overlap in resource use Interspecific competition occurs Resource use by one species reduces its availability to the other One or more species is adversely affected Alternate process hypotheses do not explain observed patterns of population density and distribution


Slide39: Competition 1. Species Interactions Mechanisms Effects 2. Types and characteristics of competition 3. Models of competition Lotka-Voltera Tilman 4. Competition in natural populations Fundamental versus realized niches Character displacement Coexistence-the paradox of competition Does competition ever occur? 5. Adaptive strategies r and K Grime’s plant competition strategies


Slide40: r K


Adaptive strategiesr- selection: Adaptive strategies r- selection Occurs when individuals never reach their carrying capacity Individuals want to grow and reproduce quickly to take advantage of the resources Also might be in a situation where a population doesn’t reach K because of high death rates In any case, this situation favors rapid growth of the population


Adaptive strategiesK-selection: Adaptive strategies K-selection Carrying capacity is reached Competition for resources (both interspecific and intraspecific) favors those individuals that have high fitness, which includes longevity


Slide43: r-selected short-lived small weak waste a lot of energy less intelligent, experienced... have large litters reproduce at an early age fast maturation little care for offspring strong sex drive small size at birth low density situations K-selected long-lived large strong or well-protected energy efficient more intelligent, experienced... have small litters reproduce at a late age slow maturation much care for offspring weak sex drive large size at birth high density situations Comparison of characteristics of r & K selected organisms


 - selection:  - selection Where one organism prevents access to a resource by another organism Most types of interference phenomenon fall into this category


Grime’s Theory of Plant Selection: Grime’s Theory of Plant Selection Plants are subjected to two categories of factors influencing their growth and reproduction Shortages of resources  stress Light, water, nutrients, temperature, other physical/chemical limitation Disturbance Grazing, fire, frost, wind, erosion, ice storms, etc.


Grime’s Theory of Plant Selection: Grime’s Theory of Plant Selection


Grime’s Plant Types: Grime’s Plant Types Competitive (K) strategy – low stress and disturbance Dense leaf canopy, rapid growth, low seed production, short life spans – get big fast to dominate resources Stress tolerant strategy – high stress, low disturbance Small leaves, evergreens, long-lived, low seed production Ruderal (r or weed) strategy – low stress, high disturbance Common in disturbances, small size, rapid growth, often annual plants, high seed production