Unformatted text preview: Competition February 19, 2009 Notes Lecture podcasts: counting discrepancy resolved as of today. Previous several (since the morning special discussion on Jan 23) are off by 1; so use dates. Info on Midterm Familiar routine Today Natural history of competition Experiments testing outcomes of competition Mathematical models of competition BIS 2B - Section B Midterm 2
Monday, February 23rd Notes on Midterm 21 Scantron Questions 4 short answer questions 3 Bonus questions Content: Natural Selection through Predator Prey interactions (last Friday) 5:10 - 6:00 pm
If your last name begins with: AL MS TZ go to go to go to Chem 194 Everson 176 Hunt 100 NOTE: I WILL NOT BE HERE FRIDAY during office hours.... 2:30-4. Thursday PM 2:30- Competition: when individuals use a shared resource that is in short supply (not enough resource for all) Results in decreased rates of growth, survival or reproduction that alter population growth rates. Intraspecific competition 1 Mechanisms of Competition I. Exploitative competition If I am using it, then you can't Over what do species compete? Leibig's Law of the Minimum
Organisms are limited by the resource that is most limiting: in short supply: the scarcest resource. Corollary: competition is fiercest for the most limiting resource.
Developed in 1820's relevance today in thinking about how and when organisms compete. Examples Mechanisms of Competition II. Interference competition Examples? Bare ground established and maintained by allelopathic chemicals exuded by plants that inhibit germination and establishment of other plants It can be difficult to distinguish types of competition. What are these animals doing? Mechanisms of Competition III. Apparent competition
predator + prey + prey Apparent Competition 2 A different classification of competitive interactions More types of competitive interactions, modified for mobile animals:
Territoriality: mobile animals defend space for the resources in that space. (e.g., nesting birds, scent marking dingos). (like pre-emption or possibly overgrowth) Predicted Outcome of Competition
1. Option 1: Competition BETWEEN species works much like intraspecific competition. Different species, however, may have very different competitive abilities (one may grow faster, etc.) Paramecium 2. Option 2:
Competitive exclusion principle: two species that use the same resources in the same way cannot coexist. One will drive the other to extinction. (Gause) Strong competitive asymmetries are common Once again, the conundrum of diversity If we expect competition to lead to competitive exclusion, then why do ecosystems support so many species? Spatial variability in resource supply (niche space) Temporal variation in resource supply (niche space) Gurevitch et al. 3 Some apparently similar species co-exist... how? Coexistence and space: Competition among barnacles on the Scottish coast
Chthamalus upper limit Balanus upper limit Chthamalus zone When species co-exist they must some how differ in the way they couse resources... these Parameciums "partitioned" the tubes vertically. resources... partitioned" Balanus zone
Connell 1961 Why doesn't Chthamalus live where Balanus lives? Experiment: remove Balanus from areas of the lower intertidal and see what happens Chthamalus settlement Chthamalus settlement Do lower intertidal species competitively displace stress tolerant species to high intertidal locations? Thus far we know the following:
1. Organisms compete for limiting resources Chthamalus is fine when Balanus is excluded from lower tidal height Competition Intensity 2. There are many mechanisms by which species can compete. 3. Competitive exclusion of one species occurs when two species compete for the same resources in the same way at the same place and time. One is a more efficient competitor and excludes the other*.
* What happens if they are exactly the same? Lower distribution limit set by interspecific competition + 1954 1955 Low Intertidal
Why don't Balanus exclude Chthamalus from the higher intertidal too? Connell 1961 4. BUT: species can coexist when they segregate in space or time or when they use different resources in different ways. 4 Mechanisms that prevent competitive exclusion Spatial structure: Temporal variability Different species specialize in different habitats Different species specialize in different conditions, and these conditions vary in time. Even if species are exactly the same, chance events will eventually lead to one excluding the other Non-equilibrium systems disturbance maintained. Mathematical models of coexistence. Gettin mathy wit it We begin with the fundamentals
dN/dt = rN[1-(N/K)] dN/dt = rN[(K-N)/K]
The logistic equation intraspecific competition Again dN/dt = rN[1-(N/K)] dN/dt = rN[(K-N)/K]
The logistic equation intraspecific competition Remember - this is just exponential growth, reduced to zero as N approaches K Adding interspecific competition dN1/dt = r1N1[(K1-N1 - a12N2)/K1] N1 = number of individuals of species 1 r1 = intrinsic growth rate of species 1 K1 = carrying capacity of species 1 N2 = number of individuals of species 2 a12 = competition coefficient indicating the effect of species 2 on species 1. Term to account for use of resources (reduction of K) by another species (N2) * If a12 = 1 then species are identical in resource use, and intraspecific competition is the same as interspecific competition Now we can figure out how species might coexist in nature despite competition.
Step 1. Set dN1/dt = r1N1[(K1-N1 - a12N2)/K1] = 0 A zero net growth isocline (ZNGI)
K1/a12 dN1/dt = 0 N2 N2 N1 N1 K1 5 Now for species 2.
Step 2. Set dN2/dt = r2N2[(K2-N2 a21N1)/Kk] = 0 K2 Now for species 2
dN2/dt = 0 N2 N2 N1 N1 K2/a21 Now combine the two
K1/a12 K2 N2 N2 Option 1
species 1 wins dN1/dt = 0 N1 N1 K2/a21 K1 Option 2
species 2 wins K2 K1/a12 N2 N2 dN2/dt = 0 K2 K1/a12 Option 3 Stable equilibrium dN2/dt = 0 N1 K1 K2/a21 N1 K1 K2/a21 6 Option 4
K2 K1/a12 N2 dN2/dt = 0 unstable equilibrium Coexistence among competitors occurs when: 1. 2. 3. Interspecific competition is weak relative to intraspecific competition. competition Species are more self-limiting than other species limiting. selfWhen a12 and a21 << 1 . THESE ARE NOT 3 different criteria, they are the saying the same thing using three different sets of words. K2/a21 N1 K1 LIMITING SIMILARITY AND THE NICHE First, lets (re)define the concept of a niche:
moisture From the previous slide: If a is << 1 for both (very little competitive overlap): species very likely to coexist If a is >> 1 for either species, one species will usually outcompete the other If a = 1, then the outcome will depend on initial abundance and K of each species Convince yourself:
moisture predation competition disease competition dN1/dt = r1N1[(K1-N1 - a12N2)/K1] dN2/dt = r2N2[(K2-N2 - a21N1)/K2]
Performance Not enough to grow nutrients
Fundamental niche: defined by physical conditions under which a species can persist Realized niche: alteration of fundamental niche by species interactions nutrients How much less than 1 does a have to be for coexistence to occur? Rephrase: What is the limit to the similarity of a that will allow coexistence? Web simulation Too much becomes toxic Performance Lo nutrients Hi Lo Realized niche: species actually occurs over a narrower range of nutrients due to competition from other species nutrients Hi No niche overlap, no interspecific competition, a = 0
Performance Performance Complete niche overlap, strong interspecific competition, a = 1 How might interspecific competition cause the evolution of resource partitioning?
Performance Niche axis (e.g., prey size) High niche overlap, strong interspecific competition, a close to, but less than 1
Performance Performance prey size Low niche overlap, weak interspecific competition, a << 1 prey size Complete niche overlap, a > 1 for effect of blue species on the red one
Performance Performance prey size prey size prey size 7 How might interspecific competition cause the evolution of resource partitioning?
Performance Character displacement is the process of evolution toward niche divergence as a result of interspecific competition Compare populations living in the same location as another species (sympatry) vs. those living in an area without the other species (allopatry)
Geospiza fortis Start with 2 species with high niche overlap and assume resource use is genetically controlled prey size Individuals of the red species that eat small prey will have reduced fitness relative to those that eat larger prey because they experience strong interspecific competition Individuals of the red species that eat large prey will have greater fitness relative to those that eat small prey because they experience only weaker interspecific competition in sympatry Performance Because individuals that eat larger prey left more offspring than those that eat smaller prey, the mean phenotype has shifted to a higher prey size in the next generation prey size Geospiza fuliginosa Character displacement in beak size of populations of Galapagos finches The "ghost of competition past"... The following minimum criteria must be met for character displacement to have occurred: 1. 2. 3. 4. 5. 6. Morphological differences between a pair of sympatric species are greater than the difference between allopatric populations of the same species These morphological differences have a genetic basis Variation in the character has a measurable effect on resource use There must be competition for the resource in question and competition must be directly correlated with the character Differences in the character cannot be explained by differences in the resources available to sympatric vs. allopatric populations (habitat differences). Differences must have evolved in place and not be the result of sympatric and allopatric populations arising from different founder stock (founder effects). Galapagos finches - partitioning of food resources is manifested in the evolution of different beak sizes... leading to reduced competition. Species packing: adding new species shrinks the niche width of all species. To compensate species often become more specialized, and more efficient at using a narrower range of resources. Is there a limit to the number of species that can be "packed" into an area and still coexist? If the resource axis is finite, adding species will increase the competition coefficent between pairs of species, decreasing the likelihood of coexistence Corollary: as you add species, the probability of establishment of new species should decrease: Invasion success should decrease with increasing numbers of resident species, and successful invaders should be those that are most different from established species (least overlap).
Few species = Many "empty niches", susceptible to invasion. Resource
Performance Of course, there may be many resource axes operating at once Species can also coexist a number of other ways due to tradeoffs -strong competitors are susceptible to predators -good colonizers are often poor competitors (r vs. K) The niche of an organism includes not only its resource utilization and competition with other species, but its response to environmental conditions, predators, etc.
predation competition Performance moisture Diet proportion
disease competition Many species = Few "empty niches" resistant to invasion. Resource nutrients 8 Modern issues in competition How can so many non-native species `fit' into their recipient communities? When do non-native species displace natives? Empty niches, or competitive advantage? Dov Sax and competitive exclusion Climate change and species distribution shifts What will be the role of competition 9 ...
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- Spring '09
- Interspecific competition, Competitive exclusion principle, Biological interactions