Biology 205 11

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Biology 205Ecology and Adaptation

Lecture 10: Population Life HistoryCompetition

Dr. Erik D. Davenport

Outline• Offspring Number Versus Size• Life History Variation Among Species• Life History Classification

– r - Selected– k - Selected

• Resource Competition• Niches• Mathematic and Laboratory Models for competition

– Lotka-Volterra model• Competition and Niches

Different life history – why?

• Principle of Allocation: Only certain amount of energy is available to a living organism. If organisms use energy for one function such as growth, the amount of energy available for other functions is reduced.

Main concept 1:Offspring Number Versus Size

Because all organisms have access to limited energy and other resources, there is a trade-

off between the number and size of offspring; those that produce larger offspring are

constrained to produce fewer; while those that produce smaller offspring may produce

larger numbers.

Egg Size and Number in Fish• Fish show more variation in life-history than any

other group of animals.• Darter fish as an example….

Seed Size and Number in Plants

• In addition to the number of the seed– Plant growth form will influence seed size– Dispersal mode might influence seed size.

• Westoby et.al. recognized four plant growth forms:– Graminoids: Grass and grass-like

plants.– Forbs: Herbaceous, non-

graminoids.– Woody Plants: Woody thickening of

tissues.– Climbers: Climbing plants and

vines.– Woody plant and climbers

produced 10x the mass of seeds than either graminoids or forbs.

Westoby et.al. recognized six seed dispersal strategies:– Unassisted: No specialized

structures.– Adhesion: Hooks, spines, or

barbs.– Wind: Wings, hair, (resistance

structures).– Ant: Oil surface coating

(elaisome).– Vertebrate: Fleshy coating

(aril).– Scatterhoarded:

Gathered,stored in caches.9

Main concept 2:

Adult survival rate vs. reproduction ageWhen adult survival is lower, organisms begin

reproducing at an younger age and invest a greater proportion of their energy budget into reproduction;

where adult survival is higher, organism defer reproduction to a later age and allocate a smaller proportion of their resources to reproduction.

What will be the results on individual body size?

Life History Variation Among Species

• Shine and Charnov pointed out vertebrate energy budgets are different before and after sexual maturity.– Before - maintenance or growth.– After - maintenance, growth, or reproduction.– Individuals delaying reproduction will grow faster

and reach a larger size.

Life History Variation

• Bertschy and Fox studied the influence of adult survival on pumpkinseed sunfish life histories.– Findings supported theory that when adult

survival is lower, natural selection will favor allocating greater resources to reproduction.

– Smaller or bigger body size???

Concept 3: Life History Classification

• MacArthur and Wilson– r selection (per capita rate of increase)

• Characteristic high population growth rate.– K selection (carrying capacity)

• Characteristic efficient resource use.• Pianka : r and K are ends of a spectrum, while

most organisms are in-between.– r selection: Unpredictable environments.– K selection: Predictable environments.

r and K: Fundamental Contrasts• Per captia Rate of Increase:

– Highest in r selected species.• Competitive Ability:

– Highest in K selected species.• Reproduction:

– r: Numerous individuals rapidly produced.– K: Fewer larger individuals slowly produced.

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IMPORTANT!!!!

Population interactionscompetition

Modes of Competition

• Interference: – Direct aggressive interaction between individuals.

• Intraspecific: – Competition with members of own species.

• Interspecific: – Competition between individuals of two species -

reduces fitness of both.

Intraspecific Competition Among Herbaceous Plants

• Plant growth rates and weights have been found to increase in low density populations.

• Competition for resources is more intense at higher population densities.

• Usually leads to mortality among competing plants.

• Self-Thinning: As the population of the trees develops, more and more biomass is composed of fewer and fewer individuals, this process is called self-thinning. Why??????

• Self-thinning appears to result from intraspecific competition for limited resources, as a local population of plants develops, individual plants take up increasing quantities of nutrients, water, and space for which some individuals compete more successfully, the loser in this competition for resources die, and population density decrease as a consequence. Over time, the population is composed of fewer and fewer large individuals.

Plant population density declines more rapidly then biomass increase

Niches (very very important!!)

• Niche: Summarizes environmental factors that influence growth, survival, and reproduction of a species.

• Gause: Principle of Competitive Exclusion– Two species with identical niches cannot coexist

indefinitely.– One will be a better competitor and thus have

higher fitness and eventually exclude the other.

Niches

• Hutchinson defined niche as:– n-dimensional hyper-volume– n equates the number of environmental factors

important to survival and reproduction of a species.

– Fundamental niche – hyper-volume– Realized niche includes interactions such as

competition that may restrict environments where a species may live.

Feeding Niches of Galapagos Finches

– Grant found differences in beak size among ground finches translates directly into diet.

– Size of seeds eaten can be estimated by measuring beak depths.

– Individuals with deepest beaks fed on hardest seeds.

– After 1977 drought, the remaining seeds were very hard. Thus, mortality was most heavy in birds with smaller beaks.

Feeding Niches of Galapagos Finches

Mathematical and Laboratory Models

• Metz summarized models:– Abstractions and simplifications, not

facsimiles of nature.– Man-made construct; partly empirical and

partly deductive.– Used to provide insights into natural

phenomena.

What model is used for the intraspecific competition??

dN/dt = rN(1-N/K)

• r = per capita rate of increase under ideal conditions.

• Logistic population growth model.• What term in this model describe the

intraspecific competition???

Lotka Volterra

• Effect of interspecific competition on population growth of each species:

– dN1 / dt = rm1N1 ((K1-N1- α 12N2) / K1)

– dN2 / dt = rm2N2 ((K2-N2- α 21N1) / K2)

• α12: Effect of individual of species 2 on rate of pop. growth of species 1.

• α21: Effect of individual of species 1 on rate of pop. growth of species 2.

Lotka-Volterra

• The Lotka-Volterra model incorporates interspecific competition by using a parameter called α.

• α is the coefficient of competition (or competition coefficient) and measures the competitive effect of one species on another. (IMPORTANT!!!!)

• For example: α12 is the effect of species 2 on species 1, α21 is the effect of species 1 on species 2.

Predicting the outcome of competition

• The outcome of competition, according to the Lotka-Volterra model, is ultimately determined by

• carrying capacity (Ki) and • the competition coefficient (aij) of the two

species

examples

• Green sunfish Bluegill• K1 = 600 K2 = 600• r1 = 0.10 r2 = 0.10• α12 = 1.50 α21 = 0.90

• What is the results of this completion?

Lab Experiments

• Gause demonstrated resource limitation with Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of Bacillus pyocyaneus.– When grown alone, carrying capacity determined

by intraspecific competition.– When grown together, P. caudatum quickly

declined.– Reduced resource supplies increased competition.

Paramecia Lab Experiments

Flour Beetle Experiments

• Tribolium beetles infest stored grain products.– Park studied interspecific competition between T.

confusum and T. castaneum under varied environmental conditions.

– Growing the two species together suggested interspecific competition restricts the realized niches of both species to fewer environmental conditions.

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Competition and Niches

• Competition can restrict species to their realized niches.– But if competitive interactions are strong and

pervasive enough, they may produce an evolutionary response in the competitor population.

– Changes fundamental niche.

Niche Overlap and Competition Between Barnacles

• Connell discovered interspecific competition in barnacles. Balanus plays a role in determining lower limit of Chthamalus within intertidal zone.– Did not account for all observed patterns.

Character Displacement

• Because degree of competition is assumed to depend upon degree of niche overlap, interspecific competition has been predicted to lead to directional selection for reduced niche overlap.

Character Displacement

Biology 205 11

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