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Resource competition among >2 species

Resource competition among >2 species

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Resource competition among >2 species

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  1. Resource competition among >2 species One resource species with lowest R* excludes all others • example: species 1 excludes all others

  2. Resource competition among >2 species Two resources, essential Constant, homogeneous environment Two resources - two coexisting species at equilibrium which two species depends on resource ratios each species is best competitor for a particular ratio of resources

  3. 1 1 & 2 sp.2 2 R2 sp.1 2 & 3 3 3& 4 2 4 1 3 4 2 sp.3 3 sp.4 R1 Resource competition, >2 species

  4. New effect: spatial variation • Suppose resource ratios vary locally • natural heterogeneity in soil nutrients • consequences for coexistence? • When there is local spatial variation in resource ratios, >2 species can coexist • with local spatial segregation (patchiness) • More species than resources

  5. Variation in resource ratios sp.2 R2 sp.1 1, 2, 3, & 4 2 1 3 4 2 sp.3 3 sp.4 R1

  6. Spatial variation • Local variation fosters diversity • More species than resources possible • Dependent on extent of variation • Plant communities • often 100’s or 1000’s species • only about 12 essential resources • often patchy • Variance in Resource Ratios Hypothesis (VRR)

  7. What is the effect of nutrient enrichment? • Relationship of diversity & productivity • Unimodal vs. Monotonic • Mechanisms producing relationships • Unimodal • particularly decrease in diversity with productivity

  8. 2 & 3 4 only 1 only 1, 2, 3, & 4 sp.2 sp.1 R2 Enrichment and coexistence 2 1 3 sp.3 4 2 3 sp.4 R1

  9. Nutrient enrichment • Increase all resources uniformly • local variation in resource ratios allows coexistence of fewer species • Increase one resource • necessarily makes resource ratios more extreme • raises, then lowers number of coexisting species • Assumes resources increase without increasing variation • “Paradox of enrichment” • enrichment = reduced diversity

  10. Switching resources • Does VRR predict coexistence of many species on 2 switching resources? • species don’t specialize on ratios • each species consumes one resource or the other only • At equilibrium there are 2 species, each consuming and limited by one resourse • Fundamental difference between animal and plant communities

  11. 1 & 4 Switching resources R2 sp.4 sp.3 sp.2 4 sp.1 1 R1

  12. Plants vs. Animals • Plants use essential resources • VRR predicts high species:resources ratio • Animals use switching resources • Theory predicts species:resources ratio = 1

  13. Coexistence and evolution • Competitive coevolution • 2 spp. competing for 1 resource cannot coexist • if individuals vary in resource use • if that variation is heritable • competition creates selection • May select for increasing efficiency • selection for better resource use (lower R* ) • a “race” to be most efficient • end result is still exclusion

  14. Coexistence and evolution • Competition may select for divergence in resource use • individuals exploiting an alternative resource favored (not affected by competition) • alternative resources could be different spatially, temporally, in size • for substitutable or switching resources • evolution of divergence may avoid exclusion

  15. selection against freq. of use size of prey time freq. of use size of prey Example: Divergence in prey size

  16. R2 sp. 1 unstable sp. 2 sp. 2 sp. 1 Evolution of divergence in resource use R1

  17. R2 sp. 1 stable sp. 2 R2 R2 sp. 1 sp. 1 stable unstable sp. 2 sp. 2 R1 Evolution of divergence in resource use

  18. Competitive character displacement • Competition selects for divergence in a morphological feature • presumably results in divergence of resource use • often held to be the best evidence for the importance of competition • Example: Sitta nuthatches

  19. Nuthatches • Example: Sitta nuthatches • Asia & Europe • Ranges include regions of allopatry (no contact) • also regions of sympatry (co-occur) • Sitta neurenmayer (Europe) • Sitta tephronata (Asia) • Sympatry in Iran

  20. Nuthatches • Bill size • related to prey size • data suggest character displacement on bill size • S. neurenmeyer S. tephronta • Allopat. 25 mm 25 mm • Sympat. 22 mm 28 mm

  21. S. tephronata bill length (mm) S. neurenmayer site (longitude) Prediction of character displacement

  22. S. tephronata bill length (mm) S. neurenmayer site (longitude) Actual pattern (Grant 1972)

  23. Nuthatches • No shift in cline of bill size when region of sympatry is reached • Bill sizes vary geographically in a continuous fashion • Not much evidence for character displacement

  24. Hydrobia snails • intertidal mud snails • particle feeders (diatoms, sediment) • Allopatry • H. ventricosamean length = 3.1 mm • H. ulvaemean length = 3.3 mm • Sympatry • H. ventricosamean length = 2.8 mm • H. ulvaemean length = 4.5 mm

  25. Hydrobia snailsQuestions • Character displacement? • Competition for food particles? • Levinton - does particle size affect growth? • larger species does best on larger particles? • Result: No difference in growth for different particle sizes

  26. Hydrobiasnails: More questions • H. ulvae& H. ventricosasympatric in lagoons • H. ulvaealone in intertidal • Lagoon H. ulvae • alone … 1.2 X larger than intertidal H. ulvae • w/ H. ventricosa … 1.4 X larger than intertidal H. ulvae • size difference due to physical environment? • lagoons: low reproduction, high growth

  27. Character displacement • Classic cases of character displacement now questioned • Probably not a widespread phenomenon • Morphology (size) presumed related to resource use • Competition presumed to be the driving force • Examples of size differences reducing competition?

  28. St. Maarten A. gingivinus SVL = 41 mm A. wattsi SVL = 38 mm St. Eustatius A. bimaculatus SVL = 53 mm A. wattsi SVL = 40 mm Caribbean AnolisPacala & Roughgarden 1985

  29. Predict less competition on St. Eustatius Note: size strongly correlated with prey size Caribbean Anolis

  30. 60 Ag 100 Aw 60 Ag 60 Ab 100 Aw 60 Ab 60 Ag 100 Aw 60 Ag 60 Ab 100 Aw 60 Ab St. Maarten St. Eustatius Experiment 12 X 12 m enclosures; fenced 1.5 m; clear lizards

  31. St. Maarten A. gingivinus+ A. wattsi less food in stomach lower growth rate (0.5X) perch height higher (2X) compared to A. gingivinusalone Interspecific effect strong St. Eustatius A. bimaculatus+ A. wattsi same amount in stomach same growth rate same perch height compared to A. bimaculatusalone Interspecific effect absent Caribbean Anolis

  32. Alternative interpretation • Suppose competition is absent on St. Eustatius • large resource base, abundant food • predators reduce density • A. bimaculatusenclosures • escapes occurred over time • density: 60  45  30 lizards • 1 mo 2 mo • as density drops growth increases; competition

  33. Conclusion • Size difference  reduced competition • One case, but it shows this effect is possible • Authors do NOT claim size difference evolved due to competition • Has not established that size would evolve in response to competition

  34. Morphological evolution & competition (Schluter 1994)

  35. Sticklebacks • species complex • extreme body forms • limnetic - feed on plankton (e.g., Daphnia) • benthic - feed on benthic invertebrates see also Robinson & Wilson 1994

  36. Sticklebacks • Morphological intermediates exist • 1 sp. in a lake -- typically intermediate morph • 2 spp. in a lake -- typically 2 morphs • Morphology is related to feeding efficiency and growth • Hypothesis: evolved morphological divergence due to competition (Character displacement)

  37. intermediate X intermediate intermediate X limnetic intermediate X benthic Morphology Experiment • 23 X 23 m ponds • Target species intermediate in morphology • produced by hybridization

  38. Target time Limnetic Morphology Target Limnetic Morphology Hypothesis • Competition with a limnetic will have greatest effect on survival and growth of forms morphologically similar to limnetic

  39. intermediate X intermediate intermediate X limnetic intermediate X benthic Morphology Experiment • Hybrids add variation on which selection can work

  40. Implication • If hypothesis is supported, selection for character divergence is occurring via competition

  41. Experimental 1800 target 1200 limnetic Control 1800 target Experiment X 2 ponds

  42. Data collection • 3 months • Collect fish, measure Target • Growth rate reduced by density • competition occurs • Regression of growth vs. morphology • Slope = growth differential between more benthic and more limnetic

  43. Control Growth Competitor I x B I x I I x L Results morphology

  44. Results • Growth differential • significant for 1 experimental group • nearly so for a 2nd experimental group • clearly not significant for both controls • Survival differential • some evidence for an effect in 1 pond • Target individuals with limnetic morphology fare worst

  45. Conclusions • Experimental evidence for character displacement • Caveats: • pseudoreplication • statistical weakness

  46. Lake whitefish Coregonus lavaretus dwarf, limnetic benthic

  47. Null models in community ecology • Experiments • show that a process occurs • may show it can cause effects on distribution, abundance, fitness of a limited set of species • Does that process structure the community as a whole? • experiments rarely can test that • If interspecific competition is important, what patterns would be predicted for communities?

  48. Community patterns • Competition favors differences in resource use among co-occurring species • Predict: co-occurring species should be more different in resource use than expected if species were placed together randomly. • Should be present across similar species within a community

  49. G. E. Hutchinson • Co-occurring European Corixids • Body lengths – ratio of larger to smaller tended to be >1.3 • Morphology as a surrogate for resource use • Origin of idea of limiting similarity

  50. Morphological pattern • Predict: co-occurring species should be more different in morphology than expected if species were placed together randomly. • "Community-wide character displacement" • How do you tell? • Null models or Neutral models of communities • Morin 98-103; Chase & Leibold 117-122