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Experimental studies

Experimental studies. Evidence is cumulative Density manipulations are now the standard Not always feasible spatial scale ethics Reviews of experiments Connell 1983 Schoener 1983 Gurevitch et al. 1992. Schoener 1983 Prevalence of competition. 164 experimental studies

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Experimental studies

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  1. Experimental studies Evidence is cumulative Density manipulations are now the standard Not always feasible spatial scale ethics Reviews of experiments Connell 1983 Schoener 1983 Gurevitch et al. 1992

  2. Schoener 1983Prevalence of competition • 164 experimental studies • 90% demonstrated competition • Freshwater 91% • Marine 94% • Terrestrial 89% • 90% of all species compete??? • 90% of cases where hypothesis of competition is reasonable, competion can be demonstrated

  3. Connell 1983Prevalence of competition • Frequency of experiments rather than studies or species • incorporates more variation in time & space • more conservative • 69 studies 86% found competition • 200 species 55% showed competition • spp. w/ >1 expt. 43% of expts. / spp.

  4. Schoener 1983Trophic status and competition • Hairston, Smith, Slobodkin - HSS (1960): prevalence of competition varies with trophic level in terrestrial systems • producers, carnivores, decomposers, frugivores, granivores, nectarivores should often compete • phytophagous herbivores should rarely compete

  5. Schoener 1983Trophicstatus • Terrestrial carnivores 67% compete • 21 all tests, 0 some tests, 14 no tests • Terrestrial producers 84% compete • 74 all tests, 45 some tests, 22 no tests • Nectar & grain feeders 88% compete • 21 all tests, 2 some tests, 3 no tests • Phytophagous herbivores 50% compete • 5 all tests, 1 some tests, 6 no tests

  6. Connell 1983Trophic status • Terrestrial carnivores compete in 11% of expts. • Terrestrial plants compete in 30% of expts. • Nectar/Grain feeders compete in 17% of expts. • Terrestrial herbivores compete in 23% of expts. • Lower % than Schoener • Less support for HSS • competition rare for carnivores, more common for herbivores

  7. Schoener 1983Asymmetrical competition • Lawton & Hassell (1981): Competition is typically asymmetrical; exclusion • Asymmetrical 51 studies • Possibly asymmetrical 24 studies • Symmetrical 10 studies • Support Lawton & Hassell • Exclusion should be common

  8. Connell 1983Asymmetrical competition • 44 pairs of species no competition • 33 pairs of species strong asymmetry • 21 pairs of species symmetry • 61% of cases demonstrating competition were strongly asymmetrical • But, 27% of asymmetrical cases showed reversals (time or space) • Exclusion?

  9. Connell 1983Inter- vs. intraspecific competition • if inter- > intra- predict exclusion • 123 experiments determined both • no competition 26% of experiments • inter-  intraspecific 18% of experiments • inter- > intraspecific 17% of experiments • inter- < intraspecific 39% of experiments • Connell expects exclusion to be rare

  10. Connell & Schoener • Agree that: • competition is often found when observations lead to the hypothesis of competition • competition it is often asymmetrical • competition is common for plants & marine systems • HSS hypothesis not well supported • Disagree about likelihood of exclusion

  11. Connell & Schoener • Method: enumerate studies or experiments that found significant effects • More recent approach – meta-analysis • statistical evaluation of standardized effect sizes across multiple studies • does not rely on significance of effect • effects can contribute to evidence even if individually they are not significant

  12. Gurevitch et al. 1993meta-analysis • Estimate effect size for a single study • where: • YeandYcare treatment means for experimental (i.e., raised or lowered density) and control • s is the pooled standard deviation for the effect, and Ne and Ncare numbers of individuals in experimental and control treatments (summed over replicates

  13. Gurevitch et al. 1993meta-analysis • m = Ne+ Nc - 2 • J(m) is a correction factor for small sample sizes • variance of d is:

  14. Gurevitch et al. 1993meta-analysis • cumulative effect size, summed across all studies is: • variance of d+ is: • and lower and upper confidence limits for d+ are: • where Ca/2 is a 2-tailed critical value from a standard normal distribution

  15. Gurevitch et al. 1993meta-analysis • Meta-analysis provides: • explicit tests of whether d+= 0 • tests for heterogeneity of effects among classes of studies (e.g., between trophic levels) • details given in paper

  16. Gurevitch et al. 1993meta-analysis • Results • Competition had large collective effect: d+ = 0.80 (95% CI = 0.77 – 0.83) • heterogeneity was very high • competitive effects differed substantially among trophic levels, but NOT in the way we would expect based on HSS

  17. Gurevitch et al. 1993meta-analysis • Results: trophic levels

  18. Gurevitch et al. 1993meta-analysis • Results: habitat • effect size did not differ among terrestrial, freshwater, and marine systems for producers • for carnivores results were unclear • Results: inter- vs. intra-specific competition • for both carnivores & producers, strengths of inter- and intra-specific competition were not different

  19. Gurevitch et al. 1993meta-analysis • Addressed a number of non-ecological issues concerning design of studies • duration • replication • caging and other enclosures

  20. Three examples • Connell 1961 - barnacles • Dunham 1981 - lizards • Horton & Wise 1983 - orb spiders

  21. Connell 1961 Distributions of Balanus & Chthamalus Chthamalus Balanus highest high tide Chthamalus Balanus ROCK Adults Larvae Adults Larvae lowest low tide

  22. Experiments • Densities of Balanus = 49 / cm2 • Choose plots = 88 cm2, divide in half • randomly assign 1/2 for removal of Balanus • other 1/2 control, count only • Rocks with young adults of one species • transplant Balanusto high & low intertidal • transplant Chthamalusto high & low intertidal • Follow marked individuals over years

  23. Experimental result #1 • Without Balanus, Chthamalussurvives well at all intertidal levels • intraspecific competition among Chthamalus rarely resulted in death. • With Balanuspresent, Chthamalus is completely eliminated • Local competitive exclusion of Chthamalus below the high water mark

  24. Experimental result #2 • Balanus individuals grow rapidly • Shell undercuts, crushes adjacent Chthamalus • Competition for space; Balanuswins Undercut Crushed

  25. Experimental results #3 • Balanus does not survive in the high intertidal, regardless of Chthamalus • Chthamalus tolerates dry conditions • Balanus upper limit set by physical environment • Chthamalus has a refuge from competition

  26. Experimental results #4 • Very low intertidal -- near low tide mark • Neither species survives well unless protected from the snail Thais • tends to prefer Balanus • rarely goes above mean water line

  27. Experimental conclusions • Balanus • upper limit set by physical environment • lower limit set by Thais predation • Chthamalus • upper limit probably set by physical environment • lower limit set by interspecific competition • Asymmetry

  28. Sceloporus occidentalis Dunham 1981Competition between insectivorous lizards • Sceloporus merriami • Urosaurus ornatus

  29. Experiment • Chihuahuan desert of Texas • 6 plots: • 2 remove Sceloporus • 2 remove Urosaurus • 2 control • Census monthly, marked individuals • Population size • Survival

  30. Experiment • 3-year study • short relative to generation time • population growth correlates • reproduction, lipid store, growth • foraging success (gut contents) • food availability

  31. Experimental results • Population density • Remove Urosaurus … No effect on Sceloporus • Remove Sceloporus … Urosaurusdensity  • Survival (1st year ind.; Table 5) • Remove Urosaurus… Sceloporus survival  in 1/3 years (1975) • Remove Sceloporus … Urosaurus survival  in 1/3 years (1974; 1976 it declined) • Survival (adults; Table 6)

  32. Experimental results • Individual growth • Remove Urosaurus … No effect on Sceloporus • Remove Sceloporus … Urosaurusgrowth  in 1/3 years • Foraging success • Remove Urosaurus… No effect on Sceloporus • Remove Sceloporus … Urosaurus body mass & lipid store  in 1/3 years

  33. Experimental conclusions • Intensity of competition varies year to year • Resource competition • foraging success implicated • aggressive encounters rarely observed • Rain, arthropod abundance varies year to year • Sceloporus effect on Urosaurus in years with lower food abundance • Urosauruseffect on Sceloporus minor in most years … asymmetry

  34. Horton & Wise 1983Competition among garden spiders “Although occasionally afflicted with an irrational fascination with vertebrates, most ecologists, after detached deliberation, would name the spider as a typical terrestrial carnivore.” — David Wise 1983

  35. Horton & Wise 1983Competition among garden spiders • Argiopeaurantia, Argiopetrifasciata • Large orb weaving garden spiders • Webs of A. aurantia lower in some studies • Potential responses to density manipulations • Web location, height • Prey type, size • Survival • Growth of individuals

  36. Density remove A. trifasciata A. aurantia N = 2 N = 2 CONTROL HIGH 2-3X remove A. aurantia A. trifasciata N = 2 N = 2 Both N = 2 N = 2 Experiment • 12 X 12 m plots • 1.5 m mowed

  37. Experimental results • A. aurantia built web lower • no effects of presence/absence of A. trifasciata • density affected height in 1/5 censuses • Species differed in prey taxa captured • no effects of density or competitor on proportion of webs with prey • no effects on frequency of insect orders

  38. Experimental results • Emigration from plots • no effects of density or competitor • Individual growth rate • no effects of competitor • Survival • density affected A. trifasciatain 1/2 years • density did not affect A. aurantia • no effects of competitor

  39. Experimental conclusions • Interspecific competition undetectable • typical of spiders: food limitation without resource competition • add food, population increases • remove other species, no effect • food may be hard to get, but not depleted by spiders

  40. Three studies • Illustrate the range of outcomes • Interspecific competition ranges from • strong & consistent • to variable • to absent • When it occurs, it is often asymmetrical • Correlates of population growth

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