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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 • 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 Connell 1983 Prevalence of competition • Frequency of experiments rather than studies or species • incorporates more variation in time & space • more conservative – 69 studies – 200 species – spp. w/ >1 expt. 86% found competition 55% showed competition 43% of expts. / spp. Schoener 1983 Trophic 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 Schoener 1983 Trophic status • Terrestrial carnivores 67% compete – 21 all tests, 0 some tests, 14 no tests • Terrestrial producers84% 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 Connell 1983 Trophic 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 Schoener 1983 Asymmetrical competition • Lawton & Hassell (1981): Competition is typically asymmetrical; exclusion – Asymmetrical – Possibly asymmetrical – Symmetrical 51 studies 24 studies 10 studies • Support Lawton & Hassell • Exclusion should be common Connell 1983 Asymmetrical competition • 44 pairs of species • 33 pairs of species • 21 pairs of species no competition strong asymmetry symmetry – 61% of cases demonstrating competition were strongly asymmetrical – But, 27% of asymmetrical cases showed reversals (time or space) – Exclusion? Connell 1983 Inter- vs. intraspecific competition • if inter- > intra- predict exclusion • 123 experiments determined both – – – – no competition inter- intraspecific inter- > intraspecific inter- < intraspecific 26% of experiments 18% of experiments 17% of experiments 39% of experiments • Connell expects exclusion to be rare 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 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 Gurevitch et al. 1993 meta-analysis • Estimate effect size for a single study where: – Ye and Yc are treatment means for experimental (i.e., raised or lowered density) and control – s is the pooled standard deviation for the effect, – and Ne and Nc are numbers of individuals in experimental and control treatments (summed over replicates Gurevitch et al. 1993 meta-analysis • m = Ne+ Nc - 2 J(m) is a correction factor for small sample sizes variance of d is: Gurevitch et al. 1993 meta-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 Gurevitch et al. 1993 meta-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 Gurevitch et al. 1993 meta-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 Gurevitch et al. 1993 meta-analysis • Results: trophic levels Gurevitch et al. 1993 meta-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 interand intra-specific competition were not different Gurevitch et al. 1993 meta-analysis • Addressed a number of non-ecological issues concerning design of studies – duration – replication – caging and other enclosures Three examples • Connell 1961 - barnacles • Dunham 1981 - lizards • Horton & Wise 1983 - orb spiders Connell 1961 Distributions of Balanus & Chthamalus highest high tide Balanus Chthamalus Chthamalus ROCK lowest low tide Adults Larvae Adults Larvae 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 Balanus to high & low intertidal – transplant Chthamalus to high & low intertidal • Follow marked individuals over years Experimental result #1 • Without Balanus, Chthamalus survives well at all intertidal levels – intraspecific competition among Chthamalus rarely resulted in death. • With Balanus present, Chthamalus is completely eliminated • Local competitive exclusion of Chthamalus below the high water mark Experimental result #2 Crushed Undercut • Balanus individuals grow rapidly • Shell undercuts, crushes adjacent Chthamalus • Competition for space; Balanus wins 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 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 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 Dunham 1981 Competition between insectivorous lizards • Sceloporus merriami • Urosaurus ornatus Sceloporus occidentalis Experiment • Chihuahuan desert of Texas • 6 plots: – 2 remove Sceloporus – 2 remove Urosaurus – 2 control • Census monthly, marked individuals • Population size • Survival Experiment • 3-year study • short relative to generation time • population growth correlates – reproduction, lipid store, growth – foraging success (gut contents) – food availability Experimental results • Population density – Remove Urosaurus … No effect on Sceloporus – Remove Sceloporus … Urosaurus density • 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) Experimental results • Individual growth – Remove Urosaurus … No effect on Sceloporus – Remove Sceloporus … Urosaurus growth in 1/3 years • Foraging success – Remove Urosaurus… No effect on Sceloporus – Remove Sceloporus … Urosaurus body mass & lipid store in 1/3 years 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 • Urosaurus effect on Sceloporus minor in most years … asymmetry Horton & Wise 1983 Competition 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 Horton & Wise 1983 Competition among garden spiders • • • • Argiope aurantia, Argiope trifasciata 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 Experiment Density CONTROL HIGH 2-3X N=2 remove A. trifasciata A. trifasciata N = 2 N=2 remove A. aurantia Both N = 2 N=2 A. aurantia N = 2 12 X 12 m plots 1.5 m mowed 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 Experimental results • Emigration from plots – no effects of density or competitor • Individual growth rate – no effects of competitor • Survival – density affected A. trifasciata in 1/2 years – density did not affect A. aurantia – no effects of competitor 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 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