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 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

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.

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

Schoener 1983Trophic status

• 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

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

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

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?

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

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. 1993meta-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. 1993meta-analysis

• m = Ne+ Nc - 2 J(m) is a correction factor for small sample sizes

variance of d is:

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

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

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

Gurevitch et al. 1993meta-analysis

• Results: trophic levels

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

Gurevitch et al. 1993meta-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

lowest low tide

highest high tide Balanus Chthamalus

Adults Larvae Adults Larvae

Balanus

Chthamalus

ROCK

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

• Balanus individuals grow rapidly• Shell undercuts, crushes adjacent Chthamalus• Competition for space; Balanus wins

UndercutCrushed

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 1981Competition between insectivorous lizards

• Sceloporus merriami• Urosaurus ornatus

Sceloporusoccidentalis

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 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

Horton & Wise 1983Competition 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

A. aurantia

A. trifasciata

Both

N = 2 N = 2

N = 2

N = 2

N = 2

N = 2

removeA. trifasciata

removeA. aurantia

DensityCONTROL HIGH 2-3X

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