Download Community Ecology BSC 405

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
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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
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•
Argiope aurantia, Argiope trifasciata
Large orb weaving garden spiders
Webs of A. aurantia lower in some studies
Potential responses to density manipulations
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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