Transcript Laboratory Experiments for Inter

Chapter 13
Competition
Modes of Competition
Interference vs. exploitation:
– Direct aggressive interaction between
individuals
– Using up resource
Intraspecific:
– Competition with members of their own species
Interspecific:
– Competition between individuals of two species –
reduces fitness of both
Studies of Intra-Specific
Competition
Resource Competition
Intraspecific Competition - Herbaceous Plants
– Tilman + Cowan (1989):
– Indiangrass grown at two densities
7 per pot; 100 per pot
Different Nitrogen
– Plant growth rates and weights increase in low
density populations
Resource competition
Competition for resources is more intense
at higher population densities
– Usually leads to mortality among competing
plants
Intraspecific Competition Among
Planthoppers
Denno and Roderick (1992)
Experiment: 3,11,40 leafhoppers per cage
with salt marsh grass
– Attributed prevalence of competition among
leafhoppers to habit of aggregating, rapid
growth, and mobile nature of food supply
Fig 13.6
Niches
Niche:
– summarizes environmental factors that influence
growth, survival, and reproduction of a species
Gause:
Principle of Competitive
Exclusion
– Two species with identical niches cannot
coexist indefinitely
One will be a better competitor and thus have
higher fitness and eventually exclude the other
Niches
Hutchinson:
– defined niche as:
– n-dimensional hyper-volume
n is number of environmental factors important to
survival and reproduction of a species
Niche
– Fundamental niche – hypervolume
All of these environmental factors
– Realized niche: includes interactions such as
competition that may restrict environments
where a species may live
Feeding Niches of Galapagos
Finches
– Grant (1986): differences in beak size among
ground finches - translated directly into diet
Fig 13.8
Size of seeds eaten estimated by measuring beak
depths
– Individuals with deepest beaks fed on hardest seeds
Fig 13.9
After 1977 drought, remaining seeds were very hard
– thus mortality was most heavy in birds with smaller
beaks
Fig 13.10
Fig 13.11
Mathematical and Laboratory
Models
Models:
– Abstractions and simplifications, not
facsimiles of nature
– Man-made construct; partly empirical and
partly deductive
– Used to provide insights into natural
phenomena
Lotka Volterra
Effect of interspecific competition on pop.
growth of each species:
– dN1 / dt = rm1N1 ((K1-N1-12N2) / K1)
– dN2 / dt = rm2N2 ((K2-N2-  21N1) / K2)
 12: effect of individual of species 2 on rate of pop.
growth of species 1
 21: effect of individual of species 1 on rate of pop.
growth of species 2
Laboratory Experiments for
Inter-specific Competition
Paramecia Lab Experiments
Gause tested Lotka-Volterra predictions
He demonstrated resource limitation with
Paramecium caudatum and Paramecium
aurelia in presence of two different
concentrations of Bacillus pyocyaneus
– When grown alone, carrying capacity
determined by intraspecific competition
– When grown together, P. caudatum quickly
declined
Fig 13.15
Competition and Niches
Competition can restrict species to fewer
environmental conditions
– But if competitive interactions are strong and
pervasive, may produce evolutionary
response in competitor population
Changes fundamental niche
Competition Examples
Tansley (1971):
2 species of bedstraw
that grow in different
soils.
Mutual competitive
exclusion?
Experiments
Tansley’s results suggested interspecific
competition restricts realized niche of each
of two species of bedstraw (Galium spp.) to
a narrower range of soil types
Fig 13.18
Field Experiments for Interspecific Competition
Removal Studies
Connell (1961)
Found interspecific competition in barnacles
Balanus play role in determining lower limit of
Chthamalus within intertidal zone
– Exposure to air did not account for all observed
patterns
Fig 13.20
Competition Examples
Brown and Munger (1985) studied
competition among rodents in Chihuahuan
Desert
Largescale experiments: 20 ha site
– 24 plots that were 50 x 50 m
Definitions of mammal groups:
Granivores: seed eaters
– Large = Dipodomys - 100 g kangaroo rats
– Small = Perognathus - 15 g pocket mice
Insectivores: eat insects
– Onychomys - 30 g grasshopper mice
Experiments in enclosures:
Wire mesh buried, and too small for
rodents
Holes in fence initially to allow all in
Then exclude large kangaroo rats
– Little guys should increase if they were
competing for food
Fig 13.23
Character Displacement
Because degree of competition is
assumed to depend upon degree of niche
overlap,
– interspecific competition is predicted to lead to
directional selection for reduced niche overlap
Character displacement:
Fig 13.25
allopatry
sympatry
Character Displacement
Taper and Case: Necessary criteria:
– Morphological differences between sympatric
species greater than differences between
allopatric populations
– Differences between sympatric and allopatric
populations have genetic basis
– Differences between sympatric and allopatric
pops. evolved in place:
not derived from different founder pops already
differing in the character
Taper and Case: Characteristics
– Variation in character must have known effect on
use of resources
– Must be demonstrated competition for the
resource and competition must be directly
correlated with character similarity
– Differences in character cannot be explained by
differences in resources available to each of the
populations
Summary
Studies of intraspecific competition provide
evidence for resource limitation
A niche reflects the environmental
requirements of a species
Mathematical and laboratory models provide
theoretical foundation for studying
competitive interactions in nature
Competition can have significant ecological
and evolutionary influences on the niches of
species