Download Interspecific Competition

Interspecific Competition
BIOL400
14 September 2015
Environmental Resource
 Consumed
or utilized item in the
environment, which results in population
increase if it becomes more abundant






Food
Space (sessile organisms)
Sunlight
Soil nutrients and water
Refugia
Etc.
Limitation
 It
is only important to study resources
which limit rate of population growth of
species using them
Mechanisms of Competition
 Exploitative/Scramble



Out-consume others for food
Pre-empt space
Grow over, to intercept sunlight or raining
aquatic detritus
 Contest/Interference

Drive other competitors away
• with chemicals (plants)
• with aggressive behavior (animals)
Principle of Competitive Exclusion
 No
two species using the same limiting
resource can coexist over the long term
Experimental Demonstrations
of Competitive Exclusion
 H.F.
Gause—1920s and 1930s
 Used two species of ciliated protozoan
(Paramecium)
 P. aurelia always survived, P. caudatum
always went extinct
HANDOUT—Gause’s
Paramecium Studies
Experimental Demonstrations
of Competitive Exclusion
 Gause’s
results mimicked by researchers
using short-lived organisms

Flour beetles
• Winner moisture- or temperature-dependent




Mice
Drosophila
Diatoms
Etc.
Figs. 10.11 & 10.12 pp. 171-172
29.1C
32.3C
HANDOUT—Tilman et al.
Diatom Studies
Experimental Demonstrations
of Competitive Exclusion
 Coexistence,
without competitive
exclusion, occurred in some pairings
Lotka-Volterra Equations
 Elaborations
of logistic population growth
 Logistic model assumes growth rate is
reduced by the extent to which members
of a population compete for limited
resources intraspecifically
 L-V model introduces the effect on
resource availability of the members of
second, competing species
Text p. 166
 and 
 Competition
coefficients
 Extent to which a member of a second
species impinges on the ability of a
member of the first to harvest resources
 The higher  and , the more intense the
competition between species
Isocline
(Figs. 10.2-10.4)
 Points
on the graph—combinations of N1
and N2—at which present population
growth rate for one is zero

e.g., r1[(K1-N1-N2)/K1] = 0
 Starting
points above the isocline go down
to it, while starting points below the
isocline go upward to it
Fig. 10.2 p. 166
Isocline Example
= 1000,  = 0.60
 Realized r1 = 0 when [N1 + (N2)] = K1
 r1 = 0 when:
 K1





N1 = 1000
N1 = 800
N1 = 500
N1 = 200
N1 = 0
and N2 = 0
and N2 = 333
and N2 = 833
and N2 = 1333
and N2 = 1667
Fig. 10.3 p. 166
Fig. 10.4a p. 167
"Good" for species 1—K1 > K2/
"Bad" for species 2—K2 < K1/
Fig. 10.4b p. 167
"Bad" for species 1—K1 < K2/
"Good" for species 2—K2 > K1/
Fig. 10.4c p. 167
"Bad" for both species—stable equilibrium
of species abundances
Fig. 10.4d p. 167
"Good" for both species—outcome of
competition is unpredictable exclusion
Schoener (1983)
 164
experimental tests for effects of
interspecific competition



148 (90%) showed one species affecting N
and r of another
Of 390 spp. tested, 76% showed competition
effects in at least some conditions tested
57% always showed effects
 But—surely
NOT a random sample of
study species
Gurevitch et al. (1992)



Effect Size =
difference in biomass
between experimental
species with and
without competitors,
divided by pooled
standard deviations of
the two treatments
218 competition
experiments
Large effect sizes,
esp. for herbivores
Fig. 10.18 p. 177
Niche

Grinnell (1917)—
literally a subdivision
of habitat
 Elton (1927)—"role"
in an ecosystem
 Hutchinson (1958)—
mathematically, an ndimensional
hypervolume
Fig. 10.13 p. 173
Restating the Gaussian Principle
of Competitive Exclusion Using
Hutchinsonian Terminology
 Species
with high overlap in their
hypervolumes (high  and ):

Exclusion of one or the other
 Species

with little overlap (low  and ):
Resource partitioning and coexistence
Resource Partitioning
 Closely
related or ecologically similar
species that coexist generally differ in
resource use, reducing competition
 Concept stems from doctoral work of
Robert MacArthur
Fig. 10.14 p. 174
Niche Complementarity
 Two
species that overlap greatly in one
major type of resource (e.g., diet) will tend
to overlap little in another (e.g.,
microhabitat)
HANDOUT—Cody 1968
HANDOUT—Lindeman 2000
Slider, Trachemys scripta
River cooter, Pseudemys concinna
Ouachita map turtle, Graptemys ouachitensis
Mississippi map turtle, Graptemys pseudogeographica
Smooth Softshell, Apalone mutica