LECTURE 14
CH 16
COMPETITION
MAJOR CONCEPTS
1) Facilitation is the alternative to competition; it is understudied.
2) Consumers compete by using a resource that reduces availability to others.
3) Competition occurs through exploitation of shared resources or direct interference.
4) Responses of plant and animal species to intraspecific competition include decrease in
growth, delay in reproduction, and decrease in reproduction and survival.
5) The theory of competition and coexistence is an extension of logistic growth models.
6) The niche reflects the environmental requirements of species; its dimensions shrink
when faced with interspecific competition.
7) Organisms evolve in response to competition and may not be competing today; many
types of evidence are used to deduce that (past) competition explains current traits.
8) The principle of competitive exclusion arose when species failed to coexist in the lab.
9) Habitat productivity can influence plant competition.
10) Consumers of competitors can influence the outcome of competition.
11) Experimental field studies demonstrate widespread competition in nature.
PART 1
Facilitation as alternative to competition
Sp 1 aids recruitment or survival of sp 2 14.10
Not altruism; no cost to facilitator
Understudied relative to competition
Competition: Use or defense of a limiting resource that reduces availability to others
Resource: any substance or factor that is:
a) consumed by an organism
b) supports increased population growth rates as its availability increases.
Superior competitors can persist at lower resource levels. 16.2
Types of competition pg. 338-339
Exploitation (scramble; indirect): use by 1 makes unavailable to another
Vs. Interference (direct): active defense by 1 makes unavailable to another
Intraspecific (within species) vs. interspecific (between species)
Expect intrasp > intersp because individuals more similar pg. 331; 16.4
Allelopathy
Chemical interference competition by plants 16.14, 16.15, 16.16
Chemical exudates directly inhibit growth of competitor
Circumstantial and laboratory evidence
Difficulty of demonstrating experimentally in field
Alternative explanations of patterns (e.g. seed predation)
Types of resources (a condition, e.g. temperature, is NOT a resource)
Plants
Abiotic (light, water, nutrients)
Biotic (symbionts, pollinators, seed dispersers)
Animals
Abiotic (space, territory, hiding places) 16.3
Biotic (food)
Renewable (constantly regenerated) pg. 332
Non-renewable (occur in fixed amounts and can be fully re-used)
Liebig’s Law of Minimum pg. 332
Population limited by the most scarce single resource
Assumes no interaction between resources in determining response
Can be violated because of interaction and synergism between resources 16.5, 16.6
Law of constant yield: if exceed K in planting density, no increase in population size
above K
Consequences of competition for population growth:
Intraspecific competition in plants  density-dependence of population growth
Decreases growth and size hierarchies develop (competition as selective force)
Decreases reproduction (births)
Increases mortality (deaths) (‘self-thinning’ of population)
Intraspecific competition in animals  density-dependence of population growth
Decreases growth
Increases age of sexual maturity (delays births)
Decreases births + increases mortality lowers r
Logistic equation extended to include theory of competition and coexistence
Based on intraspecific competition decreases per capita pop. growth rate
via density-dependent birth and death rates; causes r to decrease
Population size continues to increase until N = K, but
rate of pop growth slows as N approaches K (as (K-N)/K approaches 0)
at K, r = 0 16,8
Additions to logistic growth equations
Modified to include interspecific competition; 16.9
add effects of sp 1 on 2; sp 2 on 1
Competition coefficients: conversion factor to convert sp 2 individuals into
equivalent # of sp 1
Presence of sp 2 decreases size of K available to sp 1
 = effect of sp2 on sp 1 N1 =  N2 = saturating effect
 = effect of sp1 on sp2
N2 =  N1 = saturating effect
 +  = proportionality constants of effect of 1 sp on 2nd sp
dN1/dt = r1N1 (K1 - N1 -  N2) / K1
dN2dt = r2N2 (K2 – N2 -  N1) / K2
PART 2
Niche pg. 423-4
Ranges of conditions and resource qualities within which species persists
Niche axis
Niche breadth
Multiple niche axes  hypervolume
In practice, study 1 or 2 axes
Fundamental niche (without competitors)
Vs. Realized niche (with competitors) pg.200
Intraspecific competition  selects for broader niche
Interspecific competition  may select for narrower niche (leads to specialization)
Interspecific competition
‘Ghost of competition past’: today’s organisms no longer in competition because of
past selection to avoid competition
Often lose evidence of competitive exclusion because poor competitor is gone
Types of evidence used to deduce that (past) competition explains current traits
Niche separation (resource partitioning); reduced niche overlap
Habitat shift
Character displacement pg. 359 17.16, 17.17
Competitive exclusion: two species can’t coexist on same limiting resource 16.7
Classic experiments of interspecific competition
Winner of competition can depend on environmental conditions
Two species can coexist if limited by different resources 16.10
Competitive release when one competitor is gone
Predation can influence outcome of competition
Predators can maintain prey species diversity by reducing superior competitors 16.17, 16.18
Apparent competition: Herbivores or predators can determine the competitive balance 16.19, 16.20
Field studies: how demonstrate competition?
Introduce (add) species it excludes related species
Removal of species  competitive release (increase in remaining competitors)
Compare intra-vs. interspecific competition in field 16.1
Both can occur simultaneously
Evidence of asymmetric competition 16.11
Sp 1 controls Sp 2 in Sp 2’s habitat, but not vice versa
Occurs when different factors limit the populations of competitors
Possibility of tradeoffs: superior competitive ability in favorable habitat may be linked
to poor tolerance of stressful conditions; and vice versa
Summary 1-16!