Recap
Competition model and outcomes
Asymmetric competition
Habitat productivity
13.4 Habitat productivity can influence
competition between plant species
Two hypotheses:
1. Plants compete more intensively when mineral nutrients are
less abundant in the soil (By Grubb and Tilman)
Plants compete more intensively when nutrients are less.
High nutrients are less likely to limit plant population;
thus the intraspecific competition is weak.
2. Competition is less intense when water and nutrients are
less abundant (Grime and Keddy)
Competition for light is more important than
competition for nutrients; limit in water and nutrients would
limit the population growth to a certain point that individual
plants are widely spread and do not compete for light.
Difference between these hypotheses lies in the relative
importance placed on belowground and aboveground
competition for resources --Light or nutrient. (Debate)
Habitat productivity can influence
competition between plant species
Smooth cordgrass
saltmeadow cordgrass, black grass, alder
Habitat productivity can influence
competition between plant species
Saltmeadow vs Smooth
Blackgrass vs saltmeadow
Fertilization alters the outcome of competition by removing
nutrient limitation on stress-tolerant plants, expand, away from
water.
13.5 Competition may occur through
direct interference


Exploitation: indirectly influencing each
other by consuming the same resources
(eat same grass by zebras , compete for
water uptake by trees, indirectly)
Interference: direct influencing each other
by preventing others to occupy a habit or
access resources (birds, bees chase birds
and bees, animals release toxic
chemicals).
 Meadow vole (wet) and mountain vole
(dry). (Asymmetric competition also)
Allelopathy (chemical competition)
Figure 16.14 Some plants
(eucalyptus) compete by chemical
means.
Clumps of shrubby Salvia plants
(mint) are usually surrounded by
bare zones separating the sage from
neighboring grassy areas ( Figure
16.15)
Australian ironwood trees
Consumers can influence the outcome of competition
Keystone predator
Starfish prey on
mussels, barnacles,
limpets, and chitons
Remove starfish,
what would happen?
Species diversity
increase or decrease?
Why?
Grazing on
plant
diversity?
Predator can influence the
outcome of prey competition
Peter Morin, Rutgers
Salamander
Frog or toad tadpole
(300 each of 3 species)
Apparent competition
Combined populations of
two prey species support a
larger predator population
neither can support alone.
As a result, two prey
populations reduced, gives
outward appearance of
interspecific competition.
Experimental supports:
Nettle aphid, grass aphid and ladybug beetle (Smith and Smith,
page 359)
Brought nettle aphid plants to grass aphid plants together
suppressed both population, as a results of larger ladybug beetle
population.
Apparent competition mediated by
pathogens (microbes)
Corals can be indirectly harmed by the presence of algae
Antibiotics can reverse the negative effects of algae
on coral growth
Smith et al. 2006
BIOL 4120: Principles of Ecology
Lecture 14: Evolution of
Species Interaction
Dafeng Hui
Office: Harned Hall 320
Phone: 963-5777
Email: dhui@tnstate.edu
A story of European rabbits in
Australia
Invasive species and its control.
Rabbit and myxoma virus.
Coevolution can occur between hostparasite, predator and prey and
competitors



Predators exert a selective pressure on prey —
any characteristic that enables individual prey to
avoid being detected and captured by a predator
will increase its fitness
Natural selection should
• Function to preserve “smarter,” more evasive
prey
• Produce “smarter,” more skilled predators
Coevolution: as prey species evolve ways to
avoid being caught, predators evolve more
effective means to capture them


Coevolution is different from adaptation to
environments
Biological agent:
 Stimulate mutual evolutionary responses in the
traits of interacting populations
 Predator and prey
 Adaptations to environment have no
reciprocal effect
 Foster diversity of adaptations, rather than
promote similarity
 Avoid competition, predate on same prey
etc
 Adaptations to physical stress lead to same
traits  convergence.
Outline (Chapter 17)
14.1 Adaptations in response to predation
demonstrate selection by biological agents
14.2 Antagonists evolve in response to each other
14.3 Coevolution in plant-pathogen systems reveals
genotype-genotype interactions
14.4 Consumer and resource populations can achieve
an evolutionary steady state
14.5 Competitive ability responds to selection
14.6 Coevolution involves mutual evolutionary
responses by interacting populations
14.1 Adaptations in response to predation
demonstrate selection by biological agents

Coloration is an example of a trait that can evolve in
prey under selection from predators

Prey: use coloration to avoid predation

Predators: can increase the fitness (adaptation) by
increasing the efficiency to find well-camouflaged
prey or avoid prey with coloration that signals
noxious qualities
Crypsis versus warning coloration
• Cryptic coloration (colors and patterns, object
resemblance))
 Hide in normal environment
• Moths on trees. Flounder
• Most palatable, edible animals
• Warning coloration or aposematism (bold
colors with patterns that serve as warning to
would-be predators)
 Learnt behavior due to bad experience
• Bees and wasps
• Snake
• Skunk (black and white stripes)
• Noxious animals
Why aren’t all potential prey noxious or
unpalatable?
Many palatable organisms have evolved cryptic
appearances to avoid detection by predators
A Katydid, a stick insect, and a
lantern fly
Mantid
Many unpalatable organisms have evolved
warning coloration
Aggregate to enhance the
warning signal.
Mimicry
Copy coloration of toxic species
• Batesian mimicry
Edible species mimic inedible species,
non-venomous mimic venomous
species
Snakes
• Mullerian mimicry
Unrelated species have a shared color
patterns that function to keep
predators away
Bumblebees, Social wasps
Tropical butterfly
Batesian mimics are palatable prey organisms
that resemble noxious ones
Poisonous
coral snake
Scarlet king
snake
Mullerian mimics are unpalatable species adopt
a single pattern of warming coloration.
Mantid (b) and moth (c) have both evolved to resemble a wasp (a).
Mullerian mimics are unpalatable organisms a
pattern of warning coloration
Warning
coloration
Mullerian
mimicry
Patterns of black and orange “tiger stripes” or black, red and
yellow patterns serve as warning signals.
Predator Defenses
Cryptic coloration, warning coloration,
Mullerian mimicry, Batesian mimicry

Predators may use cryptic coloration to blend into
background and use deception by resemble the
prey
14.2 Antagonists evolve in response to
each other




Coevolution of pathogen and host by
Charles Mode (1958)
Pathogen virulence was controlled by
V gene
Host resistance was controlled by R
gene
Fitness of host and fitness of
pathogen were each contingent on
the genotype of the other






Mode’s Conceptual Model:
When host is susceptible (rr),
selection favors virulent pathogens
(VV or Vv)
Virulent pathogens cause selection
for host resistance (RR or Rr)
When host is resistance, selection
favors avirulent pathogen (vv)
When pathogen is avirulent,
selection favors susceptible hosts (rr)
Cycling r(host)->V(pathogen)>R(host)->v (pathogen)->r(host)
Evolution in houseflies and their
parasitoids
David Pimentel et al.
Cornell University (1960s)
One control population
Not attacked by wasp
One treatment population
Attacked by wasp
Train over three years
Reproduction rate for wasp
dropped from 135 to 39
progeny per female in the
treatment cage, longevity
decreased from 7 to 4 days.
Then start experiment, allow
population size changes
Pimentel’s classic experiment tested for a host
evolutionary to a parasitoid
Only wasp
evolve,
control
Wasp: parasitoid,
Both
evolve,
experiment
Fly: host
Population changes in Pimentel’s parasitoid-host
system demonstrated that populations evolve in
response to each other
14.3 Coevolution in plant-pathogen
systems reveals genotype-genotype
interactions
Genetic basis of coevolution
Crops and diseases
Wheat and wheat rust
Wheat rust (fungi):
Many different strains
Wheat: May be resistant or
susceptible to a strain
Genotype-genotype
interaction: differences in the
expression of genotypes in
one species depending on the
genotypes of another species.
Proportion of wheat rust strain populations in
Canada (Green 1975)
Genetic variation in a host may parallel genetic
variation in a pathogen
Scale insects are sedentary and local populations evolve
independently on individual trees (Alstad and Edmunds 1978)
14.5 Competitive ability responds to
selection
Competitors exert selection pressure on each
other
1. Diverge from one another in terms of the
resources they consume
2. Selection for increased efficiency of resource
use
Demonstrating genetic variation in
competitive ability
Francisco Ayala
Two fruit fly species: Drosophila serrata and D. nebulosa
Cage study in a lab
Co-existence: 20-30% D. serrata and 70-80% D. nebulosa
If remove D. serrata from these populations and put in a cage
with unselected stocks of D. nebulosa, D. serrata showed
superior competitive ability
One general conclusion: Sparse population can evolve the
ability to compete against other species more rapidly than
dense populations.
Competiton study between fly
populations
David Pimentel
House fly and
blowfly
Similar life cycle
and food
requirements
Initially, each
species won twice
Using this facility,
Pimentel showed
that two species coexist for 70 weeks.
A rare competitor can evolve superior
competitive ability
A species that is being excluded by another species and becoming rare,
can evolve increased interspecific competitive ability rapidly to regain
control.
Character displacement
Sympatric species:
Two species coexist
within the same
geographic area
Allopatric species:
Two species’s geographic
ranges do not overlap.
If competition causes
divergence, then the
sympatric populations of
species 1 and 2 in area B
will differ more from each
other than the allopatric
populations of those
speices in areas A and C:
Character displacement.
Beak sizes of
finches illustrate
character
displacement
Darwin’s Finches
Character
displacement
Character displacement

The outcome of the
competition was a shift
in feeding niches. When
the shift involves
features of the species’
morphology, behavior,
or physiology, it is called
character displacement.
14.6 Coevolution involves mutual
evolutionary responses by interacting
populations
Coevolution implies reciprocal evolutionary responses between pairs
of populations, as we see in predator-prey, pathogen-host.
Pollinator and flower (humingbird and flower)
Ants and aphids
Aphids suck plant juices and produce large volumes of excreta
from which they either do not or can’t extract all the nutrients. Thus
honeydew production may simply reflect their diet, rather than having
evolved to encourage protection by ants.
Ants: voracious generalists and attack any insects they
encounter. May not need special adaptations to deter the predators of
aphids.
Why don’t ants eat aphids they tend? (this restraint is an
evolved trait of ants that facilitate ant-homopteran mutualism.)
Plant defenses and herbivore responses
Amount of biomass eaten by herbivores:
6-10% forest,
30-50% grassland
Outbreak of grasshopper, gypsy moths etc can kill.
Normally not kill, even stimulate growth.







Plants defend themselves by producing
secondary compounds
Legumes (pea family) seeds contain SCs that
inhibit the digestive enzymes of insect
herbivores
But adult bruchid beetles lay eggs on
seedpods. Larvae can consume and grow
Soybeans are resistant to attack even by
bruchid species (produces chemicals that
inhibit development of bruchid larvae)
A tropical legume also contain a nonprotein
amino acid that is toxic to most insect.
But one bruchid can feed on the plants,
possesses enzymes that degrade the amino
acid.
<Every defense, a new counterattack devised>
Yucca moth and yucca, an obligate
mutualism
Moth will lay
eggs into the
ovary of the
flower, then
crawls to the
top of the
pistil of the
flower and
deposits a bit
of pollen on
the stigma to
make sure
plant will
Yucca, agave family
produce
Semelparous, huge flower
seeds.
Female also carry around pollens to other
plants after lay eggs
Phylogenetic trees can reveal preadaptation
(traits that become useful for a purpose other than
for which they evolved)
Case study: Global Change
Invasive plant species, the role of herbivores
and Meta-Analysis
Wide spread of invasive species: lack of natural enemies (no
predators, parasites, pathogens)
Results are not conclusive.
John Parker, Georgia
Tech
Question: How native
and introduced
herbivores affect the
abundance of native
and introduced plants.
Papers: 63 studies
Herbivores: bison,
deer, rabbits
One specific study of the herbivores:
Pampas grass (an introduced species to CA) and native jackrabbits
John Lambrinos,
University of
California
Exclude rabbits
decreased grass
survival to 60%,
allow grazing
decreased the
survival to 5%.
Grazing of invasive
species by native
herbivores decreased
survivorship.
What do the results of the metaanalysis tell us?
1. What invasive plant spp
encounter matters (escape of
native herbivores may be not a
reason)
2. Native species survive better with
native herbivores (co-evo)
3. Single study can’t reveal this
information, as only native or
invasive spp used.
The End
14.4 Consumer and resource populations
can achieve an evolutionary steady state
Consumer and resource
populations evolve
continually in response to
selection by their
antagonists, what’s the
end result of the
interaction? Do evolution
ever stop?
Strict or diffuse
relationships: one host
(wheat-rust) or prey or
multiple
Evolve toward a steady
state in which evolution
continues
The End