How do species interact with one
another to make stable
Ecological Communities?
Ecological Effects of Species 1 on
Species 2:
(A) Effect is Positive (+) if species 1
increases the numbers of species 2.
(B) Effect is Negative (-) if species 1
decreases the numbers of species 2.
Species 2
+/- Ecological Effects of
One species on the other
+
-
+
Mutualism
Predation
-
Predation
Competition
Species 1
Species 2
Ecological Effects of
One species on the other
+
-
+
Mutualism
Predation
-
Predation
Competition
Species 1
Mutualism is an interaction between
two (or more) species that is
beneficial (+) to both (all) species.
Mutualism is an interaction between
two (or more) species that is
beneficial (+) to both (all) species.
Algae: + effects on fungi: algal photosynthesis produces sugars and
oxygen for the fungus.
Fungus: + effects on algae: fungus absorbs nutrients from the
atmosphere and produces CO2 which permits the alga
to photosynthesize. Fungus also protects the alga from drying
out.
Mutualism is an interaction between
two (or more) species that is
beneficial (+) to both (all) species.
Beetles: + effects on fungi: the beetle ‘plants’ the fungal spores
and maintains optimal humidity for fungal growth.
Fungus: + effects on beetle: fungus provides nutrition for
the beetle.
Ant-Aphid mutualism
Ants: protect the aphid
from predators.
Aphids: provide plant
sugars for the ants
Species 2
Ecological Effects of
One species on the other
+
-
+
Mutualism
Predation
-
Predation
Competition
Species 1
Competition occurs when of two
species each require the same limited
resource. The availability of the
resource to one species is negatively
influenced by the presence of the
other species. It is a "-/-" interaction.
Gause’s Competitive Exclusion Principle:
When two species make similar demands
on a limited resource, then one or the other
species will go extinct as a result of
competition for the resource.
Paramecium caudatum
Paramecium aurelia
Gause’s Experiments
Single Species Populations: each survives
indefinitely when reared alone.
Competition Populations:
P. aurelia out-competes
P. Caudatum when reared
Together.
Competition occurs when of two
species each require the same limited
resource. The availability of the
resource to one species is negatively
influenced by the presence of the
other species. It is a "-/-" interaction.
Tribolium
confusum
Thomas Park’s
experiments
Tribolium
castaneum
Single Species Equilibrium Population
Sizes when reared ALONE
Climate
T. castaneum T. confusum
Cold-Dry
21
208
Cold-Wet
99
225
Warm-Dry
150
237
Warm-Wet
401
264
Hot-Dry
77
190
Hot-Wet
306
329
Predict the
Winner in
Competition
Single Species Equilibrium Population
Sizes when reared ALONE
Climate
T. castaneum T. confusum
Predicted
Winner in
Competition
Cold-Dry
21
208
confusum
Cold-Wet
99
225
confusum
Warm-Dry
150
237
confusum
Warm-Wet
401
264
castaneum
Hot-Dry
77
190
confusum
Hot-Wet
306
329
?Toss Up
Observed Competitive Outcomes:
Percent Wins when raised together
Climate
T. castaneum T. confusum
Predicted
Winner in
Competition
Cold-Dry
0%
100%
confusum
Cold-Wet
30%
70%
confusum
Warm-Dry
13%
87%
confusum
Warm-Wet
86%
14%
castaneum
Hot-Dry
10%
90%
confusum
Hot-Wet
100%
0%
Toss Up
Unusual Outcomes based on Single Species Predictions
Observed Competitive Outcomes:
Percent Wins
Climate
T. castaneum T. confusum
Predicted
Winner in
Competition
Cold-Dry
0%
100%
confusum
Cold-Wet
30%
70%
confusum
Warm-Dry
13%
87%
confusum
Warm-Wet
86%
14%
castaneum
Hot-Dry
10%
90%
confusum
Hot-Wet
100%
0%
Toss Up
Gause’s Competitive Exclusion Principle:
When two species make similar demands
on a limited resource, then one or the other
species will go extinct as a result of
competition for the resource.
With T. castaneum and T. confusum,
One species won and the other went extinct
in every one of the 170
competition populations
Where they were raised together.
Changing the Climate from
Hot-Wet to Cold-Dry
Changed the identity of the winning species
from T. castaneum to T. confusum.
Stochastic Outcome: In Intermediate Climates
each species won in at
least some of the competition populations.
The outcome of competition was not completely
Predictable.
Changing the Hot-Wet Environment by
ADDING a thrid species, the pathogen,
Adelina tribolii
Changed the identity of the winning species
from 100% T. castaneum
to 80% T. confusum.
Predator-Prey Arms Races:
Reciprocal Co-Evolution of Offense and
Defense
Evolution of Garter Snake (Predator)
Exploitation Newt (Prey)
Evolution of Newt (Prey) Defense
against Garter Snake (Predator) predation
Species 2
Ecological Effects of
One species on the other
+
-
+
Mutualism
Predation
-
Predation
Competition
Species 1
Arms-Race Co-evolution
Selection by Predator
on Prey
Exploitative
Ability of
Predator
Selection by Prey
on Predator
Defensive
Ability
of Prey
Life-Dinner Principle
Predator is hunting for its dinner. If it fails in an encounter
with a prey, it loses only a meal and the effect on predator
fitness is relatively small.
Prey is running for its life. If it fails in an encounter with a
predator, it loses its life and the effect on prey fitness is
very large.
Natural Selection on the Prey species to evolve defenses
is STRONGER than Natural Selection on the Predator
Species to evolve hunting ability.
Arms-Race Co-evolution is Typically
Asymmetrical
Selection by Predator
on Prey is Strong
Exploitative
Ability of
Predator
Selection by Prey
on Predator is
Weak
Defensive
Ability
of Prey
Intensity of Coevolution depends upon the
Reciprocity of the fitness effects of Predator on Prey
and Prey on Predator.
Life-Dinner Principle suggests a lack of reciprocity of
fitness effects, and thus the intensity of coevolution
resulting from the arms race is weak.
However, when Prey are Dangerous or Toxic,
then Dinner for the Predator means a risk of Death.
This Reciprocity of the fitness effects means
a STRONG Arms Race
Tetrodotoxin in skin of Newt.
Na+ channel blocker, causes
paralysis.
Toxic to most animals.
Found in crabs, fugu fishes,
annelid worms and algae.
Possibly produced by
symbiotic bacteria
Species of Newt
Skin Toxicity in
“Mouse Units”
Taricha
granulosa
Taricha
torosa
Taricha
rivularis
Notophthalmus
viridescens
25,000
1,000 –2,500
1,000 –2,500
20
Range of Taricha (prey) species
T. granulosa
T. granulosa,
T. torosa,
T. rivularis
T. granulosa,
T. torosa
T. torosa
Range of Thamnophis sirtalis
Benton
Study Sites
Tenmile
Bioassay of Predator
Resistance to Tetrodotoxin
1.
Measure baseline speed
2.
Inject known dose of TTX.
3.
Measure post-injection speed.
4.
“TTX resistance” is the % reduction
in speed after injection of toxin.
Nonresistant
T. sirtalis
Predator Resistance (% reduction)

Resistant
T. sirtalis


100




 
Coluber


‘”Super” Resistant
T. sirtalis


 



 



 












 





 


 


 

50



 
 

































 
 












 


0
0.01
0.1
1

 




10




100
1000
Prey Toxin [mouse units of TTX]
10000
Geographic Variation in
Newt Toxicity
Geographic Variation in
Snake Resistance
NonResistant
T. granulosa
Weakly
Resistant
Strongly
Resistant
T. granulosa,
T. torosa,
T. rivularis
T. granulosa,
T. torosa
T. torosa
Super
Resistant