Download Mutualisms

FOUR main kinds of species interactions; all play
a role in the formation of community structure...
Competition (-,-)
Predation, parasitism (+,-)
Mutualism (+,+)
Commensalism (+,0)
Mutualism:
Any long-term association between two
species that confers mutual fitness benefits
to individual members of both species.
Facultative mutualisms: Each species gains a benefit
from the presence of the other, but each can still survive
without the other. “Generalist” mutualisms.
Obligate mutualisms: Each species can only live
in the presence of the other. “Exclusive” mutualisms.
A) Plant-animal mutualisms:
- most common (90%)
1. defensive / protective mutualisms
2. dispersive mutualisms
1. Defensive / protective mutualisms
-animal species receives food and/or shelter
from mutualistic plant species in return for
defending plant
e.g. ants and “swollen thorn” acacias
- ants protect acacias by attacking herbivores and other plants
-acacia plants save energy by not
producing expensive alkaloids
- ants gain shelter and food
2. Dispersive mutualisms
i) seed dispersal
ii) pollination
i) Seed dispersal mutualisms
- birds and mammals disperse seeds via defecation
- seed burial by mammals
- animals benefit from fruit
that surrounds seed
-plants benefit by having
seeds moved to favorable
germination sites
Most seed dispersal mutualisms are facultative;
however, a few are obligate…
Obligate seed dispersal mutualisms:
e.g. Mistletoe Bird
An obligate mutualism is an interaction in which at least
one species cannot survive without the presence of the other
species; a facultative mutualism is an interaction that
benefits both species but is not required by either species.
One reason facultative mutualisms are more common is that
A) in an obligate mutualism, if the partner becomes extinct, the
obligate mutualist will also become extinct.
B) in a facultative mutualism, if the partner becomes extinct, the
facultative mutualist will also become extinct.
C) in an obligate mutualism, the obligate mutualist is obliged to
support its partner.
D) in a facultative mutualism, neither partner can depend on the
other partner.
E) facultative mutualisms require longer periods of time to evolve.
Historical obligate seed dispersal mutualism?
e.g. Dodo and Calvaria Tree
- extinction of Dodo once thought to threaten Calvaria tree
Problem for plants:
- many seed dispersers are also seed predators
Solutions:
-‘mast’ seeding
- ‘choose’ dispersal agent
Mast seeding:
- all trees of a particular species in a given area
produce large seed crops simultaneously
- alternating years of high and low production
e.g. Oak tree
production of acorns
-squirrels can’t
retrieve all
Toxic or distasteful fruits are one way for a plant
to ‘choose’ its dispersal agents.
e.g. chili peppers
and birds
Capsaicinoids distasteful to mammals,
but very tasty to birds!
ii) Pollination mutualisms
Phlox family adaptations
to many different pollinators
What are the advantages of exclusive
pollination mutualisms?
For plants?
For animals?
What are the disadvantages of exclusive
pollination mutualisms?
If one member goes extinct, the other will too.
STDs!
e.g. nostril mites
Insects are the most specialized pollinators and
have evolved more obligate mutualisms than
birds or mammals. Why?
A) Because there are so many different species, they are
able to pollinate a greater variety of flowering plants.
B) Because they have short life cycles, short generation
times, and many offspring.
C) Because they have small brains and therefore cannot
learn to recognize many different plant species.
D) Because they can move quickly from plant to plant
and therefore can remember the last species visited.
Species-specific Coevolution:
A mutual evolutionary influence between
only two interacting species.
Two classic examples of species-specific
coevolution:
Madagascar Star Orchid
-discovered by Du Petit-Thouars (1822)
-white, night-scented
-nectar tube 30 cm long
-Darwin predicted moth pollinator
with proboscis slightly shorter
Predicta hawkmoth
- discovered 40 yrs later
- proboscis 25 cm long
Nilsson (1988) experimentally shortened nectar tubes.
Moth’s head does not contact pollen; so flower produces
fewer seeds.
Another classic example of
species-specific coevolution:
Figs and Fig Wasps...(video clip)
B) Animal-animal mutualisms:
1. cleaner mutualisms
a) aquatic
b) terrestrial
Aquatic Cleaner System: Cleaner Fish
Cleaner Wrasse plus “clients”
Do cleaner fish really clean?
1) Exclusion experiment on coral reef:
-fish on reefs without cleaners had
4.5X more parasites
YES
2) Diet choice experiment:
- choice of fish mucus or parasites
- cleaners preferred client mucus
NO
Terrestrial Cleaner System: Tickbirds
Red-billed Oxpecker on Impala
Do oxpeckers really clean?
Commensalism: +, 0
Shark with remora
Cape buffalo with cattle egrets
Commensalism: +, 0
e.g. Antbirds and army ants