Download types of benefits: related terms: commensalism

Bio205H5: lecture 11
mutualism & community structure
mutualism: interactions between species
that benefit both parties
mutualism ≠ altruism
mutualism = kind of “reciprocal parasitism”:
each obtains what is needed from mutualist
at lowest cost to itself
related terms:
types of benefits:
• transportation: transport gametes or
individuals
• nutrition: exchange of nutrients
• protection: attack or remove
predators, competitors,
parasites
(may be a
combination)
commensalism
commensalism: one species receives
benefit; other not affected
symbiosis: long-term, intimate association
may include
• parasitism (+ / -)
• commensalism (+ / 0)
• mutualism (+ / +)
• one species benefits; other is unaffected
• e.g. human bot fly (Dermatobia hominis)
& mosquitos:
myiasis: larval stages of flies inhabiting
humans or animals
• bot fly: large insect (10-15 mm); loud
• unlikely to effectively deposit eggs on host
1. diffuse mutualism: interactions among
many species
e.g. most mycorrhizae; pollinators and
seed dispersers
2. direct mutualism:
• facultative mutualism: species can
survive without mutualistic partner
• obligate mutualism: species’ survival
depends on mutualistic partner
coevolution - reciprocal evolutionary
responses between species
• bot fly captures mosquito; transfers eggs
• eggs hatch when mosquito alights
• phoresis: one species uses another only
as means of transportation
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diffuse mutualism
• lichen: mutualistic relationship between
fungi and alga or cyanobacteria
Rhizobium and legumes:
• not always mutualistic!
• in habitats where lichen occur, neither partner
can exist alone
• photosynthetic part provides carbohydrates;
fungal hyphae protect from water loss
• fungi may be parasitic (kill algal cells)
mycorrhizae:
• cause legumes to form nodules on roots
• nodules harbor and protect bacteria;
bacteria fix nitrogen
arbuscular mycorrhizae:
penetrate root cortex cells
ectomycorrhizae:
surround root cortex cells
provide plant with nitrogen & phosphorous;
receive carbon
seed dispersers:
• may eat fruit; seed passed unharmed
• eat some, but not all seeds
• heavy seeds must be transported to prevent
competition with parent plant
2
direct mutualism:
tightly-coupled relationship
• e.g. bottle gentian
• flower stays closed
• nectary can only be
accessed by large
bumblebees
• faithful pollinators; reliable
source of nectar
evolution of mutualism:
1. many evolve out of initially antagonistic
pair-wise interactions
e.g. yucca moths:
evolution of mutualism:
• moth deposits eggs in ovary, but does not
fertilize plant
• result: few seeds develop, little food for
moth larvae
• moth deposits eggs in ovary; delivers
“pollen ball”
• result: many seeds develop; ample food
for moth larvae; plant successful
• carrion beetles bring mites to source of fly
eggs
• mites consume competitor’s eggs
2. mutualism may involve 3 or more species,
including an antagonistic pair:
a) carrion beetles, phoretic mites & bluebottle
flies
b) oropendulas, cowbirds, bees & botflies
c) ants, treehoppers & spiders
• cowbirds are nest parasites
• tolerated when nests far from
bees & wasps
• without bees, botfly infestation
common
• cowbird chick eats botfly
larvae
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• ants harvest honeydew from treehoppers
• do ants protect treehoppers from jumping
spiders?
if mutualism is beneficial to both,
shouldn’t it always evolve?
• not always – it may be profitable to cheat!
results depend on density of jumping spiders: when
density low, treehoppers do not benefit
vs.
mutualism → parasitism
interactions between species are fluid!
evolution of mutualism from
parasitism:
1. indirect transmission: parasite
attacks host; leaves (success of host &
parasite separate)
nectar robbers
deceptive flowers
2. direct transmission: parasite’s
offspring stay on host (fate of host
affects fate of parasite)
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1. indirect transmission: if parasite
“cooperates”, another parasite may
attack and damage host
• parasite not assured of benefit; better to
be antagonistic
2. direct transmission: better for
parasite to cooperate (fate of offspring
depends on fate of host)
• cooperation of parasite → cooperation of
host
•
mutualism will evolve if mutualists have
greater fitness than non-mutualists
•
under what conditions does it benefit the
sunflower to produce extra-floral
nectaries?
modeling the costs and benefits of
mutualism:
• e.g. facultative mutualism between ants
and sunflowers: extra-floral nectaries feed
ants; ants protect from seed predators
let:
H = amount of plant tissue damage without
any defenses (measure of threat of seed
predation)
D = amount of protection plant has without
ants (e.g. chemical defenses)
A = amount of protection offered by ants
IA = investment in extrafloral nectaries
ID = investment in other defenses
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mutualistic plants will evolve if:
p ⎡⎣ H (1 − D ) A⎤⎦ > I A
help
provided
by ants
risk of
damage
proportion
of pop
that attract enough
ants to mount a
defense
risk of
damage
without
ants
cost of being
a
mutualist
community structure
• communities contain assemblages of
interacting species
• affected by abiotic properties
• are communities highly structured, or are
they loose assemblages of species?
evolution of mutualism:
• mutualism often evolves from other types
of interactions
• involves various types of benefits
• may be facultative or obligate; may be
direct or diffuse
• may involve several species including an
antagonist
• evolution depends on relative costs &
benefits (may vary in space and time)
2 views:
Clements: communities are
stable, integrated and
orderly
• communities pass though
predictable seral stages
• culminates in climax
community
• nature of climax
community set by
climate; does not change
Gleason: communities are
neither stable nor predictable
• plant and animal communities
are ephemeral
• abiotic factors and history
determine what species are
found in a community (which
colonists first arrive, etc.)
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conclusion:
• total of 61 species found in all ponds
• each pond had only 31 – 39 species
• some species are good dispersers and
colonists; found in all ponds
• arrival of certain predators or competitors
early in process affects what can establish
after
• both hypotheses partially supported
interactions in food webs
strong and weak interactions:
• feeding activity of a few species has a
dominant influence on community
structure
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• at least 3 species of gallfly
• at least 14 species of parasitoid wasp
attack gallflies
• how do they coexist?
do consumers increase or decrease
diversity of prey species?
prefers Enteromorpha
Littorina
intertidal snail
eats Chondrus if
nothing else available
keystone species:
low density:
Enteromorpha
crowds out
other algae
medium density:
prevents
exclusion;
increases diversity
high density:
intense grazing
reduces diversity
• species that has a great impact on the
structuring of a community
• e.g. bison: small fraction of total mass of
grassland species
• maintain diversity of forbs
(herbaceous flowering plants)
by grazing down grasses
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next lecture:
• trophic interactions, disturbance &
succession
• wrapping it all up!
• review of midterm; Q&A after next week’s
lecture
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