lecture8translated - College of Forestry, University of Guangxi

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Lecture 8
Exploitative and Mutualistic
Species Interactions
Principles of Ecology
College of Forestry, Guangxi University
Eben Goodale
The midterm
• Not fully graded, but clearly was difficult.
• Don’t worry too much … grades will be
• Biggest problems the essay.
• Because writing may be difficult to you
especially in a timed framework, I will reassign
the essays for homework.
Midterm homework
• On website, find two short answer and essay
questions. Hand in your work next Saturday.
• For essays, pick one essay topic that you did NOT
do in class exam.
• VERY IMPORTANT. This assignment is to be done
by yourself and in your own words. If I find that
your answers are similar to someone else’s or
copied from some source, this is PLAGIARISM(抄
袭) and I will give you a 0.
• Grades from this assignment will be combined
with the midterm.
Where we are in class
• What levels of organization have we covered
• Individuals form populations; populations
form species; these species interact.
• What is meant by competition being a - / interaction?
Today: more species interactions
• Predator / Prey (+ / - )
• Parasites (+ / - )
• Mutualists ( + / +) or Commensalists (+ / 0)
Types of exploitive interactions
1) Predators: kill prey
2) Herbivores: usually don’t kill the plants
3) Parasites(寄生虫): don’t usually kill host
 Pathogens(病原菌;病原体): induce disease
 Parasitoids: insect predators that put eggs in
insect host, then grow and kill host
Adaptations of prey to avoid predators
Armor(装甲), or other physical
defense (like running speed)
Aposematism: this animal is toxic
and advertised that to predators
with bright colors.
Cryptic(隐藏,隐蔽的 ): looks like background
Mimicry(拟态): looks like
somethingthat is dangerous
Movie here
on caterpillar
that acts like snake
And counter-measures by predators
• Predators can use
crypsis and mimicry
• Evolutionary “armsrace”(防卫装备).
Herbivores / plants also show arms
• Many plants produce
“secondary compounds”
which are toxins(毒素)
that repel herbivores.
• Some herbivores,
especially insects, have
evolved to resist these
secondary compounds.
• This may be reason that
there are a lot of
Milkweed plant produced toxin.
Monarch caterpillar resistant to toxin.
Eating milkweed makes caterpillar
and butterfly poisonous to birds.
Besides refuges, another prey strategy:
swamping(食物来源) the predator
• Many taxa produce offspring on masse, too
many for the predator to quickly react to.
13 year
Acorn masts
Effects on prey by predator
can be severe
Cactoblastis cactorum
And prickly pear
The cactoblastis
monument in
Australia … one of
world’s only
monuments to An example of ‘biological control’:(生物防治)
Introducing predators to control
an insect…
invasive prey population
Herbivores can alter structure of
• Darwin first realized that
in many pastures there
are both grasses and
small trees and cows eat
• Both grasses adapted to
regrowing, trees not.
• Without cows, trees
rapidly replace grasses
• True in natural
grasslands, too
Predators too can have
community-wide effect
• Foxes introduces to
Aleutian islands.
• Eat birds.
• Fewer birds = lesser
guano (bird droppings,
a very good fertilizer)
• Less guano = less
• Shift in plant
Predator change balance of
• Famous research of
Robert Paine.
• Showed that if remove
a predator, one kind of
mussel (shellfish)
outcompetes all other
species in intertidal
• Predator keeps diversity
of system
From Paine 1966
Taken together, preys’ effect on predators
and predators’ effect on prey can lead to
The famous
Snowshoe-lynx cycles
Hudson Bay Co
Note how blue peaks
Usually after red ones
Taken together, preys’ effect on predators
and predators’ effect on prey can lead to
But not so simple….
Oscillations not just
caused by hare-lynx
What other factors?
Hares overshoot their
own carrying capacity
causing plant die-offs,
plants to increase toxins,
and disease epidemics.
Oscillations: a mathematical model
Exponential growth equation
= rmax (N)
= rmax (N) (1 -
P stands for
= r (N)
- aNP
Logarithmic growth equation
N stands for
= baNP- mP
a = efficiency with which
predators kill prey
When N = 0, predators die off
due to their mortality (m).
When N are present, predator increase in #,
in accordance with their efficiency in killing
prey, and their efficiency in converting prey
into new offspring (b)
Oscillations: a mathematical model
= r (N)
0 = r(N) – aNP
aNP = rN
P = r/a
- aNP
= baNP- mP
0 = baNP – mP
mP = baNP
N = m/ba
Oscillations: a mathematical model
Oscillations: hard to get to in lab
Experiments by Huffaker (1950s):
1) Apples and oranges.
2) Mites eat oranges, increase
in number.
3) Add predator, predator
increases for a while,
then both populations go
Oscillations: hard to get to in lab
Prey disperse to unoccupied places,
increase in #.
Predators find prey, eat them all but
not before a few disperse.
Population cycles.
2nd experiment:
Sticky substance added that
Partially blocks predator’s
movement. Small sticks on top
of oranges allow prey to “balloon”.
Today: more species interactions
• Predator / Prey (+ / - )
• Parasites (+ / - )
• Mutualists ( + / +) or Commensalists (+ / 0)
Parasitoids … aliens!(异形)
Picture from ‘Alien’ (1979)
Example of worm that grows inside cricket,
Changes behavior of cricket so cricket jumps
Into water, and then hatches out (movie).
More pathogen and parasites’ clever tactics(策
略) for transmission(传播途径)
infects snails
and makes them
seek light.
Snails climb to the
top of grasses
where they are
consumed by birds,
the next host
in the fluke’s
More pathogen and parasites’ clever
One recently
reported parasitoid
hatches out of host
but doesn’t kill it …
Instead affects host
so that it stands
around and defends
parasitoid cocoons!
From Grosman
et al., 2008,
Some generalizations(补充,推广) about
• They usually reproduce
more rapidly than
• They usually only
interact with one of a
few hosts in their
• High specialization.
• > 50% of all species on
earth parasites?
Some parasites are pathogens
• Pathogens
• Malaria(疟疾)
is a good
• Diseases, just like
the parasites we
talked about
before, need to
ensure their
Sneeze conservatively
Placed at 150 km/hour,
~ 40,000 microscopic droplets
Malaria replicates in blood so increases chance
it gets into mosquito
The idea of evolutionary medicine
• A paradox(疑难问题): why would a
pathogen want to kill its host? It will kill itself,
• But if a pathogen is very good at transmission,
then it can afford to kill host.
• This idea predicts that very “virulent”(致死的)
pathogens are ones that transmit to many
hosts quickly.
The idea of evolutionary medicine
Some evidence for this idea:
diseases =
very high
What does this idea
suggest for how disease
can be stopped?
Perhaps if we lower transmission rate,
we also lower disease virulence.
Freeman (2004)
Model of disease transmission
• How is a disease transmitted?: a model.
I = # infected, S = # susceptible (not infected)
• dI/dt … change in infected individuals over
time = βSI – mI
β = transmission coefficient. How easily spreads.
m = rate that infected people become
uninfected (by dieing or recovering).
Model of disease transmission
dI/dt = βSI – mI
What should we do now with this equation?
What happens when dI/dt > 0
βSI – mI > 0
βSI > mI
βS > m  S > m/β. When ST = m/β, disease will spread.
For the disease to spread, we need to have a certain amount (a
threshold 阈值, ST) of susceptible people.
Ways to combat disease:
-Reduce S by killing animals (bird flu)
-Reduce S by immunization (human disease)
-Increase m by helping people recover.
-Reduce β by teaching people not to transmit (hand-washing)
Turning to mutualism / commensalism
• Commensalism(偏利作用) (+ / 0) is
• Where a tree shades a small plant:
Small plant benefits, tree unaffected.
• Mutualism is different in that for both sides
benefit > cost. ( + / +)
• Mutualism involves co-evolution
Mutualism Can Vary in Intensity
– Mutualism can be obligatory(强制性的,专性的),
which means that one or the other or both the mutualists
can’t survive without the other partner.
– Or mutualism can be facultative(兼性), which means
that the partners sometimes engage in the relationship
and sometimes don’t
Example, facultative
Corals and zooxanthellae
Ants and
Very important mutualisms
• Mycorrhizae and plants
• Nitrogen fixing bacteria and plants (especially
• Plants and pollinators
• Plants and seed-dispersers
• Plants and protectors like ants
• Corals and zooxanthellae
• Corals and protectors like crabs
• Mixed groups of birds and mammals
Some amazing mutualisms:
Mycorrhizae 菌根
Mutualism between plants and fungi that interact with plant roots.
Fungi get sugar from plants, help plants absorb nutrients from soil.
80% flowering plants, all conifers have mycorrhizae.
Mycorrizae can coat outside of plants or even be inside plant cell
walls in very complex morphology.
Some amazing mutualisms: Corals 珊瑚
Zooxanthellae :
Mutualistic partner of
coral; photosynthesize
giving coral sugars.
Coral gives them
protection, nitrogen
Some amazing mutualisms: Corals
Corals: a worrisome
“Bleaching events” where
zooxanthellae are expelled.
Appears to be related to
water temperature.
Some amazing mutualisms: fig trees(榕树)
Found throughout tropics.
Many grow as parasites on other trees,
eventually killing them, but becoming
huge trees themselves.
Year-round source of food for many animals
Some amazing mutualisms: fig trees
But figs have no flowers!
Their flowers are actually inside the fig.
How are they pollinated?
Figs are pollinated only
by small wasps
Fig wasps, in turn, only
live inside figs
Wasp lays eggs inside fig and simultaneously
pollinates them.
Some amazing mutualisms:
cleaner fish
Evolution of mutualism … an example from
Increasing complexity and mutuality
From Kostans 2002
Evolution of mutualism … an example from
Beginning steps:
One animals listens to calls of another animal
Eavesdropping. Example downy woodpeckers listen to the alarm
calls(警报) of chickadees
From Kostans 2002
Evolution of mutualism … an example from
Next step:
Two animals listen to each other.
Example: both yellow-bellied marmots(土拨鼠) and golden-mantled
Ground squirrels(松鼠) listen to each others alarm calls.
From Kostans 2002
Evolution of mutualism … an example from
Next step:
One animal gains from response of other animal to its calls
Example: caterpillars make call that attacts ants to them.
Actually mimics a call ants make. In this case, ants benefit by
gather secretions of caterpillars (so true mutualism, although
communication asymmetric).
From Kostans 2002
Evolution of mutualism … an example from
Final step:
Both animals gains from their mutual responses to each others calls
Example: humans/ratels and honeyguides(向蜜鸟,蜜鴷)
Read this article.
It is not too difficult.
Concentrate on what
evidence the authors
give to demonstrate
it’s a mutualism.
From Kostans 2002
Evolution of Mutualism:
Cheaters a Problem
• What happens if there are
• Cheaters might negatively
affect their partners, hence
negatively affecting
• Some mutualists have
mechanisms to prevent
• Example yucca moth.
Pollinates yucca and leaves
some eggs. But if leaves too
many eggs, whole flower
Mutualists can co-speciate(协同进化)
Evolutionary history
of a clade of
orchids is
closely related
to the evolutionary
history of
its pollinators
Mutualisms can have major effects on
• For example, if cleaner
fish removed, species
diversity of fish goes
• Likewise if mycorrhizae
fungi are eliminated,
plants may not be able
to live in some
Mutualisms may occur more in
stressful conditions
This slide shows
that the “Relative
Neighbor Effect (RNE)” –
the effect that a
plant has on its neighbor –
is positive at high
elevations (high stress)
but negative at low elevations
• For next Saturday: Take-home portion of
• Read summary of Chapters 16, 17 (I will e-mail
you them).
• Read Isack and Reyes (honey-guides).
• I will look for primary reading for next
Key concepts
• Predator-prey interactions
and herbivory / plant
relationship involve coevolutionary arms-race.
• Predation /herbivory has
major effects on
distribution/abundance of
prey and even on
• Parasites are frequent in
nature and adapted to
transmission between hosts
• Commensalism is when one
species benefits and the
other is unaffected and is
also common.
• Mutualism is when both
partners benefit and
involves coevolution.
• Mutualisms vary in
intensity; very intense ones
are obligate, where
partners cannot live
without eachother.
Tips for listening, reading
Reading to write: