Lecture 8Exploitative and Mutualistic

Species Interactions

Principles of EcologyCollege of Forestry, Guangxi University

Eben Goodale

The midterm

• Not fully graded, but clearly was difficult.• Don’t worry too much … grades will be

curved.• 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

Review

• What levels of organization have we covered now?

• 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 prey2) Herbivores: usually don’t kill the plants3) 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 predatorsArmor （装甲） , or other physical

defense (like running speed)

Aposematism: this animal is toxicand 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 hereon caterpillarthat acts like snake

And counter-measures by predators

• Predators can use crypsis and mimicry themselves.

• Evolutionary “arms-race” （防卫装备） .

Herbivores / plants also show arms race

• 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 specialist （专业，专家） herbivores.

Milkweed plant produced toxin.Monarch caterpillar resistant to toxin.Eating milkweed makes caterpillarand 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.

Acorn masts

13 year cicadas

Effects on prey by predator can be severe

Cactoblastis cactorumAnd prickly pear

The cactoblastismonument in Australia … one ofworld’s only monuments toan insect…

An example of ‘biological control’: （生物防治）Introducing predators to control invasive prey population

Herbivores can alter structure of community

• Darwin first realized that in many pastures there are both grasses and small trees and cows eat both.

• 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 grasses

• Shift in plant composition

Predator change balance of competition

• Famous research of Robert Paine.

• Showed that if remove a predator, one kind of mussel (shellfish) outcompetes all other species in intertidal zone.

• Predator keeps diversity of system

From Paine 1966

Taken together, preys’ effect on predators and predators’ effect on prey can lead to

oscillations

Hudson Bay Co

The famousSnowshoe-lynx cycles

Note how blue peaksUsually after red ones

Hare

Lynx

Taken together, preys’ effect on predators and predators’ effect on prey can lead to

oscillations

But not so simple….

Oscillations not justcaused by hare-lynxrelationship…

What other factors?

Hares overshoot theirown carrying capacitycausing plant die-offs, plants to increase toxins,and disease epidemics.

Oscillations: a mathematical model

dN

dt

= rmax (N) Exponential growth equation

K

dN

dt

= rmax (N) (1 - )N

Logarithmic growth equation

dP

dt

= baNP- mP When N = 0, predators die offdue to their mortality (m).When N are present, predator increase in #,in accordance with their efficiency in killing prey, and their efficiency in converting preyinto new offspring (b)

a = efficiency with whichpredators kill prey

N stands forPrey

P stands forPredator

dN

dt

= r (N) - aNP

Lotka Volterra

Oscillations: a mathematical model

dP

dt

= baNP- mP dN

dt

= r (N) - aNP

0 = r(N) – aNPaNP = rNP = r/a

0 = baNP – mPmP = baNPN = m/ba

Oscillations: a mathematical model

Oscillations: hard to get to in labExperiments by Huffaker (1950s):1) Apples and oranges.2) Mites eat oranges, increasein number.3) Add predator, predator increases for a while, then both populations goextinct.

Oscillations: hard to get to in lab2nd experiment: Sticky substance added that Partially blocks predator’s movement. Small sticks on top of oranges allow prey to “balloon”.

Prey disperse to unoccupied places, increase in #.Predators find prey, eat them all but not before a few disperse.Population cycles.

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）

Snail-fluke （蜗牛 - 吸虫）infects snails and makes themseek light.Snails climb to the top of grasseswhere they areconsumed by birds,the next hostin the fluke’slifecycle

More pathogen and parasites’ clever tactics

One recentlyreported parasitoidhatches out of hostbut doesn’t kill it …Instead affects hostso that it stands around and defendsparasitoid cocoons!

From Grosmanet al., 2008,(PLoS)

Some generalizations （补充，推广） about parasites

• They usually reproduce more rapidly than hosts.

• They usually only interact with one of a few hosts in their lifetime.

• High specialization.（选择专向性）

• > 50% of all species on earth parasites?

Some parasites are pathogensSneeze conservativelyPlaced at 150 km/hour,~ 40,000 microscopic droplets

Malaria replicates in blood so increases chanceit gets into mosquito

• Pathogens produce diseases.

• Malaria （疟疾） is a good example.

• Diseases, just like the parasites we talked about before, need to ensure their transmission

The idea of evolutionary medicine

• A paradox （疑难问题） : why would a pathogen want to kill its host? It will kill itself, too!

• 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.

（医学发展）

The idea of evolutionary medicine

Some evidence for this idea:

Freeman (2004)

Waterborne （水环境，水基）diseases =very hightransmissionrates

What does this ideasuggest for how diseasecan be stopped?

Perhaps if we lower transmission rate,we also lower disease virulence.

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 – mIβ = transmission coefficient. How easily spreads.m = rate that infected people become

uninfected (by dieing or recovering).

Model of disease transmissiondI/dt = βSI – mIWhat should we do now with this equation?What happens when dI/dt > 0βSI – mI > 0βSI > mIβS > 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 everywhere.

• 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

Exampleobligatory Relationship?

Example, facultative relationship?

Corals and zooxanthellae

Ants andsunflowers

Very important mutualisms

• Mycorrhizae and plants• Nitrogen fixing bacteria and plants (especially

legumes)• 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: CoralsCorals: a worrisome development…“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 becominghuge trees themselves.

Year-round source of food for many animals

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 simultaneouslypollinates them.

Some amazing mutualisms: fig trees

Some amazing mutualisms: cleaner fish

Evolution of mutualism … an example from communication （交流，信息传递）

From Kostans 2002Increasing complexity and mutuality

Evolution of mutualism … an example from communication

From Kostans 2002

Beginning steps:

One animals listens to calls of another animal

Eavesdropping. Example downy woodpeckers listen to the alarmcalls （警报） of chickadees

Evolution of mutualism … an example from communication

From Kostans 2002

Next step:

Two animals listen to each other.

Example: both yellow-bellied marmots （土拨鼠） and golden-mantledGround squirrels （松鼠） listen to each others alarm calls.

Evolution of mutualism … an example from communication

From Kostans 2002

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 bygather secretions of caterpillars (so true mutualism, althoughcommunication asymmetric).

Evolution of mutualism … an example from communication

From Kostans 2002

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 whatevidence the authors give to demonstrateit’s a mutualism.

Evolution of Mutualism: Cheaters a Problem

• What happens if there are cheaters （伪装） ?

• Cheaters might negatively affect their partners, hence negatively affecting themselves.

• Some mutualists have mechanisms to prevent over-exploitation （过度开发） .

• Example yucca moth. Pollinates yucca and leaves some eggs. But if leaves too many eggs, whole flower drops.

Mutualists can co-speciate （协同进化） together

Evolutionary historyof a clade oforchids isclosely relatedto the evolutionaryhistory ofits pollinators

Mutualisms can have major effects on communities

• For example, if cleaner fish removed, species diversity of fish goes down.

• Likewise if mycorrhizae fungi are eliminated, plants may not be able to live in some environments.

Mutualisms may occur more in stressful conditions

This slide shows that the “RelativeNeighbor Effect (RNE)” –the effect that a plant has on its neighbor –is positive at high elevations (high stress)but negative at low elevations

Homework

• For next Saturday: Take-home portion of midterm.

• 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

Saturday.

Key concepts

• Predator-prey interactions and herbivory / plant relationship involve co-evolutionary arms-race.

• Predation /herbivory has major effects on distribution/abundance of prey and even on communities.

• 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

Listening:http://www.artsci.utoronto.ca/current/advising/ell/pdfs/ELL_Strengthening_Listening_Comprehension.pdf

Reading to write:http://www.artsci.utoronto.ca/current/advising/ell/pdfs/Reading_to_Write_Previewing.pdf