Midterm Exam Review Questions• Short essays (60-70% of exam) drawn from list of

questions distributed Tuesday on the website• Stratified (although non-random) across the chapters• Address the question--no bonus for extraneous stuff• Only textbook and lecture material• Closed book exam without notes, etc.

Midterm Exam Review Session• Sunday, 1-4pm EST• 53 Church St., Room L01• Bring photo ID for access to building• Email questions to Anne (ammcguir@fas.harvard.edu)• To access the live stream of the review sessions, students should go to http://cm.dce.harvard.edu/classroom/ and click on ENVR E-140 at around 12:55 p.m. on the day of the review.

• A window will open with a button that says "Join". Students click on the button and join the classroom. There is a one or two second delay in the stream.

• If the page opens to "No videos are available at this time" it means they're probably too early for the class. They'll need to refresh the page periodically until they get the "Join" button and then click on the "Join" button.

• The sessions will also be added to the course video page.

OFT, Predator-Prey & Parasite-Host Relationships (continued from last lecture)

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Habitat Preference and Switching -determined by relative profitabilities of encountering and harvesting prey in different habitats

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… but habitat preferences change when predator

present

-trade-off between foraging efficiency & predation risk

The Lotka-Volterra predator-prey model predicts coupled cycles in predator and prey abundance; - hare-lynx & hare-food oscillations linked together

But even in simple communities---a complex web of interactions

Huffaker’s mites:

a) herbivorous mite prey without its mite predator

b) simple system of coupled oscillations

c) complex environment (orange patches separated by complex travel routes)allows sustained oscillations

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…and snowshoe hares do not cycle in patchy habitats!

Disease transmission threshold & critical population size

• Ro: basic reproduction rate = average number of new infections per infected host introduced into a susceptible population

• Transmission threshold: infection will spread when Ro >= 1

• Ro increases with – L, the period of time host remains infected– S, number of susceptible individuals in population– B, transmission coefficient-

• If microparasites are highly infectious (large B) or have long periods of infectiousness (large L) then:

– have high Ro values – & small transmission thresholds of population size

• If powerful immune response, so very transient in individuals (small L), then– Large critical population size (e.g., measles -= 300,000)

• S drops during epidemic as susceptibles die or develop immunity

Disease dynamics and cycles: immunity & death reduces % of “susceptibles” & so lowers incidence; but then susceptibles increase through births & immigrants

a) 1-2 year cycles in measles (pre-vaccination) b) 3-4 year cycles in whooping cough (mass vaccination > 1956) (England & Wales)

Epidemic Curves of Infection:parasite will spread if critical susceptible population is exceeded

a) Legionaire’s disease & b) foot-and-mouth disease (cattle & sheep)

Population ecology of parasites:- Density-dependent regulation of flea population on blue tits

- Density-dependent regulation of nematodes in eels

Predator-mediated coexistence of prey species:predation or grazing can interrupt competitive exclusion Ex.1: pigmy

owls prevent competitive dominance of Coal Tits on Scandinavian

islands

Ex. 20= no grazing,1=light, etc.4=very heavy grazing

Intermediate densities of periwinkles maintain high algal diversity in New England tide pools - Enteromorpha alga competitively exclude others at low densities- At high densities, periwinkles broaden diet- effect is due to preference for competitive dominant

… but Carcinus crabs prey on periwinkle juveniles in Enteromorpha canopy…

a) tide pools vs. b) emergent substrates

Impact of parasites on community structure is underappreciated

-Kevin Lafferty’s salt marsh food web

Coevolution & Mutualisms

• (skip over chap 8.1-8.2 for now)

• Coevolutionary arms races

• Symbiosis: Mutualism and Parasitism

• Mutualisms

Interspecific Competition

• Influence on Community Structure (chap 6.4)

• Importance of Competition

Ecology of Mutualisms & Parasitism

• Symbiosis: association between species in which one lives on or in the other

• Parasite: lives intimately associated with host organism(s), deriving resources from and harming the host

• The “habitat” of parasites are host organisms (egg or larval stages may use different host types or survive in the soil)

• Microparasites: small, numerous and reproduce within host (bacteria, viruses, etc.)

• Macroparasites: large, grow within host but produce infective stages that disperse to new hosts

– Live on the host body or within body cavity – Parasitic worms, nematodes, etc.

• Parasites harm the host: have negative effects on growth, reproduction and/or survivorship

Coevolutionary Arms Races- predator/prey-grazer/plant-specialists are more prone to arms races

a) specialists have lower mortality on toxic chemicals

b) generalists lower mortality on chemicals not provoking coevolutionary response

(e.g., tannins, lignin)

Coevolutionary Arms Races

- parasites & hosts

- continual evolution of new influenza strains

-Canadian Plains Indiansevolution of resistance to TB

Coevolution of host and parasite: Control of rabbits in Australia & Britain by selection for increased transmissibility in Myxoma virus

strains (thereby lowering virulence)

Mutualists & Commensuals

• Mutualism: association which benefits both species (have positive effects on growth, reproduction and/or survivorship for both populations)

• Mutualisms are cases of reciprocal exploitation in which both partners have net benefit

– mycorrhiza- associations between fungus and plant root that extracts soil nutrients

– Pollination, seed dispersal, etc…

• Commensuals are symbionts that have no measurable cost or benefit to host– Effects may depend on the “load” of parasites or commensuals and depend on nutritional status

and health of host organism

Higher plants as specialized niches for arthropods, worms & protozoa

- not shown are nematodes, bacteria & viruses (!)

Higher animals as specialized niches for arthropods, worms, protozoa, bacteria & viruses

fungi- brownworms- bluearthropods- greenprotozoa-yellow

Mutualistic protectors: cleaner fishcleaner: Labroides dimidiatus

client: Hemigymnus melapterus

Modified plant parts for mutualists:

a) beltian bodies of bullhorn Acacia tended by ants

b) hollow thorns enclosing colonies

Experimental removal of predatory symbiotic ants reveals mutualistic benefit in reducing herbivory

tree: Tachigali myrmecophila ant: Pseudomyrmex concolor

Farming mutualisms in aphids & ants, cost of mutualism: a)ants lower predation on aphids, but b) if no predators, ant-excluded colonies lower aphid growth & reproduction Tuberculatis quericola Formica yessensis

-Pollination - reward is nectar or pollenspecialized (at least temporarily) to ensure pollen transfer to conspecificslimit production so visit sequential individuals

Seed Dispersal- reward is pulp or seedsno need for temporal specialization, so can be generalists

Fig wasp symbiosis & fig tree phenology: - intraspecific randomized fruiting to maintain pollinating agaonid wasp species

Fruiting Phenology- as a result of coevolution with pollinating wasps, figs always available for vertebrate frugivores

- figs are “keystone species” in rainforests

u l a s o nd j f may u l a s o nd j f may u l a s o nd j f may u l a s o nd j f may u l a s o nd j f ma0

1

2

3

1985 1986 1987 1988 1989 1990

fruiting trees >15 cm dbh /0.1 ha

Ficus subtenda1 f-r--f-r-f-r-f-r--f-r---f-r2 r--f-r-f-r-f-r--f-r-f-r--f-r3 -f-r--f-r-f-r--f-r---f-r--f-r

Ficus stupenda1 f----r--f-----r----f-------r----f-2--f-----r----f-----r---f------r---3-r---f-----r---f-----r--f------r--

Parasites vs. Mutualists

• Many symbiotic species (specialized)

• Complex life cycles (e.g., alternating hosts)

• Dispersal important (transmission & infection)

• Sex common to generate variation in evolutionary wars

• Few symbiotic (generalized; e.g., lichens, rhizobial bacteria)

• Simple life cycles

• Vectors for transmission rare

• Sex uncommon as evolutionary conflict reduced

Life cycle of the parasitic cestode worm Diphyllobothrium

- birth rates determined by hosts

Life cycle of malaria-causing Plasmodium protozoa: from vertebrate red blood cells to mosquito gut to salivary glands

Behavioral responses of Hosts to Parasites: Manipulation to benefit the parasite

-Anopheles mosquitos feed on more hosts when infected with the malaria parasite Plasmodium

-Gall growth to harbor larvae induced by different species of Andricdus wasps

The vertebrate immune response changes habitat quality for specific parasites (and toxins); production of antigens promotes immunity to subsequent

colonization attempts

Mutualistic microbiota in vertebrate guts - microbes (mostly protozoa & bacteria) receive plant foods & live in high pH, regulated shelters- vertebrate hosts receive food from otherwise indigestible leaves- food in form of short-chain fatty acids (SCFAs), protein & vitamins

rabbit

sheep

zebra

kangaroo

Corals contain photosynthetic algae called zooanthellae

-intracellular symbionts that photosynthesize

-expelled by stressed corals (bleaching)

the symbiotic, mutualistic nitrogen-fixing bacteria of Rhizobium, infecting root nodules (especially legumes)

Legume trees are common on N-poor soils, and provide protein-rich leaves & seeds (Koompassia)

Effects of symbiotic Rhyzobium vs. N on soybeans & grass:change in competitive outcomes

Most mycorrhizas increase P & N uptake by host, but some may protect from pathogens:

P uptake in Hyacynthoides Vulpia protected from fungus by Glomus