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					BIOL 4120: Principles of Ecology

Lecture 14: Parasitism and Mutualism
Dafeng Hui Room: Harned Hall 320 Phone: 963-5777 Email: dhui@tnstate.edu

Outline (chapter 15)
15.1 Parasite draw resources from host organisms 15.2 Hosts provide diverse habitats for parasites 15.3 Direct transmission between host organisms 15.4 Multiple hosts and stages 15.5 Hosts respond to parasitic invasions 15.6 Parasite can impact host survival and reproduction 15.7 Parasites may regulate host population 15.8 Parasites can evolve into a positive relationship 15.9 Symbiotic mutualisms are involved in the transfer of nutrients 15.10 Some symbiotic mutualisms are defensive 15.11 Mutualisms may be nonsymbiotic 15.12 Mutualisms are often necessary for pollination, seed dispersal 15.13 Mutualism can influence population dynamics 15.14 A simple model

15.1 Parasites draw resources from host organisms
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Parasitism: a relationship of two organisms living together (symbiosis) and one derives its nourishment at the expense of the other Parasite and host Parasitism has • Negative effect on hosts • But do not usually kill hosts Parasite consists of a wide range of organisms, including • Virus, bacteria, protists, fungi, plants, and invertebrates (include arthropods)

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Parasites draw resources from host organisms
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According to size, parasites may be classified as • Microparasites  Small size and short generation time  Viruses, bacteria, fungi, protozoa etc  May cause disease  Usually direct transmission from host to host: Air, water, etc • Macroparastites  Relatively large, have comparatively long generation time  Liver flukes, lice, ticks, mistletoe (shrub), etc  Do not complete an entire life history in one host, usually more than one host  Both direct and indirect transmission, later involves a vector such as a mosquito for malaria

15.2 Hosts provide diverse habitats for parasites
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Hosts are the habitats for parasites Depends on the places: • Ectoparasites: live on the skin within the protective cover of feathers and hair • Endoparasites: live within the host
Examples: Fleas, ticks, are ectoparasites Live flukes, lung flukes, flatworms, are endoparasites

Hosts provide diverse habitats for parasites
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Animal host: • Ectoparasites: live on the legs, on the upper and lower body surfaces, and even on the mouthparts • Endoparasites: live in the bloodstream, heart, brain, digestive tract, nasal tracts, lungs, gonads, bladder, pancreas, eyes, gills of fish, muscle tissue, or other sites Plant host: • Ecotparasites: live on the roots and stems, flowers, pollen or fruit • Endoparasites: penetrate the roots, bark to live in the woody tissue beneath, within leaves. For parasites to survive and multiply, parasites have to escape (when a host dies) and infect another host Process of transmission from one host to another can occur by either direct or indirect means and can involve adaptations by parasites to virtually all aspects of feeding, social and mating behaviors in host species.

15.3 Direct transmission can occur between host organisms
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Direct Transmission From host to host, no intermediate organism involved Most mircoparasites are transmitted directly Two approaches: • Direct contact with a carrier (host)  Smallpox virus and the variety of bacterial and viral parasites associated with sexually transmitted diseases (HIV) • Indirect through the air, water, or other substrate  Influenza (airborne)  SARS (bird flu)

Direct transmission can occur between host organisms
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Macroparasites of animal and plants can move from infected hosts to uninfected hosts by direct transmission Animals • Ectoparasites  Fleas, ticks on birds and mammals • Endoparasites  Roundworms in mammals (life cycle in textbook, page 313)

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Plants • Holoparasites and hemiparasite • squawroot (parasitizes the roots of oaks) and beechdrops (parasitizes the root of beech trees)

15.4 Indirect transmission involves an intermediate vector
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Host  vector (intermediate organism)  host

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Animal black-legged tick

• Lyme disease (major arthropod-borne disease in US) • Caused by bacteria, Borrelia burgdorferi • It lives in the blood-stream of vertebrates, from birds to deer and humans • Depends on tick for transmission from one host to another.

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Mosquito
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Malaria (still kills 0.7 to 2.7 million people in Africa) Caused by 4 species of protozoan parasites (Plasmodium) Blood-stream Infected female mosquitoes

Indirect transmission involves an intermediate vector
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Plant Elm bark beetles

• Fungi • Cause devastating Dutch elm disease from tree to tree

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Birds (through seed dispersal)

• Mistletoes (Phoradendron spp.) • Hemiparasites (can do photosynthesis, but draw water and nutrients from host) • Birds feed on the mistletoe fruits, seeds pass through digestive system, are deposited on trees. Sticky seeds attach to limbs and send out rootlets that embrace the limb and enter the sapwood.

Birds and mistletoe

15.5 Transmission can involve multiple hosts and stages
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Life history of an organism involves several stages • Growth and reproduction or juvenile (prereproduction), reproduction, and postreproduction • Some parasites can’t complete their entire life cycle in a single host species Definitive host: host species in which the parasite becomes an adult and reaches maturity (where adults reproduce) Intermediate host: other hosts which harbor some developmental phase of parasites (where juveniles grow) Intermediate hosts can be one, two or even three

Transmission can involve multiple hosts and stages

Deer picked infected snails. In stomach, larvae leave the snail, enter abdominal membranes, travel via spinal cord to brain. Mate and lay eggs, larvae and eggs pass through the bloodstream to lung, cough, swallow, and passed out through feces. Picked up by snails.

The life history of a macroparasite, the meningeal worm Parelaphostrongylus tenuis.

White-tailed deer, moose, and elk.
Transmission is indirect, involving snails as intermediate host.

15.6 Host respond to parasitic invasions
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Adaptations of host species to minimize the impact of parasite Host response to parasite • Avoidance
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• Reduce parasitic invasion • Combat parasitic infection once it has occurred

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• Inflammatory response in animals
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Birds and mammals, grooming Birds, preening Deer seek dense and shaded places to avoid deerflies

• Gall formation in plants
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Stimulate secretion of histamines (chemical alarm signals), induce increased blood flow to site, resulting inflammatory Abnormal growth structure Cut off contact of fungus with health tissue

Host respond to parasitic invasions
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Host response to parasite (cont.) • Avoidance • Inflammatory response in animals • Gall formation in plants • Immune response in animals  When a foreign object such as a virus and bacteria (antigen), enter bloodstream, elicits an immune response  White cells produce antibodies  Antibodies target antigens present on the parasite’s surface, helping to counter their effect

Host respond to parasitic invasions
• Immune response in animals
Antibodies expensive to produce  Immune system has a remarkable “memory”  Vaccination
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Sometimes, immune system does not work • HIV and AIDS as an example • HIV (human immunodeficiency virus), the causal agent of AIDS • Transmitted sexually or through the use of shared needle, or by infected donor blood.

15.7 Parasites affect host survival and reproduction
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Parasites affect host survival and reproduction • Malaria in humans • Malaria on western fence lizard
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Can also reduce reproductive success of males

Clutch size is 20% smaller

• Secondary sex characteristics such as bright and ornate plumage of male birds • Infection influence the attraction

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Parasites can increase mortality that can result from indirect consequences of infection

Infection of the California killfish

15.7 Parasites may regulate host populations
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Parasites can be major regulators of population • Plants
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• Humans
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Chestnut blight, nearly exterminated the American chestnut Dutch elm disease, nearly removed American elm from North America Black Death in14th century Smallpox in 18th century Cholera in 19th century

Parasites may act as a selective agent of mortality, infecting only a subset of the population.

Parasites may regulate host populations
• Black Death in14th century
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• Smallpox in 18th century
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Needs lots of rats and some concentration of human population Not a major problem to Romans or Chinese civilizations due to good urban planning
Needs even high human population due to direct transmission Halted by human evolution
• Vaccination

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• Cholera in 19th century
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Needs even higher density and water/food transmitted Halted by human evolution
• Clean water supply and bacteriology

• AIDS in 21st century
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Needs even higher population density

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Sexual transmission requires large number of contact, cf syphilis in 19th century Major effect on population once infection rate reaches 2%-5% Halted by
• Exponential growth in southern Africa • Affects human productivity directly
• Change in human behavior? • Science?

• Never a problem in low density central African origin except to some villages

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15.9 Parasitism can evolve into a positive relationship
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Parasitism: • Parasites gain benefits from hosts; hosts suffer. Commensalism: • A relationship between two species in which one species benefits without significantly affecting the other Mutualism: • A relationship between members of two species that benefit both. • Individuals of both species enhance their survival, growth, or reproduction

Commensalism
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Defined as an ecological relationship in which one species benefits from other species, which is itself not affected one way or the other by the relationship This is thus a “+, 0” relationship Example: Spanish moss (epiphyte) on trees

Commensalism between cattle (as food beaters) and cattle egrets (three white birds, one sitting on cow) in Jamaica

Mutualism
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Mutualism: Interactions between individuals of different species that benefit both partners.
• Facultative Mutualism occurs when a species can live without its mutualistic partner. • Obligate Mutualism occurs when a species is dependent on a mutualistic relationship.
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Lichens: symbiotic association A fungus and an alga combined to form a spongy body (thallus).

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Mutualism: • Individuals of both species enhance their survival, growth, or reproduction • can be symbiotic or nonsymbiotic • At least one member of the pair becomes totally dependent on the other

Mutualism – more examples
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Plants and pollinators Plant and mycorrhizal fungi
• Ash tree and mycorrhizal fungi

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Corals and zooxanthellae Phainopepla and mistletoe

15.10 Symbiotic mutualisms are involved in the transfer of nutrients
Herbivores: Digestion system of cow Chamber of ruminant’s stomach contain large populations of bacteria and protozoa that carry out the process of fermentation. Anaerobic process Plants: Nitrogen fixation N-fixing bacteria: Rhizobium Legumes: clover, beans, peas Plants attract bacteria through the release of exudates and enzymes from the roots. Infection and form of root nodules Reduce N2 to ammonia (nitrogen fixation)

Plant roots and mycorrhizal fungi Fungi assist the plant with the uptake of nutrient from the soil (extended water and nutrients absorption) Plant provides the fungi with carbon, a source of energy.

Endomycorrhizae (a)
And ectomycorrhizae (b)

15.11 Some symbiotic mutualisms are defensive
Example one: Plant and fungi Plant provide food to fungi in the form of photosynthates Fungi defend the host plant against grazing by producing alkaloids compounds in the tissue of host grasses (tastes bitter, toxic). Example two: cleaning mutualism Cleaner-shrimp and cleaner-fish
Moray eel

Red-billed oxpecker

15.12 Mutualisms may be nonsymbiotic
Not necessary to be symbiotic Example: plant-pollinator relationship (insects, birds) Seed dispersal (birds, ants)

wasp and orchid

bleeding heart and elaiosome

cedar waxwing

15.13 Mutualism can influence population dynamics
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Symbiotic mutualism: depends on each other remove one, another can’t grow well or die Nonsymbiotic mutalism
• Difficult to study
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Mutualism can involve multiple species and affect the community

Example: pollination, could result extinction, but most of cases, subtle effects

• Oaks
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Truffles Voles Pigs (search truffles) Humans (eat truffles)

15.14 A simple model of mutualistic interactions
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Lotka-Voltrra mutualism model
species 1

dN1 K1  N1   21 N 2  r1 N1 ( ) dt K1 dN 2 K 2  N 2  12 N1  r2 N 2 ( ) dt K2

species 2

• Very similar to two species competition model • Alphas are positive interaction coefficients

A model of mutualistic interactions
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Solve the equation (zero isolines)
• dN1/dt=0, N2=(K1-N1)/(a2,1) • dN2/dt=0, N2=(k2+(a2,1)N1)
r1=3.22, k1=1000,alpha12=0.5 r2=3.22, k2=100, alpha21=0.6

Stable co-existence

With higher K values

Population trajectories
When projected, it is showed that carrying capacity of each species is increased by the presence of another mutualist.

End

Mutualisms can be classified ecologically:
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Trophic--specialized partnerships for obtaining energy and nutrients • Corals (algae & zoozanthellae) • Nitrogen-fixing bacteria (e.g., rhizobium & plant) • Ectotrophic mycorrhizae & plants • Lichens (fungus & alga) Defensive--partnerships providing protection against herbivores, predators, or parasites • Cleaner fish • Ant-Acacia (ants protect against herbivores) Dispersive--partnerships in which animals disperse pollen or seeds of plants, generally for food reward • Flower-pollinator • Fruit-seed disperser


				
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