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Exploitative Interactions: Exploitative Interactions: Predation, Herbivory,Predation, Herbivory,

Parasitism, and Disease Parasitism, and Disease

Chapter 14Chapter 14

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OutlineOutline

IntroductionIntroduction Complex InteractionsComplex Interactions Exploitation and AbundanceExploitation and Abundance Population FluctuationsPopulation Fluctuations

ModelsModels RefugesRefuges

Prey DensityPrey Density SizeSize

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IntroductionIntroduction

ExploitationExploitation: Interaction between : Interaction between populations that enhances fitness of one populations that enhances fitness of one individual while reducing fitness of the individual while reducing fitness of the exploited individual.exploited individual. PredatorsPredators kill and consume other organisms. kill and consume other organisms. ParasitesParasites live on host tissue and reduce host live on host tissue and reduce host

fitness, but do not generally kill the host.fitness, but do not generally kill the host.• ParasitoidParasitoid is an insect larva that consumes the is an insect larva that consumes the

host.host. PathogensPathogens induce disease. induce disease.

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Parasites That Alter Host Parasites That Alter Host BehaviorBehavior

Spiny-headed worms (Acanthocephalans) Spiny-headed worms (Acanthocephalans) changes behavior of amphipods in ways that changes behavior of amphipods in ways that make it more likely that infected amphipods will make it more likely that infected amphipods will be eaten by a suitable vertebrate host.be eaten by a suitable vertebrate host. Infected amphipods swim toward light (positive Infected amphipods swim toward light (positive

phototaxis), which is usually indicative of shallow phototaxis), which is usually indicative of shallow water, and thus closer to predators.water, and thus closer to predators.

• Only when the worm reaches the appropriate stage in life.Only when the worm reaches the appropriate stage in life.

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Parasites That Alter Host Parasites That Alter Host BehaviorBehavior

In a terrestrial In a terrestrial example, a spiny-example, a spiny-headed worm headed worm infects a pill bug.infects a pill bug.

Infected pill bugs Infected pill bugs leave shelter to leave shelter to wander out in the wander out in the open where they open where they are eaten by are eaten by starlings.starlings.

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Parasites That Alter Host BehaviorParasites That Alter Host Behavior

Experiments Experiments showed that showed that infected isopods infected isopods were more likely to were more likely to be eaten by be eaten by starlings.starlings. Likely due to Likely due to

behavior.behavior.

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Parasites That Alter Host Parasites That Alter Host BehaviorBehavior

Rust fungus Rust fungus Puccinia Puccinia monoicamonoica manipulates manipulates growth of host growth of host mustard plants mustard plants ((ArabisArabis spp.). spp.).

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Parasites That Alter Host Parasites That Alter Host BehaviorBehavior

PucciniaPuccinia infects infects ArabisArabis rosettes and invades rosettes and invades actively dividing actively dividing meristemic tissuemeristemic tissue.. Rosettes rapidly Rosettes rapidly

elongate and become elongate and become topped by a cluster of topped by a cluster of bright yellow leaves.bright yellow leaves.

Pseudo-flowers are Pseudo-flowers are fungal structures fungal structures including sugar-including sugar-containing spermatial containing spermatial fluids.fluids.

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Parasites That Alter Host BehaviorParasites That Alter Host Behavior

The combination of the yellow color and The combination of the yellow color and sugary fluids attracts pollinators.sugary fluids attracts pollinators. Carry rust spermatia (fungal reproductive Carry rust spermatia (fungal reproductive

cells) to other pseudo-flowers.cells) to other pseudo-flowers. Host plant generally dies.Host plant generally dies.

Check out this recent Check out this recent blog-post by Carl by Carl Zimmer on this subject!Zimmer on this subject!

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Entangling Exploitation with Entangling Exploitation with CompetitionCompetition

ParkPark found the presence/absence of a found the presence/absence of a protozoan parasite (protozoan parasite (Adeline triboliiAdeline tribolii) ) influences competition in flour beetles influences competition in flour beetles ((TriboliumTribolium).).

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Entangling Exploitation with Entangling Exploitation with CompetitionCompetition

AdelinaAdelina lives as an lives as an intracellular parasite.intracellular parasite. Reduces density of Reduces density of T. T.

castaneumcastaneum but has but has little effect on little effect on T. T. confusumconfusum..

T. castaneumT. castaneum is usually is usually the strongest the strongest competitor, but with the competitor, but with the presence of presence of AdelinaAdelina, , T. T. confusumconfusum becomes becomes strongest competitor.strongest competitor.

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Exploitation and AbundanceExploitation and Abundance

Predators, parasites, and pathogens Predators, parasites, and pathogens influence the distribution, abundance, and influence the distribution, abundance, and structure of prey and host populations.structure of prey and host populations.

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Herbivorous Stream Insect and Herbivorous Stream Insect and Its Algal FoodIts Algal Food

Lamberti and Lamberti and Resh studied Resh studied influence of influence of caddisfly larvae caddisfly larvae ((Helicopsyche Helicopsyche borealisborealis) on algal ) on algal and bacterial and bacterial populations on populations on which it feeds.which it feeds.

Results suggest Results suggest larvae reduce the larvae reduce the abundance of their abundance of their food supply.food supply.

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Herbivorous Stream Insect and Herbivorous Stream Insect and Its Algal FoodIts Algal Food

In a follow up study, a set of tiles was raised off In a follow up study, a set of tiles was raised off the stream bed in a way that prevented the stream bed in a way that prevented colonization of colonization of HelicopsycheHelicopsyche, but not other , but not other invertebrates.invertebrates.

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Herbivorous Stream Insect and Its Herbivorous Stream Insect and Its Algal FoodAlgal Food

The results show that bacterial & algal The results show that bacterial & algal populations were reduced on the streambed tiles populations were reduced on the streambed tiles as compared to the elevated tiles.as compared to the elevated tiles. HelicopsycheHelicopsyche reduces populations of its food. reduces populations of its food.

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Introduced Cactus and Herbivorous Introduced Cactus and Herbivorous MothMoth

Mid 1800Mid 1800’’s: prickly pear s: prickly pear cactus cactus Opuntia strictaOpuntia stricta was was introduced to Australia.introduced to Australia. Established populations in Established populations in

the wild with no natural the wild with no natural enemies.enemies.

• Government sought an insect Government sought an insect herbivore to reduce the herbivore to reduce the population.population.

• Moth Moth Cactoblastis cactorumCactoblastis cactorum found to be effective predator.found to be effective predator.

Also disperses pathogensAlso disperses pathogens Reduced by 3 orders of Reduced by 3 orders of

magnitude in 2 years.magnitude in 2 years.

• Equilibrium between the two.Equilibrium between the two.

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A Pathogenic Parasite, a Predator, A Pathogenic Parasite, a Predator, and Its Preyand Its Prey

Foxes in Sweden Foxes in Sweden infected with mange infected with mange mites in 1975.mites in 1975. Results in hair loss, Results in hair loss,

skin deterioration, & skin deterioration, & death.death.

Spread throughout Spread throughout Sweden in a decade.Sweden in a decade.

Population of foxes Population of foxes reduced by 70%.reduced by 70%.

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A Pathogenic Parasite, a Predator, A Pathogenic Parasite, a Predator, and Its Preyand Its Prey

Ecologists studied the Ecologists studied the effects of population effects of population reduction of foxes on reduction of foxes on their prey.their prey. Prey species Prey species

population sizes population sizes increased following the increased following the reduction of foxes.reduction of foxes.

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DynamicsDynamics

Predator-prey, host-parasite, and host-Predator-prey, host-parasite, and host-pathogen relations are dynamic.pathogen relations are dynamic. Temporal dynamics – populations of Temporal dynamics – populations of

predators and prey are not static, they cycle in predators and prey are not static, they cycle in abundance over time.abundance over time.

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Cycles of Abundance in Snowshoe Cycles of Abundance in Snowshoe Hares and Their PredatorsHares and Their Predators

Snowshoe Hares (Snowshoe Hares (Lepus americanusLepus americanus) and ) and Lynx (Lynx (Lynx canadensisLynx canadensis) both have extensive ) both have extensive trapping records that allow us to study trapping records that allow us to study population sizes over the past 200 years.population sizes over the past 200 years. Elton proposed abundance cycles driven by Elton proposed abundance cycles driven by

variation in solar radiation.variation in solar radiation. Keith suggested overpopulation theories:Keith suggested overpopulation theories:

• Decimation by disease and parasitism.Decimation by disease and parasitism.• Physiological stress at high density.Physiological stress at high density.• Starvation due to reduced food.Starvation due to reduced food.• Suggested long term studies.Suggested long term studies.

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Population FluctuationsPopulation Fluctuations

The data show that lynx and hare The data show that lynx and hare populations fluctuate with a 10 year cycle.populations fluctuate with a 10 year cycle.

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Snowshoe Hares - Role of Food Snowshoe Hares - Role of Food SupplySupply

Hares live in boreal forests dominated by Hares live in boreal forests dominated by conifers.conifers. Dense growth of understory shrubs.Dense growth of understory shrubs.

In winter, they browse on buds and stems In winter, they browse on buds and stems of shrubs and saplings such as aspen and of shrubs and saplings such as aspen and spruce.spruce. One population reduced food biomass from One population reduced food biomass from

530 kg/ha in late Nov. to 160 kg/ha in late 530 kg/ha in late Nov. to 160 kg/ha in late March.March.

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Snowshoe Hares - Role of Food Snowshoe Hares - Role of Food SupplySupply

Shoots produced after heavy browsing can Shoots produced after heavy browsing can increase levels of plant chemical increase levels of plant chemical defenses.defenses. Reducing usable food supplies.Reducing usable food supplies.

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Snowshoe Hares - Role of Snowshoe Hares - Role of PredatorsPredators

Lynx (Classic specialist predator)Lynx (Classic specialist predator) Coyotes & other generalist predators may Coyotes & other generalist predators may

also play a large role.also play a large role. Predation can account for 60-98% of Predation can account for 60-98% of

mortality during peak densities.mortality during peak densities.

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Snowshoe Hares - Role of Snowshoe Hares - Role of PredatorsPredators

Complementary:Complementary: Hare populations increase, causing food Hare populations increase, causing food

supplies to decrease. Starvation and weight supplies to decrease. Starvation and weight loss may lead to increased predation, all of loss may lead to increased predation, all of which decrease hare populations.which decrease hare populations.

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Experimental Test of Food and Experimental Test of Food and Predation ImpactsPredation Impacts

A large-scale, long-A large-scale, long-term experiment was term experiment was designed to sort out designed to sort out the impacts of food the impacts of food and predation on and predation on snowshoe hare snowshoe hare population cycles.population cycles.

Populations of all Populations of all three trophic levels three trophic levels need to be studied need to be studied simultaneously.simultaneously.

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Population Cycles in Mathematical and Population Cycles in Mathematical and Laboratory ModelsLaboratory Models

Mathematical and laboratory models offer Mathematical and laboratory models offer population ecologists the opportunity to population ecologists the opportunity to manipulate variables that they cannot manipulate variables that they cannot control in the field.control in the field.

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Population Cycles in Mathematical Population Cycles in Mathematical and Laboratory Modelsand Laboratory Models

The Lotka-Volterra model assumes the host The Lotka-Volterra model assumes the host population grows exponentially, and population grows exponentially, and population size is limited by parasites, population size is limited by parasites, pathogens, and predators.pathogens, and predators.

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Model BehaviorModel Behavior

Host exponential growth often opposed by Host exponential growth often opposed by exploitation.exploitation. Host reproduction immediately translated into Host reproduction immediately translated into

destruction by predator.destruction by predator. Increased predation = more predators.Increased predation = more predators. More predators = higher exploitation rate.More predators = higher exploitation rate. Larger predator population eventually reduces Larger predator population eventually reduces

host population, in turn reducing predator host population, in turn reducing predator population.population.

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Model BehaviorModel Behavior

Reciprocal effects Reciprocal effects produce oscillations in produce oscillations in two populations.two populations. Although the Although the

assumptions of eternal assumptions of eternal oscillations and that oscillations and that neither host nor exploiter neither host nor exploiter populations are subject populations are subject to carrying capacities are to carrying capacities are unrealistic, L-V models unrealistic, L-V models made valuable made valuable contributions to the field.contributions to the field.

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Laboratory ModelsLaboratory Models

UtidaUtida found reciprocal found reciprocal interactions in adzuki bean interactions in adzuki bean weevils, weevils, Callosobruchus Callosobruchus chinensis,chinensis, over several over several generations.generations.

GauseGause found similar patterns found similar patterns in in P. aureliaP. aurelia..

Most laboratory experiments Most laboratory experiments have failed in that most have have failed in that most have led to the extinction of one led to the extinction of one population within a relatively population within a relatively short period. short period.

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RefugesRefuges

To persist in the face of exploitation, hosts To persist in the face of exploitation, hosts and prey need and prey need refugesrefuges..

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RefugesRefuges Gause attempted to Gause attempted to

produce population produce population cycles with cycles with Paramecium Paramecium caudatumcaudatum and and Didinium Didinium nasutumnasutum.. DidiniumDidinium quickly quickly

consumed all consumed all ParameciumParamecium and went extinct. (Both and went extinct. (Both populations extinct)populations extinct)

• Added sediment for Added sediment for ParameciumParamecium refuge. refuge.

• Few Few Paramecium Paramecium survived survived after after DidiniumDidinium extinction. extinction.

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RefugesRefuges Huffaker studied six-spotted mite Huffaker studied six-spotted mite

Eotetranychus sexmaculatusEotetranychus sexmaculatus and and predatory mite predatory mite Typhlodromus occidentalisTyphlodromus occidentalis.. Separated oranges and rubber balls with Separated oranges and rubber balls with

partial barriers to mite dispersal.partial barriers to mite dispersal.

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RefugesRefuges

TyphlodromusTyphlodromus (pred) crawls while (pred) crawls while Eotetranychus Eotetranychus (prey) balloons.(prey) balloons.

Provision of small Provision of small wooden posts to wooden posts to serve as launching serve as launching pads maintained pads maintained population population oscillations oscillations spanning 6 months.spanning 6 months.

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Variety of Refuges - SpaceVariety of Refuges - Space

Spatial refuges – places where members Spatial refuges – places where members of the exploited population have some of the exploited population have some protection from predators and parasitoids.protection from predators and parasitoids. BurrowsBurrows TreesTrees AirAir Water or landWater or land

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Variety of Refuges - NumbersVariety of Refuges - Numbers Living in a large group provides a Living in a large group provides a ““refuge.refuge.”” PredatorPredator’’s response to increased prey density:s response to increased prey density:

Prey consumedPrey consumed x x PredatorsPredators = = Prey ConsumedPrey Consumed

PredatorPredator AreaArea Area Area

Wide variety of organisms employ predator Wide variety of organisms employ predator satiation defense.satiation defense. Prey can reduce individual probability of being eaten Prey can reduce individual probability of being eaten

by living in dense populations.by living in dense populations.

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Predator Satiation by an Predator Satiation by an Australian TreeAustralian Tree

Synchronous widespread Synchronous widespread seed and fruit production seed and fruit production is known as is known as mastingmasting.. Janzen proposed that Janzen proposed that

seed predation is a major seed predation is a major selective force favoring selective force favoring mast crop production.mast crop production.

OO’’Dowd and Gill Dowd and Gill determined synchronous determined synchronous seed dispersal by seed dispersal by Eucalyptus Eucalyptus reduces losses reduces losses of seeds to ants.of seeds to ants.

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Predator Satiation by Periodical Predator Satiation by Periodical CicadasCicadas

Periodical cicadas Periodical cicadas Magicicada sppMagicicada spp. . emerge as adults emerge as adults every 13-17 years.every 13-17 years. Densities can Densities can

approach 4x10approach 4x1066 ind / ind / ha.ha.

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Predator Satiation by Periodical Predator Satiation by Periodical CicadasCicadas

WilliamsWilliams estimated 1,063,000 cicadas emerged estimated 1,063,000 cicadas emerged from 16 ha study site.from 16 ha study site. 50% emerged during four consecutive nights.50% emerged during four consecutive nights. Losses to birds was only 15% of production.Losses to birds was only 15% of production.

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Size As A RefugeSize As A Refuge

If large individuals are ignored by If large individuals are ignored by predators, then large size may offer a form predators, then large size may offer a form of refuge.of refuge.

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Size As A RefugeSize As A Refuge Large mussels are eaten Large mussels are eaten

infrequently by sea stars.infrequently by sea stars. If mussels can avoid predation If mussels can avoid predation

long enough to reach 10-12 long enough to reach 10-12 cm, it will be immune from cm, it will be immune from most sea stars.most sea stars.

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Size As A RefugeSize As A Refuge

Peckarsky Peckarsky observed observed mayflies (Family mayflies (Family Ephenerellidae) Ephenerellidae) making themselves making themselves look larger in the face look larger in the face of foraging stoneflies.of foraging stoneflies. In terms of optimal In terms of optimal

foraging theory, large foraging theory, large size equates to lower size equates to lower profitability.profitability.