LSM2251-08 Species Interactions II

8/3/2019 LSM2251-08 Species Interactions II

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Species Interactions (2)

ExploitativeInteractions

LSM2251

Ecology and Environments

A/P Li Daiqin


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Introduction

Exploitation: Interaction betweenpopulations that enhances fitness of oneindividual while reducing fitness of theexploited individual. That is, one organism

makes its living at the expense of another. Predators kill and consume other organisms.

Parasites live on host tissue and reduce hostfitness, but do not generally kill the host. Parasitoid is an insect larva that consumes the host.

Pathogens induce disease.


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Parasitism

Parasitism-one partner benefits, the other is harmed in someway.

It is a very common life strategy and affects a wide varietyof fish, but few fish are parasites themselves.

Ectoparasites (externalparasites) are usuallyfrom the order Isopoda.They have flattened

bodies and complexsucker organisms forattachment to host fishas well as sharp teeth.


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Parasites that alter host behavior

Spiny-headed worm (Acanthocephalans) changes behavior offreshwater amphipods in ways that make it more likely thatinfected amphipods will be eaten by a suitable vertebrate host(e.g. ducks).

Infected amphipods swim toward light (positive phototaxis),which is usually indicative of shallow water, and thus closer topredators.

Another Acanthocephalans causes the terrestrial isopodArmadillidium vulgare to leave shelter and thus become morevulnerable to predation by the European starling (Sturnus

vulgaris).


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http://www.google.com.sg


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Molles, M.C. Jr. 2010. Ecology: concepts and applications, 3rd ed. McGraw-Hill.


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Field observations: Only 1% of pill bug infected by worms, but 40% of

starlings were infected; suggesting somethingenhancing rate of transmission.

Prediction: parasitic worms would alter behaviour of pillbugs (Moore 1983, 1984a,b).

Lab observations: two groups - uninfected and infectedpill bugs.

Infected pill bugs spent less time in sheltered areas whileuninfected pill bugs spent more time dim environments.

Suggesting that infected pill bugs would be easily seenand eaten by starlings

Feed experiments: a mixture of 10 infected and 10uninfected pill bugs was offered to starlings.

Results: starlings ate more infected pill bugs that uninfectedones.


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A plant pathogen that mimicsflowers

Rust fungus Puccinia monoica manipulatesgrowth of host mustard plants (Arabis spp.). Puccinia infectsArabis rosettes and invades

actively dividing meristematic tissue.

Rosettes rapidly elongate and become topped by a clusterof bright yellow leaves. Pseudo-flowers are fungal structures including sugar-

containing spermatial fluids.

Attract pollinators


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A plant pathogen that mimicsflowers


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Exploitation with competition: parasitesinfluence competition between flourbeetles

Tribolium confusumTribolium castaneum


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Exploitation with competition

Park(1948) found the presence/absenceof a protozoan parasite (Adeline tribolii)influences competition in flour beetles(Tribolium).

Adelina lives as an intercellular parasite. Reduces density ofT. castaneum but has little effect

on T. confusum.

T. castaneum is usually the strongest competitor, butwith the presence ofAdelina, T. confusum becomesstrongest competitor.


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Do Predators Control Preypopulations?

Does a predator control a prey populationor prey population size determine apredator population?


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Do Predators Control Preypopulations?

Does a predator control a prey populationor prey population size determine apredator population?

Snowshoe Hares (Lepus americanus) andLynx (Lynx canadensis).

Extensive trapping records. Elton proposed abundance cycles driven by

variation in solar radiation.

Keith suggested overpopulation theories: Decimation by disease and parasitism. Physiological stress at high density. Starvation due to reduced food.


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Population fluctuations


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Snowshoe hares - role ofpredators

Lynx (Classic specialist predator) Coyotes may also play a large role.

Predation can account for 60-98% of mortalityduring peak densities.

Complementary: Hare populations increase, causing food

supplies to decrease. Starvation and weightloss may lead to increased predation, all ofwhich decrease hare populations.


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Population cycles in mathematicaland laboratory models

Lotka-Volterra assumes host populationgrows exponentially, and population size islimited by parasites, pathogens, andpredators:

dNh/dt= rhNh pNhNp

rhNh = Exponential growth by host population. Opposed by:

p = rate of parasitism / predation. Nh = Number of hosts. Np = Number of parasites / predators.


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Lotka-Volterra assumes parasite/predatorgrowth rate is determined by rate ofconversion of food into offspring minusmortality rate of parasite population:

dNp/dt= cpNhNp - dpNp

cpNhNp= Conversion rate of hosts intooffspring.

pNhNp = Rate at which exploiters destroyhosts.

c= Conversion factor

Population cycles in mathematicaland laboratory models


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Model behaviour

Host exponential growth often opposed byexploitation.

Host reproduction immediately translated intodestruction by predator.

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

host population, in turn reducing predator

population.


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Reciprocal effects produce oscillations intwo populations.

Although the assumptions of eternaloscillations and that neither host nor exploiter

populations are subject to carrying capacitiesare unrealistic, L-V models made valuablecontributions to the field.

Model behaviour


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Exploitative interactions:Summary

Exploitative interactions weave populations into aweb of relationships that defy easy generalisation.

Predators, parasites, and pathogens influence thedistribution, abundance, and structure of prey andhost populations.

Predator-prey, host-parasite, and host-pathogenrelationships are dynamic.

Predators and parasites as natural enemies havebeen used to control populations of insects thatattack crops or to to control invasive weeds.

LSM2251-08 Species Interactions II

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