Exploitative Interactions: Predation, Herbivory, Parasitism, and Disease Chapter 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Outline • • • • • Introduction Complex Interactions Exploitation and Abundance Dynamics  Models Refuges  Prey Density  Size  Ecology of Fear 2

Introduction • Exploitation : Interaction between populations that enhances fitness of one individual while reducing fitness of the exploited individual.

 Predators organisms.

kill and consume other  Parasites live on host tissue and reduce host fitness, but do not generally kill the host.

 Parasitoid is an insect larva that consumes the host.

 Pathogens induce disease.

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Complex Interactions • Parasites That Alter Host Behavior  Spring-Headed Worm (Acanthocephalans) changes behavior of amphipods in ways that make it more likely that infected amphipods will be eaten by a suitable vertebrate host.

 Infected amphipods swim toward light, which is usually indicative of shallow water, and thus closer to predators.

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Parasites That Alter Host Behavior 5

Parasites That Alter Host Behavior • Rust fungus

Puccinia monoica

manipulates growth of host mustard plants (

Arabis

spp.).

 Puccinia infects Arabis rosettes and invades actively dividing meristemic tissue .

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

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

– Attract pollinators 6

Parasites That Alter Host Behavior 7

Entangling Exploitation with Competition •

Park

found the presence/absence of a protozoan parasite (

Adeline tribolii

) influences competition in flour beetles (

Tribolium

).



Adelina

lives as an intercellular parasite.

 Reduces density of

T. castaneum

has little effect on

T. confusum

.

but 

T. castaneum

is usually the strongest competitor, but with the presence of

Adelina

,

T. confusum

strongest competitor.

becomes 8

Exploitation and Abundance • Introduced Cactus and Herbivorous Moth  Mid 1800’s:prickly pear cactus

Opuntia stricta

was introduced to Australia.

 Established populations in the wild.

 Government asked for assistance in control.

 Moth

Cactoblastis cactorum

found to be effective predator.

– Reduced by 3 orders of magnitude in 2 years.

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Exploitation and Abundance 10

Herbivorous Stream Insect and Its Algal Food •

Lamberti

and

Resh

studied influence of caddisfly (

Helicopsyche borealis

) on algal and bacterial populations on which it feeds.

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

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Herbivorous Stream Insect and Its Algal Food 12

• Cycles of Abundance in Snowshoe Hares and Their Predators Snowshoe Hares (

Lepus americanus

) and Lynx (

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 14

Snowshoe Hares - Role of Food Supply • • • Live in boreal forests dominated by conifers.

 Dense growth of understory shrubs.

In winter, browse on buds and stems of shrubs and saplings such as aspen and spruce.

 One population reduced food biomass from 530 kg/ha in late Nov. to 160 kg/ha in late March.

Shoots produced after heavy browsing can increase levels of plant chemical defenses.

 Reducing usable food supplies.

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Snowshoe Hares - Role of Predators •  Lynx (Classic specialist predator)  Coyotes may also play a large role.

 Predation can account for 60-98% of mortality during peak densities.

Complementary:  Hare populations increase, causing food supplies to decrease. Starvation and weight loss may lead to increased predation, all of which decrease hare populations.

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• • Population Cycles in Mathematical and Laboratory Models Lotka Volterra assumes host population grows exponentially, and population size is limited by parasites, pathogens, and predators: dN h /d t = r h N h – pN h N p r h N h = Exponential growth by host population.

 Opposed by:  P = rate of parasitism / predation.

 N h = Number of hosts.

 N p = Number of parasites / predators.

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• • • • Population Cycles in Mathematical and Laboratory Models Lotka Volterra assumes parasite/predator growth rate is determined by rate of conversion of food into offspring minus mortality rate of parasitoid population: dN p /d t = cpN h N p -d p N p cpN h N p = Conversion rate of hosts into offspring.

pN h N p hosts.

= Rate at which exploiters destroy C = Conversion factor 18

Model Behavior • Host exponential growth often opposed by exploitation.

 Host reproduction immediately translated into destruction 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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Model Behavior • Reciprocal effects produce oscillations in two populations.

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

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Model Behavior 21

Laboratory Models • •

Utida

found reciprocal interactions in adzuki bean weevils

Callosobruchus chinensis

over several generations.



Gause

found similar patterns in

P. aurelia

.

Most laboratory experiments have failed in that most have led to the extinction of one population within a relatively short period. 22

Refuges • • To persist in the face of exploitation, hosts and prey need refuges.

Gause

attempted to produce population cycles with

P. caudatum

and

Didinium nasutum

.



Didinium

quickly consumed all

Paramecium

and went extinct. (Both populations extinct)  Added sediment for

Paramecium

refuge.

 Few

Paramecium

survived after

Didinium

extinction.

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Refuges 24

Refuges • Huffaker studied six-spotted mite

Eotetranychus sexmaculatus

and predatory mite

Typhlodromus occidentalis

.

 Separated oranges and rubber balls with partial barriers to mite dispersal.



Typhlodromus

balloons.

crawls while

Eotetranychus

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

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Refuges 26

Protection in Numbers • • Living in a large group provides a “refuge.” Predator’s response to increased prey density: Prey consumed x Predators = Prey Consumed Predator Area Area • Wide variety of organisms employ predator satiation defense.

 Prey can reduce individual probability of being eaten by living in dense populations.

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Predator Satiation by Periodical Cicadas • • Periodical cicadas

Magicicada spp

. emerge as adults every 13-17 years.

 Densities can approach 4x10 6 ind / ha.

Williams

estimated 1,063,000 cicadas emerged from 16 ha study site.

 50% emerged during four consecutive nights.

 Losses to birds was only 15% of production.

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Size As A Refuge • If large individuals are ignored by predators, then large size may offer a form of refuge.



Peckarsky

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

 In terms of optimal foraging theory, large size equates to lower profitability.

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Ecology of Fear and Refuges • • The presence of predators can alter the behavior of prey to avoid high-risk locations.

 These behavioral effects are termed “the ecology of fear”

Ripple and Beschta

found that increasing wolf populations in Yellowstone National Park have affected their prey’s distribution  Elk are more vulnerable to wolf attack in riparian habitat and have reduced their foraging in this habitat. 30

Ecology of Fear and Refuges 31

Review • • • • • Introduction Complex Interactions Exploitation and Abundance Dynamics  Models Refuges  Prey Density  Size  Ecology of Fear 32

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