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Exploitation: Predation, Herbivory, Parasitism, and Disease

Chapter 14

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Introduction

• Exploitation: Interaction between populations that enhances fitness of one individual while reducing fitness of the exploited individual.– Predators kill and consume other organisms.– 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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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

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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 pollenators

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Parasites That Alter Host Behavior

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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 but has little effect on

T. confusum.• T. castaneum is usually the strongest competitor, but

with the presence of Adelina, T. confusum becomes strongest competitor.

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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

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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

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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:

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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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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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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1818

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 crawls while Eotetranychus

balloons.– Provision of small wooden posts to serve as

launching pads maintained population oscillations spanning 6 months.

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Other kinds of Refuge?

2020

Predator Satiation by Periodical Cicadas

• Periodical cicadas Magicicada spp. emerge as adults every 13-17 years.– Densities can approach 4x106 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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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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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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