Exploitation: Predation, Herbivory, Parasitism, and Disease

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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 1800sprickly 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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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?
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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.
• Predators 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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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
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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
• dNp/dt cpNhNp-dpNp
• cpNhNp Conversion rate of hosts into offspring.
• pNhNp Rate at which exploiters destroy hosts.
• C Conversion factor

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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.

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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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Refuges
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Predator Satiation by an Australian Tree

• Synchronous widespread seed and fruit production
  is known as masting.
• Janzen proposed that seed predation is a major
  selective force favoring mast crop production.
• ODowd and Gill determined synchronous seed
  dispersal by Eucalyptus reduces losses of seeds
  to ants.