Exploitative Interactions: Predation, Herbivory, Parasitism, and Disease

1
Exploitative Interactions Predation, Herbivory,
Parasitism, and Disease

• Chapter 14

2
Outline

• Introduction
• Complex Interactions
• Exploitation and Abundance
• Population Fluctuations
• Models
• Refuges
• Prey Density
• Size

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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

• Spiny-headed worms (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 (positive
  phototaxis), which is usually indicative of
  shallow water, and thus closer to predators.
• Only when the worm reaches the appropriate stage
  in life.

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Parasites That Alter Host Behavior

• In a terrestrial example, a spiny-headed worm
  infects a pill bug.
• Infected pill bugs leave shelter to wander out in
  the open where they are eaten by starlings.

6
Parasites That Alter Host Behavior

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

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Parasites That Alter Host Behavior

• Rust fungus Puccinia monoica manipulates growth
  of host mustard plants (Arabis spp.).

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Parasites That Alter Host Behavior

• 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.

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Parasites That Alter Host Behavior

• The combination of the yellow color and sugary
  fluids attracts pollinators.
• Carry rust spermatia (fungal reproductive cells)
  to other pseudo-flowers.
• Host plant generally dies.
• Check out this recent blog-post by Carl Zimmer on
  this subject!

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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).

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Entangling Exploitation with Competition

• Adelina lives as an intracellular 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

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

13
Herbivorous Stream Insect and Its Algal Food

• Lamberti and Resh studied influence of caddisfly
  larvae (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

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

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Herbivorous Stream Insect and Its Algal Food

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

16
Introduced Cactus and Herbivorous Moth

• Mid 1800s prickly pear cactus Opuntia stricta
  was introduced to Australia.
• Established populations in the wild with no
  natural enemies.
• Government sought an insect herbivore to reduce
  the population.
• Moth Cactoblastis cactorum found to be effective
  predator.
• Also disperses pathogens
• Reduced by 3 orders of magnitude in 2 years.
• Equilibrium between the two.

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A Pathogenic Parasite, a Predator, and Its Prey

• Foxes in Sweden infected with mange mites in
  1975.
• Results in hair loss, skin deterioration,
  death.
• Spread throughout Sweden in a decade.
• Population of foxes reduced by 70.

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A Pathogenic Parasite, a Predator, and Its Prey

• Ecologists studied the effects of population
  reduction of foxes on their prey.
• Prey species population sizes increased following
  the reduction of foxes.

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Dynamics

• Predator-prey, host-parasite, and host-pathogen
  relations are dynamic.
• Temporal dynamics populations of predators and
  prey are not static, they cycle in abundance over
  time.

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Cycles of Abundance in Snowshoe Hares and Their
Predators

• Snowshoe Hares (Lepus americanus) and Lynx (Lynx
  canadensis) both have extensive trapping records
  that allow us to study population sizes over the
  past 200 years.
• 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.
• Suggested long term studies.

21
Population Fluctuations

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

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Snowshoe Hares - Role of Food Supply

• Hares live in boreal forests dominated by
  conifers.
• Dense growth of understory shrubs.
• In winter, they 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.

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Snowshoe Hares - Role of Food Supply

• 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 other generalist predators may also
  play a large role.
• Predation can account for 60-98 of mortality
  during peak densities.

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Snowshoe Hares - Role of Predators

• 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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Experimental Test of Food and Predation Impacts

• A large-scale, long-term experiment was designed
  to sort out the impacts of food and predation on
  snowshoe hare population cycles.
• Populations of all three trophic levels need to
  be studied simultaneously.

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Population Cycles in Mathematical and Laboratory
Models

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

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Population Cycles in Mathematical and Laboratory
Models

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

31
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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Refuges

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

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Refuges

• Gause attempted to produce population cycles with
  Paramecium 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.

34
Refuges

• Huffaker studied six-spotted mite Eotetranychus
  sexmaculatus and predatory mite Typhlodromus
  occidentalis.
• Separated oranges and rubber balls with partial
  barriers to mite dispersal.

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Refuges

• Typhlodromus (pred) crawls while Eotetranychus
  (prey) balloons.
• Provision of small wooden posts to serve as
  launching pads maintained population oscillations
  spanning 6 months.

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Variety of Refuges - Space

• Spatial refuges places where members of the
  exploited population have some protection from
  predators and parasitoids.
• Burrows
• Trees
• Air
• Water or land

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Variety of Refuges - 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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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.

39
Predator Satiation by Periodical Cicadas

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

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Predator Satiation by Periodical Cicadas

• 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.

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Size As A Refuge

• Large mussels are eaten infrequently by sea
  stars.
• If mussels can avoid predation long enough to
  reach 10-12 cm, it will be immune from most sea
  stars.

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Size As A 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.