Exploitative Interactions: Predation, Herbivory,
Parasitism, and Disease
Chapter 14
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Outline
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Introduction
Complex Interactions
Exploitation and Abundance
Dynamics
 Models
Refuges
 Prey Density
 Size
 Ecology of Fear
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Introduction
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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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Complex Interactions
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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
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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
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Parasites That Alter Host Behavior
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Entangling Exploitation with Competition
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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
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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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Herbivorous Stream Insect and Its Algal Food
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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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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
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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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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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Model Behavior
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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
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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
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Laboratory Models
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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
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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
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Refuges
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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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Refuges
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Protection in Numbers
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Living in a large group provides a “refuge.”
Predator’s response to increased prey density:
Prey consumed x Predators = Prey Consumed
Predator
Area
Area
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Wide variety of organisms employ predator
satiation defense.
 Prey can reduce individual probability of
being eaten by living in dense populations.27
Predator Satiation by Periodical Cicadas
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Periodical cicadas Magicicada spp. emerge
as adults every 13-17 years.
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 Densities can approach 4x10 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
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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
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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.
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Ecology of Fear and Refuges
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Review
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Introduction
Complex Interactions
Exploitation and Abundance
Dynamics
 Models
Refuges
 Prey Density
 Size
 Ecology of Fear
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