Exploitative Interactions:
Predation, Herbivory,
Parasitism, and Disease
Chapter 14
1
Outline
 Introduction
 Complex
Interactions
 Exploitation and Abundance
 Population Fluctuations

Models
 Refuges


Prey Density
Size
2
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.
3
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.
4
Parasites That Alter Host
Behavior

In a terrestrial
example, a spinyheaded worm
infects a pill bug.
 Infected pill bugs
leave shelter to
wander out in the
open where they
are eaten by
starlings.
5
Parasites That Alter Host Behavior

Experiments
showed that
infected isopods
were more likely to
be eaten by
starlings.

Likely due to
behavior.
6
Parasites That Alter Host
Behavior

Rust fungus Puccinia
monoica manipulates
growth of host
mustard plants
(Arabis spp.).
7
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 sugarcontaining spermatial
fluids.
8
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!
9
Entangling Exploitation with
Competition
 Park
found the presence/absence of a
protozoan parasite (Adeline tribolii)
influences competition in flour beetles
(Tribolium).
10
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.
11
Exploitation and Abundance
 Predators,
parasites, and pathogens
influence the distribution, abundance, and
structure of prey and host populations.
12
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.
13
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.
14
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.
15
Introduced Cactus and Herbivorous
Moth

Mid 1800’s: 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.
16
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%.
17
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.
18
Dynamics
 Predator-prey,
host-parasite, and hostpathogen relations are dynamic.

Temporal dynamics – populations of
predators and prey are not static, they cycle in
abundance over time.
19
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.
20
Population Fluctuations
 The
data show that lynx and hare
populations fluctuate with a 10 year cycle.
21
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.
22
Snowshoe Hares - Role of Food
Supply
 Shoots
produced after heavy browsing can
increase levels of plant chemical
defenses.

Reducing usable food supplies.
23
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.
24
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.
25
Experimental Test of Food and
Predation Impacts

A large-scale, longterm 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.
26
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.
27
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.
28
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.
29
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.
30
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.
31
Refuges
 To
persist in the face of exploitation, hosts
and prey need refuges.
32
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.
33
Refuges
 Huffaker
studied six-spotted mite
Eotetranychus sexmaculatus and
predatory mite Typhlodromus occidentalis.

Separated oranges and rubber balls with
partial barriers to mite dispersal.
34
Refuges

Typhlodromus
(pred) crawls while
Eotetranychus
(prey) balloons.
 Provision of small
wooden posts to
serve as launching
pads maintained
population
oscillations
spanning 6 months.
35
Variety of Refuges - Space
refuges – places where members
of the exploited population have some
protection from predators and parasitoids.
 Spatial




Burrows
Trees
Air
Water or land
36
Variety of Refuges - Numbers
in a large group provides a “refuge.”
 Predator’s response to increased prey
density:
 Living
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.
37
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.
O’Dowd and Gill
determined synchronous
seed dispersal by
Eucalyptus reduces losses
of seeds to ants.
38
Predator Satiation by Periodical
Cicadas

Periodical cicadas
Magicicada spp.
emerge as adults
every 13-17 years.

Densities can
approach 4x106 ind /
ha.
39
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.
40
Size As A Refuge
 If
large individuals are ignored by
predators, then large size may offer a form
of refuge.
41
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.
42
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.
43