Download lecture.10 - Cal State LA

Exploitative Interactions
Predation / Herbivory / Parasitism
Consumer-Resource Interactions
• Occur along a continuum of harmfulness to
the resource…reduced fitness—death.
• Common thread: one species exploits another,
one benefits and one suffers.
Consumer-Resource Interactions
• Predators: kill and eat other organisms
Consumer-Resource Interactions
• Parasite: lives on the tissue of its host, but
does not usually kill it
Consumer-Resource Interactions
• Parasites can alter the behavior of their hosts
and sometimes kill their host…
Consumer-Resource Interactions
• Herbivores: consume plants but do not usually
kill them
Consumer-Resource Interactions
• Consumer species can influence the
distribution and abundance of their resource
species
Consumer-Resource Interactions
• Consumer species can influence the distribution and
abundance of their resource species
• Introduced invasive species often successful due to
release from consumer species present in their
native range
• Biological control / integrated pest management
often utilize a specific consumer species to control
pests
Consumer-Resource Interactions
• St. John’s wort / Chrysolina beetles
Consumer-Resource Interactions
• California sea otters, purple urchins, and kelp
Consumer-Resource Interactions
• Predatory / prey & other consumer-resource
interactions are dynamic
• Consumers influence the abundance of
resource populations…
• The abundance of resource populations
affects the size of consumer populations
• These dynamics only make sense if all
trophic levels are considered
Consumer-Resource Interactions:
Predator/Prey Cycles
• Increase in primary producers (plants) provides more food
for primary consumers (herbivores)
• Herbivores show a numerical response to increased food
supply and increase in numbers
• Secondary consumers (predators) also show a numerical
response to increased availability of prey (herbivores)
• Eventually, depletion of their food supply and predation
pressure cause the herbivore populations to decline in
number
• Reduced availability of prey also causes the numbers of
predators to decline
…fewer herbivores means more plants can grow…more food
available…replay cycle
Consumer-Resource Interactions
Consumer-Resource Interactions:
Predator/Prey Cycles
• Predator-prey cycles have a built in time delay
because of the time required for both
populations to produce offspring
• Ongoing occurrence of cycles is generally
stable, but the periodicity and intensity of
cycles can vary due to environmental
conditions
Consumer-Resource Interactions:
Predator/Prey Cycles
• Owl/vole populations in Scandinavia:
• Northern Scandinavia – 4 year cycle
• Snow cover in winter protects voles from
owl predation, lessens effects of
predation and extends cycle
• Southern Scandinavia – 1 year cycle
• Less snow cover, owls can hunt through
much of the winter, quicker cycle
Consumer-Resource Interactions:
Predator/Prey Cycles
• Climate change
– warmer
winters in
Northern
Scandinavia
Consumer-Resource Interactions:
Host/Pathogen Cycles
• Host immunity slows spread of pathogen, causes
time delays in epidemics
• Measles – highly contagious disease, but
stimulates life-long immunity
• Produces epidemics at 2-year intervals in
unvaccinated human populations
• Two years required for population to
accumulate a high enough density of
susceptible infants to sustain a measles
outbreak
Consumer-Resource Interactions:
Host/Pathogen Cycles
• Host density also an important factor:
• as host population density increases, rate of
transmission between hosts also increases
• But…if pathogens increase host mortality or
impair host reproduction, host population
densities will drop low enough to break the
chain of transmission, and host densities will
again begin to rise
Consumer-Resource Interactions:
Host/Pathogen Cycles
• Forest tent caterpillars – periodic population
booms can defoliate stands of trees over
thousands of square kilometers
• Usually brought under control by a virulent
pathogen that causes high mortality of
caterpillars at high host densities
• Most places, infestation lasts 2 years before virus
can bring the host population under control
Consumer-Resource Interactions:
Host/Pathogen Cycles
• Forest tent caterpillars – periodic population
booms can defoliate stands of trees over
thousands of square kilometers
• Usually brought under control by a virulent
pathogen that causes high mortality of
caterpillars at high host densities
• Most places, infestation lasts 2 years before virus
can bring the host population under control
Consumer-Resource Interactions:
Host/Pathogen Cycles
• ..but, in some places, it takes up to 9 years for
the virus to stop the infestation…
Lotka-Volterra model for cyclic
predator-prey interactions
Assumes exponential growth of the resource
population, limited only by consumers:
• Growth of prey population =
[the intrinsic growth rate of the prey population] –
[the removal of prey individuals by predators]
Lotka-Volterra model for cyclic
predator-prey interactions
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
Lotka-Volterra model for cyclic
predator-prey interactions
Consumer population growth rate =
[rate of conversion of food into offspring] –
[mortality rate of consumer population]
dNp/dt = cpNhNp - dpNp
cpNhNp = Rate at which consumer species converts food
(resource species) into offspring
(c = conversion factor)
dpNp= Death rate of consumer species
Lotka-Volterra model for cyclic
predator-prey interactions
Lotka-Volterra model for cyclic
predator-prey interactions
• Elements of reality that the model does not
incorporate:
• Time lags in predator/prey response
• Carrying capacities for predator and prey
populations
• No functional response in predator
Lotka-Volterra model for cyclic
predator-prey interactions
• Experiments that seek to replicate the situation
in the Lotka-Volterra model generally fail
• In order for cycles to occur, some destabilizing
factor must be present to drive the system
(e.g., time delay in the response of population
to a change in its food supply)
• To extend predator prey cycles, stabilizing
factors (generally more than one) have to be in
place to balance these destabilizing factors
Lotka-Volterra model for cyclic
predator-prey interactions
• Stabilizing factors include:
• Predator inefficiency (or enhanced prey escape/defense)
• Density-dependent limitation on either the predator or
the prey population by external factors
• Alternative food source for the predator
• Refuges for the prey at low prey densities
• Reduced time delays in predator responses to changes in
prey abundance
• Influx of new replacement individuals from source
population
Consumer-resource systems can have
more than one stable state
• Alternative stable states can arise when different
factors limit populations at low and high densities
• At low densities, individuals of the resource
population may be so difficult to locate that the
consumer species will switch to another resource
Consumer-resource systems can have
more than one stable state
• At low densities, individuals of the resource population may
be so difficult to locate that the consumer species will
switch to another resource
• At low densities, the resource species will tend to grow
faster than the consumer species can remove it, because
resources are not limiting
• As the resource population grows, however, the consumer
species will focus on the increasingly abundant food supply
and eventually bring the population under control at a low
stable equilibrium well below its carrying capacity –
Consumer-imposed equilibrium
Consumer-resource systems can have
more than one stable state
• If a resource population is at a size well above its consumerimposed equilibrium, consumer efficiency should go up as
the population density increases
• At some point, however, consumers themselves become
satiated (type II or III functional response) or the consumer
population becomes limited by external factors, such as
suitable nest sites or their own predators.
• If this point is reached, the resource population can escape
consumer control and continue to increase until it reaches
the size limit imposed by its own carrying capacity – a
resource-imposed equilibrium