Ecological Basis of Biological Control

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Ecological Basis of Biological Control
Read Chapter 2 in Bellows and Fisher Chapter 3 in
Huffaker Messenger, DeBach Rosen Now we know
the players, but what is the game? We all know
the adage about the progeny of one pair of flies
covering the world in one year. This is based
on exponential growth, as proposed by
Malthus.We are not knee deep in flies because
exponential growth cannot be sustained because
of natural control.We will briefly look at the
factors that operate in natural controland how
we can manipulate some of the factors that limit
population growth so we can use them in
biological control.
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Ecological Basis of Biological Control (Page 70)
This graph depicts the goals of classical
biological control, but the ideas extend to
conservation and augmentation.
But, in ecological terms what are we doing and
why?
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Ecological Basis of Biological Control (Page 71)
A few introductory definitions Density
independent mortality factors kill the same
proportion of the population, regardless of
population density For example, if 20C kills
90 of the individuals of a species, it should
kill 9 out 10, 90 out of 100, etc. Density
dependent mortality factors kill different
proportions of the population at different
densities Several options exist here, we will
look at them
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Ecological Basis of Biological Control (Page 71)
Density dependent mortality factors How does
the percent kill relate to population
density? Direct density dependent mortality
factors kill a higher proportion of the
population at higher densities most
specialized natural enemies Inverse density
dependent mortality factors kill a lower
proportion of the population at higher
densities some generalist predators on
specific prey species
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Ecological Basis of Biological Control (Page 71)
Density dependent mortality factors Is the
mortality factor affected too? Nonreciprocal
density dependent mortality factors are not
affected by changes in host/prey density
abiotic factors, non-living foods, some
generalist nat. en. Reciprocal density
dependent mortality factors are affected by
changes in host/prey density, e. g.
predators kill prey, fewer prey leads to
starvation of predators, fewer predators
allows prey to increase, allows predators to
increase and kill more prey specialized
natural enemies, some living foods
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Ecological Basis of Biological Control (Page 71)
Density dependent mortality factors Are
responses immediate? There is a time lag in many
systems involving density dependent mortality
factors, e. g. most natural enemies - Death
of the host/prey is often not immediate, i. e.
parasitoids, so the prey population does not
respond immediately to changes in the
natural enemy population - Natural enemy
population does not respond immediately to
changes in host/prey density, starvation kills
slowly, reproduction takes time Thus, nat.
en. are delayed density dependent mortality
factors
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Ecological Basis of Biological Control (Page 71)
Density dependent mortality factors Thus, for
those who like to know long, impressive
phrases, typical, specialized natural enemies
arereciprocal, delayed, direct density
dependent mortality factors Now one last
complication, density independent mortality
factors can interact with other features of the
environment, so that mortality appears to be
density dependent. Recall the example of 90
mortality at 20C. If the speciesis normally
sheltered and the population exceeds the number
of shelters available, the mortality rate may
increase, e. g. 1000 individuals and 800
shelters could yield 92 mortality (90 of 800 in
shelters, plus 200 unsheltered individuals).
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Ecological Basis of Biological Control (Page 72)
A Summary of Categories of Mortality Factors
with Examples
Natural Control Density independent
factors Density dependent factors
Physical Biological Nonreciprocal
Reciprocal
Temperature Host Suitability Some Foods
Parasites Humidity Food Quality Space
Predators Air Movement Territoriality
Pathogens Exposure Herbivores Soil
pH Some Foods
(BC agents, manipulable, apply biotic
pressure)
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Ecological Basis of Biological Control
How do natural enemies control a
population? Natural enemies can show three types
of responses to changing host/prey
densities Functional response a change in prey
consumption rate Developmental response an
immature predator that consumes more
food can grow faster and larger Numerical
response a change in predator distribution,
a change in predator reproductive rate The three
responses are interlinked.
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Ecological Basis of Biological Control
Functional response a change in prey consumption
rate with a change in prey density Holling
(1959) developed this concept with his disk
experiment. He looked at the number of
individual prey consumed per unitof time related
to prey density. Two factors were
important searching time handling
time He developed a curve to describe this
relationship. He also looked at earlier models
to determine their predictions for the
functional response.
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Ecological Basis of Biological Control
Nicholson, Type Iignores handling time and
satiation
Functional response
No. of preykilled/unit oftime
Holling, Type II
Thompson ignores searching time
Sigmoid, Type IIIincludes learning, host
switching
Note Final heights ofHolling, Thompson
andSigmoid curves are notsignificant, chosen
for convenience
Prey density
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Ecological Basis of Biological Control
Types of Functional Responses Type
I Type II Type III
common in insects
prey switching,some insects, many vertebrates
hypothetical
All show the basis of the developmental response,
more foodeaten at higher prey densities. However
, it is easier to understand their impact on the
target population if we look at the data in
another way.
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Ecological Basis of Biological Control
Functional Response Another perspective
Note Final heights ofHolling, Thompson
andSigmoid curves are notsignificant, chosen
for convenience
of preykilled/unit of time
Nicholson, Type Iignores handling time and
satiation
Sigmoid, Type IIIincludes learning, host
switching
Holling, Type II
Thompson ignores searching time
Prey density
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Ecological Basis of Biological Control
Functional Response Summary
of preykilled/time
No. of preykilled/time
Prey density
Prey density
Only Type III curves ever have a slope greater
than one. Only they ever yield an increasing
of the target population being killed as prey
population increases. Thus, only they are
capable of limiting prey population growth, then
only within a narrow range of prey densities
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Ecological Basis of Biological Control
Developmental Response Because predators will
eat more as food becomes moreabundant (to a
point), the predators often grow larger and
faster. Growing larger will lead to adults that
consume more preybefore satiation limits food
intake, enhancing functional response. Growing
larger will yield more fecund females, enhancing
the numerical response. Growing faster will
shorten the generation time, enhancing
the numerical response.
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Ecological Basis of Biological Control
Numerical Response The numerical response is
seen in two ways - Aggregation in areas where
prey is abundant - Increased reproductive rate
The direct impact of aggregation is usually
minor, but it is critical for initiating the
entire sequence of responses. The increase in
reproductive rate is essential for regulation of
the target population. (recall the limited
impact of functional response)
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Ecological Basis of Biological Control
Numerical Response Aggregation Predators change
search behavior in the presence of prey, turning
more frequently and at sharper angles (area
restricted search) Combined with the tendency to
search less, this leads to predators aggregating
in areas with abundant prey Predators are also
likely to stay longer in such areas because they
dont reach their giving up time The net
effect is to lead to more predators occurring
in areas with more prey and, thus consuming more
prey
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Ecological Basis of Biological Control
Numerical Response Reproduction Well-fed
insects develop faster leading to earlier
maturity and, thus shorter generation
times Well-fed insects are also larger and more
fecund, if food remains abundant for oogenesis
they will lay more eggs Shorter generation times
and higher fecundity both contribute to a more
rapid population increase Ultimately More
predators consume more prey This is the critical
mechanism for a predator population to control a
prey population, avoiding the limits of handling
time and satiation
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Ecological Basis of Biological Control
We have seen how natural enemies respond to
changing prey densities and how this causes them
to function as reciprocal, delayed, direct
density dependent mortality factors. How does
this impact the target population? The
adjustable nature of density dependent
mortality factorsis what tends to limit the
fluctuations in population densityaround the
mean carrying capacity. Biotic pressure from
natural enemies is an important component of
density dependent mortality.
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Ecological Basis of Biological Control
The impact of density dependent mortality yields
the familiarcurves for predator and prey
populations through time
Pelts receivedby the HudsonBay Companyfrom
trappersin northern Canada
This is the original hare-lynx work of MacLulich
(1937) that began the examination of
predator-prey systems.
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Ecological Basis of Biological Control
This led to the following idea
More hares (delayed) Fewer lynx
(delayed) More food for lynx Fewer hares
More lynx (delayed) This formed the
now familiar cornerstone of our ideas on
predator-prey interactions.
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Ecological Basis of Biological Control
This portion of the graph seems to show just
such a simple, reciprocal, delayed density
dependent relationship and led to the
preeminence of this idea.
But few things in biology are that simple.
Recent studies have shown that additional
factors are involved.
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Ecological Basis of Biological Control
This portion of the graph shows a deviationfrom
the previous pattern that occurredbetween 1845
and 1860. How? Why?
Hare Lynx
?
?
Recent studies have shown that increasing hare
populations impact aspen seeding populations,
inducing changes in plant chemistry that
negatively impact hare growth and
survival. Declining hare populations may then
lead to lower lynx populations due to food
shortage. Does this fit here?
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Ecological Basis of Biological Control
This more complex picture is important for
understanding population and community ecology
and shows the complexity of interactions that
can arise between biological control and host
plant resistance, but it does not alter the goals
or ecological mechanisms of biological
control.What we strive to do is to increase the
biotic pressure on thetarget population by
introducing (classical) or manipulating
(conservation and augmentation) natural enemy
population(s). By increasing the biotic
pressure, we lower the environmental carrying
capacity for the pest species and thus the mean
population density of the pest. With luck, the
new mean population density is below the economic
threshold for the pest .
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Ecological Basis of Biological Control
Thus, this graph also depicts the ecological
bases and processes of biological control.
Add density dependent mortality factor
increase biotic pressure
reduced carrying capacity