Outbreaks

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Outbreaks
An outbreak is the abrupt and massive increase in population size of animals and plants.
Schistocera gregaria outbreak 2013 in Egypt
Barbosa et al. 2012
Coccinella septempunctata outbreak 2011 in Northern Germany
Lemmus lemmus
Lemming
outbreaks are
triggered by winter
breeds and by
changes in survival
that cause
additional breeds
Ims et al. (2011)
Proc Natl Acad Sci
USA. 108: 1970–
1974.
Korpimaeki et al.
2004, Bioscience
54: 1071-1079.
Spruce budworm Choristoneura fumiferana
Cameraria ohridella
Jellyfish blooms in Eastern Asian seas
Nemopilema nomurai
Causes for Nemopilema blooms are increased water temperatues, over fishing, polluted
waters, and saltier waters, dead zones, and redirected ocean currents.
Blooms are made by anthropogenic factors
The Rocky Mountain locust (Melanoplus
spretus) ranged through the western half of
the USA and part of Canada until the end of
the 19th century.
It was a typical prairy species.
The last living species was seen in 1902.
During the last half of the Nineteens century it
had several mass outbreaks and constant high
population sizes.
Probably the species died out by prairy
irrigation of settlers.
Extinction was human caused.
Mast years in plants as a special form of gradation
Many trees have more or less
regulalry mast years (Oak,
Beech, castan, but also fruit
trees. Mast years occur in cycles
of five to ten years.
2013 was in Poland a mast year
for apples.
A chronogram of oak masts in the Southern Apalachian
(Speer 2001,
http://web.utk.edu/~grissino/downloads/James%20Speer%20dissertation.pdf)
Common ecological characteristics (life history trades) of outbreak species
• Phytophages (rarely
paraisoids or predators)
• r strategists
Number of species
Monophagous
Polyphagous
Coniferous host
Deciduous host
Body size
34
13
19
26
6
Nonoutbreak
species
176
79
92
115
56
1.0-1.95
2.0-2.95
> 3.0 mm
Facultative multivoltine
Strictly multivoltine
1
6
24
8
8
58
56
45
16
7
Outbreak
species
• High reproductive output
• Short reproduction times
• Multiple annual breeds
• High dispersal rates
• Regulated by predators
• Polyphages
These are not sufficient conditions for an outbreak species!
P(c2)
0.56
0.11
<0.0001
0.22
Data from Koricheva et al. 2012,
Insect Outbreaks Revisited
Causes for defoliation by herbivore insect outbreaks
(Mattson et al. 1991)
• Defoliation severity increases directly with homogeneity of the forest composition.
• Defoliation severity increases with the average amount of exposure of the individual
tree crowns.
• Defoliation severity increases, though not necessarily linearly, with tree age.
• Defoliation severity increases with warm, dry weather during the growing season.
• Defoliation severity increases with the folivore's predilection for polyphagy.
• The effects of defoliation on tree vigor are cumulative and not linear.
Classification of outbreak species
Stable erruptions
Unstable high
equilibrium
Pulse erruptions
Permanent erruptions
Cyclic erruptions
Stable high equilibrium
Stable low equilibrium
Population
Unstable low equilibrium
Average outbreaks of herbivores last 2 to 4 years,
outbreak duration rarely exceeds 10 years.
Sustained eruptions
Cyclic eruptions
Bark beetles (Scolytidae)
Larch Tortrix (Zeiraphera griseana)
Pulse eruptions
Gypsy moth Lymantria dispar
The gypsy moth
develops on over 300
differed tree species
including
gymnosperms and
angiosperms
Population size
Temporal pattern of outbreaks
Population size
Time
Upper population
limit
Predators
control
populations
Habitat conditions
amplify population
growth
Starvation and
disease reduce
populations
Outbreak level
Time
Mechanisms of outbreaks
Environmental factors
• Favourable weather conditions
• New resources
• Threshold effects
Intensive modelling showed that the direct impact of environmental conditions
is generally much too small to explain the magnitude of outbreaks.
Outbreaks are caused by ecolgical factors that amplify reproduction rates.
 =  + −1
 − −1

Thed discrete Pearl - Verhulst model of
population growth
A high increase is population size is
linked to a high reproductive output.
Any factor combination that increases r
might be an amplifier for outbreaks.
Rain might serve as an amplifier
Abundance
Schistocera gregaria
Swarming
Larger
gregarious
form
Smaller
stationary form
Drought
Rain
Drought
Swarming
Smaller
stationary
form
Larger
gregarious form
Rain
Drought
Time
Consumption rate
Important amplifiers are:
Escape from enemies
Relative mortality caused by generalist
predators of type II or type III functional
response decreases with increasing prey
density.
The greater is the population density,
the faster it grows.
Type III
US state Maine
population outbreak
1924
Prey density
Type II
Prey density
N
Reproductive
output
Type I
1996
Gypsy moth Lymantria dispar
Data from Williams and Liebhold (1995)
Lymantria dispar
Random regional weather conditions
Time
Physiological oak mast cycles
Time
Mast failures
Time
Small mammal
population cycles
Time
Mast failures cause breakdown of
small mammal population during
winter
Low spring predation of
small mammals after
mast failures cause
outbreaks of the gypsy
moth winter
Time
Important amplifiers are:
Threshold effects
Some bark beetles (Scolytidae) might
succeed in attacking a healthy tree only
when the number of beetles is large.
When the density of adults is high, then
they cause considerable damage and the
tree looses its resistance to developing
larvae.
Beetle abundance
After reaching a certain threshold density population increase becomes positively
density dependent and results in an outbreak.
Threshold
Tree damage
Important amplifiers are:
Habitat effects
Population of spider mites grow very fast at high temperature.
They live on plant leaves where local temperature is lower than the ambient temperature.
T. urticae is
extremely
polyphagous
16
90
12
70
8
50
15
20 25 30
Temperature
Number of eggs
Tetranychus urticae
Developmental time
During the draught, plant transpiration is reduced, and thus, the temperature of leaves
increases causing rapid reproduction of spider mites.
Citation
35
Higher temperature increases fecundity
and decreases developmental times
leading to accelerated pupolation growth
Drought
Increased temperature
Decreased humidity
Plants
Increased
Temperature
Stress metabolites
Osmolytes
Sugars
Secondary compounds
Decreased
Growth
Resistance
Water content
Natural enemies
Increased
Decreased
Abundance
Phytophagous insect
Increased
Decreased
Resources
Adult survival
Plant utilisation
Larval survival
Enemy escape
Growth of symbionts
Rate of reproduction
Outbreak
Important amplifiers are:
Habitat effects
Pine sawflies, Diprion pini, have >50% of their population in
a prolonged diapause lasting from one to five years.
Drought may cause reactivation of a large
proportion of diapausing sawflies.
Diaprion outbreak in Germany was finished by
the outbreak of the red backed vole
Abundance
Diprion pini
Clethrionomys glareolus
1964
1965
Time
1966
Turced 1966,
Waldhygiene 6: 181-182
Outbreaks collapse usually due to one of the following
mechanisms:
•Destruction of resources
•Natural enemies
•Unfavorable weather
The Clark and Holling (1979) model of insect outbreaks
a is related to the
strength of biotic
interaction
C. S. Holling 1930r is the intrinsic
growth rate
−1 2
 − −1
 = −1 + −1
−

1 + −1 2
Logistic growth
Interaction
b is related to the
effects
behaviour of the
species
How to derive the model?
 =  −  
 =  
Predation P is
proportional do
prey density N and
to effective search
time TS.
Effective search
time TS is the
difference
between total
search time T and
handling time TH.


=
 1 +  
Predator efficacy or pedator rate
Consumer
abundance
The model describes Holling’s
type II functional response.
Resource
abundance
 =   −   

=
1 +  
Holling’s disc equation
 − −1
 = −1
−

 = 
Searching time is
 =  proportional to prey
density
−1 2
 − −1
 = −1 + −1
−

1 + −1 2
Monocultures
Do monocultures increase the probability of outbreaks?
Monocultures are often devoid
of natural enemies.
Outbreak species asre of opf
higher density.
Monocultures provide high
resource densities
?
Outbreak species are often
polyphagous.
Tropical forests often face
severe insect outbreaks.
The proportion of potential
outbreak species is higher in
tropical forests.
Do outbreaks harm ecological systems?
?
In terms of economy: yes.
In terms of ecosystem functiong:
probably no
Outbreaks lead to higher resource
turnover.
Post-outbreak systems increase in
species richness.
Outbreak might lead to
evolutionay innovations.
Leptinotarsa decemlineata
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