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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
Ims et al. (2011) Proc Natl Acad Sci USA. 108: 1970–1974.Korpimaeki et al. 2004, Bioscience 54: 1071-1079.
Lemming outbreaks are triggered by winter breeds and by changes in survival that cause additional breeds
Lemmus lemmus
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
• High reproductive output
• Short reproduction times
• Multiple annual breeds
• High dispersal rates
• Regulated by predators
• Polyphages
Outbreak species
Non-outbreak species
P(c2)
Number of species 34 176Monophagous 13 79
0.56Polyphagous 19 92Coniferous host 26 115
0.11Deciduous host 6 56Body size
1.0-1.95 1 58<0.00012.0-2.95 6 56
> 3.0 mm 24 45Facultative multivoltine 8 16
0.22Strictly multivoltine 8 7
These are not sufficient conditions for an outbreak species! Data from Koricheva et al. 2012, Insect Outbreaks Revisited
• 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.
Causes for defoliation by herbivore insect outbreaks (Mattson et al. 1991)
Population
Stable high equilibrium Unstable high equilibrium
Stable low equilibrium Stable erruptions Pulse erruptions
Unstable low equilibrium Permanent erruptions Cyclic erruptions
Classification of outbreak species
Average outbreaks of herbivores last 2 to 4 years, outbreak duration rarely exceeds 10 years.
Sustained eruptions
Bark beetles (Scolytidae)
Cyclic eruptions
Pulse eruptions
Gypsy moth Lymantria dispar
Larch Tortrix (Zeiraphera griseana)
The gypsy moth develops on over 300 differed tree species including gymnosperms and angiosperms
Temporal pattern of outbreaks
Predators control populations
Outbreak level
Habitat conditions amplify population growth
Upper population limit
Starvation and disease reduce populations
Time
Popu
latio
n si
zePo
pula
tion
size
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.
𝑁𝑡=𝑁𝑡+𝑟𝑁 𝑡−1
𝐾−𝑁 𝑡−1
𝐾
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.
Schistocera gregaria
TimeDrought Drought DroughtRain Rain
Smaller stationary form
Larger gregarious form
Larger gregarious form
Smaller stationary form
Abun
danc
e
Swarming Swarming
Rain might serve as an amplifier
Important amplifiers are:
Gypsy moth Lymantria dispar
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. Prey density
Cons
umpti
on ra
te Type I Type II
Type III
Prey density
Repr
oduc
tive
outp
ut
Data from Williams and Liebhold (1995)
1924 1996
US state Maine population outbreak
N
Random regional weather conditions
Time
Time
Physiological oak mast cyclesTime
Mast failures
Time
Small mammal population cycles
Mast failures cause breakdown of small mammal population during winter
Time
Low spring predation of small mammals after mast failures cause outbreaks of the gypsy moth winter
Lymantria dispar
Threshold effects
After reaching a certain threshold density population increase becomes positively density dependent and results in an outbreak.
Important amplifiers are:
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.
Tree damage
Beet
le a
bund
ance Threshold
Habitat effects
Important amplifiers are:
Population of spider mites grow very fast at high temperature. They live on plant leaves where local temperature is lower than the ambient temperature.
During the draught, plant transpiration is reduced, and thus, the temperature of leaves increases causing rapid reproduction of spider mites.
Tetranychus urticae
Temperature15 20 25 30 35
Dev
elop
men
tal ti
me
16
12
850
70
90
Num
ber of eggsT. urticae is extremely polyphagous
Higher temperature increases fecundity and decreases developmental times leading to accelerated pupolation growth
Citation
DroughtIncreased temperature Decreased humidity
PlantsIncreased DecreasedTemperature GrowthStress metabolites ResistanceOsmolytes Water contentSugarsSecondary compounds
Natural enemiesIncreased Decreased- Abundance
Phytophagous insectIncreased DecreasedResources Adult survivalPlant utilisation Larval survivalEnemy escapeGrowth of symbiontsRate of reproduction
Outbreak
Pine sawflies, Diprion pini, have >50% of their population in a prolonged diapause lasting from one to five years.
Habitat effects
Important amplifiers are:
Diprion pini
Clethrionomys glareolus
Time
Abun
danc
e
Diaprion outbreak in Germany was finished by the outbreak of the red backed vole
1964 19661965Turced 1966,
Waldhygiene 6: 181-182
Drought may cause reactivation of a large proportion of diapausing sawflies.
Outbreaks collapse usually due to one of the following mechanisms:
•Destruction of resources•Natural enemies•Unfavorable weather
𝑁𝑡=𝑁𝑡− 1+𝑟𝑁 𝑡− 1
𝐾−𝑁 𝑡−1
𝐾−
𝛼𝑁𝑡 −1❑2
1+𝛽𝑁𝑡 −1❑2
The Clark and Holling (1979) model of insect outbreaks
Logistic growth Interaction effects
a is related to the strength of biotic interaction
b is related to the behaviour of the species
r is the intrinsic growth rate
C. S. Holling 1930-
𝑃=𝑠𝑇 𝑆𝑁 𝑇 𝑆=𝑇 −𝑇𝐻 𝑃 𝑃=𝑠 (𝑇 −𝑇𝐻 𝑃 )𝑁
𝑃=𝑠𝑁𝑇
1+𝑠𝑇𝐻𝑁
Holling’s disc equation
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.
𝑃𝑁
=𝛼𝑇
1+𝛼𝑇𝐻𝑁
Predator efficacy or pedator rate
The model describes Holling’s type II functional response.
How to derive the model?
Resource abundance
Cons
umer
ab
unda
nce
𝑁𝑡=𝑟𝑁 𝑡−1
𝐾 −𝑁𝑡 −1
𝐾−𝑃
𝑇𝐻=𝑜𝑁Searching time is proportional to prey density
𝑇=𝑞𝑁
𝑁𝑡=𝑁𝑡− 1+𝑟𝑁 𝑡− 1
𝐾−𝑁 𝑡−1
𝐾−
𝛼𝑁𝑡 −1❑2
1+𝛽𝑁𝑡 −1❑2
Monocultures
?
Do monocultures increase the probability of outbreaks?
Outbreak species are often polyphagous.Tropical forests often face severe insect outbreaks. The proportion of potential outbreak species is higher in tropical forests.
Monocultures are often devoid of natural enemies.Outbreak species asre of opf higher density.Monocultures provide high resource densities
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