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