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
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) 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( 2 ) Number of species34176 Monophagous1379 0.56 Polyphagous1992 Coniferous host26115 0.11 Deciduous host656 Body size 1.0-1.95158 <0.0001 2.0-2.95656 > 3.0 mm2445 Facultative multivoltine816 0.22 Strictly multivoltine87 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 equilibriumStable erruptionsPulse erruptions Unstable low equilibriumPermanent erruptionsCyclic 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 Population 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. 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 Time Drought Rain Smaller stationary form Larger gregarious form Smaller stationary form Abundance 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 Consumption rate Type I Type II Type III Prey density Reproductive output Data from Williams and Liebhold (1995) 19241996 US state Maine population outbreak N
Random regional weather conditions Time Physiological oak mast cycles Time 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 damageBeetle abundance 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 Temperature 1520253035 Developmental time 16 12 8 50 70 90 Number of eggs T. urticae is extremely polyphagous Higher temperature increases fecundity and decreases developmental times leading to accelerated pupolation growth Citation
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 Abundance Diaprion outbreak in Germany was finished by the outbreak of the red backed vole 1964 1966 1965 Turced 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
The Clark and Holling (1979) model of insect outbreaks Logistic growth Interaction effects is related to the strength of biotic interaction is related to the behaviour of the species r is the intrinsic growth rate C. S. Holling 1930-
Hollings disc equation Predation P is proportional do prey density N and to effective search time T S. Effective search time T S is the difference between total search time T and handling time T H. Predator efficacy or pedator rate The model describes Hollings type II functional response. How to derive the model? Resource abundance Consumer abundance Searching time is proportional to prey density
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