Ecological Mechanisms of Outbreaks

   Population outbreaks are characterized by rapid change in population density over
   several orders of magnitude. Only a small number of species have outbreaks: e.g.,
   some insect pests, pathogens, and rodents. Population outbreaks often cause
   serious ecological and economic problems. Examples of outbreak species:
   [1]locusts, [2]southern pine beetle, [3]spruce budworm, [4]gypsy moth

   Two kinds of outbreaks:
   1. Introduction of a species to a new area
   2. Growth of a native population.

   The second case is most interesting because it is important to understand why a
   population suddenly increases in its density. Usually an outbreak goes through
   the following phases:

                                     [phases.gif]

   The building phase in insect pests often goes unnoticed because the effect of
   pests on host plants is still very small. Regular monitoring helps to detect
   population growth before the pest species devastates host plants over large
   areas.

   Initially ecologists tried to explain outbreaks by direct impact of environmental
   factors. However, the magnitude of change in these factors was always much
   smaller than the magnitude of change in population density. Attempts to find a
   "releasing factor" usually fail.

   Thus, there should be an "amlipier" of a small initial disturbance.

   Examples of "amplifiers":

   1. Inverse density-dependence (positive feedback)

   1.1. Escape from natural enemies: gypsy moth.
          Mortality caused by generalist predators with a type II or III functional
          response decreases with increasing prey density. Thus, the greater is the
          population density, the faster it grows. Escape from natural enemies may
          also result from a delay in the numerical response of natural enemies
          (pine sawflies, gypsy moth).

   1.2. Group effect: bark beetles, sawyer beetle (Cerambicidae), locusts.
          Bark beetles succeed in attacking a healthy tree only when the number of
          beetles is large. Adults of the sawyer beetle, Monochamus urussovi, feed
          on small branches of Siberian fir. When the density of adults is high,
          then they cause considerable damage and the tree looses its resistance to
          developing larvae of this species. Locusts change their behavior at high
          population density, and their reproduction rate increases.

   2. Density-independent processes.

   2.1. Plant response to disturbance: spider mites.
          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.

   2.2. Insect physiological response to disturbance: sawflies.
          Pine sawflies, Diprion pini, have >50% of their population in a prolonged
          diapause lasting from 1 to 5 years. Draught may cause reactivation of a
          large proportion of diapausing sawflies. This effect is combined with
          subsequent escape from natural enemies.

   These amplifiers can be triggered only at specific state of the population
   system. When an outbreak is already in progress, additional disturbances have
   almost no effect. Only when an outbreak cycle is finished, then the population
   may again respond to another disturbance. In some cases, even small disturbances
   cause an outbreak, and then the population is permanently in an outbreak cycle.

   Outbreaks collapse usually due to one of the following mechanisms:
     * Destruction of resources
     * Natural enemies
     * Unfavorable weather

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   [8]Alexei Sharov 04/17/97

References

   1. http://www.ent.agri.umn.edu/academics/classes/ipm/chapters/showler.htm
   2. http://viner.ento.vt.edu/~salom/SPBinfodirect/spbinfodirect.html
   3. http://alexei.nfshost.com/PopEcol/lec13/chorist.html
   4. http://www.gypsymoth.ento.vt.edu/slides/morelaid.jpeg
   5. http://alexei.nfshost.com/PopEcol/lec13/outbreak.html
   6. http://alexei.nfshost.com/PopEcol/lec13/outbreak.html
   7. http://alexei.nfshost.com/PopEcol/lec13/model.html
   8. http://alexei.nfshost.com/~sharov/alexei.html