Disturbances

• Why disturbances?

  • changes in the environment

  • differential effects on populations

  • might have long-lasting effects

• Definition

  • Disturbance as a factor

    • a change in environmental conditions that results in changes in the ecosystem

    • Pickett and White (1985): a disturbance is any relatively discrete event in time that disrupts ecosystem, community, or population structure, and that changes resources, availability of substratum, or the physical environment

    • Resh et al (1998): frequency and intensity are outside the predictable range

    • A change in environmental conditions that results in changes in the ecosystem

    • Pickett and White (1985): A disturbance is any relatively discrete event in time that disrupts ecosystem, community, or population structure, and that changes resources, availability of substratum, or the physical environment

    • Resh et al. (1988): frequency and intensity are outside the predictable range

  • Disturbance as a driver

    • changes in environmental conditions create opportunities for some species

    • Disturbance as driver of change

      • forest gaps

      • control over competitors

Disturbance and community diversity

• Global Climate change → Disturbance → Spatial heterogeneity and Biotic interactions (predator-prey, competition, other) → community diversity

• Alternative stable states

Intermediate disturbance hypothesis

• 

• The hypothesis that more species are present in a community that occasionally experiences disturbances than in a community that experiences frequent or rare disturbances.

Disturbance and biodiversity

• Tropics:

  • tropical forest is not at equilibrium

  • “If equilibrium is reached, a lesser degree of diversity may be sustained by niche diversification or by a compensatory mortality that favors inferior competitiors”

  • “tropical forests and reefs are subject to severe disturbances often enough that equilibrium may never be attained”

Intermediate disturbance hypothesis

Forest community example

• Bongers et al. (2009). The intermediate disturbance hypothesis applies to tropical forests, but disturbance contributes little to tree diversity. Ecology letters, 12(8), 798- 805.

• Forest in Ghana

• 2504-1 hectare plots

• 331,567 trees

• DI as the percentage of trees in a plot that belong to the pioneer species.

• Bongers et al (2009): Our results generally support the IDH as an explanation of why diversity varies across sites. Species richness is most sensitive in its response to disturbance levels at the drier parts of the rainfall gradient … We conclude that disturbance is less important for species richness patterns in wet tropical rain forests than previously thought.