Disturbance

Disturbance

1. Types of Disturbance

   • Natural disasters
     • floods, volcanic activity, landslides, earthquakes, storms, human activity
   • Disturbances – discrete event in time and space altering the ecosystem structures and causes a change in resource availability. Any physical force that removes biomass from the community (Grime 1977)
   • Type: Timing – frequency and when relative to other events, Severity and Scale

2. Components

   • Intensity
     • The proportion of total biomass killed
     • Often inversely related to frequency
     • How strong the force is – energy/area/time
   • Frequency
     • Number of disturbances in given time interval
     • Detmines how far succession can proceed
     • Annual patterns can give predictable frequencies for certain types of disturbances such as fire or floods.
   • Timing
     • The coincidence of the disturbance with important cycles or events in the ecosystem affected by disturbance
   • Disturbance Area
     • Absolute and relative size of disturbance area and the shape have important effects on recolonization/succession

3. Effects of Disturbance

   • Forest fire – loss of vegetation, loss of nutrient availability, the ground surface has no direct insulation from sun (shade from vegetation), species dynamic interactions change

4. biotic communities

   • Change in population
   • Loss of modification of habitat
   • Change in conditions (recourses, light, nutrients, microclimate)
   • Creates opportunities for other species
   • Drives increases in biodiversity by initiating succession of communities
   • Greater biodiversity may increase the resilience of an ecosystem to disturbance

5. Intermediate disturbance hypothesis

   • Connell, 1978, Huston 1979) Disturbance maintains biodiversity
   • Sousa (1979) disturbance frequency affects algal diversity
     • the disturbance was strong algae was affected, smaller boulders easiest to turn over by waves so species number was low.
   • Criticisms
     • Growing evidence that most empirical studies rarely find predicted hump-shape
     • Where competitive exclusion is rare it doesn’t hold
     • Theoretical refutations – theoretical mechanisms are logically valid

6. Disturbance heterogeneity promotes diversity

   • From a conservation perspective, heterogeneity is desirable because the greater it is, the more niches are available
   • Therefore important to understand the natural disturbance regime
   • Also important to understand effect of disturbance on different species

7. Stability: Resistance & resiliance

   • Resistance: the ability of a community or ecosystem to maintain structure and/or function in the face of potential disturbance
     • With a high resistance there is a decrease in resistance after multiple disturbances
       • Low resistance, high resilience is harder to bounce back from an takes longer to
         • Re
   • Resilience: the ability of a community or ecosystem to return to its original conditions following disturbance
     • For resilience, each disturbance decreases the community state of the ecosystem but the impact of disturbances decreases over time
       • Low resistance low resilience will take a long time if it even increases in diversity
       • Resilience Mechanisms - Forest Fires:
         • Nutrients released to soil, Regrowth by remnant roots and seeds, Invasions from neighbouring undisturbed ecosystems (biodiversity benefits), Rapid restoration of energy flow and nutrient cycling.
       • Determines resilience to disturbance
         • Evolutionary history, Adapted to disturbance, (Vegetation can recover fast, bark thickness relates resistance to fire, below ground roots and underground stems, re-sprouting from epicormic buds) and (Influence by species, age, fire intensity, fire height)
   • The extent of resistance or resilience to a given disturbance will depend on the adaptations of the organisms affected
   • This depends on their historic exposure to that disturbance over evolutionary time – whether they have developed adaptations to cope with the disturbance.
   • Humans are greatly altering disturbance cycles
     • Suppressing Forrest fire
       • Changed type of fires as more vegetation closer together, Fires are more intense so trees are killed, Ponderosa pine forests – requires forest fires, density of trees increased and increase in Douglas fir than ponderosa pines so habitat is lost, Can get a shift in dominant ecosystem (Savanna to Forest e.g.), Change in ecosystem processes, biodiversity, resources, ecosystem services, Can be hard to reverse.
   • comparison example
     • Fire Grassland, California. Resistance is low, Recovery is fast. Dry forest, Hawaii. Non-native grasses that were introduced that are easy burning and causes fires, Native trees were not used to coping with fire so they have a low resilience.
   • Resistance or resilience: Traits that enable fire survival (Avoidance), Many species actively use the disturbance (birds feed on insects fleeing).

8. Life history traits & response to disturbance

   • Determine how population responds to disturbance
   • For plants: Noble & Slayer (1980) developed ‘vital attribute system’ to predict population responses to disturbance
   • Functional groups based on four processes: Means of arrival at disturbed site, Means of persistence within the site, Pattern of establishment of new species.
   • Three critical time periods: Time to reach maturity, Time for all plants to die out in a site, Time for all propagules to die out in a site.

9. Recovery from disturbance

   • Agricultural abandonment, Costa Rica – dry forest recovery
   • Gradual species accumulation and convergence of composition towards that of old-growth forests BUT capacity for recovery at a sufficient rate to match threats is uncertain
   • Need to try meet growth to threats
   • Loss of Disturbance
     • 50% of digging marsupial species in Australia now extinct. Tasmania: 5812 diggings/hectare (11% of forest floor) which helps trap organic material -> higher fertility and soil moisture.

10. Effects on food webs

    • Plants grow rapidly because no herbivors
    • decrease in plants because theres no predators but herbivors
    • predators rise so plants are able to rise