Lecture 6
(Quiz 1) – short lecture
Disturbance
• Any discrete event that disrupts ecosystem, community, or population
structure and changes resource availability or physical environment
• Natural vs Anthropogenic
• Biotic versus Abiotic
• Scale, intensity, frequency, duration, predictability
• Disturbance
• Endogenous vs. exogenous
• A basic tenet of community ecology
• Disturbance creates vacant space, releases resources, alters species
interactions
• Long held that disturbance necessary for invasive plants to establish
(Disturbance facilitates invasion)
• Gross oversimplification!
Effects of Disturbance are Variable
► Effects are often complex and system dependent
► Some successful invasions occur in undisturbed ecosystems
The Importance of Disturbance
• Very difficult to disentangle disturbance from other factors such as
propagule pressure
• Some experiments suggest that disturbance AND propagule pressure can act
synergistically.
Invasive Species as a Disturbance Factor
• Passengers versus Drivers
• Two sides of the coin – invasive species are passively taking advantage of
existing or ongoing disturbance = ‘passenger’.
E.g., dams alter hydrological regimes and facilitate the establishment of some
invasive fish
• Invasive species as ‘drivers’ – they themselves are the disturbance, altering
the ecosystem through their activities.
These species are often ‘ecosystem engineers’ altering resource availability for
other species (e.g. nitrogen fixing plants)
Invasional Meltdown Hypothesis
Simberloff and Von Holle (1999)
• Positive interactions among introduced species
• With  number of introductions, even if not successful, the ecosystem is
incrementally weakened, facilitating the invasion of other species.
• As established invaders , future invasions succeed more easily, AND may
have larger impact.