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.