Ecosystems normally experience gradual change and evolve in response to climatic conditions, and human influences. But sometimes they experience catastrophic change from which they may not recover. Some systems are more resistant to such abrupt changes than others, and some systems become less resistant due to a variety of factors. If an ecosystem is very resistant to such changes we say it is "resilient". Scheffer et al (2001) give examples of catastrophic shifts and suggest that sustainable management of ecosystems requires that we understand and maintain the level of resilience of such systems.
Reference
Scheffer, M., S. Carpenter, J. A. Foley, C. Folke, and B. Walker, 2001: Catastrophic shifts in ecosystems. Nature, 413, 591-596.