The resistance and resilience of perennial grasses and a small shrub to a natural disturbance (drought) were measured on stress gradients that were produced by domestic livestock in desert grassland ecosystems in the northern Chihuahuan Desert of New Mexico, USA. Both survivorship of grasses and a sub-shrub (Gutierrezia sarothrae) during a drought (a measure of resistance) and re-establishment of plant cover following a drought (a measure of resilience) were reduced in the intensely stressed ecosystems in comparison to the lightly stressed ecosystems. The compromise of the resistance and resilience characteristics of an ecosystem can be used as a «fitness» test for the health of ecosystems. Fitness tests can provide early warning of ecosystem degeneration and allow intervention to reduce or eliminate anthropogenic stress on the ecosystems.