Survival and growth of native and exotic composites in response to a nutrient gradient

Mortality and performance (total biomass) of five exotic (weedy) and five native annual Asteraceae in Western Australia were compared along a mineral nutrient gradient. Mortality did not differ between exotic and native species but varied greatly within each group. The exotic species responded more positively to higher nutrient additions than the native species, indicating that the exotics might have a competitive advantage in a nutrient-enhanced situation (e.g., after fire). The concept of phenotypic inertia (PI), combining survival and performance over a range of environments, is introduced as a means of evaluating the range of growing conditions tolerated. The two species with highest PI (one exotic and one native) had the lowest mortality and uniformly moderate biomass for the nine nutrient treatments. The species with the lowest PI (a native) had complete failure at the two highest nutrient levels and low variable growth in the remainder. PI has potential as a measure of environmental tolerance although, in the present study, the exotic species were not clearly superior to the native species in their responses to a wide range of soil nutrient levels.