Decline in biotic resistance and the reorganization of an annual plant community by an exotic invader

Background/Question/Methods

Species interactions are considered an important mechanism that limits the abundance and distribution of exotic species, a process known as biotic resistance. Many studies documenting strong biotic resistance have been conducted over short time scales. However, given time-lags inherent in exponential growth and natural population fluctuations, the strength of biotic resistance may change over time.  Here, we use a thirty-year dataset to show an eventual decline in the strength of biotic resistance.

Results/Conclusions

For over 20 years, native granivorous rodents limited the abundance of the exotic annual plant, Erodium cicutarium: the invader first increased in abundance on plots where rodents had been removed, and remained more abundant on those plots compared to control plots that had an intact rodent community. After 20 years, E. cicutarium increased on control plots as well. This coincided with a natural decline in the main consumer, kangaroo rats. Eventually E. cicutarium became the dominant species in the annual plant community and the increase negatively affected the abundance and diversity of native annuals. Strong biotic control for more than 20 years could not ultimately prevent the reorganization of the annual plant community from one dominated by a high diversity of natives to one dominated by a single exotic invader.