Invasive non-native annuals use novel source-sink strategies in North American deserts

Background/Question/Methods

Desert ecosystems are currently showing a dramatic increase in the arrival and spread of non-native species. Traditionally, annual plants have patchy distributions in desert scrub ecosystems, being restricted to nutrient rich areas beneath shrubs, and are not found in open areas between shrubs. We hypothesize that some of the invasive species are able to spread into the inter-shrub space by using novel population strategies that differ from native species. The “traditional” population strategy, which we call the “source focus strategy” where populations tend to remain in their focus habitat (shrub understory), and the new strategy, which appears to have been adopted by some non-native plants and we call the “source sink strategy”. We investigated the density and fecundity of a number of native and non-native annuals in relation to shrubs in the Mojave and the Sonoran Desert in order to test our prediction that non-native species indeed follow different strategies.

Results/Conclusions

Amsinckia, a native annual, was more abundant on the north sides of shrubs than the south sides. On the north side the density and reproduction were greatest beneath shrubs, and lowest in the open areas.  On the south sides of shrubs, there was not a distinct pattern in density or reproduction. The non-native annual grass Schismus behaved differently on the north sides of shrubs than on the south sides, and also acted differently between deserts. In the Sonoran, on the north sides of shrubs, Schismus density was highest under the shrub canopy (UC) and very similar at the canopy drip-line (CD), the open areas near shrubs (ON), and the open areas further from shrubs (OF). Reproduction, was inversely proportional to density.  On the south sides of shrubs, there was no distinct pattern in Schismus density. In the Mojave desert, Schismus density was dramatically higher in the ON and OF plots on both sides of the shrubs than in the UC and CD plots.  Overall more plots in the Mojave Desert had Schismus than in the Sonoran Desert. These data suggest that in the Sonoran the non-native Schismus behaves similarly to natives, with higher density beneath shrubs, whereas in the Mojave, Schismus appears to prefer the open areas where historically few plants grew.