Competitive abilities of native and invasive species in the Mojave Desert

Background/Question/Methods

In the desert southwest, invasive species pose as a major threat, because they can outcompete native species and provide fuel for wildfire. In the Mojave Desert, native plant species exhibit a patchy distribution due to the evenly spaced distribution of shrubs, resulting in a soil nutrient gradient. Nutrients are highest under the shrub canopy, and decline in the concentration into the interspace, creating “fertility islands.”

Annual plant species, both native and invasive, respond differently to fertility islands.  While invasives can survive in the interspace, natives often thrive directly under the shrub. To test competitive abilities of invasive and native species along the nutrient gradient, we investigated the relationship of available area (determined by Thiessen polygons) to plant biomass. We expected that higher nutrient levels would allow growth of more natives which should then outcompete the invasives. Alternatively, invasives will thrive and outcompete natives in the lower nutrient soil.

In a greenhouse study, we investigated the competitive relationship between native and invasive species at three nutrient levels, chosen to represent specific parts of the nutrient gradient. Two native species (Phacelia distans and Lasthenia californica) and two invasive species (Bromus rubens  and Schismus arabicus) were planted and then monitored for 14 weeks.

Results/Conclusions

Surprisingly, species thrived better in the low and medium nutrient levels, which showed three fold more growth than that of high nutrient level. At all nutrient levels, invasive species persisted in the experiment long after native species died. This could indicate that invasive species have the capacity to thrive under any of these nutrient conditions. We did not find a strong correlation between biomass and plant available area in plant populations at week one and week six, indicating that self-thinning did not occur in these greenhouse populations.