Spatial variability in the mechanism by which an invasive plant affects native seedlings

Background/Question/Methods

Whether invasive plants pose a threat to biodiversity has been hotly debated. Native extinctions due to competition have not been widely recorded, but competitive exclusion may be occurring slowly, resulting in an extinction debt. Predicting the outcome of competition requires a better understanding of the mechanisms by which invasive plants interact with the native vegetation.

While these mechanisms are increasingly studied, their variability in time and space is often overlooked. Apparent competition through native predators may provide an advantage to an invasive species only in early stages of the invasion, or only under certain circumstances of resource availability, for example.

To measure the impact of an invasive bunchgrass on native seedling recruitment due to resource competition and to changes in small mammal granivore and herbivore behavior, we established a series of small experimental plots at three sites on the west side of Tucson, Arizona. These sites were separated by tens of kilometers, with distinct rainfall and invasion histories, though similar substrates and native vegetation communities. Half the plots established in the invaded area had grass removed, and an equal number of plots in native vegetation were established near each invaded site.  Experimental plots contained two mammal exclosures.

Results/Conclusions

Two sites showed emergence and survival patterns indicating resource competition from the invasive grass is excluding native species, while at a third site, the invasive grass increased small mammal foraging on native seeds and seedlings. At Sites 1 and 2, very few native seedlings emerged in plots containing grass, or within the experimental removals surrounded by dense grass. This suggests resource competition precluded the emergence of native seedlings. Mammal exclosures did increase the emergence and survival of native seedlings in nearby native vegetation, though, demonstrating the presence and potential impacts of herbivores in the area. At Site 3, native seedlings emerged readily in the invaded area, with or without removal, and mammal exclosures increased emergence and mean survival time primarily in the invaded area. Native granivores and herbivores may release the grass from competition by foraging near it.  Live trapping, camera trapping, and removal of seeds and seedlings from trays confirmed mammal presence and activity.

We demonstrated the magnitude and mechanisms by which invasive grass impacts native vegetation varies between invasion sites. Further work should determine the factors that determine these impacts and whether this variation is sufficient to change the long term trajectories of exclusion for native plants.