Propagule Pressure and Soil Disturbance Diminish Plant Community Resistance to Invasion Across Habitat Types

Questions

Community resistance to non-native plant invasions results from intrinsic habitat characteristics, propagule pressure, and the presence of disturbance. Species identity further complicates this relationship due to pre-existing adaptations. Despite these mechanisms being understood in isolation, their interplay is rarely explored in natural field communities. Furthermore, while survey studies have reported levels of invasion across habitat types, few have quantified differences in intrinsic invasibility experimentally.

Location

Roy Berg Kinsella Research Ranch, Alberta, Canada.

Methods

We manipulated soil disturbance and propagule pressure in three habitat types (aspen forest, shrub vegetation, and prairie grassland) and examined their impact on the germination success of three pairs of phylogenetically similar native and non-native plant species (Bromus ciliatus/B. inermis, Elymus trachycaulus/Agropyron cristatum, Poa secunda/P. pratensis) for 3 months after seed addition.

Results

Habitats played a crucial role in determining resistance to invasion, with aspen forest exhibiting the highest germination rates and invasibility and prairie grassland the lowest. High propagule pressure significantly increased invasibility across all habitat types and genera, and its impact was most pronounced when combined with soil disturbance, though this was contingent on genus. Invasive Bromus had higher germination compared to its native congener, even in the absence of disturbance. However, native Elymus and Poa species had equal or greater germination compared to their non-native counterparts.

Conclusions

Our results underline that propagule pressure, disturbance, and species identity interact as drivers of plant community invasibility. Furthermore, our study demonstrated that habitat types differ in their intrinsic resistance to invasions. While aspen forests have greater invasibility, grasslands are more invaded than their resistance suggests. Thus, invasibility contrasts with levels of invasion reported in field surveys, supporting previous suggestions that these attributes do not always align.