Question: What is the combined effect of two drivers of local biodiversity changes (presence of a potentially invasive species and seasonal drought) on the performance of seedlings of plants from gypsum habitats under experimental conditions?

Location: A controlled microcosm reconstruction of natural assemblages of gypsum plant communities from central Spain.

Methods: We evaluated the effects of a potentially invasive grass (Lolium rigidum) and water stress on the survival, height growth and biomass of five woody species (Colutea hispanica, Gypsophila struthium, Thymus lacaitae, Lepidium subulatum and Helianthemum squamatum) from semi-arid gypsum ecosystems. Seedlings of the five species were grown with or without the potential invader and under three watering regimes: early stress – simulating an advanced summer, late stress – simulating the characteristic timing of current summer drought and well-watered.

Results: Seedling survival and performance were negatively affected by the presence of the potential invader. Early stress had larger impacts on the gypsum species than late stress. No interactions were found between factors for any of the study variables, and responses to both factors were found to be species-specific.

Conclusions: The lack of interactions between factors indicates that the presence of the potentially invasive grass and water stress had additive effects in our study system. The negative impact of early water stress draws attention to the possible consequences of the advances of summer drought predicted for Mediterranean ecosystems. Finally, the differential responses found for the study species suggest that plant communities will not respond as a unit to global change, leading to significant changes in species composition and dominance.

Nomenclature: Tutin et al. (2001).