Back to the basics: using density series to test regulation versus limitation for invasive plants

Numerous hypotheses have been invoked to explain invasion in plant communities. Here, we use a fundamental tool from plant ecology, density series in situ, to explore whether a reasonable starting point for highly successful invasive plant species is to consider regulation (biotic effects) and limitation (environmental constraints). To explore the utility of density series to understanding invasion we present a case study using C. solstitialis , a perfect candidate since it is a prolific seed producer and invasive in many grasslands globally. Using seed addition into existing vegetation in three grasslands with densities of up to 6500 seeds per m 2 , we found no evidence for regulation via intra or interspecific interference but large differences amongst sites. This strongly suggests that in this species limitations imposed by the environment are the only constraints. Hence, we propose that an excellent starting point for ecologists studying invasion should involve back to the basics experiments on novel species to determine whether other hypotheses need be invoked.