Maia Raymundo¹, John Dwyer² and Margie Mayfield¹

¹The Ecology Centre, University of Queensland, St. Lucia, QLD 4072, Australia
²CSIRO Land and Water, EcoSciences Precinct, Dutton Park, QLD 4001, Australia

e-mail: raymundo.mlh@gmail.com

Plant population growth is well known to be impacted by a range of factors, including seed production, dispersal, competition, herbivory, and predation, operating at various life-history stages. The recruitment phase (i.e. germination, survivorship and growth) has been identified as a bottleneck for population growth and is thus one of the key stages for understanding plant community structure and diversity. Numerous ecological models and empirical studies have focused on determining the mechanisms affecting survival during recruitment and successive life stages. However, the causes of recruitment limitation and the factors limiting subsequent population growth is particularly understudied in invaded communities. The annual understory of Southwest Australia’s York gum woodland system is an ideal system for studying the complex processes involved in plant community assembly of invaded systems because of the relatively short growing seasons and heavily structured invasion patterns along biotic and abiotic gradients. Quantifying the extent to which abiotic and biotic filters influence the assembly of invaded communities and whether dispersal limitation is a factor in determining local heterogeneity is vital to understanding invasive species’ persistence through time. We tested the extent to which dispersal and biotic and abiotic filtering impose constraints on germination and survival of native annual forb populations in invaded communities through a series of seed addition experiments along biotic and abiotic gradients associated with invasion. Our results showed that though focal species used in seed augmentation were common throughout the plots, diversity was almost five times higher in seed-augmented subplots than control subplots. This suggests that dispersal plays a vital role in influencing the diversity and heterogeneity of these invaded communities, particularly as natives are dispersal-limited and their introduction to a community increases overall diversity even in exotic dominated sites. We found that native annual forbs were generally seed limited, as indicated by the higher number of germinants in seed-augmented plots compared to control plots (Fig.1). However, the mean effect size for seed limitation were all below 35%, showing that only a small fraction of the viable seeds germinated, which suggests that microsite limitation may be more important for all focal species at the post-dispersal stage. Overall, post-dispersal processes were the strongest constraints to seedling emergence and overall population size. Canopy cover and soil phosphorus, both structuring factors in these communities, did not strongly influence population sizes of added focal species at the seedling and adult stages. Exotic densities also did not strongly limit population sizes across all species. The lack of strong trends indicates that native species may be more resilient to soil eutrophication and invasion in terms of population size after seedling emergence.