Comparative Seed and Dispersal Ecology of Three Exotic SubtropicalAsparagusSpecies

Abstract

AbstractThe genusAsparagusincludes at least six invasive species in Australia.Asparagus aethiopicusandA. africanusare invasive in subtropical Australia, and a third species,A. virgatusis naturalized and demonstrates localized spread in south east Queensland. To better understand how the attributes of these species contribute to their invasiveness, we compared fruit and seed traits, germination, seedling emergence, seed survival, and time-to-maturity. We further investigated dispersal ecology ofA. africanus, examining the diet of a local frugivore, the figbird (Sphecotheres viridis) and the effect of gut passage on seedling emergence. Overall,A. aethiopicuswas superior in germination and emergence, with the highest mean germination (98.8%) and emergence (94.5%) under optimal conditions and higher emergence (mean of 73.3%) across all treatments. In contrast,A. africanushad the lowest germination under optimal conditions (71.7%) and low mean seedling emergence (49.5%), but had fruits with the highest relative yield (ratio of dry pulp to fruit fresh weight) that were favored by a local frugivore. Figbirds consumed large numbers ofA. africanusfruits (∼30% of all non-Ficusfruits), and seedling germination was not significantly affected by gut passage compared to unprocessed fruits.Asparagus virgatusgerminated poorly under cool, light conditions (1.4%) despite a high optimum mean (95.0%) and had low mean performance across emergence treatments (36.3%). The species also had fruits with a low pulp return for frugivores. For all species, seed survival declined rapidly in the first 12 mo and fell to < 3.2% viability at 36 mo. On the basis of the traits considered,A. virgatusis unlikely to have the invasive potential of its congeners. Uniformly short seed survival times suggest that weed managers do not have to contend with a substantial persistent soil-stored seed bank, but frugivore-mediated dispersal beyond existing infestations will present a considerable management challenge.