Demographic, dispersal, predation and genetic data reveal the potential vulnerability of an endangered rainforest shrub, Triunia robusta (Proteaceae)

Abstract

We investigated the population ecology of Triunia robusta (C.T. White) Foreman, an endemic rainforest shrub of south-east Queensland, Australia. Two-time demographic data from 1999 and 2010 were used to estimate the species life span and changes in demographic factors over the 11 year period. The potential dispersal vectors and their activities were monitored, and the effects of predation on seed and seedling mortality were quantified. Published genetic data was used to assess the gene flow distance in years. On average, T. robusta has a life span of 103 years, with a generation time of 44 years. Larger populations (>200) increased in size since 1999, whereas smaller populations retained the same or slightly reduced numbers. Small, isolated populations in the northern distribution range showed substantially lower reproductive rates. Local rodents and marsupials were considered responsible for the majority of observed secondary seed dispersal (<10.3 m) and predation activities. Post-predation mortality was high (82%), with only 12% surviving to become seedlings. The empirical evidence of short-distance dispersal, limited gene flow, high post-predation mortality rates and relatively low reproductive rates, combined with potential absence of primary dispersers suggests that critically small and isolated populations may be highly vulnerable.