CANCELLED - Old paradigms and new threats: Propagule pressure, predation and invasion success

Background/Question/Methods

Biological invasions continue to pose a serious threat to global diversity, and present a unique and complex issue for ecologists and managers alike. Recent models incorporating propagule pressure have had some success at explaining invader distributions, but there remain relatively few experimental studies in marine systems that have manipulated propagule pressure directly and quantified its effects. As such, although the basic positive relationship between propagule pressure and invasion success is well supported, the shape of the ‘dose-response’ relationship is unknown. Additionally, heterogeneity of resource use through space and time is a typical feature of all ecosystems and multiple inoculations may increase invasion success by enabling non-indigenous species to take advantage of this heterogeneity.

We use the Australian invasive oyster, Crassostrea gigas, to first examine the ‘dose-response’ relationship between propagule pressure and recruitment, as well as survival and persistence. We use a novel technique allowing a quantitative manipulation of propagule pressure into sub-tidal sessile invertebrate communities. We further test the hypothesis that increasing frequency of larval arrival, as opposed to increasing intensity, leads to greater recruitment and persistence of C. gigas populations when total propagule pressure remains constant.  

Results/Conclusions

Our results show that initial recruitment of C. gigas increases linearly with increasing exposure to a larger ‘innoculation’ of C. gigas larvae, suggesting recruitment limitation. Over time, competition modifies this function, such that the probability of individual larval survival, after 3 months, increases when the initial larval inoculation is small ie. with low propagule pressure. Predation acts to further reduce incipient populations to extremely low numbers, regardless of initial inoculation size.  Furthermore, we show that smaller more frequent ‘inoculations’ of larvae into this system over time, leads to greater recruitment and persistence of C. gigas. The theoretical interest in these results is complemented with considerable applied significance, as the control of marine propagules is one of the few viable options for controlling the spread of exotic species throughout the world.