Manipulating the intrinsic parameters of propagule pressure: implications for bio‐invasion

Abstract

Much effort has been devoted to understanding the factors promoting species colonisation, and processes that may cause small incipient populations to fail. Post colonisation and community level processes are proving to be less reliable predictors of initial population size and propagule pressure is now posited as a key predictor of species establishment. Yet, empirical studies manipulating the intrinsic parameters of propagule pressure, such as arrival frequency (number), and intensity (size) are lacking. Understanding colonisation and incipient population survival is particularly important in the field of bio‐invasions. Propagule pressure is now often cited as a key determinant of invasion success, yet we have few empirical tests manipulating the intrinsic parameters of propagule pressure. We conducted a series of factorial field experiments utilizing larvae of the cosmopolitan invasive oyster Crassostrea gigas . A newly developed technique was used to quantitatively alter the size of propagules and the frequency at which they arrive. When total propagule pressure was held constant, and the size and arrival frequency was altered, frequent small inoculations dramatically increased incipient population size >65% relative to infrequent large introductions. We also found that smaller, less dense inoculations resulted in >85% greater proportional settlement and colonisation of this cosmopolitan species. Predation altered the overall survival of incipient oyster populations, but did not alter this pattern. Our findings may help explain patterns of ballast water and hull‐fouling introductions, as ports and harbours that receive frequent exposure to invasive propagules from the same source location will be at greater risk of invasion.