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

Abstract

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 bioinvasions. 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.

Decomposing propagule pressure: the effects of propagule size and propagule frequency on invasion success

“…These limitations of inference from historical data demonstrate that experimental studies are important to improve our understanding of how the temporal distribution of introduced individuals affects invasion success. Apart from the experimental Daphnia introductions by Drake et al (2005), we know of only one other such study: Hedge et al (2012) performed colonization experiments with larvae of the pacific oyster Crassostrea gigas and found higher population growth with frequent small introduction events compared to fewer large introduction events, a phenomenon that they attributed to reduced intraspecific competition or increased facilitation if individuals arrive at different time points.…”

The interacting effects of diversity and propagule pressure on early colonization and population size

“…Propagule pressure relates the numerical size (or the abundance of arriving individual organisms) and arrival rates of propagules, which are in turn comprised of individual organisms (Lockwood, Cassey & Blackburn , ). We would argue that this term also encompasses the diversity and quality of arriving propagules (Hedge, O'Connor & Johnston ). Numerous studies have now demonstrated the strong effects of individual quality on settlement probability and postsettlement fitness (Pechenik, Wendt & Jarrett ), and effects resulting from the diversity of arriving propagules should not be ignored (Lockwood, Cassey & Blackburn ).…”

“…Where empirical studies have been conducted, however, the relationship between propagule pressure and colonization is usually expressed as a simple dose‐response function relating propagule size with the subsequent number of individuals surviving until census (Ruiz et al . ; Clark & Johnston , ; but see Hedge, O'Connor & Johnston ). Further, despite the intensity of study that interspecific diversity has engendered over the past few decades (Loreau et al .…”

“…Propagule pressure relates the numerical size (or the abundance of arriving individual organisms) and arrival rates of propagules, which are in turn comprised of individual organisms (Lockwood, Cassey & Blackburn 2005, expressed as a simple dose-response function relating propagule size with the subsequent number of individuals surviving until census (Ruiz et al 2000;Clark & Johnston 2005, 2009; but see Hedge, O'Connor & Johnston 2012). Further, despite the intensity of study that interspecific diversity has engendered over the past few decades Hooper et al 2005; and references therein), we have a limited understanding of the role of a genetic and phenotypic diversity in altering ecological processes such as colonization.…”

Propagule size and dispersal costs mediate establishment success of an invasive species

“…While density‐dependent decreases in individual performance are common in invasive species (Burgess and Marshall , Hedge et al. ), it is the direction of the interaction that is surprising. Poorer‐quality individuals often experience negative‐density dependent effects more strongly than higher‐quality individuals (Rubenstein , Clutton‐Brock et al.…”