Propagule pressure is a measure of the number of viable individual organisms in an introduced population (2). This value is equivalent to the number of individuals introduced in a single introduction event and the number of introduction events within a certain amount of time (2).
Propagule pressure is a crucial component of determining the success and failure probability of an invasive by a non-native species in a new environment (2). Multiple introductions of a species can greatly increase the chances of a species establishing itself along with a greater amount of initial individuals (1). Introduced arthropod and marine species often have high propagule pressure because they are easily moved through accidental human transport via trade of commodities or ballast water from ships (1). This traffic operating at high frequencies from major ports can also ensure multiple introductions of a species (1).
These pressures can also have negative effects on an introduced population. If the multiple introductions are derived from the same place it can limit the number of genotypes available (2). This can make the population collapse if the introduction frequency decreases or a change occurs in the new environment.
References: (1). Bacon, S.J., Aebi, A., Calanca, P., Bacher, S., Quarantine arthropod invasions in Europe: the role of climate, hosts and propagule pressure. Diversity and Distributions, (Diversity Distrib.) (2014) 20, 84–94
(2). Roman, J. and Darling, J.A., Paradox lost: genetic diversity and the success of aquatic invasions. TRENDS in Ecology and Evolution, July 27, 2007. Vol.22 No.9