Propagule pressure

Propagule pressure (also termed ‘introduction effort’) is a composite measure of the number of individuals of a species released into a region to which they are not native. It incorporates estimates of the absolute number of individuals involved in any one release event (propagule size) and the number of discrete release events (propagule number). As the number of releases and/or the number of individuals released increases, propagule pressure also increases.

References

Lockwood JL, Cassey P, Blackburn T (2005) The role of propagule pressure in explainingspecies invasions. "Trends in Ecology and Evolution", 20, 5, 223-228.