Propagule pressure nodes and the invasion process

The invasion process consists of a series of emigration (active or passive dispersal from a region) and immigration (arrival if active, or release if passive, dispersal) phenomena. The arrival-release step constitutes inoculation, but not necessarily introduction. For passive dispersal, initial emigration consists of (1) uptake (entrainment) from a species pool followed by (2) transport; immigration consists of (3) discharge (vector exit), (4) surviving release, (5) reproduction, and (6) establishment (colonization), in turn followed by (7) the next emigration pulse (range expansion). Propagule pressure is manifested at three points along this pathway, punctuated by four survival windows. Each of the seven steps is accompanied by a probability estimate based on microscale timing (such as the probability of interfacing with a vector at a given moment), species biology (physiological probabilities of surviving transport, reproductive requirements, and other qualities), and the nature of environmental regimes (chemical, physical, and biological) in the recipient region. The breadth and depth of variables in this framework assure that species invasions will continue if vectors are available.