Problems of scale in assessing the role of propagule pressure in influencing introduction outcomes illustrated by Common Pheasant (Phasianus colchicus) introductions

Abstract

Species that are introduced into novel habitats have potentially devasting effects on ecosystem services and functions. Predicting which species and introduction events represent the largest threats are important scientific and management goals. It has been suggested that propagule pressure, the sum of individuals introduced, can be used to predict the probability of establishment success. Previous analyses of this hypothesis have often improperly used historical data by combining introductions over large spatial extents and over long time periods. We use the historical record for Common Pheasant releases in the USA to evaluate the process at scales more relevant to the problem of understanding population establishment. Introduction success varied widely among regions with histories of Common Pheasant introductions in the USA. In a series of logistic regression models involving data from the Foreign Game Investigation Program, we found no evidence to support propagule pressure when the ‘state’ in which the releases occurred was included as a random effect. There are many examples of states with large numbers of Common Pheasants that were introduced but failed to establish self-sustaining populations. David Watmough. Dreamstime.com.