The relationship between propagule pressure and establishment success in alien bird populations: a re-analysis of Moulton & Cropper (2019).

Tim M Blackburn,Phillip Cassey,Julie L Lockwood,Richard P Duncan

doi:10.7717/peerj.8766

Abstract

A recent analysis by Moulton & Cropper (2019) of a global dataset on alien bird population introductions claims to find no evidence that establishment success is a function of the size of the founding population. Here, we re-analyse Moulton & Cropper’s data and show that this conclusion is based on flawed statistical methods—their data in fact confirm a strong positive relationship between founding population size and establishment success. We also refute several non-statistical arguments against the likelihood of such an effect presented by Moulton & Cropper. We conclude that a core tenet of population biology—that small populations are more prone to extinction—applies to alien populations beyond their native geographic range limits as much as to native populations within them.

Highlights

A fundamental ecological tenet is that small populations with few individuals are more likely to go extinct than larger populations
The dataset published by Sol et al (2012) includes information on a wide variety of traits that might affect establishment success, but we focus on the subset of 832 separate introductions of bird populations worldwide analysed by Moulton & Cropper (2019), and presented by them as an appendix to their paper
A simple logistic regression shows a strong positive relationship between establishment probability and founding population size across the 832 introduction events in the data used by Moulton & Cropper

Summary

Introduction

A fundamental ecological tenet is that small populations with few individuals are more likely to go extinct than larger populations This outcome arises because small populations are more susceptible to demographic, environmental and genetic accidents (i.e., stochasticity), and to Allee effects, that can cause them to die out, regardless of how suited they are to the environment they inhabit (Caughley & Gunn, 1996). This finding, termed the small population problem, is core to understanding the establishment success or failure of introduced alien populations (Lockwood, Hoopes & Marchetti, 2013). The relationship between founding population size (often termed propagule pressure; Lockwood, Cassey & Blackburn, 2005) and establishment success is sufficiently general that it is considered a ‘‘null model for biological invasions’’ (Colautti, Grigorovich & MacIsaac, 2006)

Methods

Results

Conclusion