Successful population establishment from small introductions appears to be less common than believed

Area of Science:

• Ecology
• Conservation Biology
• Ornithology

Background:

• Small populations face high extinction risks, yet claims of successful introductions from few individuals persist.
• Evidence for the viability of small populations often relies on successful species introductions outside their native range.

Purpose of the Study:

• To evaluate the validity of purported successful bird introductions from small founding populations (<30 individuals).
• To assess the reliability of evidence supporting the establishment of bird species from limited introductions.

Main Methods:

• Reviewed introduction compendia and original sources for deliberate bird translocations.
• Assessed 23 cases of purported successful establishments from small introductions (<30 individuals).
• Evaluated evidence quality, focusing on the number of individuals released and potential confounding factors.

Main Results:

• Only two of the 23 purported successful introductions were substantiated.
• Most cases were rejected or deemed ambiguous due to insufficient data on release numbers or evidence of supplemental introductions/natural invasion.
• Findings indicate that successful establishment from small introductions is frequently overstated.

Conclusions:

• The success rate of bird introductions from small populations is lower than often reported.
• Results support the link between propagule pressure and establishment success, reinforcing the theory of small population viability.
• Future analyses should exclude studies with unknown or unreliable introduction efforts to improve accuracy.