Diluting the founder effect: cryptic invasions expand a marine invader's range.

Area of Science:

• Marine Biology
• Invasive Species Ecology
• Population Genetics

Background:

• The European green crab (Carcinus maenas) invasion in the US began in 1817, followed by northward range expansion.
• Recent population explosions in the Canadian Maritimes challenge linear invasion models.
• Previous hypotheses attributed expansion to warming seas or cold adaptation.

Purpose of the Study:

• To investigate the genetic diversity of Canadian green crab populations.
• To determine the origins of the recent population boom in the Canadian Maritimes.
• To assess the role of multiple introductions and gene flow in invasive species range expansion.

Main Methods:

• Genetic analysis of Canadian green crab populations.
• Comparison of genetic diversity between northern and southern populations.
• Inference of introduction events and pathways.

Main Results:

• Canadian populations exhibit higher genetic diversity than southern populations.
• Evidence suggests multiple introductions of European green crabs into the Maritimes since the 1980s.
• New genetic lineages, likely from northern Europe, are colonizing previously unsuitable cold-water areas.

Conclusions:

• Multiple introductions, facilitated by ballast water discharge, are the primary driver of the green crab's recent expansion in the Canadian Maritimes.
• Increased genetic diversity from new introductions enhances invasive species' persistence and range expansion capabilities.
• Invasion dynamics are more complex than linear progression, involving episodic events and significant gene flow.