Local-scale invasion pathways and small founder numbers in introduced Sacramento pikeminnow (Ptychocheilus grandis)

Abstract

Given the general pattern of invasions with severe ecological consequences commonly resulting from multiple introductions of large numbers of individuals on the intercontinental scale, we explored an example of a highly successful, ecologically significant invader introduced over a short distance, possibly via minimal propagule pressure. The Sacramento pikeminnow (Ptychocheilus grandis) has been introduced to two coastal rivers in northern California where it poses a risk to threatened and endangered fishes. We assayed variation in seven microsatellite loci and one mitochondrial DNA gene to identify the source populations and estimate founder numbers for these introductions. Our analysis suggests that successful invasion of the Eel River was likely the result of a single transfer of 3–4 effective founders from nearby within the species’ native range: Clear Lake or its outflow Cache Creek. The other introduced population (Elk River), known from only seven individuals, likely represents secondary expansion from the introduced Eel River population. Our findings highlight the threat posed by close-range invaders and the ability of some fishes to rapidly invade ecologically suitable areas despite small effective founding numbers.