Estimating the propagule size of a cryptogenic crested newt population

Abstract

Encountering cryptogenic populations that are either native or introduced is a common but underreported phenomenon in field biology. Such local species' occurrences of unknown origin hamper our understanding of species' natural distribution ranges, and pose a problem to conservation management decisions. Genetic tools are frequently used to infer the ancestry of natural or invasive populations based on spatial geographical variation. Here we describe the occurrence of cryptogenic crested newts Triturus cristatus c. 25 km away from their main range and situated in an area where an introduction has taken place half a century ago. We first verify the suitability of a coalescent-based analysis that reconstructs the number of founder individuals of a putative propagule from a source pool, based on previously published data documenting a well-known introduction (the Laysan finch on the Hawaiian archipelago). After showing the validity of the approach, we apply this analysis to the case of T. cristatus, revealing that the number of effective founders would have been of the same order as the current effective carrying capacity of the population. We thus argue that the local T. cristatus occurrence is in fact of natural origin, with the documented introduction having little or no impact to the current gene pool. The result has major implications to our understanding of the species' habitat requirements in a zone of parapatry with a closely related species.

An isolated crested newt population in Dutch coastal dunes: distribution relict or introduction?

“…Such a scenario has been hypothesized to explain the presence of isolated northern crested newt populations in the Highlands of Scotland (O'Brien and Hall, 2012). A study using a variable nuclear DNA marker system, such as RAD-sequencing, could potentially shed more light on the origin of the Dutch coastal dune population (see e.g., Arntzen et al, 2010).…”

Stabilization of a salamander moving hybrid zone

“…footnote to Table in Vallée, ), but molecular genetic data indicate that the local presence is natural. The arguments underlying this claim are as follows: (1) assignment tests indicate a closer genetic relationship with other northern T. cristatus populations than with the presumed source of the introduction near the city of Laval in the center of the département, and (2) the genetic variability of the Pré‐en‐Pail T. cristatus population is such that the propagule size of an introduction must have been large, which is unlikely (Arntzen et al., ). The alternative scenario is the northward dispersal of T. cristatus to have reached the Pré‐en‐Pail area along the Mayenne river.…”

“…This showed the (northward) advance of T. cristatus, the regression of T. marmoratus, and the continued presence of hybrids (Schoorl & Zuiderwijk, 1981;Vallée, 1959) (Figure 1). Second, T. marmoratus is surrounded by T. cristatus in enclaves and other persisting occurrences in areas of species replacement (Arntzen & Wallis, 1991;Arntzen, 1996; see also Arntzen, Burke, & Jehle, 2010). Third, genetic variation thought to result from hybridization is significantly higher in T. cristatus than in T. marmoratus (Arntzen & Wallis, 1991).…”

How did the toad get over the sea to Skye? Tracing the colonisation of Scottish inshore islands by common toads (Bufo bufo)

“…Such small-scale introductions would however be reflected in the genetic make-up of populations (e.g. low allelic richness or lack of haplotype diversity, see Arntzen et al, 2010;Tingley et al, 2015), and would not explain the population on the uninhabited Crowlin Island.…”