Patch analysis of atlas data reveals pattern and process of species replacement

The dynamics of species range borders may be difficult to study due to the local rarity of populations and individuals. This hindrance applies less when range borders are parapatric contact zones. Analysis of species in parapatry has the further advantages of no (inferred) absence data and that shortcomings in data gathering such as uneven sampling apply to the counterpart species about equally. The large-bodied newts Triturus cristatus and T. marmaratus are spatially segregated within a wide area of range overlap in the west of France. They locally show abutting or slightly overlapping distributions with many isolated occurrences (here called ‘patches’) of either species within the continuous range section of the other. Historical and genetic data suggest that T. cristatus has been superseding T. marmoratus. Species replacement should be also discernible from local species distributions, with more, larger and more distantly positioned patches in the receding than the advancing species. Atlas data for France and from the French region Pays de la Loire largely confirm these predictions. The data also indicate that T. marmoratus patches may be void of T. cristatus, suggesting that they are persisting strongholds, whereas the more admixed T. cristatus patches are in flux. The species’ differential ecological signature is that of forested, hilly terrain for T. marmoratus and open, flat terrain for T. cristatus. Accordingly, the main dispersal route for T. cristatus towards the Atlantic coast has been through the valley of the Loire River, with a secondary, intraspecific contact zone at the Normandy coast. A literature survey revealed several other species pairs of European herpetofauna that may be amenable to patch analysis, as will be species pairs in other groups of organisms characterized by limited, habitat dependent dispersal. In such efforts the availability of species occurrence data from atlases and digital databases is an indispensable asset.

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