A two-species distribution model for parapatric newts, with inferences on their history of spatial replacement

Abstract

AbstractRelated species often engage in abutting or overlapping contact zones with various strengths of interspecific competition. Biotic interactions such as these preclude the registration of the full profile of environmental variables that would describe the otherwise larger species ranges. Here, I advocate to forego full range species distribution modelling and instead focus on the ecography of the contact zone, for example with ‘two-species distribution models’ (TSDMs), in which presence data are contrasted against the background of environmental data. The newts Triturus cristatus and Triturus marmoratus meet in the west of France. A countrywide TSDM suggests that the contact zone of the species is located at a climatic gradient, in line with their north-eastern vs. south-western ranges. The species are also ecologically segregated by elevation and forestation, which is in line with a documented movement of the contact zone caused by hedgerow removal in lowland areas. Hindcasts for the Holocene suggest that the species contact zone was positioned at either the same place as at present or more to the south, depending on the amount of forestation. A forecast under climate warming predicts a fast movement to the north, but this scenario is deemed unrealistic. One reason is that recent habitat loss compromises dispersal and range expansion. Other species pairs to which TSDMs have been applied are listed for comparison.