Assessing the range expansion of a generalist carnivore by a novel approach combining questionnaires to experts and dynamic occupancy models

Despite the ongoing global biodiversity crisis, the geographic ranges of many mesopredators are expanding, largely driven by their generalist life histories and the decline of apex predators. Quantifying these expansions and identifying their underlying drivers remain challenging and require innovative methodological approaches. We aimed at quantifying the factors driving the recent spatial expansion of the Egyptian mongoose ( Herpestes ichneumon ) at the European edge of its range. We combined occurrence data from three periods (2000–2009, 2010–2015 and 2016–2018) in the central Iberian Peninsula (Spain), collected via questionnaires to environmental rangers, with Bayesian dynamic occupancy models. Responses were obtained from 66% of rangers, covering 91.6% of municipalities (93.9% of the total area), ensuring broad spatial representation. Detection probability increased with the proportion of time rangers devoted to field work. Initial mongoose occupancy showed unimodal relationships with elevation and scrubland cover, peaking at low to intermediate elevations and at intermediate scrubland proportions. Colonization probability was highest in warmer areas, near already occupied sites, and in areas with higher scrubland cover. The Egyptian mongoose has undergone substantial expansion in central Spain between 2000 and 2018, with an estimated 262% increase in occupied municipalities (from 6.8 ± 1.5% to 24.6 ± 2.7%). Our study suggests that integrating expert-based questionnaires with dynamic occupancy models may be an effective approach for quantifying range expansions and assessing environmental drivers of colonization.