Long-term population monitoring reveals an alarming decline in the endangered endemic Reunion Harrier Circus maillardi

Abstract

Insular species are disproportionately vulnerable to extinction due to small population sizes, geographic isolation, and high susceptibility to stochastic and anthropogenic pressures. The Reunion Harrier Circus maillardi, the last breeding raptor on La Reunion (Indian Ocean), is currently listed as Endangered (EN) on the IUCN Red List, although population decline has so far only been suspected rather than robustly quantified. Concerns have recently emerged regarding the demographic impacts of secondary poisoning from anticoagulant rodenticides used for rat control in agricultural landscapes, in addition to habitat loss, fragmentation, and reduced genetic diversity. Here, we analysed data from three standardised island-wide breeding surveys (1998--2000, 2009--2010, 2017--2019), encompassing 346 sampling points and totalling over 1,500 h of observation, to quantify population trends and their spatial variation. Using generalized additive mixed models accounting for spatial structure and heterogeneous survey designs, we detected a significant decline of -46% [-62%; -27%] in the relative abundance of breeding pairs at the island scale over 21 years, equivalent to three generations, supporting the species' current Endangered status. The decline was steeper between 1998 and 2009 (-33% [-52%; -9%]) than between 2009 and 2017 (-18% [-42%;+14%]). The decrease was more markedly pronounced in the eastern part of the island (-57% [-71%; -40%]) than in the west (-29% [-53%; + 0.8%]), reflecting a complete reversal of the historical east-west abundance gradient. Combined with previous evidence of inbreeding, high mutational load, and rodenticide exposure, these results suggest the population is undergoing an extinction vortex requiring immediate, spatially targeted conservation actions, including reduced rodenticide exposure and genetic management. These findings also underscore the need for sustained monitoring and improved demographic knowledge to support the implementation of the ongoing National Species Action Plan