Changes in habitat connectivity for range-restricted birds reflect patterns of woodland invasion

Habitat fragmentation and landscape change are common causes of concern for species persistence, especially for habitat specialists. The composition of the matrix surrounding habitat fragments influences connectivity between them, which affects gene flow across the landscape. This can further impact populations in various ways. The Shola Sky Island landscapes naturally comprise a biphasic, forest-grassland mosaic ecosystem unique to the high-altitude regions of the Western Ghats of India. Earlier, this mosaic consisted of patches of native cloud forests embedded in a large grassland matrix. Over the last few decades, however, extensive invasion (up to 60%) of timber species into native grasslands has inverted this mosaic, i.e., small patches of grasslands are nested in a woodland matrix. We attempt to study the effects of these modifications on functional habitat connectivity in this region by modelling species movement using a circuit theory-based algorithm. We do this for seven Shola endemic, range-restricted bird species; six forest-specialist and one grassland-specialist species, based on a decade of field data. We consider a range of species-environment relationships and dispersal capacities for a past, relatively uninvaded landscape and a present, highly modified landscape. We used bird occupancy data (presence/absence from a total of 720 grid cells from targeted occupancy surveys for forest species and 744 presence locations for grassland species from occupancy surveys combined with opportunistic records) along with remotely sensed landscape, vegetation, climatic and topographic variables. We find that connectivity has increased overall for forest specialists, but has reduced for the grassland species. This pattern is concordant with regions where woodland cover from invasive timber species has expanded over approximately two decades. We also identify species-specific areas of low and high connectivity, which may have implications for gene flow within the landscape. This would help focus conservation efforts and predict how future landscape change might affect species persistence.