The effects of habitat loss and fragmentation on updated estimates of the population of the Agulhas Long-billed Lark Certhilauda brevirostris, a South African endemic

Abstract

The Agulhas Long-billed Lark Certhilauda brevirostris is a South African endemic and is restricted to the fynbos habitat of the Agulhas Plain in the southwestern corner of South Africa. The 1990, 2014, and 2018 land use land cover (LULC) databases prepared for South Africa were used to determine the LULC categories that best describe suitable habitat for this bird using beta regressions. Previously published densities of Agulhas Long-billed Lark and the surface area of the LULC categories considered suitable habitat for the birds were used to estimate their historical and current population sizes, and the rates of change in the bird’s population. Beta regression models were compiled from the bird’s adjusted reporting rate and the total surface area, and three other landscape metrics, of the per pentad LULC categories considered suitable habitat for them. These models revealed that although the Agulhas Long-billed Lark is adversely affected by the loss of its natural fynbos habitat these adverse effects have been offset by an increase in the availability of fallow fields in summer. This confirms previous observations that agriculture may have allowed the Agulhas Long-billed Lark to increase in numbers. The Agulhas Long-billed Lark’s extent of occurrence (EOO) was estimated at 1 322 509 ha (n = 197 pentads) with an area of occupancy (AOO) estimated to be 1 076 854 ha in 2020. The decline in habitat available to the birds was estimated to be 3.5% over three generations (10.5 years). The present study provides improved estimates of the population of the Agulhas Long-billed Lark of between 146 344 and 264 691 individuals, with between 60 753 and 109 883 individuals in natural habitat, and between 85 591 and 154 808 individuals in transformed habitat. The results of the present study indicate that the Agulhas Long-billed Lark’s Near Threatened status should be reviewed.