Abstract
Previous studies demonstrate that old-growth forest remnants and vegetation regenerating after anthropogenic disturbance provide habitat for birds in a human modified coastal dune forest landscape in northern KwaZulu-Natal, South Africa. However, occurrence does not ensure persistence. Based on a 13-year monitoring database we calculated population trends for 37 bird species and general trends in overall bird density in different vegetation types. We evaluated species' characteristics as covariates of population trend and assessed changes in rainfall and proportional area and survey coverage per vegetation type. 76% of species assessed have declined, 57% significantly so at an average rate of 13.9% per year. Overall, bird density has fallen at 12.2% per year across old-growth forest and woody regenerating vegetation types. Changes in proportional area and coverage per vegetation type may partly explain trends for a few species but are unlikely to account for most. Below average rainfall may have contributed to bird declines. However, other possibilities warrant further investigation. Species with larger range extents tended to decline more sharply than did others, and these species may be responding to environmental changes on a broader geographical scale. Our results cast doubt on the future persistence of birds in this human modified landscape. More research is needed to elucidate the mechanisms driving population decline in the study area and to investigate whether the declines identified here are more widespread across the region and perhaps the continent.
Highlights
Coastal dune forest is one of South Africa’s rarest vegetation types; restricted to the eastern coast, it covers less than 1000 km2
We investigated how species’ characteristics known to be associated with extinction proneness of forest birds—i.e. clutch size, habitat affinity, diet, tolerance of human modified landscapes, and range extent—related to population trend and assessed changes in rainfall, proportional area of vegetation types, and survey coverage per vegetation type as possible determinants of population and general trends
Between 1997 and 2009, birds were surveyed via transect counts in 9 survey years at two relatively pristine old-growth coastal dune forest sites and nine regenerating forest sites of known age (Table S1) within a mining lease area maintained by Richards Bay Minerals (RBM)
Summary
Coastal dune forest is one of South Africa’s rarest vegetation types; restricted to the eastern coast, it covers less than 1000 km. Isolated stands of protected coastal dune forests may be insufficient for their long-term conservation [4] because dispersal ability of many tree species is constrained by distance between forest patches [6] Due to their vagility and role in seed dispersal [7], birds may enhance connectivity of coastal dune forest fragments (see [6]). Land-use options that incorporate coastal dune forest elements such as remnant forest patches in agricultural landscapes or active regeneration after anthropogenic disturbances may allow bird populations to persist beyond protected areas. This may be the case in South Africa’s northern coastal dune forests
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