Data from: Primary rainforest amount at the landscape scale mitigates bird biodiversity loss and biotic homogenization

1.Tropical conservation strategies traditionally focus on large tracts of
pristine forests, but given rapid primary forest decline, understanding
the role of secondary forest remnants for biodiversity maintenance is
critical. Until now, the interactive effects of changes in forest amount,
configuration and disturbance history (secondary vs. primary forest) on
the conservation value of tropical landscapes has remained unknown,
hampering the incorporation of these global change drivers into local and
global conservation planning. 2.We disentangled effects of landscape wide
forest amount, fragment size, and forest age (old growth versus secondary
forest) on abundance, α-diversity, β-diversity (biotic homogenisation) and
community shifts of bird communities in human-dominated landscapes of
southern Costa Rica. Utilizing two complementary methods, yielding 6900
individual detections and 223 species, we characterized bird communities
in 49 forest fragments representing independent gradients in fragment size
(<5 ha vs >30 ha) and forest amount (5%-80%) in the
surrounding landscape (within 1000 m). 3.Abundance and α-diversity of
forest specialists and insectivores declined by half in small fragments,
but only in landscapes with little old growth forest. Conversely,
secondary forest at the landscape scale showed no such compensation
effect. Similarly, a null-model approach indicated significant biotic
homogenisation in small versus large fragments, but only in landscapes
with little old growth forest, suggesting forest amount and configuration
interactively affect β-diversity in tropical human-dominated landscapes.
Finally, dramatic abundance-based community shifts relative to intact
forests are largely a result of landscape-scale loss of old growth rather
than changes in overall forest cover. 4.Policy implications. Our study
provides strong evidence that retaining old growth within tropical human
modified landscapes can simultaneously curb erosion of avian forest
specialist α-diversity, mitigate collapse of β-diversity (biotic
homogenisation) and dampen detrimental avian community shifts. However,
secondary forests play, at best, a subordinate role to mitigate these
processes. To maintain tropical forest biodiversity, retaining old growth
forest within landscapes should be first priority, highlighting a
land-sparing approach.