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.

1. 热带保护策略传统上以大片未受干扰的原生林为核心，但鉴于原生林正快速缩减，明确次生林剩余斑块在生物多样性维持中的作用至关重要。迄今为止，森林面积、景观格局以及干扰历史（次生林vs.原生林）的变化对热带景观保护价值的交互效应仍未明晰，这阻碍了将这些全球变化驱动因子纳入地方及全球保护规划之中。
2. 本研究厘清了哥斯达黎加南部人类主导景观中，景观全域森林面积、斑块面积以及林龄（成熟原始林（old growth forest）vs.次生林（secondary forest））对鸟类群落多度、α多样性（α-diversity）、β多样性（β-diversity，即生物均质化（biotic homogenisation））以及群落偏移的影响。本研究采用两种互补的调查方法，累计获取6900次个体检测记录与223个鸟类物种，对49个森林斑块的鸟类群落进行了表征；这些斑块在斑块面积（<5公顷与>30公顷）以及周边1000米范围内的景观森林占比（5%~80%）上形成独立梯度变化。
3. 森林专性物种与食虫鸟类的多度及α多样性在小斑块中下降了一半，但这一现象仅出现于成熟原始林占比极低的景观中。与之相反，景观尺度上的次生林并未表现出此类补偿效应。同样，零模型分析结果显示，仅在成熟原始林占比极低的景观中，小斑块与大斑块间存在显著的生物均质化现象，这表明森林面积与景观格局会对热带人类主导景观的β多样性产生交互影响。最后，相较于完好原生林，基于多度的群落结构出现剧烈偏移，这一现象主要源于景观尺度上成熟原始林的丧失，而非整体森林覆盖度的变化。
4. 政策启示：本研究提供了强有力的证据，表明在热带人类改造景观中保留成熟原始林，可同时遏制鸟类森林专性物种α多样性的流失、缓解β多样性的衰退（即生物均质化），并抑制有害的鸟类群落偏移。然而，次生林在缓解这些过程中至多仅能发挥次要作用。为维持热带森林生物多样性，景观内保留成熟原始林应作为首要任务，这凸显了土地节约（land-sparing）保护策略的重要性。