Protection Reduces Loss of Natural Land-Cover at Sites of Conservation Importance across Africa

There is an emerging consensus that protected areas are key in reducing adverse land-cover change, but their efficacy remains difficult to quantify. Many previous assessments of protected area effectiveness have compared changes between sets of protected and unprotected sites that differ systematically in other potentially confounding respects (e.g. altitude, accessibility), have considered only forest loss or changes at single sites, or have analysed changes derived from land-cover data of low spatial resolution. We assessed the effectiveness of protection in reducing land-cover change in Important Bird Areas (IBAs) across Africa using a dedicated visual interpretation of higher resolution satellite imagery. We compared rates of change in natural land-cover over a c. 20-year period from around 1990 at a large number of points across 45 protected IBAs to those from 48 unprotected IBAs. A matching algorithm was used to select sample points to control for potentially confounding differences between protected and unprotected IBAs. The rate of loss of natural land-cover at sample points within protected IBAs was just 42% of that at matched points in unprotected IBAs. Conversion was especially marked in forests, but protection reduced rates of forest loss by a similar relative amount. Rates of conversion increased from the centre to the edges of both protected and unprotected IBAs, but rates of loss in 20-km buffer zones surrounding protected IBAs and unprotected IBAs were similar, with no evidence of displacement of conversion from within protected areas to their immediate surrounds (leakage).学界已逐步形成共识：保护区是减缓不利土地覆被变化的关键手段，但其保护成效仍难以量化。此前针对保护区保护成效的诸多评估研究，要么在海拔、可达性等存在系统性混杂因素的保护区与非保护样点间对比变化，要么仅关注单一样点的森林损失或变化，亦或是采用空间分辨率较低的土地覆被数据开展变化分析。本研究针对非洲全境的重点鸟区（Important Bird Areas, IBAs），通过高分辨率卫星影像的针对性目视解译，评估了保护区在减缓土地覆被变化方面的成效。我们对比了1990年前后起约20年间，45个受保护重点鸟区与48个未受保护重点鸟区的大量样点的自然土地覆被变化速率。本研究采用匹配算法筛选样点，以控制受保护与未受保护重点鸟区之间可能存在的混杂差异。研究结果显示，受保护重点鸟区内部样点的自然土地覆被损失率仅为未受保护重点鸟区匹配样点的42%。森林区域的土地覆被转换尤为显著，但保护区使森林损失率以相近的相对幅度降低。受保护与未受保护重点鸟区的土地覆被转换速率均从核心区域向边缘递增，但两者周边20公里缓冲区内的损失速率相近，未发现有证据表明土地转换从保护区内部转移至周边紧邻区域（漏损，leakage）。