Drought reduces the value of both artificial and natural wetlands for gulls breeding in the Mediterranean region

Increasing water extraction and climate change combine to increase the
impact of droughts in Mediterranean wetlands. Natural wetlands are being
increasingly transformed into artificial wetlands, notably ricefields and
aquaculture ponds. Since waterbirds are highly dependent on aquatic
resources throughout their life cycles, changes in wetland habitats can
significantly affect their use and movements. Studies relying on count
data can highlight the importance of artificial wetlands for waterbirds,
but analysis of movements using GPS tracking data facilitates a full
assessment of their value compared to natural wetlands. We used
GPS-tracking to study the habitat use of nesting black-headed gulls,
Chroicocephalus ridibundus, in the iconic Doñana wetlands in Spain. In
this area, the flooding of remaining natural wetlands has been reduced by
overextraction of groundwater and by prolonged drought, whereas adjacent
fish ponds have maintained stable water levels. We tagged 65 adults, and
studied habitat selection of nesting birds (n = 30 bird-years), combining
flooding information from Landsat images with land-use maps. Size of
colonies, fledging success and proportion of tagged birds that nested all
increased in years of higher rainfall. In the wetter 2024, when flooded
natural marshes were available, gull home ranges shifted to include those
areas. When away from the nest, fish ponds were the preferred habitat in
2022 and 2023, and natural marshes in 2024. Although ricefields are used
by gulls in winter they are almost unused during the nesting period. In
the driest year 2022, home range sizes were smallest. But due to
successive drought years, availability of freshwater habitats was even
lower in 2023, and gulls responded to rainfall events by flying up to 54
km to exploit other, distant, temporarily flooded, agricultural habitats.
The importance of artificial wetlands for waterbird conservation can be
easily overestimated from count data alone. The value of extensive fish
ponds as a breeding habitat depends on maintaining the availability of
natural marshes as a foraging habitat. This requires reducing impacts from
groundwater extraction. Given recent abandonment of aquaculture, some fish
ponds should be restored into natural marshes.

不断加剧的水资源抽取与气候变化共同加剧了地中海湿地的干旱影响。自然湿地正日益被改造为人工湿地，尤以稻田和水产养殖塘为甚。由于水鸟在整个生命周期中高度依赖水生资源，湿地生境的变化会显著影响其栖息利用与活动迁徙。基于计数数据开展的研究可凸显人工湿地对水鸟的重要性，但借助GPS追踪数据开展活动分析，则能全面评估其相较于自然湿地的价值。我们依托GPS追踪技术，研究了西班牙多尼亚纳（Doñana）标志性湿地中繁殖期的黑头鸥（Chroicocephalus ridibundus）的生境利用情况。该区域内，剩余自然湿地的淹水状况因地下水过度抽取与长期干旱大幅缩减，而邻近的鱼塘却维持了稳定的水位。我们共标记了65只成鸟，并结合Landsat卫星影像的淹水信息与土地利用地图，对繁殖鸟类的生境选择展开研究（共纳入30个繁殖年度样本）。繁殖群规模、雏鸟出飞成功率以及标记个体的繁殖比例，均在降雨量更高的年份有所提升。在降水更为充沛的2024年，当自然沼泽得以淹水时，鸥类的家域范围扩展至该区域。离巢活动期间，鸥类在2022年与2023年优先选择鱼塘作为生境，而在2024年则优先利用自然沼泽。尽管稻田在冬季会被黑头鸥利用，但在繁殖期几乎未被使用。在最为干旱的2022年，鸥类的家域面积最小。但由于连续多年干旱，2023年淡水生境的可获得性进一步降低，鸥类会借助降雨事件飞行最远达54公里，以利用其他偏远的临时淹水农业生境。仅依靠计数数据极易高估人工湿地对水鸟保护的重要性。规模化鱼塘作为繁殖生境的价值，依赖于自然沼泽作为觅食生境的可获得性，这要求降低地下水抽取带来的负面影响。鉴于当前水产养殖的废弃现状，部分鱼塘应被修复为自然沼泽。