Long-term impact of protected colonial birds on a jeopardized cork oak population: conservation bias leads to restoration failure

1. Conservation bias towards flagship species sometimes threatens other species of chief concern. Long-term studies of potential harm by favoured species on other sensitive species, though seldom adopted, are required to fairly evaluate the suitability of management and restoration efforts. 2. We illustrate the potential detrimental outcomes of conservation biased towards birds by investigating the long-term (1963-2009) impact of a large waterbird colony (up to 13 000 pairs) on a remnant cork oak Quercus suber population at a World Biosphere Reserve in south-western Spain (the Donana National Park). To this end, we compared changes in performance (growth, crown vigour and survival) of oaks occupied and unoccupied by the waterbird colony. 3. After 46 years of occupation, the risk of death to centenarian oaks in the area occupied by the colony was over twofold higher than for trees outside the area. Non-centenarian planted and naturally regenerated oaks showed similar trends, leading to restoration failure. 4. Synthesis and applications. Our long-term study reveals that waterbirds and centenarian oaks cannot coexist, at the most local scale, but they can at a regional scale including within the Donana area. We propose immediate planting efforts in suitable colony-free areas, while managers evaluate the feasibility of relocating colonial waterbirds to an alternative location. To preserve the Donana oak genetic pool, such reforestation should be accomplished using local seeds and seedlings. New trees should not be planted in close proximity of colony-occupied trees since it significantly reduces their survival. Donana stakeholders should both overcome current conservation bias in favour of birds and enter into a process of settlement to best preserve the overall biodiversity of the system.