Gulls contribute to olive seed dispersal within and among islands in a Mediterranean coastal area

Aim: To analyse the role of non-frugivorous birds on seed dispersal, seed dispersal by gulls is expected to be especially instrumental in island ecosystems, as these have a smaller subset of frugivores when compared to the mainland, and because long-distance dispersal is required for plant colonization. Here we investigated the seed dispersal of olives by gulls among ten islands of the same archipelago to reveal if gulls contribute to long-distance seed dispersal including different islands, and how gulls' adaptation to domestic olives and individual differences in foraging activities affect their seed dispersal pattern. Location: Balearic Islands in the Western Mediterranean Sea, Spain Taxon: Yellow-legged gulls ( Larus michahellis), Domestic and wild Olives ( Olea europaea and O. europaea var. sylvestris) Methods: We developed seed dispersal models of the two ecotypes of olives dispersed by gulls across an archipelago, based on GPS tracking data, gut passage time, and seed viability. Results: Mean dispersal distances were 7.67 (±12.48) km in wild and 12.57 (±13.08) km in domestic olives. Seven-point one percent of wild and 8.5% of domestic olives were dispersed among islands. Among these, 8.2% of domestic seeds were transported from large to small islands where gull colonies are located, whereas wild olives were dispersed in more variable directions. Such dispersal pattern of two olive ecotypes were consistent despite the differences in dispersal distances among individuals. Main conclusions: Gulls contributed to long-distance olive seed dispersal including different islands. The seed dispersal of domestic olives to longer distances with specific directions may facilitate colonization and expansion of that variant if the conditions of seed deposition sites are suitable. Our findings indicate that gulls are relevant vectors for long-distance dispersal of large fleshy fruits in island ecosystems where specialist large frugivores are absent.

研究目的：本研究旨在分析非食果鸟类（non-frugivorous birds）在种子扩散（seed dispersal）中的作用。相较于大陆生态系统，岛屿生态系统的食果动物（frugivores）类群更为匮乏，且植物定殖（plant colonization）依赖长距离扩散（long-distance dispersal），因此鸥类介导的种子扩散被认为在岛屿生态系统中尤为关键。本研究针对同一群岛（archipelago）内的10座岛屿，调查了鸥类对橄榄种子的扩散情况，以明确鸥类是否可介导不同岛屿间的长距离种子扩散，以及鸥类对栽培橄榄的适应性、个体觅食活动差异如何影响其种子扩散模式。
研究地点：西班牙西地中海巴利阿里群岛（Balearic Islands）
研究类群：黄腿鸥（Yellow-legged gulls, Larus michahellis）、栽培与野生橄榄（Domestic and wild Olives, Olea europaea 与 O. europaea var. sylvestris）
研究方法：本研究基于GPS追踪数据、肠道滞留时间与种子活力，构建了群岛范围内鸥类传播的两种橄榄生态型的种子扩散模型。
研究结果：野生橄榄的平均扩散距离为7.67（±12.48）km，栽培橄榄的平均扩散距离为12.57（±13.08）km。7.1%的野生橄榄与8.5%的栽培橄榄可在群岛内不同岛屿间完成扩散。其中，8.2%的栽培种子被从大岛搬运至鸥类繁殖群落所在的小岛，而野生橄榄的扩散方向更为多变。尽管个体间的扩散距离存在差异，但两种橄榄生态型的扩散模式均保持一致。
主要结论：鸥类可介导橄榄种子在不同岛屿间进行长距离扩散。若种子沉积位点的环境条件适宜，栽培橄榄通过长距离定向扩散可促进该生态型的定殖与扩张。本研究结果表明，在缺乏特化大型食果动物的岛屿生态系统中，鸥类是大型肉质果实长距离扩散的重要媒介。