Wandering Albatross observed and simulated GPS tracks from Crozet and Marion islands (2016-2019)

Sexual competition is increasingly recognized as an important selective pressure driving species distributions. However, few studies have investigated the relative importance of inter- vs. intrapopulation competition in relation to habitat availability and selection.

To explain spatial segregation between sexes that often occurs in non-territorial and central place foragers, such as seabirds, two hypotheses are commonly used. The ‘competitive exclusion’ hypothesis states that dominant individuals should exclude subordinate individuals through direct competition whereas the ’niche divergence’ hypothesis states that segregation occurs due to past competition and habitat specialization.

We tested these hypotheses in two populations of an extreme wide-ranging and sexually dimorphic seabird, investigating the relative role of intrapopulation and interpopulation competition in influencing sex-specific distribution and habitat preferences.

Using GPS loggers, we tracked 192 wandering albatrosses Diomedea exulans during four consecutive years (2016-2019), from two neighbouring populations in the Southern Ocean (Prince Edward and Crozet archipelagos). We simulated pseudo-tracks to create a null spatial distribution and used Kernel Density Estimates (KDE) and Resource Selection Functions (RSF) to distinguish the relative importance of within vs. between population competition.

KDE showed that only intrapopulation sexual segregation was significant for each monitoring year, and that tracks between the two colonies resulted in greater overlap than expected from the null distribution, especially for the females. RSF confirmed these results and highlighted key at-sea foraging areas, even if the estimated of at-sea densities were extremely low. These differences in selected areas between sites and sexes were, however, associated with high interannual variability in habitat preferences, with no clear specific preferences per site and sex.

Our results suggest that even with low at-sea population densities, historic intrapopulation competition in wide-ranging seabirds may have led to sexual dimorphism and niche specialization, favouring the ‘niche divergence’ hypothesis. In this study, we provide a protocol to study competition within as well as between populations of central place foragers. This is relevant for understanding their distribution patterns and population regulation, which could potentially improve management of threatened populations.

Methods
Wandering albatrosses from Possession Island, Crozet Archipelago (46°24’S, 51°46’E, henceforth abbreviated as ‘Crozet’) and Marion Island, Prince Edward Archipelago (46°54’S, 37°48’E, henceforth abbreviated as ‘Marion’), were individually sexed from field observations or from genetic analyses. GPS loggers (CatLog-S, Catnip Technologies, Hong Kong, and Igot-U GPS, Mobile Action Technology, at Marion, and Igot-U and X-GPS at Crozet, Weimerskirch et al. 2018) were deployed on incubating birds and attached to the back feathers using Tesa© tape and left on birds for one (180 individuals) or two (12 individuals) at-sea trips. Tracking was conducted synchronously at Marion and Crozet. A total of 192 birds (Crozet: 66 males and 55 females totaling 121 birds; Marion: 44 males and 27 females totaling 71 birds, Table 1) were tracked during four subsequent years (2016-2019).

性竞争日益被视为驱动物种分布格局形成的重要选择压力。然而，鲜有研究探讨种群间竞争与种群内竞争相对于栖息地可获得性与栖息地选择的相对重要性。

为解释非领地性中心地觅食者（如海鸟）中常见的性别空间分离现象，学界常用两种假说。“竞争排除假说”认为优势个体可通过直接竞争排挤从属个体；而“生态位分化假说”则提出，性别分离源于过往竞争与栖息地特化。

本研究以一种分布范围极广且存在两性异形的海鸟为研究对象，在两个种群中检验了上述两种假说，探讨了种群内与种群间竞争在影响性别特异性分布与栖息地偏好方面的相对作用。

本研究依托GPS定位记录仪（GPS logger），于2016至2019年连续四年对南大洋两个相邻种群（爱德华王子群岛与克罗泽群岛）的192只漂泊信天翁（*Diomedea exulans*）进行了追踪。研究通过模拟伪轨迹构建零空间分布模型，并结合核密度估计（Kernel Density Estimates, KDE）与资源选择函数（Resource Selection Functions, RSF），以区分种群内与种群间竞争的相对重要性。

核密度分析结果显示，仅种群内的性别分离在各监测年均具有显著性；且两个繁殖群的觅食轨迹重叠度高于零分布模型的预期值，雌性群体的这一特征尤为明显。尽管海上种群密度的估算值极低，但资源选择函数的分析结果验证了上述结论，并明确了关键的海上觅食区域。不过，不同繁殖地与不同性别的觅食选择区域差异，与栖息地偏好的年际高变异性相关，并未呈现出特定繁殖地与性别对应的明确偏好。

本研究结果表明，即便海上种群密度较低，广布海鸟的历史种群内竞争或已推动了两性异形与生态位特化，这一发现支持“生态位分化假说”。本研究为探究中心地觅食者的种群内与种群间竞争提供了一套标准化方案。该方案有助于理解此类物种的分布格局与种群调控机制，有望为受威胁种群的保护管理提供支撑。

研究方法
研究对象为克罗泽群岛波塞申岛（南纬46°24′，东经51°46′，以下简称“克罗泽种群”）与爱德华王子群岛马里恩岛（南纬46°54′，东经37°48′，以下简称“马里恩种群”）的漂泊信天翁，通过野外观察或遗传分析对个体进行性别鉴定。研究为孵卵期个体部署了GPS定位记录仪：马里恩种群使用的设备为香港Catnip Technologies公司的CatLog-S以及Mobile Action Technology公司的Igot-U GPS；克罗泽种群使用的设备为Igot-U与X-GPS（Weimerskirch等，2018）。记录仪通过Tesa©胶带固定于背部羽毛，佩戴时长对应1次（180只个体）或2次（12只个体）海上觅食航程。马里恩与克罗泽种群的追踪工作同步开展。2016至2019年连续四年间，共计追踪了192只个体：克罗泽种群共121只（雄性66只，雌性55只）；马里恩种群共71只（雄性44只，雌性27只），详见表1。