Dispersal patterns of the endangered Crested Ibis suggest high breeding densities drive natal dispersal

ABSTRACT Although dispersal is a key driver of population dynamics and species distributions, we still know little about how it affects the dynamics of endangered and restricted-range species. Density-dependent effects on dispersal in particular may be critical for the range expansion of recovering populations. After 1981, when the last 7 wild individuals of Crested Ibis (Nipponia nippon) were discovered in China, the species remained confined to a single location (“original nesting area”) until 2000 (<24 breeding pairs). Then, the breeding population began a significant spatial and numerical expansion toward new breeding sites (“secondary nesting areas”). Our analyses of long-term (1993–2017) individual resighting data (n = 193) found that natal dispersal was common (∼77.2%), whereas breeding dispersal was rare (∼2.1%). Breeding density was negatively related to productivity, and natal dispersal was more likely toward low-density areas. The Weibull distribution provided the best fit to the observed dispersal distances, indicating the presence of long-distance natal dispersers. Although we cannot rule out the effect of dispersal mechanisms inherent in the species regardless of density, our findings suggest that the probability of natal dispersal is density-dependent, implying a role for intraspecific competition in the Crested Ibis spreading in the region. We expect the Crested Ibis to spread throughout the region if conservation programs can identify and protect potential reintroduction areas at a large spatial scale to allow dispersing Crested Ibis to settle into new and low-density breeding areas. LAY SUMMARY Dispersal is a key driver of population dynamics and species distribution. Studying dispersal is challenging because it requires long-term individual-based data on a large spatial scale. We used long-term data (1993–2017) from a population of the endangered Crested Ibis (Nipponia nippon) in Central China to investigate the role of density in productivity and dispersal. Breeding density was negatively related to productivity, and natal dispersal was more likely toward low-density areas. Some individuals dispersed over long distances, and those born farther away from the original nesting area were more likely to engage in natal dispersal. Our findings suggest that intraspecific competition may play an important role in the population dynamics and expanding species range. Future conservation programs should identify and protect potential reintroduction areas at a large spatial scale to allow the settlement of dispersing Crested Ibis into new and low-density breeding areas. 摘要： 扩散是影响种群动态和空间分布的关键驱动因素之一，然而我们对狭域分布濒危物种的扩散机制了解甚少。密度对 扩散的制约效应，可能是限制濒危物种种群分布范围扩大的重要因素。自1981年7只朱鹮(Nipponia nippon)被重新发 现以来，其野生种群一直分布在单一的繁殖巢区（即原始巢区，original nesting area）。然而在2000年以后，该种群繁 殖巢的空间分布范围和总体数量都出现明显的扩张现象，逐渐扩散到新的繁殖巢区（即二级巢区，secondary nesting areas）。本研究分析了1993-2017年长期的环志再现数据（n = 193），结果表明，出生扩散在朱鹮野生种群中较为普遍 （约占77.2%），繁殖扩散发生频次较低（约占2.1%）；繁殖密度与出飞雏鸟数量负相关，新生个体倾向于扩散至低密 度巢区；朱鹮出生扩散距离符合威布尔分布（Weibull distribution），推断部分朱鹮个体会进行远距离出生扩散。尽管 本研究不能完全排除繁殖密度以外的其它扩散机制，但研究结果表明，朱鹮出生扩散一定程度上受到繁殖密度的制 约，种内竞争对于种群分布区的快速扩张起到了关键作用。因此，在制定保护方案时，我们应当在更大的空间范围进 行潜在再引入地点的筛选和保护管理，以保障繁殖个体可以扩散至周边低密度巢区。