Predator exclosures increase nest success but reduce adult survival and increase dispersal distance of Piping Plovers, indicating exclosures should be used with caution

Diagnosing unsuccessful population outcomes for endangered species
requires understanding relationships among vital rates, ecological
conditions, and management variables, including unintended consequences of
management actions. The federally-threatened Piping Plover (Charadrius
melodus) has remained below recovery goals, despite intensive management,
including the use of nest exclosures to protect eggs from predation. We
studied ecological factors and management actions affecting nest success,
survival, site fidelity, and dispersal of the New Jersey, USA population
of Piping Plovers over a seven-year period, focused on evaluating the
impact of exclosures on demography. While exclosures increased nest
success by 62% over a 34-day period, exclosed nests were 4.7 times more
likely to be abandoned, which was likely a consequence of adult mortality.
Abandoned nests were associated with lower adult survival, particularly
for males, but there was evidence that site fidelity was greater for birds
whose last nest was exclosed vs. unexclosed. Regardless of exclosure
status, females who abandoned their first nesting attempt dispersed 10
times farther between attempts than those whose first nest attempts were
lost to other causes. Moreover, females that abandoned their last nesting
attempts dispersed farther than females that lost their last nest to
predation or flooding. This difference was more substantial for males. Our
results corroborate studies documenting adverse impacts of exclosures on
survival, and, for the first time, demonstrate that surviving mates
(particularly females) emigrate from the breeding site, resulting in a
realized loss of a local breeding pair. Further, we used an online
population project model (i.e., PiperEx) to demonstrate that exclosures
are not expected to improve growth rates in New Jersey. We conclude that
there is a tradeoff among increased nest survival, reduced adult survival
and increased emigration rates, and we encourage managers to consider
whether exclosures are worth the protection of eggs from predators using
online decision support tools.

针对濒危物种种群存续失败的诊断研究，需明晰生命率（vital rates）、生态条件与管理变量间的关联，其中亦涵盖管理措施所带来的非预期后果。尽管已采取包括使用巢防护围栏（nest exclosures）防范卵被捕食在内的高强度管理措施，但被美国联邦列为受威胁物种的笛鸻（Charadrius melodus）种群始终未达恢复目标。本研究针对美国新泽西州的笛鸻种群开展了为期7年的调查，探究影响其巢成功率、个体存活、繁殖位点保真度与扩散行为的生态因子及管理措施，核心目标为评估防护围栏对种群统计学特征的影响。尽管防护围栏可使巢成功率在34天周期内提升62%，但被围栏保护的巢被亲鸟弃巢的概率是未围栏巢的4.7倍，这一现象大概率与成鸟死亡相关。弃巢事件与成鸟存活率降低存在显著关联，其中雄性成鸟的存活率下降尤为明显；但研究数据同时显示，末次繁殖巢采用围栏保护的个体，其繁殖位点保真度高于末次巢未采用围栏保护的个体。无论是否采用围栏保护，首次繁殖尝试遭弃的雌性个体，其两次繁殖尝试间的扩散距离是首次繁殖因其他因素失败的雌性个体的10倍。此外，末次繁殖尝试遭弃的雌性个体，其扩散距离亦长于末次巢因捕食或洪水而失败的雌性个体；这一差异在雄性个体中表现得更为突出。本研究结果佐证了现有关于防护围栏对个体存活存在负面影响的研究结论，并首次证实：幸存的配偶（尤其是雌性）会从繁殖位点迁出，进而造成当地繁殖配对的实际损失。进一步而言，本研究借助在线种群预测模型（PiperEx）证实，防护围栏无法提升新泽西州笛鸻种群的增长率。综上，本研究认为巢存活率提升、成鸟存活率降低与迁出率升高三者间存在权衡关系；同时我们呼吁管理方借助在线决策支持工具，评估采用防护围栏防范卵被捕食是否具备实际应用价值。