Natural coral recovery despite negative population growth

Abstract

Demographic processes that ensure the recovery and resilience of marine populations are critical as climate change sends an increasing proportion on a trajectory of decline. Yet for some populations, recovery potential remains high. We conducted annual monitoring over 9 years (2012–2020) to assess the recovery of coral populations belonging to the genus Pocillopora. These populations experienced a catastrophic collapse following a severe typhoon in 2009. From the start of the monitoring period, high initial recruitment led to the establishment of a juvenile population that rapidly transitioned to sexually mature adults, which dominated the population within 6 years after the disturbance. As a result, coral cover increased from 1.1% to 20.2% during this time. To identify key demographic drivers of recovery and population growth rates (λ), we applied kernel-resampled integral projection models (IPMs), constructing eight successive models to examine annual change. IPMs were able to capture reproductive traits as key demographic drivers over the initial 3 years, while individual growth was a continuous key demographic driver throughout the entire monitoring period. IPMs further detected a pulse of reproductive output subsequent to two further Category 5 typhoon events during the monitoring period, exemplifying key mechanisms of resilience for coral populations impacted by disturbance. Despite rapid recovery, (i.e., increased coral cover, individual colony growth, low mortality), IPMs estimated predominantly negative values of λ, indicating a declining population. Indeed, while λ translates to a change in the number of individuals, the recovery of coral populations can also be driven by an increase in the size of surviving colonies. Our results illustrate that accumulating long-term data on historical dynamics and applying IPMs to extract demographic drivers are crucial for future predictions that are based on comprehensive and robust understandings of ecological change.

越來越多物種的族群數因氣候變遷而日益下降，故海洋物種復原力與韌性的族群統計過程至關緊要。話雖如此，某些物種仍舊充滿復原潛力。在2009年，鹿角珊瑚屬珊瑚族群在威力強大的颱風襲擊之下而嚴重崩壞，我們針對此族群進行長達9年的(2012–2020)年度監控，評估復原情況。在監控初期，大量的入添建立了幼年珊瑚族群。這些幼年珊瑚快速進入性成熟期，在干擾事件後的6年內成為主導族群，珊瑚覆蓋率也從1.1% 成長至20.2%。為獲取珊瑚礁復原力和韌性的關鍵族群驅動因素以及族群成長率(λ)，我們應用核心重採樣積分投影(IPMs)，並建立8組連續模型以檢視每一年的變化。IPMs結果指出，繁殖性狀為首3年的關鍵族群驅動因素，而個體成長則是整段監測期間持續性的關鍵族群驅動因素。在研究期間，IPMs更近一步檢測到此族群在2次五級颱風侵襲後分別有短促的繁殖輸出，體現珊瑚族群在干擾事件後的關鍵復原調節機制。即便族群快速恢復(珊覆蓋率增加、個體族群成長、低死亡率)，IPMs估算的λ卻多為負值，象徵族群正在縮減。的確，λ代表個體數量改變，但倖存珊瑚殖民地的擴張也能驅動珊瑚族群的復原。若要做出以完善生態變遷認知為基礎的預測，長期搜集歷史動態資料以及應用IPMs提取族群驅動因素，至關重要。