海龜食草行為對臺灣小琉球肚仔坪潮間帶海草床之影響

Abstract

海草床是地球上最重要的生態系之一，它們提供許多資源與功能，例如做為其他海洋生物的避難、棲息和覓食場所，以及捕集大氣中二氧化碳與碳封存。許多海洋生物如綠蠵龜（Chelonia mydas）會食用大量的海草與海藻，影響海草生態系統動態甚巨，而高密度的綠蠵龜已被證實會對沿岸海草床造成危害並導致衰退。在臺灣小琉球的肚仔坪潮間帶，曾經有以泰來草（Thalassia hemprichii）為主的廣闊海草床，但是現在近乎消失，而綠蠵龜仍然在漲潮時進入肚仔坪潮間帶，並食用區域中殘留的零星海草。本研究的目的為調查綠蠵龜對肚仔坪潮間帶海草床的影響。研究包含兩個排除實驗，觀察綠蠵龜的食草行為，對海草（葉片長度、葉片豐度與生物量）與藻類（覆蓋度與生物量）的影響。實驗期間為2021年11月至2023年5月，每三個月進行一次調查，比較實驗組（30 × 30 × 15公分的籠子，部分或完全排除草性動物）與對照組（30 × 30公分的開放區塊）間差異，同時也調查草食性動物（綠蠵龜、海膽、魚類）的數量。研究結果顯示，海草葉片長度在兩個實驗組與對照組以及兩個實驗組之間存在顯著的差異，海草葉片豐度與藻類覆蓋度僅在兩個實驗組與對照組之間有差異。然而，除了海草根與莖的生物量外，海草葉、整體與藻類的生物量在各組之間沒有顯著的差異。結果顯示排除草食性動物後，海草葉片長度、葉片豐度與藻類覆蓋度都有顯著的增加，本研究揭露了綠蠵龜的覓食行為是導致當下肚仔坪潮間帶海草床退化的主要原因，而其他草食性動物的影響則僅限於減少海草葉片長度。由於小琉球綠蠵龜密度極高，在有限資源的情況下，保育海龜或維護海草生態系統成為兩難的困境與挑戰，未來還需要更多相關研究以理解綠蠵龜與海草生態系統之間的交互作用，以尋找創新的生態管理解決方案。

Seagrass meadows are among the most valued ecosystems on Earth. They provide many goods and services such as sheltered habitats, feeding grounds, carbon sequestration and storage. Many organisms feed on seagrasses, which are critical to ecosystem dynamics. Some herbivores graze exclusively on seagrass, while others are more flexible in their diet, accepting other food items. Green sea turtles (Chelonia mydas) consume large amounts of seagrass and algae. The high density of green sea turtles observed in some meadows has been shown to contribute to their degradation. In the Duozaiping Intertidal Zone (DIZ) on Liu-Qiu Island, Taiwan, there was once an extensive seagrass meadow dominated by Thalassia hemprichii, but this has now almost disappeared. Sea turtles enter the DIZ at high tide and consume the sporadic foliage remaining in the area. The aim of this study is to investigate the impact of green sea turtles on the condition of seagrass meadows at the DIZ. Two exclusion experiments were conducted in parallel to investigate the effects of green sea turtle grazing on seagrasses (leaf length, leaf abundance, and biomass) and algae (cover and biomass). In each experiment, conditions in the treatment groups (30 × 30 × 15 cm cages, partial or complete exclusion of herbivores) were compared with those in control groups (30 × 30 cm, open plots) from November 2021 to May 2023. The abundance of herbivores (green sea turtles, sea urchins, fish) was also monitored every three months along the course of the experiment. The results showed that there were significant differences in seagrass leaf length between the two treatments and the control, as well as between treatments. Seagrass leaf abundance and algal cover differed between the two treatments and the control, only. In addition, no significant differences were found in either seagrass or algal biomass between treatments, with the exception of the underground seagrass biomass. The results demonstrate a significant increase in seagrass leaf length, leaf abundance, and algal cover following herbivore exclusion. In particular, they evidence that green sea turtle grazing activity is the primary cause of the current disturbed state of the seagrass meadow at the DIZ, while other herbivores exert only marginal effects, limited to seagrass leaf length only. The very high density of green sea turtles found at Liu-Qiu poses a challenge to the maintenance of seagrass when the resource is limited. The dilemma between sea turtles and seagrasses presents a paradox for conservation that requires a better understanding of ecological interactions to find innovative solutions for ecological management.