Expansion of invasive seagrass inhibited by nutrient and grazing stress; a case study in Aruba
Seagrass ecosystems are rapidly declining around the world as human pressures are increasing. To build the resilience of seagrass ecosystems, a holistic understanding is needed of the bottom-up and top-down influences on a seagrass community. This is ever more relevant in the Caribbean, where the recent invasion by Halophila stipulacea is believed to be enhanced by the preferential grazing of green sea turtles on the native Thalassia testudinum. In this study, we tested the response of the native and invasive seagrass in Aruba to grazing and nutrient pressures. Grazed plots were mimicked by clipping treatments, while enriched plots were mimicked with fertilizer treatments. Then the two responses, recovery and competition, were tested in each plot by applying gaps in the seagrass cover and transplanting either native or non-native seagrass. The experiments showed that in stark contrast to the native T. testudinum, the invasive H. stipulacea was easily able to recover and occupy new territory. It expanded into native habitats with an average distance of 16.6 cm after 76 days. Nonetheless, the combined stress of intensive grazing and nutrients significantly reduced the expansion of the invasive in its damaged habitat. This was contrary to the notion that the invasive is facilitated by eutrophication and grazing. On the other hand this experiment was not able to (dis)prove the competitive facilitation of the invasive seagrass under nutrient and grazing stress. The finding of this study suggests that the invasion by H. stipulacea will not easily be halted. Despite this threat, it is suggested that resource managers should put great efforts in preventing physical disturbances and mitigating eutrophication in Caribbean seagrass systems.
seagrass invasive grazing seaturtle eutrophication resilience recovery competition Caribbean