Remote sensing in seagrass ecology: coupled dynamics between migratory herbivorous birds and intertidal meadows observed by satellite during four decades

• Published in: Remote sensing in ecology and conservation Vol. 9; no. 3; pp. 420 - 433
• Main Authors: Zoffoli, Maria Laura, Gernez, Pierre, Oiry, Simon, Godet, Laurent, Dalloyau, Sébastien, Davies, Bede Ffinian Rowe, Barillé, Laurent, Scales, Kylie, Jones, Alice
• Format: Journal Article
• Language: English
• Published: Oxford John Wiley & Sons, Inc 01.06.2023; Wiley
• Subjects: Aquatic birds; Biodiversity; Biomass; Bird migration; Bird populations; Branta bernicla bernicla; trophic relationships; Coasts; Conservation; Earth observation; Ecology; Ecology, environment; Ecosystem management; Ecosystems; Environmental policy; essential biodiversity variables; Food chains; Geography; Herbivores; Humanities and Social Sciences; In situ measurement; Life Sciences; Migratory birds; Migratory species; Phenology; Population number; Protected species; Remote sensing; Satellite observation; Time series; Trophic relationships; Waterfowl; Wildlife conservation; Zostera noltei; France; Zostera noltei; trophic relationships; Earth observation; essential biodiversity variables; time-series; Branta bernicla bernicla
• ISSN: 2056-3485; 2056-3485
• DOI: 10.1002/rse2.319

Taking into account trophic relationships in seagrass meadows is crucial to explain and predict seagrass temporal trajectories, as well as for implementing and evaluating seagrass conservation policies. However, this type of interaction has been rarely investigated over the long term and at the scale of the whole seagrass habitat. In this work, reciprocal links between an intertidal seagrass species, Zostera noltei, and a herbivorous bird feeding on this seagrass species, the migratory goose Branta bernicla bernicla, were investigated using an original combination of long‐term Earth Observation (EO) and bird census data. Seagrass Essential Biodiversity Variables (EBVs) such as seagrass abundance and phenology were measured from 1985 to 2020 using high‐resolution satellite remote sensing over Bourgneuf Bay (France), and cross‐analysed with in situ measurements of bird population size during the goose wintering season. Our results showed a mutual relationship between seagrass and Brent geese over the four last decades, suggesting that the relationship between the two species extends beyond a simple grass—herbivore consumptive effect. We provided evidence of two types of interactions: (i) a bottom‐up control where the late‐summer seagrass abundance drives the wintering population of herbivorous geese and (ii) an indirect top‐down effect of Brent goose on seagrass habitat, where seagrass development is positively influenced by the bird population during the previous wintering season. Such a mutualistic relationship has strong implications for biodiversity conservation because protecting one species is beneficial to the other one, as demonstrated here by the positive trajectories observed from 1985 to 2020 in both seagrass and bird populations. Importantly, we also demonstrated here that exploring the synergy between EO and in situ bird data can benefit seagrass ecology and ecosystem management. Trophic interactions in seagrass meadows have been rarely investigated over the long‐term and at the scale of a whole seagrass habitat. In this work, reciprocal links between an intertidal seagrass species, Zostera noltei, and an herbivorous bird feeding on this seagrass species, the migratory goose Branta bernicla bernicla, were investigated using an original combination of long‐term Earth Observation (EO) and bird census data. Our results showed positive feedbacks between seagrass and herbivorous birds, and such interaction might be responsible for the increasing trajectories observed from 1985 in both populations in Bourgneuf Bay (France), where this study was developed. Such relationship has strong implications for biodiversity conservation because protecting one species is beneficial to the other one. Importantly, we also demonstrated here that exploring the synergy between EO and in situ bird data can benefit seagrass ecology and ecosystem management. Résumé Les interactions trophiques sont rarement considérées dans les études à long‐terme et macro‐échelle des habitats formés par les angiospermes marines. Pourtant, leur prise en compte permettrait de mieux comprendre les trajectoires temporelles des herbiers marins, d'anticiper leur évolution future, et d'améliorer les mesures de gestion et de protection des écosystèmes côtiers. Dans cette étude, nous avons analysé, en combinant de manière originale des séries temporelles à long‐terme issues de la télédétection spatiale et du suivi de terrain de l'avifaune marine, les relations mutuelles entre un herbier intertidal de zostère, Zostera noltei, et la bernache cravant, Branta bernicla bernicla, une oie migratrice herbivore. Les variables essentielles de biodiversité telles que l'abondance et la phénologie des zostères ont été mesurées par des satellites d'observation de la Terre à haute résolution spatiale de 1985 à 2020. Ces observations ont été analysées conjointement avec les comptages pluriannuels de la population de bernaches hivernant en Baie de Bourgneuf (France). Nous avons mis en évidence une relation entre les zostères et les bernaches lors des quatre dernières décennies, suggérant que les interactions entre les deux espèces ne sont pas limitées à une simple relation d'herbivorie. Notre interprétation met en lumière deux types d'interactions: (i) une relation de type “bottom‐up” par laquelle la population hivernante de bernaches est. régulée par l'abondance estivale de zostère, et (ii) une relation indirecte de type “top‐down” par laquelle le développement annuel de l'herbier est. influencé par la population de bernaches séjournant dans l'herbier lors de l'hiver précédent. Dans un contexte général de conservation de la biodiversité, une telle relation mutualiste est. particulièrement importante car la protection d'une des deux espèces est. bénéfique à l'autre, comme le démontre l'augmentation à long‐terme observée de 1985 à 2020 de manière parallèle de l'abondance de zostères et de bernaches. En outre, notre étude met en lumière l'intérêt d'analyser en synergie les suivis d'avifaune et les observations issues de la télédétection spatiale pour l'étude et la gestion des écosystèmes côtiers.