An interdisciplinary approach to understanding the changing distribution of mangrove crabs affected by erosion in Mayotte

International audience

Inglés. The loss of natural habitats is generally associated with a loss of biodiversity that will cause a change in the functioning of ecosystems and consequently in their capacity to provide ecosystem services. As ecosystem engineers, mangrove crabs fulfil key ecological roles in the structure of the mangrove ecosystem through burrowing and in the functioning through nutrient cycling and bioturbation. They are also of societal and economic interest that can be used by the populations. In the West Indian Ocean, there are gaps in knowledge on the identification and distribution of mangrove crab species. This challenges our understanding of the consequences of mangrove erosion on the associated biodiversity. For example, between 1950 and 2016, the mangroves of Mayotte have been affected by a moderate regression of 6%. This loss is likely to impact the distribution of crabs and consequently the structure and functioning of the ecosystem. The objective of this study is to understand the impact of mangrove erosion on the distribution of crabs through an interdisciplinary approach combining methodologies in ecology, human geography and geomorphology.The combination of these methodologies allows access to data at several temporal and spatial scales. The characterisation of erosion rates since 1950 and consequences on sedimentary movements at large temporal and spatial scales will be provided by geomorphology data. The human geography data provides access to a medium temporal scale on the characterisation of the evolution of the distribution of crabs at the scale of the bay over the last decades. At a finer scale, by replacing time by space, the ecology data will ensure a comparison of two zones representing two different instants of the mangrove erosion trajectory.This study focuses on the mangroves of Dapani and Mzouazia, located on the southern coast of Mayotte Island. We focus on the central and sea mangroves because have been in major decline since 1950. The Mzouazia mangrove covers 3 ha, consisting of Sonneratia alba in seaward fringe, followed by a fringe of Rhizophora mucronata. The Dapani mangrove covers 10 ha, is composed of a Sonneratia alba in the seaward fringe, followed by a large Rhizophora mucronata stand in the central mangrove.The methodology used in geomorphology consisted of a diachronic analysis of vertical aerial photographs between 1950 and 2020 in order to characterise the spatial evolution of the mangroves studied over 70 years. Aerial photographs taken by unmanned aerial vehicles were also acquired in order to characterise the surface changes of mangroves in the short-term between 2020 and 2022. Topographic surveys were carried out between 2020 and 2022, in order to characterise the morphology of the coastal profile and its evolution. We also have a series of topographic profiles distributed irregularly between 2005 and 2019. In human geography, we conducted semi-structured interviews with people in villages bordering the mangroves of Mzouazia and Dapani in order to better understand their local ecological knowledge regarding crabs. We targeted people over 15 years old who have been frequenting or regularly visiting the mangrove for several years or people having activities near the mangrove. We asked people about the vernacular names of the crabs present in the mangrove of their village, a recognition criteria, their location, their habitat and about the evolution of their distribution area, quantity and size.The objective of the ecology protocol was to compare the distribution of crab species across the shore, along an eroding and non-eroding mangrove section. For this purpose, two 100 m long transects were deployed per mangrove, starting from the beach ridge, and oriented perpendicular to the coastline. The Point-Centered Quarter Method was used to place 10 sampling points haphazardly along the transects where we measured the following parameters: abundance of crab species, identity of dominant mangrove trees, tree circumference, and grain size. The height on the foreshore is provided by a digital elevation model. Data have been acquired in Mzouazia and those in Dapani will be acquired in the coming weeks.The analysis of aerial photographs shows that these two mangroves have experienced a regression of -64% for Dapani and -15% for Mzouazia since 1950. On the eroding section, the morphological measurements show an accumulation of sand which reflected a significant rise of 0.55 m in the beach ridge from February 2005 to March 2022. The profiles also show a retreat of almost -10m of sediment between 2013 and 2022, reflecting the decrease in height on the foreshore. The sediment analyses performed along the transects show that muddy sediments prevail in the non-eroded sections whereas sandy sediments dominate eroded sections. This can be explained by the presence of trees, which attenuate the wave energy and favour the deposition of fine sediments, whereas in the eroded area, fine sediments are transported away and coarse sediments accumulate on the beach ridge.Preliminary results of the 19 semi-structured interviews conducted in Mzouazia show that the inhabitants are more familiar with Cardisoma carnifex and Scylla serrata, which live at the land and sea ends of the mangrove. They do not seem, however, to perceive the effects of marine erosion on the distribution of crabs. Yet, crab community distributions show a higher species abundance and richness in the non-eroded mangrove sections than in the eroded mangrove sections. Comparison with the data provided by the 18 semi-structured interviews conducted in Dapani and the ongoing study of crab distributions will allow generalising these results. The relative importance of tree presence, grain size, and elevation on the foreshore for crab community composition will also be discussed and confronted to the species used and known by inhabitants.These data provide new information on the distribution of mangrove crabs and on the factors affecting their distributions. These data may raise questions about potential changes in mangrove ecosystem functioning, and the consequences for the ecosystem services provided by eroding mangroves. As such, they will provide predictive data on the distribution of mangrove crabs that will be of interest to stakeholders and scientists. Analysis of the data will identify if environmental memory is being lost with the erosion of the mangrove.