Abstract
Identifying the ecological niche of species is necessary to improve our understanding of patterns of biodiversity. In particular, identifying the degree of niche overlap between congeneric co-occurring species helps identifying both the degree of functional redundancy and the level of taxonomic resolution meaningful for evaluating the contribution of these species to ecosystem functioning. Mangrove ecosystems provide a heterogeneous range of habitats, ranging from sediment endofauna to tree epiphytes. Evaluating habitat partitioning or overlap between mangrove dwellers thus contributes to better characterising the degree of habitat heterogeneity provided. For congeneric species that are morphologically similar, such as species of the tree dwelling gastropod genus Littoraria, identifying individual and relative species’ distributions is the first step towards evaluating whether these species have similar ecological preferences or are driven by competitive exclusion.The objective of the present study was thus to evaluate whether the different Littoraria species found in mangroves shared identical habitat niches. We considered the habitat niche at two nested spatial scales: first, the mangrove fringe (seaward or landward), characterised by different locations over the tidal gradient as well as different dominating mangrove trees; Second, within each mangrove fringe, the tree substrate (leaves or bark) over which the Littoraria can be found at low tide, which can relate to a combination of different thermal stress tolerance, predator refuge and trophic needs. For this purpose, we tested the following two hypotheses: There are no differences in relative abundances between mangrove fringes or between tree substrate; there are no differences in individual species abundances between mangrove fringes or tree substrate. The first hypothesis allowed to evaluate whether a given habitat is preferred by the four Littoraria species found coexisting in these mangroves. The second hypothesis aimed at identifying specifically the habitat niche of each species.Sampling took place in three fringing mangroves of Mayotte Island, which display similar zonation patterns due to dominant mangrove trees. The present study focused on the seaward fringe, dominated by Sonneratia alba trees, and the landward fringe, dominated by Avicennia marina trees. Within each fringe of each site, 10 trees were randomly selected along a 50 m transect parallel to the coast. Littoraria spp. were sampled by defining a standardized sampling effort of 32 min per tree, shared between two observers each focusing on either substrate, and up to 2 m high. The relative abundances between species were compared across either fringes (landward or seaward) or substrates (leaves or bark) using a chi-squared test for each spatial scale. The individual abundances of each species were compared across fringes or substrates using a nested ANOVA.Of the 6324 individuals sampled, significantly more Littoraria spp. were found on the seaward fringe irrespective of substrate and significantly more Littoraria were found on leaves irrespective of fringe. Only 5% of individuals (small or damaged) could not be identified to either of the four species found in Western Indian ocean mangroves, namely Littoraria intermedia, L. pallescens, L. scabra, and L. subvittata. Of these, L. subvittata was significantly most abundant than all other species overall, being a generalist species using equally either fringes or either substrates within each fringe. L. scabra and L. pallescence ranked second in overall abundance, being both specialist species: L. scabra being significantly more abundant on the bark of the seaward fringe while L. pallescence being significantly more abundant on the leaves of either fringe. Finally, while L. intermedia seems also a specialist of bark from the seaward fringe, it was a rare species, representing only 3% of sampled individuals. As a result, three of the four identified species display niche partitioning within the three studied mangroves. Leaves and bark turned out to be different tree substrate microhabitats, significant for niche partitioning of three of the four species. Future research understanding the physiological (thermal tolerance) or trophic (food sources, predation refuges) mechanisms involved is now needed to refine our understanding of the distribution of these species within mangroves. Finally, this study highlights the importance of nested designs for identifying spatial resolution of species distribution, which should be considered when evaluating the degree of habitat provisioning across mangroves.