Recently revisited, the concept of niche ecology has lead to the formalisation of functional and trophic niches using stable isotope ratios. Isotopic diversity indices (IDI) derived from a set of ...measures assessing the dispersion/distribution of points in the δ-space were recently suggested and increasingly used in the literature. However, three main critics emerge from the use of these IDI: 1) they fail to account for the isotopic sources overlap, 2) some indices are highly sensitive to the number of species and/or the presence of rare species, and 3) the lack of standardization prevents any spatial and temporal comparisons. Using simulations we investigated the ability of six commonly used IDI to discriminate among different trophic food web structures, with a focus on the first two critics. We tested the sensitivity of the IDI to five food web structures along a gradient of sources overlap, varying from two distinct food chains with differentiated sources to two superimposed food chains sharing two sources. For each of the food web structure we varied the number of species (from 10 to 100 species) and the type of species feeding behaviour (i.e. random or selective feeding). Values of IDI were generally larger in food webs with distinct basal sources and tended to decrease as the superimposition of the food chains increased. This was more pronounced when species displayed food preferences in comparison to food webs where species fed randomly on any prey. The number of species composing the food web also had strong effects on the metrics, including those that were supposedly less sensitive to small sample size. In all cases, computing IDI on food webs with low numbers of species always increases the uncertainty of the metrics. A threshold of ~20 species was detected above which several metrics can be safely used.
In the Guaymas Basin, the presence of cold seeps and hydrothermal vents in close proximity, similar sedimentary settings and comparable depths offers a unique opportunity to assess and compare the ...functioning of these deep-sea chemosynthetic ecosystems. The food webs of five seep and four vent assemblages were studied using stable carbon and nitrogen isotope analyses. Although the two ecosystems shared similar potential basal sources, their food webs differed: seeps relied predominantly on methanotrophy and thiotrophy via the Calvin-Benson-Bassham (CBB) cycle and vents on petroleum-derived organic matter and thiotrophy via the CBB and reductive tricarboxylic acid (rTCA) cycles. In contrast to symbiotic species, the heterotrophic fauna exhibited high trophic flexibility among assemblages, suggesting weak trophic links to the metabolic diversity of chemosynthetic primary producers. At both ecosystems, food webs did not appear to be organised through predator-prey links but rather through weak trophic relationships among co-occurring species. Examples of trophic or spatial niche differentiation highlighted the importance of species-sorting processes within chemosynthetic ecosystems. Variability in food web structure, addressed through Bayesian metrics, revealed consistent trends across ecosystems. Food-web complexity significantly decreased with increasing methane concentrations, a common proxy for the intensity of seep and vent fluid fluxes. Although high fluid-fluxes have the potential to enhance primary productivity, they generate environmental constraints that may limit microbial diversity, colonisation of consumers and the structuring role of competitive interactions, leading to an overall reduction of food-web complexity and an increase in trophic redundancy. Heterogeneity provided by foundation species was identified as an additional structuring factor. According to their biological activities, foundation species may have the potential to partly release the competitive pressure within communities of low fluid-flux habitats. Finally, ecosystem functioning in vents and seeps was highly similar despite environmental differences (e.g. physico-chemistry, dominant basal sources) suggesting that ecological niches are not specifically linked to the nature of fluids. This comparison of seep and vent functioning in the Guaymas basin thus provides further supports to the hypothesis of continuity among deep-sea chemosynthetic ecosystems.
Human trampling is one of the main anthropogenic threats to coastal communities, especially in rocky intertidal habitats. The adverse effects of human trampling have recently received increasing ...attention from conservation biologists, especially when concerning species playing key functional roles. These include biogenic reefs providing extremely productive and diverse habitats due to their structural heterogeneity and three-dimensional complexity. The degradation of such habitats could not only adversely affect the whole coastal biota, but it could also have strong socio-economic implications. This study investigated the potential impact of human trampling on biogenic reefs built by the honeycomb worm Sabellaria alveolata in north-western Portugal. Three increasing intensities of human trampling were manipulated to test for their direct effects on S. alveolata bio-constructions and indirect effects on associated benthic infauna. Experimental trampling, even at low intensity, negatively affected reefs by reducing the percentage cover of intact S. alveolata concretions over a period of two months, but it did not alter the structure of whole assemblages compared to the unmanipulated condition. Idiosyncratic responses were shown by the most conspicuous taxa, ranging from no significant effects on S. alveolata and Amphipoda to spatially and temporally variable effects on the Syllidae and Sabellidae polychaetes, the Mytilidae bivalves and the Rissoidae gastropods. When present, however, differences were always in the direction of larger abundances under the highest intensity of trampling than in the less disturbed treatments and the unmanipulated control. This study provides one of the first experimental evidence linking the intensity of human trampling to the physical damage of S. alveolata reefs. Direct implications of present findings as tools to promote a sustainable use of these systems at a local scale and to stimulate protection and management initiatives at other locations are discussed.
•Effects of human trampling on S. alveolata reefs were experimentally assessed.•Even low intensity of trampling reduced the cover of S. alveolata bio-constructions.•Trampling did not affect the abundance of S. alveolata individuals.•The overall structure of associated benthic assemblages was unaffected by trampling.•Conservation implications in terms of controlling human access are discussed.
In temperate coastal regions of Western Europe, the polychaete Sabellaria alveolata (Linné) builds large intertidal reefs of several hectares on soft-bottom substrates. These reefs are protected by ...the European Habitat Directive EEC/92/43 under the status of biogenic structures hosting a high biodiversity and providing ecological functions such as protection against coastal erosion. As an alternative to time-consuming field campaigns, a UAV-based Structure-from-Motion photogrammetric survey was carried out in October 2020 over Noirmoutier Island (France) where the second-largest known European reef is located in a tidal delta. A DJI Phantom 4 Multispectral UAV provided a topographic dataset at very high resolutions of 5 cm/pixel for the Digital Surface Model (DSM) and 2.63 cm/pixel for the multispectral orthomosaic images. The reef footprint was mapped using a combination of two topographic indices: the Topographic Openness Index and the Topographic Position Index. The reef structures covered an area of 8.15 ha, with 89% corresponding to the main reef composed of connected and continuous biogenic structures, 7.6% of large isolated structures (<60 m2), and 4.4% of small isolated reef clumps (<2 m2). To further describe the topographic complexity of the reef, the Geomorphon landform classification was used. The spatial distribution of tabular platforms considered as a healthy stage of the reef in contrast to a degraded stage was mapped with a proxy that consists in comparing the reef volume to a theoretical tabular-shaped reef volume. Epibionts colonizing the reef (macroalgae, mussels, and oysters) were also mapped by combining multispectral indices such as the Normalised Difference Vegetation Index and simple band ratios with topographic indices. A confusion matrix showed that macroalgae and mussels were satisfactorily identified but that oysters could not be detected by an automated procedure due to their spectral complexity. The topographic indices used in this work should now be further exploited to propose a health index for these large intertidal reefs.
Stable isotope ratios are used to reconstruct animal diet in trophic ecology via mixing models. Several assumptions of stable isotope mixing models are critical, i.e., constant trophic discrimination ...factor and isotopic equilibrium between the consumer and its diet. The isotopic turnover rate (λ and its counterpart the half-life) affects the dynamics of isotopic incorporation for an organism and the isotopic equilibrium assumption: λ involves a time lag between the real assimilated diet and the diet estimated by mixing models at the individual scale. Current stable isotope mixing model studies consider neither this time lag nor even the dynamics of isotopic ratios in general. We developed a mechanistic framework using a dynamic mixing model (DMM) to assess the contribution of λ to the dynamics of isotopic incorporation and to estimate the bias induced by neglecting the time lag in diet reconstruction in conventional static mixing models (SMMs). The DMM includes isotope dynamics of sources (denoted δs), λ and frequency of diet-switch (ω). The results showed a significant bias generated by the SMM compared to the DMM (up to 50% of differences). This bias can be strongly reduced in SMMs by averaging the isotopic variations of the food sources over a time window equal to twice the isotopic half-life. However, the bias will persist (∼15%) for intermediate values of the ω/λ ratio. The inferences generated using a case study highlighted that DMM enhanced estimates of consumer's diet, and this could avoid misinterpretation in ecosystem functioning, food-web structure analysis and underlying biological processes.
Sabellaria alveolata is a gregarious polychaete that uses sand particles to build three-dimensional structures known as reefs, fixed atop rocks or built on soft sediments. These structures are known ...to modify the local grain-size distribution and to host a highly diversified macrofauna, altered when the reef undergoes disturbances. The goal of this study was to investigate the different sedimentary and biological changes associated with the presence of a S. alveolata reef over two contrasting seasons (late winter and late summer), and how these changes were linked. Three different sediments were considered: the engineered sediment (the actual reef), the associated sediment (the soft sediment surrounding the reef structures) and a control soft sediment (i.e. no reef structures in close proximity). Univariate and multivariate comparisons of grain-size distribution, soft sediment characteristics (organic matter content, chlorophyll a, pheopigments and carbohydrate concentrations) and macrofauna were conducted between the different sediment types at both seasons and between the two seasons for each sediment type. A distance-based redundancy analyses (dbRDA) was used to investigate the link between the different environmental parameters and the macrofauna assemblages. Finally, we focused on a disturbance continuum of the engineered sediments proxied by an increase in the mud present in these sediments. The effects of a continuous and increasing disturbance on the associated fauna were investigated using pairwise beta diversity indices (Sørensen and Bray-Curtis dissimilarities and their decomposition into turnover and nestedness). Results showed a significant effect of the reef on the local sediment distribution (coarser sediments compared to the control) and on the benthic primary production (higher in the associated sediments). At both seasons, S. alveolata biomass and sediment principal mode were the environmental parameters which best differentiated the engineered, associated and control sediment assemblages. These two parameters are under the ecosystem engineer's influence stressing its importance in structuring benthic macrofauna. Furthermore, in late summer but not in late winter, presence/absence and abundance-based beta diversity were positively correlated to our disturbance proxy (mud content) a tendency driven by a species replacement and a rise in the associated fauna density. Our first set of results highlight the importance of S. alveolata reefs as benthic primary production enhancers via their physical structure and their biological activity. The results obtained using beta diversity indices emphasize the importance of recruitment in structuring the reef's macrofauna and – paradoxically – the ecological value of S. alveolata degraded forms as biodiversity and recruitment promoters.
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•S. alveolata is a reef-building ecosystem engineer that modifies abiotic and biotic environmental conditions.•Benthic primary production is higher in the immediate vicinity of a Sabellaria alveolata reef.•S. alveolataleads to the establishment of two assemblages: one in the reefs and another inthe soft sediment around them.•An increasing disturbance of the reefs leads to a species turnover and a total abundance increase.
Rationale
Stable isotopic analysis is extensively used in trophic ecology. Inorganic carbonates, usually originating from shell fragments, are routinely removed from samples using an acid treatment ...because they affect δ13C values. However, acid treatment can also change δ15N values. For some taxa such as molluscs, the general assumption is that acid pre‐treatment is not necessary since their shell is easily dissected from soft tissues and represents the only source of inorganic carbonates. However, other sources of non‐dietary carbon (e.g., intracellular inorganic carbon) enriched in13C thus get overlooked.
Methods
Soft tissues (foot) of the invasive gastropod Crepidula fornicata of different size classes were analysed for their δ13C and δ15N values with and without acid pre‐treatment using isotope ratio mass spectrometry. In toto microscopic investigations coupled with acid treatment, scanning electron microscopy and energy‐dispersive spectroscopy were used to highlight the presence of inorganic carbonate. A correction model was derived and applied to existing stable isotope data for C. fornicata. We used both seasonal variations in δ13C signatures and mixing model outputs to assess the error in δ13C values.
Results
Acid pre‐treatment had a significant effect on the stable isotope compositions of C. fornicata foot tissue, especially on δ13C values: isotopic differences increased with size, up to 3‰ for large females. No effect was detected for small (below ~20 mm) and motile males. In toto microscopic analysis revealed the presence of small spherules of inorganic carbonate, hence explaining the differences in δ13C values. Mixing model outputs and seasonal variation of δ13C values showed that untreated samples can lead to large misinterpretations about diet proportions and degree of trophic niche overlap, respectively.
Conclusions
Spherules of inorganic carbonate in C. fornicata soft tissues are likely to be linked with the motility of this species and their mucus production. We recommend assessing the presence of inorganic carbonate in soft tissue of sessile gastropods.
Species distributions have been profoundly affected by past climate change, and are expected to change considerably in response to future environmental change. To better apprehend how future climate ...change is likely to affect genetic diversity in marine populations, it is essential to first evaluate the processes that have shaped the current distribution of genetic diversity in the sea. The honeycomb worm is a reef-building polychaete that hosts high biodiversity. Here we show that the genetic diversity in populations of
S. alveolata
is highest towards the edges of the current species range and lowest at its center. Pleistocene glacial cycles likely led to extirpations of
S. alveolata
from central populations in the Bay of Biscay, with coalescent-based estimates of post-glacial colonization dating to the beginning of the Holocene interglacial, from 10,000 to 14,000 years ago. Meanwhile, populations in the Irish Sea and English Channel likely persisted in glacial refugia since the Eemian interglacial, 120,000 years ago. Northern populations host at least two sets of divergent haplotypes, indicating that two refugia possibly existed in the north, with Ireland being a likely second refugium. Within biogeographic regions, populations were overall well-connected, but strong genetic differentiation suggests that little exchange occurs between regions. These two unexpected reservoirs of genetic diversity at the range edges deserve greater attention as warming temperatures threaten trailing edge populations, while greater climatic variability threatens leading edge populations.
Distributional shifts in species ranges provide critical evidence of ecological responses to climate change. Assessments of climate‐driven changes typically focus on broad‐scale range shifts (e.g. ...poleward or upward), with ecological consequences at regional and local scales commonly overlooked. While these changes are informative for species presenting continuous geographic ranges, many species have discontinuous distributions—both natural (e.g. mountain or coastal species) or human‐induced (e.g. species inhabiting fragmented landscapes)—where within‐range changes can be significant. Here, we use an ecosystem engineer species (Sabellaria alveolata) with a naturally fragmented distribution as a case study to assess climate‐driven changes in within‐range occupancy across its entire global distribution. To this end, we applied landscape ecology metrics to outputs from species distribution modelling (SDM) in a novel unified framework. SDM predicted a 27.5% overall increase in the area of potentially suitable habitat under RCP 4.5 by 2050, which taken in isolation would have led to the classification of the species as a climate change winner. SDM further revealed that the latitudinal range is predicted to shrink because of decreased habitat suitability in the equatorward part of the range, not compensated by a poleward expansion. The use of landscape ecology metrics provided additional insights by identifying regions that are predicted to become increasingly fragmented in the future, potentially increasing extirpation risk by jeopardising metapopulation dynamics. This increased range fragmentation could have dramatic consequences for ecosystem structure and functioning. Importantly, the proposed framework—which brings together SDM and landscape metrics—can be widely used to study currently overlooked climate‐driven changes in species internal range structure, without requiring detailed empirical knowledge of the modelled species. This approach represents an important advancement beyond predictive envelope approaches and could reveal itself as paramount for managers whose spatial scale of action usually ranges from local to regional.
Many species have discontinuous ranges, both natural and human‐induced. Projected responses to climate‐driven range changes have so far overlooked changes in species internal range structure, providing only partial information on potential climate change effects. To overcome this limitation, we propose to combine landscape ecology metrics–traditionally used to describe landscape configuration–with predicted spatial distribution model outputs (i.e. presence–absence). We show that this novel unified framework provides additional quantitative information on species ranges that can be used for answering multiple scale‐dependent ecological questions.
Reef-building species play key roles in promoting local species richness and regulating ecosystem functions like biogeochemical fluxes. We evaluated the functioning of a habitat engineered by the ...reef-building polychaete Sabellaria alveolata, by measuring oxygen and nutrient fluxes in the reef structures and in the soft-sediments nearby. Then, we investigated the relative importance of temperature, the engineer S. alveolata, and different facets of macrofauna diversity (taxonomic, functional diversity and identity), on the reef biogeochemical fluxes using multiple linear regressions and effect sizes. The reef fluxes were more intense than the soft-sediment fluxes and mainly driven by the engineer biomass and abundance, stressing the importance of these biogenic structures. Higher water temperatures and an intermediate level of associated macrofauna functional dispersion weighted only by abundance (i.e. intermediate biological trait variability) maximized the reef’s global biogeochemical functioning. Ultimately, the physical degradation of the reefs could lead to lower levels of functioning.
•We studied the functioning of a reef habitat, linking it to macrofauna diversity•We measured oxygen and nutrient fluxes using core incubations•The reef structures had higher fluxes than the neighboring soft sediments•The reef-builder's biomass and temperature were the main drivers of reef fluxes•An intermediate level of macrofauna trait diversity maximized these fluxes
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