Understanding the patterns of connectivity is required by the Strategic Plan for Biodiversity 2011-2020 and will be used to guide the extension of marine protection measures. Despite the increasing accuracy of ocean circulation modeling, the capacity to forecast the population connectivity of sessile benthic species with dispersal larval stages can be limited due to the potential effect of demographic filters acting before or after dispersal, which modulate offspring release or settlement, respectively. We applied an interdisciplinary approach that combined demographic surveys, genetic methods (assignment tests and coalescent-based analyses) and larval transport simulations to test the relative importance of demographics and ocean currents in shaping the contemporary patterns of gene flow among populations of a Mediterranean gorgonian (Eunicella singularis) in a fragmented rocky habitat (Gulf of Lion, NW Mediterranean Sea). We show that larval transport is a dominant driver of gene flow among the populations, and significant correlations were found between the contemporary gene flow and recent larval transport when the pelagic larval durations (PLDs) ranged from 7 to 14 days. Our results suggest that PLDs that efficiently connect populations distributed over a fragmented habitat are filtered by the habitat layout within the species competency period. Moreover, a PLD ranging from 7 to 14 days is sufficient to connect the fragmented rocky substrate of the Gulf of Lion. The rocky areas located in the center of the Gulf of Lion, which are currently not protected, were identified as essential hubs for the distribution of migrants in the region. We encourage the use of a range of PLDs instead of a single value when estimating larval transport with biophysical models to identify potential connectivity patterns among a network of marine protected areas or even solely a seascape. https://www.cbd.int/sp/targets/rationale/target-11/). Hence, understanding the degree and patterns of connectivity among existing MPAs and areas where protection could be extended becomes urgent. Connectivity is also known as the mechanism through which populations of sessile marine species, and particularly coral species, can recover after local and global disturbances (Roberts 1997;Hanski 1998;Cowen et al. 2000). In the context of global change, relating contemporary connectivity to the seascape processes that shape the spatial distribution of marine species is essential for the design of effective biodiversity conservation strategies. Seascape processes that drive population connectivity of sessile species with a larval dispersal stage, such as corals or gorgonians, should include both seabed geomorphology and dynamic hydrography (Manderson 2016). Correlations at the local scale between demographic population descriptors and the environment may, indeed, not be sufficient, and the successful exchange of individuals between distant locations (i.e., connectivity) should be included. Recent studies on seascape connectivity have focused on evaluati...
Climate‐induced threats are increasingly affecting marine populations worldwide. In the last few decades, several gorgonian species have been affected by mass mortality events in the north‐west Mediterranean, putatively linked to local sea temperature increases during heatwaves. For many benthic sessile species, recovery after disturbances depends upon larval supply shaping the connections among populations. In the Ligurian Sea, genetic analyses showed that some Paramuricea clavata populations recovered after mass mortality events; however, the patterns of connectivity, and the potential role of migration in supporting the recovery of the populations of P. clavata, across its distribution range within the Ligurian Sea are still unknown. In this study, the population genetic structure and migration patterns of P. clavata populations have been analysed across seven sites in the Ligurian Sea, some of which have been affected by mass mortality events. Evidence of a population bottleneck was found in most of the populations studied. Significant genetic differentiation was found among P. clavata populations, reflecting habitat fragmentation at a regional scale, except for two populations found 20 km apart. Continuing gene flow between distant populations was also revealed. Empirical data suggest that gene flow among populations may have contributed to support their recovery from mass mortality events. The study identified populations in the central part of the Ligurian Sea that can be strategic for the regional persistence of the species. These findings highlight that the preservation of key populations could maintain connectivity and gene flow in the metapopulation, and increase the resilience of the species.
Accounting for connectivity is essential in marine spatial planning and the proper design and management of marine protected areas, given that their effectiveness depends on the patterns of dispersal and colonization between protected and non-protected areas. The genetic structure of populations is commonly used to infer connectivity among distant populations. Here, we explore how population genetic structure is affected by pre- and settlement limitations with a spatially explicit coupled metapopulation-gene flow model that simulates the effect of demographic fluctuations on the allele frequencies of a set of populations. We show that in closed populations, regardless of population growth rate, the maintenance of genetic diversity at saturating initial population density increases with species life expectancy as a result of density-dependent recruitment control. Correlatively, at low initial population density, the time at which a population begins to lose its genetic diversity is driven larval and post-settlement mortality (comprised in the recruitment success parameter)—the larger the recruitment success, the stronger the genetic drift. Different spatial structures of connectivity established for soft bottom benthic invertebrates in the Gulf of Lions (NW Mediterranean, France) lead to very different results in the spatial patterns of genetic structuration of the metapopulation, with high genetic drift in sites where the local retention rate was larger than 2%. The effect of recruitment failure and the loss of key source populations on heterozygosity confirm that transient demographic fluctuations help maintain genetic diversity in a metapopulation. This study highlights the role of intraspecific settlement limitations due to lack of space when the effective number of breeders approaches saturating capacity, causing a strong reduction in effective reproduction. The present model allows to: (i) disentangle the relative contribution of local demography and environmental connectivity in shaping seascape genetics, and (ii) perform in silico evaluations of different scenarios for marine spatial planning.
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