Magdalene N. Ngeve

Bio: Magdalene N. Ngeve is an academic researcher from Vrije Universiteit Brussel. The author has contributed to research in topics: Biological dispersal & Rhizophora racemosa. The author has an hindex of 5, co-authored 9 publications receiving 85 citations. Previous affiliations of Magdalene N. Ngeve include University of Maryland, College Park.

Contrasting Effects of Historical Sea Level Rise and Contemporary Ocean Currents on Regional Gene Flow of Rhizophora racemosa in Eastern Atlantic Mangroves

Abstract: Mangroves are seafaring taxa through their hydrochorous propagules that have the potential to disperse over long distances. Therefore, investigating their patterns of gene flow provides insights on the processes involved in the spatial genetic structuring of populations. The coastline of Cameroon has a particular geomorphological history and coastal hydrology with complex contemporary patterns of ocean currents, which we hypothesize to have effects on the spatial configuration and composition of present-day mangroves within its spans. A total of 982 trees were sampled from 33 transects (11 sites) in 4 estuaries. Using 11 polymorphic SSR markers, we investigated genetic diversity and structure of Rhizophora racemosa, a widespread species in the region. Genetic diversity was low to moderate and genetic differentiation between nearly all population pairs was significant. Bayesian clustering analysis, PCoA, estimates of contemporary migration rates and identification of barriers to gene flow were used and complemented with estimated dispersal trajectories of hourly released virtual propagules, using high-resolution surface current from a mesoscale and tide-resolving ocean simulation. These indicate that the Cameroon Volcanic Line (CVL) is not a present-day barrier to gene flow. Rather, the Inter-Bioko-Cameroon (IBC) corridor, formed due to sea level rise, allows for connectivity between two mangrove areas that were isolated during glacial times by the CVL. Genetic data and numerical ocean simulations indicated that an oceanic convergence zone near the Cameroon Estuary complex (CEC) presents a strong barrier to gene flow, resulting in genetic discontinuities between the mangrove areas on either side. This convergence did not result in higher genetic diversity at the CEC as we had hypothesized. In conclusion, the genetic structure of Rhizophora racemosa is maintained by the contrasting effects of the contemporary oceanic convergence and historical climate change-induced sea level rise.

Hidden founders? Strong bottlenecks and fine-scale genetic structure in mangrove populations of the Cameroon Estuary complex

Abstract: Fine-scale genetic structure (FSGS) is common in plants, driven by several ecological and evolutionary processes, among which is gene flow. Mangrove trees rely on ocean surface currents to spread their hydrochorous propagules through space. Since pollen dispersal is generally restricted to local scales, high level of short-distance propagule dispersal is expected to result in FSGS in Rhizophora spp. We investigated FSGS, recent bottleneck events, as well as historical and contemporary expansion patterns in Rhizophora racemosa populations from the entire coast of Cameroon, using 11 polymorphic microsatellite markers. Populations of the Cameroon Estuary complex (CEC) showed significant FSGS and significant reduction in effective population sizes (recent bottlenecks), compared to the other areas. Additionally, our results indicate stark differences between historical and contemporary expansion models. These suggest that contemporary processes such as restricted propagule dispersal, bottleneck events from high indirect and direct anthropogenic pressure, and recolonization by founders from ancient local pockets/refugia most plausibly shape the patterns of FSGS in the CEC.

Bidirectional gene flow on a mangrove river landscape and between-catchment dispersal of Rhizophora racemosa (Rhizophoraceae)

Abstract: Understanding how landscape structure shapes the genetic structure of populations of keystone species is important for their long-term management. We tested the unidirectional dispersal hypothesis on the linear river landscape of the Wouri River and the one catchment-one gene pool hypothesis on red mangrove (Rhizophora racemosa) populations of the Cameroon Estuary complex. Therefore, we conducted release–recapture experiments in the field, and sampled 649 adult trees for plant material for genetic analyses. This allowed for estimating genetic diversity and structure, as well as dispersal directionality. Genetic diversity in populations downstream did not differ significantly from upstream populations and the molecular variance of populations did not correlate with their position on the linear landscape. Contemporary and historical migration estimates indicated bidirectional dispersal, i.e. in both the downstream and the upstream direction along the Wouri River. This was confirmed by the propagule dispersal directions derived from our field experiments. Bayesian clustering analysis assigned all individuals of this estuary into one cluster, suggesting high inter-catchment connectivity. River flow currents, tides, and wind plausibly operate together to ensure the high genetic connectivity within this complex estuary.

Assessment of trace metal pollution in sediments and intertidal fauna at the coast of Cameroon

Abstract: Coastal systems act as a boundary between land and sea. Therefore, assessing pollutant concentrations at the coast will provide information on the impact that land-based anthropogenic activities have on marine ecosystems. Sediment and fauna samples from 13 stations along the whole coast of Cameroon were analyzed to assess the level of trace metal pollution in sediments and intertidal fauna. Sediments showed enrichment of As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, V, and Zn. However, pollution of greater concern was observed for Cd, Cr, Cu, Ni, and Zn at the northern stations. Some sites recorded trace metal levels higher than recommended in sediment quality guidelines. Species diversity was low, and high bioaccumulation of trace metals was observed in biological samples. Some edible gastropod species accumulated trace metals above the safety limits of the World Health Organization, European Medicine Agency, and the US Environment Protection Agency. Although industrial pollution is significant along Cameroon’s coast, natural pollution from the volcano Mount Cameroon is also of concern.

Expansion of the mangrove species Rhizophora mucronata in the Western Indian Ocean launched contrasting genetic patterns

Abstract: Estimates of population structure and gene flow allow exploring the historical and contemporary processes that determine a species’ biogeographic pattern. In mangroves, large-scale genetic studies to estimate gene flow have been conducted predominantly in the Indo-Pacific and Atlantic region. Here we examine the genetic diversity and connectivity of Rhizophora mucronata across a > 3,000 km coastal stretch in the Western Indian Ocean (WIO) including WIO islands. Based on 359 trees from 13 populations and using 17 polymorphic microsatellite loci we detected genetic breaks between populations of the (1) East African coastline, (2) Mozambique Channel Area (3) granitic Seychelles, and (4) Aldabra and northern Madagascar. Genetic structure, diversity levels, and patterns of inferred connectivity, aligned with the directionality of major ocean currents, driven by bifurcation of the South Equatorial Current, northward into the East African Coastal Current and southward into the Mozambique Channel Area. A secondary genetic break between nearby populations in the Delagoa Bight coincided with high inbreeding levels and fixed loci. Results illustrate how oceanographic processes can connect and separate mangrove populations regardless of geographic distance.

The state of the world’s mangroves in the 21st century under climate change

Abstract: Concerted mangrove research and rehabilitation efforts over the last several decades have prompted a better understanding of the important ecosystem attributes worthy of protection and a better conservation ethic toward mangrove wetlands globally. While mangroves continue to be degraded and lost in specific regions, conservation initiatives, rehabilitation efforts, natural regeneration, and climate range expansion have promoted gains in other areas, ultimately serving to curb the high mangrove habitat loss statistics from the doom and gloom of the 1980s. We highlight those trends in this article and introduce this special issue of Hydrobiologia dedicated to the important and recurring Mangrove and Macrobenthos Meeting. This collection of papers represents studies presented at the fourth such meeting (MMM4) held in St. Augustine, Florida, USA, on July 18–22, 2016. Our intent is to provide a balanced message about the global state of mangrove wetlands by describing recent reductions in net mangrove area losses and highlighting primary research studies presented at MMM4 through a collection of papers. These papers serve not only to highlight on-going global research advancements, but also provide an overview of the vast amount of data on mangrove ecosystem ecology, biology and rehabilitation that emphasizes the uniqueness of the mangrove community.

A general framework for propagule dispersal in mangroves.

Abstract: Dispersal allows species to shift their distributions in response to changing climate conditions. As a result, dispersal is considered a key process contributing to a species' long-term persistence. For many passive dispersers, fluid dynamics of wind and water fuel these movements and different species have developed remarkable adaptations for utilizing this energy to reach and colonize suitable habitats. The seafaring propagules (fruits and seeds) of mangroves represent an excellent example of such passive dispersal. Mangroves are halophytic woody plants that grow in the intertidal zones along tropical and subtropical shorelines and produce hydrochorous propagules with high dispersal potential. This results in exceptionally large coastal ranges across vast expanses of ocean and allows species to shift geographically and track the conditions to which they are adapted. This is particularly relevant given the challenges presented by rapid sea-level rise, higher frequency and intensity of storms, and changes in regional precipitation and temperature regimes. However, despite its importance, the underlying drivers of mangrove dispersal have typically been studied in isolation, and a conceptual synthesis of mangrove oceanic dispersal across spatial scales is lacking. Here, we review current knowledge on mangrove propagule dispersal across the various stages of the dispersal process. Using a general framework, we outline the mechanisms and ecological processes that are known to modulate the spatial patterns of mangrove dispersal. We show that important dispersal factors remain understudied and that adequate empirical data on the determinants of dispersal are missing for most mangrove species. This review particularly aims to provide a baseline for developing future research agendas and field campaigns, filling current knowledge gaps and increasing our understanding of the processes that shape global mangrove distributions.

Poleward expansion of mangroves is a threshold response to decreased frequency of extreme cold events

Abstract: Regional warming associated with climate change is linked with altered range and abundance of species and ecosystems worldwide. However, the ecological impacts of changes in the frequency of extreme events have not been as well documented, especially for coastal and marine environments. We used 28 y of satellite imagery to demonstrate that the area of mangrove forests has doubled at the northern end of their historic range on the east coast of Florida. This expansion is associated with a reduction in the frequency of “extreme” cold events (days colder than −4 °C), but uncorrelated with changes in mean annual temperature, mean annual precipitation, and land use. Our analyses provide evidence for a threshold response, with declining frequency of severe cold winter events allowing for poleward expansion of mangroves. Future warming may result in increases in mangrove cover beyond current latitudinal limits of mangrove forests, thereby altering the structure and function of these important coastal ecosystems.

Mangrove Forest Restoration and Rehabilitation

Abstract: We analyzed current best practices and recommendations used in the implementation of mangrove rehabilitation and restoration (R/R) projects in the Atlantic-East Pacific (AEP) and the Indo-West Pacific biogeographic regions during the last 20 years. Comprehensive literature and World Wide Web searches were performed identifying 90 sites around the world where R/R actions have been implemented. For each site, we analyzed the sources of damage/impact and classified the origin as natural (siltation, erosion, the direct and indirect effect of tropical storms or tsunamis) or anthropogenic (pollution, land use policies, overharvesting, aquaculture, altered hydrology and hydroperiod). In most cases, the causes of damage were a complex mixture associated to erosion, hydrological impairment, deforestation, siltation, and land conversion for aquaculture and other land uses. The area extension of mangrove sites undergoing restoration or just afforestation ranged from few square meters to several thousand hectares. Numerous projects were implemented without an underlying science-based approach and were often ill-prepared and unsuccessful. Although there is no “one-size-fits-all” solution to restore or rehabilitate mangrove wetlands, published studies (particularly peer reviewed) provide useful insights into designing R/R projects with clearly defined and prioritized management objectives based on a diagnostic of the source of damage/deterioration. A critical step is to develop a decision tree that serves as a guide to optimize the use of available funding in the development, implementation, and monitoring of R/R protocols to set clear objectives, goals and deadlines. These steps should be part of a robust research agenda based on sound ecological theory and reliable monitoring practices, including the participation of local communities. Any monitoring and reporting program should address spatial and temporal replication that explicitly includes reference sites near the target restoration site. The results of each R/R project, whether successful or not, should be published, as they are critical sources of data and information for further development of mangrove R/R practices and methods within the community of restoration ecology science. We urge the continental level implementation of guidelines to advance international initiatives aimed to protect and conserve one the most productive and threatened coastal ecosystems in the world.