Abstract
Dispersal plays a crucial role in the connectivity of established mangrove populations and in species range dynamics. As species ranges shift in response to climate change, range expansions can occur from incremental short-distance dispersal events and from stochastic long-distance dispersal events. Most population genetic research dealt with historically accumulated events though evidence of actual propagule dispersal allows to estimate genotypic features and origin of founders. In this study, we aim to disentangle a contemporary dispersal event. Using microsatellite markers, we genotyped 60 Rhizophora racemosa drift propagules obtained on a bare unforested coastal area in southern Cameroon, estimated their relationship to 109 adult trees from most proximate sites (which were 3–85 km away), and assessed their relative difference with 873 trees of major mangrove areas (> 300 km) along the Cameroonian coastline. Proximate mangrove populations were considered as potential source populations in assignment tests. However, drift propagules could not be assigned to any of the Cameroonian mangrove sites and were genetically isolated from Cameroonian populations. Drift propagules showed higher levels of genetic diversity and private alleles giving a higher relatedness to each other than to any putative source population. Chloroplast sequences were used to confirm the identity of drift propagules as R. racemosa. We postulate that a complex interaction of ocean currents, estuarine geomorphology, and tidal patterns explain drift propagule dispersal to an area. Most likely the investigated cohort of propagules originated from more southern mangrove areas of the West African range beyond the Cameroonian border. This study unraveled the allelic, genetic, and genotypic features of stranded propagules following a stochastic long-distance dispersal. Transboundary dispersal of these propagules highlights the need for intergovernmental efforts in the management of biodiversity.
Highlights
The seafaring propagules of mangroves depend on local hydrodynamics and ocean currents for both short- and longdistance dispersal
Measures of Genetic Diversity and Structure To assess whether the drift propagules were made of a genetically diverse pool of propagules, we summarized and compared measures of genetic diversity between drift propagules and the closest mangrove populations (Campo and Kribi) by assuming the drift propagule pool as a single population (Drift)
We monitored drift propagules on a beach area ∼3 km from the closest adult mangrove stands around Campo River
Summary
The seafaring propagules of mangroves depend on local hydrodynamics and ocean currents for both short- and longdistance dispersal. Assessing real-time dispersal events in mangroves is difficult because of the challenges of tracking drifting propagules over large-scale and dynamic seascapes (Van der Stocken et al, 2019b). The difficulty of quantifying the number of candidate dispersers, owing to how much fecundity and depredation pressure varies across space and time (Van der Stocken et al, 2019b), further compounds the challenge. Such real-time dispersal events are relevant for understanding patterns and spatial scales of connectivity and range dynamics of species. More frequent short distance dispersal events are notable for maintaining the connectivity of populations at regional to fine spatial scales (Ngeve et al, 2017a,b; Triest et al, 2020, 2021a), and contributing to range expansions (Saintilan et al, 2014; Giri and Long, 2016; Kennedy et al, 2020; Triest et al, 2021b)
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