Contrasting genetic effects of red mangrove (Rhizophora mangle L.) range expansion along West and East Florida

Abstract

Aim We evaluated underlying mechanisms and genetic effects of climatedriven range expansion of Rhizophora mangle L., a coastal foundation species, along both West (WFL) and East (EFL) Florida, USA.Location Eight sites encompassing the entire Florida R. mangle range at a regional scale.Methods We characterized R. mangle population genetic structure with a combination of genetic analyses using seven microsatellite loci and modelbased propagule transport. We tested hypotheses on the genetic effects of range expansion along both WFL and EFL. Finally, we compared WFL and EFL range edges and assessed potential factors shaping observed differences.Results Regional-scale Florida R. mangle genetic structure is shaped in part by the non-independent effects of geographical distance and ocean currents that drive asymmetric propagule transport from WFL to EFL. WFL conformed to theoretical expectations of range expansion, with pronounced divergence at the range edge, whereas EFL deviated from expectations. Significant differences in diversity and differentiation at the WFL and EFL range edges were attributed to differences in migration rates, population size and founder effects.Main conclusions Contrasting genetic landscapes at the WFL and EFL range edges are in part the product of variation in ocean circulation and demographic history. These underlying mechanisms may have potential ecological and evolutionary consequences that need to be addressed with further empirical research.

Is the central‐marginal hypothesis a general rule? Evidence from three distributions of an expanding mangrove species, Avicennia germinans (L.) L

“…Osland, Feher, et al () evaluated mangrove distributions worldwide and determined that most range limits were controlled by either temperature or precipitation, with only four geographic regions influenced by both factors: Texas‐Louisiana (TX‐LA), Pacific Mexico, Western Australia, and the Middle East. CMH predictions are supported along multiple mangrove distributions controlled by either temperature or precipitation (Arnaud‐Haond et al, ; De Ryck et al, ; Francisco, Mori, Alves, Tambarussi, & de Souza, ; Kennedy et al, ; Maguire, Saenger, Baverstock, & Henry, ; Pil et al, ; Sugai et al, ), consistent with our findings along temperature‐controlled Florida. CMH is also supported in Pacific Mexico (Ochoa‐Zavala et al, ; Sandoval‐Castro et al, , ) and Western Australia (Arnaud‐Haond et al, ; Binks et al, ), where parallel declines in temperature and precipitation limit mangrove distributions.…”

“…For instance, West Florida (WFL) and East Florida (EFL) exhibited a similar decline in neutral genetic diversity, with reductions in mean allelic richness of almost 50% from south to north, and greatest intersite differentiation at the northern range margins, consistent with latitudinal reductions in mangrove abundances along these coastlines (Osland, Feher, et al, ). However, we found a stronger effect of genetic drift at the WFL range margin, a pattern also observed in a co‐occurring mangrove species, Rhizophora mangle (Hodel, Souza Cortez, Soltis, & Soltis, ; Kennedy et al, ). Greater genetic drift at the WFL range margin may be explained by multiple factors, including greater geographic isolation from the range core, more extreme minimum annual temperatures, limited ocean‐current‐driven propagule dispersal (Kennedy et al, ), and restricted colonization due to propagule predation (Langston, Kaplan, & Angelini, ).…”

“…However, we found a stronger effect of genetic drift at the WFL range margin, a pattern also observed in a co‐occurring mangrove species, Rhizophora mangle (Hodel, Souza Cortez, Soltis, & Soltis, ; Kennedy et al, ). Greater genetic drift at the WFL range margin may be explained by multiple factors, including greater geographic isolation from the range core, more extreme minimum annual temperatures, limited ocean‐current‐driven propagule dispersal (Kennedy et al, ), and restricted colonization due to propagule predation (Langston, Kaplan, & Angelini, ).…”

“…USA mangroves also provide three natural replicates of core to margin distribution ranges. Comparing variation across multiple distributions of the same species can provide greater insights into the processes shaping genetic change (Griffin & Willi, ; Kennedy et al, ; Leydet, Grupstra, Coma, Ribes, & Hellberg, ; Micheletti & Storfer, ). For instance, West Florida (WFL) and East Florida (EFL) exhibited a similar decline in neutral genetic diversity, with reductions in mean allelic richness of almost 50% from south to north, and greatest intersite differentiation at the northern range margins, consistent with latitudinal reductions in mangrove abundances along these coastlines (Osland, Feher, et al, ).…”

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

“…Genetic studies are becoming much more common in mangrove ecology to elucidate processes that promote or inhibit mangrove dispersal (Ngeve et al, 2017). Yet, contrary to expectations, dramatic increases in the genetic diversity of mangrove trees colonizing the northeast coast of Florida have been observed as a result of increased long-distance dispersal of propagules by strong poleward-flowing ocean currents (Kennedy et al, 2016). This pattern is contrasted with mangroves from Florida's west coast where low genetic diversity was caused by the lack of strong ocean currents and limited local propagule dispersal and migration rates, resulting in founder effects (Kennedy et al, 2016).…”