A new study published in the September issue of Global Ecology and Conservation by Hanqing Tang and colleagues found that plants located at the edges of populations disperse pollen over longer distances than plants in population centres. The finding challenges the assumption that edge effects are wholly negative and suggests they may aid the recovery of declining populations.
Researchers conducted field experiments with the aquatic plant Sagittaria trifolia in China over two years to compare edge and interior plants in simulated populations. They planted edge and interior individuals in artificial gardens and examined differences in pollination, fruit production, and seed output. The results showed that while edge plants received fewer visits from pollinators, they achieved similar levels of female reproductive success as interior plants, producing comparable numbers of fruit and seeds. Despite facing lower pollinator visitation rates, the edge plants were also receiving successful pollen from more distant potential sires. This greater pollen dispersal distance and mating diversity for edge plants was consistent across the different simulated habitat types and experimental years. The paternity results revealed that while edge plants face some disadvantages, like lower pollinator visitation, they gain a substantial benefit in the form of increased pollen flow across longer distances. This allows them to maintain high genetic diversity and connectivity between fragmented populations. The edge advantage was robust and repeated in varied habitat contexts over multiple years.
Our study found that edge plants exhibited longer distances of pollen flow and sires compared to interior plants, indicating an edge advantage in gene dispersal and mating diversity. This may seem counterintuitive, as edge plants received fewer pollinator visits. However, in a spatial context, edge plants had fewer close neighbors than interior plants. Considering that both achieved the same reproductive success, edge individuals might have pollen traveled to and from more conspecifics located further away, increasing mating distances. In addition, while pollinators were attracted more by interior plants and thus tended to stay, their flying paths may have been more dispersed and unpredictable when reaching the edge, causing longer pollen transfer.Tang et al. 2023
The results suggest that edge populations help maintain the genetic diversity and connectivity vital for declining species. However, the researchers caution that more studies across diverse experimental populations are still needed to understand the impacts of edge effects fully. The current findings were limited to simulated populations of one plant species.
Nonetheless, the consistent edge advantages found across different habitats and years indicate that edge plant populations warrant special attention in conservation efforts. The study authors recommend prioritizing edge plant germplasm and reintroducing species across multiple interconnected sites.
From a metapopulation perspective, edge plants, acting as stepping stones, are also the ones that can receive more pollen from the outside and disperse more propagules to wider ranges (Lander et al., 2010). Under such scenarios, edge effects in fragmented habitats may play positive roles in promoting gene exchange both within and among populations and encouraging offspring with higher genetic diversity and adaptive potential.Tang et al. 2023
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Tang, H., Niu, K., Zhou, P., Gong, Y. and Dai, C. (2023) “Plants at population edges enjoy longer mating distances: Implications for conservation practice,” Global Ecology and Conservation, 45(e02520), p. e02520. Available at: https://doi.org/10.1016/j.gecco.2023.e02520.