Caught in transit: offshore interception of seafaring propagules from seven mangrove species

Abstract

Many organisms are transported passively and make use of the energy of natural phenomena or other organisms to disperse. However, not all species are equally likely to disperse over long distances. In mangroves, which possess seafaring propagules, it is largely unknown which species are more likely to reach the ocean and contribute to long-distance dispersal. This is because dispersal has been mainly studied under reductionist laboratory conditions and via localized release-recapture experiments. Direct interceptions of propagules at sea have hardly been attempted because of the high labor intensity. Here, we set up a local citizen-science network and engaged local fishermen to collect floating mangrove propagules over a period of 27 months. By comparing the dispersing community of propagules from the local stands in which they were produced, the open water of the bay, and the open ocean beyond the coral reef barrier, we could study the transition between local and long-distance dispersal. The composition of the dispersing community changed from the local stands toward the ocean, suggesting that this transition imposes an important selective filter for leaving the local system. With the exception of three rare species (Lumnitzera racemosa, Pemphis acidula, and Xylocarpus moluccensis), we intercepted dispersing propagules of every mangrove species occurring in the East African region. Most intercepted propagules were produced by Rhizophora mucronata and Ceriops tagal, followed by Bruguiera gymnorrhiza and Avicennia marina, which also represent the most abundant species in the nearby mangrove forest. A larger number of propagules were intercepted during the wet season, with fewer propagules recovered during the dry season. Overall, our study indicates that differences in the dispersal capacity of mangrove propagules are not straightforward and that some species may better disperse at local scales within an estuary or embayment, while others might be more suitable for dispersal over longer distances. The presence of such trade-offs may help explain why current attempts to use mangrove traits to predict mangrove species distributions at different scales have remained only moderately successful.

Propagule Dispersal Determines Mangrove Zonation at Intertidal and Estuarine Scales

“…These support our first hypothesis that the dispersal of mangrove propagules is not a passive dispersal process. Our results differ from those reports where the specific gravity of mangrove propagules during dispersal has been arbitrarily considered as unchangeable [18,30,32].…”

“…However, available research on propagule dispersal has focused on species passively dispersed by wind, and theoretical models of seed dispersal have been well established [4,16,17]. In contrast, research on propagule dispersal by water is uncommon [17,18].…”

“…Most mangrove species have buoyant, water-borne propagules, and some propagules can drift by ocean currents for a few months and for thousands of kilometers [14,21]. In some studies, the specific gravity of the mangrove propagules during dispersal has been arbitrarily considered unchangeable [14,18,30,32], and plastic drift cards or artificial drogues have been used as indicators of propagule dispersal [32]. However, in contrast to most terrestrial plants, most mangrove propagules do not hibernate [21].…”

A general framework for propagule dispersal in mangroves

“…De Ryck et al , ; Van der Stocken et al , ), other studies have accounted for the natural phenology of abscission by marking propagules when still attached to the parent tree (e.g. Chan & Husin, ; McGuinness, ) or by intercepting dispersing propagules during replicate fishing events over timespans that allow the capture of intra‐ and inter‐annual variation (Van der Stocken et al , ).…”

“…Propagule types of different mangrove species and their position relative to the water surface while floating. Propagules are from the following mangrove species: Xylocarpus granatum (fruit) (A); Sonneratia alba (fruit) (B); Heritiera littoralis (C); Avicennia marina (D); Bruguiera gymnorrhiza (E); Ceriops tagal floating horizontally (F) and vertically (G); Rhizophora mucronata floating horizontally (H) and vertically (I) (after Van der Stocken et al , ).…”