Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size: Fine-Scaled Patterns on a Tropical Shore
Arrival of drift seeds on shore following long-distance dispersal by ocean currents is affected by a number of factors, including hydrology and geological processes. Measurements of beach slope and sediment size class and collections of seeds from the high tide drift line were made following high tide at 20 randomly-selected points along a 200-m transect at San Miguel Biological Station, Cabo Blanco Absolute Reserve, on the Pacific coast of Costa Rica. Percent slope was highly variable, ranging from 1.83-12.3%. Sediments were ranked from coarse to fine: cobbles, pebbles, coarse sand, medium sand, and fine sand. Pebbles were most common. Percent slope of the beach was not independent of sediment size; larger particle sizes were found on steeper slopes, and finer particles were deposited at lower beach profiles. Thus, patterns of sedimentology well-documented at continental scales were found to apply even at a very fine scale on this shore. A total of 1049 drift seeds were collected, of which 75% belonged to a single species, Terminalia catappa (Combretaceae), “almendro.” The number of seeds per sample was highly variable, ranging from 1-391. The species richness per sample ranged from 1-22 (mean 5.7 species), and increased as a function of the number of seeds sampled. Seeds were not distributed independently of one another among samples, but showed a non-random, clumped pattern, tending to occur in clusters more often than expected by chance. Seed length ranged from 0.4-24 cm (mean 4.14 cm). Larger seeds tended to be found at sample sites with larger particle size and with steeper topography; therefore, patterns of seed dispersal on the shore were generally similar to patterns of sediment deposition. Processes shaping beach topography and sediment composition are dynamic, changing with each tide, storm event, and season. Thus, the relationships seen in this study, at one point in time, between beach slope and sediment size and between beach slope and seed deposition, must be constantly re-established over very short intervals.