







































Abstract:   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. 

Aisthesis      Volume 10,  201918

Ocean-Dispersed Drift Seeds in Relation to Beach Slope and 
Particle Size: Fine-Scaled Patterns on a Tropical Shore 

by Rojina Nekoonam

Intoduction
 Seed dispersal involves the movement of 
seeds away from the parent plant, a process which 
contributes to genetic outcrossing as well as to the 
colonization of new sites.  Plants have limited mobility 
and thus rely on a variety of dispersal mechanisms 
to transport their seeds.  Dispersal vectors include 
animals, which feed on fleshy fruits or carry sticky 
seeds on their fur, as well as explosive dispersal from 
seed pods, wind, and water.  Long-distance dispersal 
by ocean currents has been described for seeds that 
are buoyant and can tolerate extended periods of 
immersion in seawater (Nathan et al., 2008); these 
seeds may drift passively with ocean currents for 
hundreds of kilometers. Adaptations such as small 
size and light weight, internal air cavities in the seed, 
waterproof coats, and corklike or fibrous coverings, 
permit drift seeds to remain afloat for days or even 
years (Zuchowski, 2005). 

 The transport of seeds by ocean currents is 
particularly important for the colonization and 
growth of vegetation on tropical and subtropical 
shorelines. Rivers and streams also carry seeds 
downstream where they may be deposited on tropical 
beaches (Smith, 1990). It is predicted that deposition 
of drift seeds on the shore is affected by the same 
factors that affect deposition of other materials: 
hydrology, including patterns of ocean currents, tidal 
cycles, and wave action. Geology, which includes the 
rocks, particles, and processes that shape the seafloor 
bathymetry, morphology, and slope of the foreshore, 
also affects deposition of drift seeds.  
 Certain processes involving ocean currents and 
coastal sedimentology are well-established in the 
literature (Davidson et al., 2002).  Waves moving 
onshore become steep and break in shallow waters, 
and the breaking waves and resulting currents pick 



Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size

Aisthesis      Volume 10,  201919

up and move sand particles, shifting sediments and 
carving out coastlines. Beaches are dynamic; there 
is constant interaction and feedback between wave 
action and beach morphology. As waves shape the 
beaches, the form of the beach, in turn, affects wave 
action.  
 According to Davidson et al. (2002), the strength 
of wave action determines what sediments are 
deposited.  Slow-moving water with little wave 
energy can only transport the finest particles, while 
strong waves and currents are able to move all sizes 
of particles, both small and very large.  The natural 
angle of deposition of water-borne sediments is 
positively correlated with particle size. The larger the 
particle size, the steeper is the angle of deposition, 
such that tidal mudflats deposited by slow moving 
water are, as the name indicates, flat in profile, while 
boulder beaches deposited by crashing surf are 
steep.  This paradigm has been useful in contrasting 
shorelines worldwide with markedly different 
extremes of wave energy.  It is not known whether 
these patterns can be detected at a fine scale, on the 
order of a few hundred meters, on a single tropical 
shore (Davidson et al., 2002).
 The shore environment at San Miguel Biological 
Station, Cabo Blanco Absolute Reserve on the Pacific 
coast of Costa Rica shows marked heterogeneity 
in beach morphology (see Figure 1) and the 
distribution of particle sizes (see Figure 2), even 
within short distances.  This raises the possibility 
of exploring the variability in sediment patterns 
and beach slope at this very small scale, and testing 
whether seed deposition reflects a process similar to 
that of sediment deposition.  

 
 
 

 This study aims to address the following research 
questions: (1) Does the nature of sediment exchange 
contribute to the shape of the beach and therefore 
seed dispersal; and (2) Is there variation in how seeds 
are dispersed along Cabo Blanco’s sandy beaches, 
and do drift seeds arrive in a random pattern? The 
goal of this study is to assess the relationship between 
foreshore slope, sediment particle size, and ocean-
dispersed drift seeds sampled at a fine scale at San 
Miguel Biological Station.  

Methods
 The study was conducted July 11-12, 2018, at the 
San Miguel Biological Station (9° 35’N, 85 °08’W), 
Cabo Blanco Absolute Nature Reserve.  Located at the 
southernmost tip of Costa Rica’s Nicoya Peninsula, 
Puntarenas Province, the site receives around 3 m of 
rain per year, mostly falling in the rainy season from 
mid-May to mid-November (Camacho Céspedes 
and Lindquist, 2007). Vegetation is 55-year-old 
secondary forest, naturally regrown after the Reserve 
was established in 1963. The vegetation type is dry-
moist tropical coastal forest (Camacho Céspedes & 
Lindquist, 2007).  The Reserve is bordered on the 
west and south by the Pacific Ocean and on the east 
by the Gulf of Nicoya.
 There are two high tides and two low tides per 24 
hours on this coast.  Tidal amplitude is high; during 
the study period, the vertical difference between low 
tide and high tide ranged from 2.4-3.0 m (8-10 ft).  
At the highest point of each high tide, a clear drift 
line remains, with depositions of seeds, fruits, small 
pieces of wood, shells, seaweed, leaves, and a variety 
of ocean-dispersed trash (see Figure 3).   

 



Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size

Aisthesis      Volume 10,  201920

Preliminary observations of seeds in the drift line at 
San Miguel showed that the overwhelming majority 
of seeds belonged to beach almond or almendro, 
Terminalia catappa (Combretaceae); because of 
its dominance in the seed assemblage, this species 
became a special focus of the study (see Figure 
4).   Descriptions of the species (Zuchowski, 2005; 
Camacho Cespedes & Lindquist, 2007; Condit et al., 
2011) indicate that almendro is native to India but is 
now common worldwide, having been dispersed by 
ocean currents to all tropical and subtropical shores. 
The trees are tolerant of strong winds, salt spray, 
and moderately high salinity environments with 
sandy soils. Growing well in sandy coastal plains and 
low-lying areas, they are a dominant species along 
the shores in Cabo Blanco.  The seeds are generally 
4-7 cm long, 2.5-3.8 cm wide, pointed at the apex, 
and flattened, rather like an almond in shape as the 
common name suggests. The prominent keel around 
both sides of the seed add buoyancy, allowing it to 
float for long distances in the sea (Zuchowski, 2005; 
Invasive Species Specialist Group [ISSG], 2018).    
 The sampling universe was a 200 m length of 
beach near the San Miguel station buildings. This 
stretch was divided into 40 sectors measuring 5 m in 
length, each marked at its midpoint by a stake flag.  
Twenty of the forty sectors were drawn at random for 
sampling. Sites that were inaccessible due to streams 
or dense vegetation were excluded from sampling. 
The percent slope of the beach was measured at 
each flag using a line level, a 5 m length of string, 
and a meter rule; the angle was then calculated 
trigonometrically. The substrate was photographed 
at each site to permit characterization of the particle 
size of sediments.  Five size categories were used 
(listed from coarse to fine):  1- cobble; 2- pebble; 3- 

coarse sand; 4- medium sand; and 5- fine sand.  If 
more than one sediment category was present, the 
most common type at the site was recorded.  
 Seeds were collected from the drift line during 
the daytime low tide on July 12th. All seeds present 
at the randomly assigned sites were collected, placed 
in a labelled bag, and transported to the laboratory 
for sorting, counting, and measuring.  Fresh-looking 
seeds that appeared to have recently fallen from the 
branches overhead were excluded from the samples.  
For each of the 20 sample bags, the almendro seeds 
were separated from seeds of all other species, and 
the two groups of seeds were counted and measured 
separately. The total number of species in each bag 
was tallied, permitting analysis of species richness.  
All seeds were measured in length. To account for 
broken or fragmented seeds, the following procedure 
was used: if 50% of the seed was present, it counted 
as half a seed; if more than 50% was present, it was 
counted as one seed; and if less than 50% was present, 
it was discarded without counting.

Results
 The slope of the beach measured at 20 randomly 
sampled points over a 200-m length of beach at 
San Miguel ranged from 1.8-12.3%, with a mean of 
7.00% (see Figure 5).  The frequency distribution 
of sediment size classes at the 20 points is shown in 
Figure 6.  Pebbles were the most frequently observed 
category. 
 There is a strong negative correlation between 
sediment size (ranked from coarse to fine) and slope 
of the beach among these samples (r= -0.944, 18 d.f.; 
P<0.001), in which d.f. indicate degrees of freedom 
(see Figure 7). Fine particles tend to be deposited at 



Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size

Aisthesis      Volume 10,  201921

lower angles than coarse particles; the largest size 
class (cobbles) had the steepest beach profiles.  
 In the 20 replicate samples, a total of 1049 seeds 
were collected (range 1-390.5 per sample).  Of these, 
787 seeds (75.0%) were almendro, and the remaining 
253 seeds belonged to other species.  The density 
distribution per sample is shown in Figure 8a for 
seeds of all species (mean 52.4, variance 7784.38) 
and in Figure 8b for seeds of almendro alone (mean 
39.4, variance 4233.37).   
 The spatial dispersion pattern for seeds among 
the samples was analyzed by comparing the ratio 
of variance/mean for the n samples to the expected 
value (1.00) for a Poisson distribution; the Poisson 
distribution is the expected, null distribution 
for objects dispersed in a random pattern.  The 
comparison was done using a special t-test (Greig-
Smith, 1983):



Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size

Aisthesis      Volume 10,  201922

 The dispersion in both cases is non-random, 
showing a clumped pattern.  Seeds are not dispersed 
independently of one another on the shore, but occur 
more often than expected in clusters. The frequency 
distribution of species richness of seeds is shown 
in Figure 5.  Richness ranged from 1-22 species 
per sample, with a mean of 5.7 species (see Figure 
9).  Species richness is highly correlated with the 
number of seeds present in the sample (see Figure 6). 
Samples with more seeds tended also to have more 
species represented (r=0.917, 18 d.f.; P<0.001) (see 
Figure 10).  
 The frequency distribution of seed size is shown 
in Figure 11a for all seeds of all species in the 20 
samples (n=1049).  Length ranged from 0.4-24 cm 
(mean 4.14 cm).   The size distribution is shown 
in Figure 11b for seeds of all species exclusive of 
almendro (n=253), and in Figure 11c for seeds of 
almendro alone (n=787).
 The mean seed length per sample was calculated 
for all species combined (see Figure 12a) and for 
almendro seeds alone (see Figure 12b).  Mean 
length per sample for all species ranged from 3.0-
6.1 cm (mean 4.39), and for almendro seeds alone 
ranged from 3.4-6.1 cm (mean 4.72 cm). The two 



Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size

Aisthesis      Volume 10,  201923

distributions are very similar, as almendro seeds 
represent such a large proportion of the total seed 
sample.  
 The relationship between the percent slope of the 
beach and various aspects of seed deposition were 
analyzed.  There is no correlation between beach 
slope and the total number of seeds per sample 
(r=0.413, 18 d.f.; P>0.05); the total seed density is 
independent of beach slope. 
 The mean length of seeds per sample is positively 
correlated with the percent slope of the beach (see 
Figure 13); the relationship is seen when all species 
of seeds are included (r=0.549, 18 d.f.; P<0.05) and 
also when only almendro seeds are included (r= 
0.473, 18 d.f.; P<0.05).  Larger seeds tend to be found 
at sites with steeper slopes. There is no correlation 
between beach slope and either the minimum length 
of seeds per sample (r=0.226, 18 d.f.; P>0.05) or 
the maximum length of seeds per sample (r=0.349, 
18 d.f.; P>0.05).  Both the minimum seed size 
and maximum seed size at these sample sites are 
independent of percent slope of the shore.
 As is seen in Figure 14, the sediment size class 
(ranked coarse to fine) is negatively correlated with 
the mean size of seeds in the sample (r=-0.450, 18 
d.f.; P<0.05).  Larger seeds tend to be found at sites 
with larger particle size. There was no correlation 
between sediment size and either maximum seed size 
per sample (r=0.286, 18 d.f.; P>0.05) or minimum 
seed size per sample (r=0.166, 18 d.f.; P>0.05). 

Discussion
 The results of this study confirm that patterns 
seen at continental scales relating angle of deposition 
to particle size are clearly in play at scales of a few 
tens of meters.  

 Many details of hydrology and ocean currents 
along this shore remain unknown. The variability 
in wave energy, while not measured, can be inferred 
from the marked variation in foreshore slope 
and sediment size seen over even short distances. 
Previous studies have shown that seeds of tropical 
plants can be moved by ocean currents to new 
destinations, and the abundance and diversity of 
seeds arriving in a small area on the shores of San 
Miguel are evidence of that process (Nathan et al., 
2008; Smith, 1990). Almendro, which was the most 
common species in the drift seed samples, is the 
most common species in the forest bordering the 
shore at San Miguel, suggesting that inputs of these 
seeds have been a long-standing process.  
 The number of seeds varied widely from sample 
to sample, but the number was not related to beach 
slope. The count of seeds present in the drift line 
may be affected by factors which are independent of 
beach profile.  Rates of arrival on the beach could 
involve chance processes, such as the number of 
floating seeds which happen to be near the shore at 
any given time.  The seeds present at a sample point 
may include accumulated seeds from past tides.  
Seeds may also be removed by animals after they 
arrive on shore.  Almendro seeds in particular are 
favored by hermit crabs, harlequin crabs, squirrels, 
agoutis, and white-faced capuchin monkeys, and the 
drift lines themselves may be disturbed by digging of 
scavengers, such as coyotes, raccoons, and coatis (D. 
Lieberman, personal communication, July 2018).  
 The species richness per sample was recorded.  
However, species identifications (except for 
almendro) were not cross-referenced between 
samples; hence, it is not possible to determine the 
total species richness for seeds collected. 
 The clumped dispersion pattern of seeds implies 
that wave action along the shore is sufficiently 
variable to deposit seeds in a non-random pattern, 
with some spots receiving far more and others 
far fewer than would be expected by chance.  It is 
doubtful whether this clumped pattern affects the 
distribution of germinated seedlings and adult trees 
along the shore. Characteristics of wave action and 
shore morphology are dynamic, and the patterns of 
deposition may change from season to season and 
year to year. 



Ocean-Dispersed Drift Seeds in Relation to Beach Slope and Particle Size

Aisthesis      Volume 10,  2019

 Patterns of seed dispersal on this shore are 
generally similar to patterns of sediment deposition. 
Seed size was not independent of sediment size. 
The conditions that led to the deposition of coarser, 
heavier particles also led to the deposition of larger 
seeds.  This finding is reinforced by the correlations 
between sediment size and beach slope and between 
seed size and beach slope, in which larger particles 
were associated with steeper beach profiles. Perfect 
correspondence between sediment particle size and 
seed size seems unlikely, as rocks and seeds differ in 
terms of density and buoyancy. Without strong wave 
energy, sediments should tend to sink, while seeds, 
especially those capable of long-distance ocean 
dispersal, should tend to float. Thus, large buoyant 
seeds may be deposited in areas where large rocks 
are absent. 
 The pattern of beach profiles and sediment 
distribution were surveyed at a single point in time 
following a single high tide.  The processes that 
shape the coastline are remarkably dynamic, and the 
characteristics measured must be in flux, shifting 
with daily cycles of high and low tides, lunar cycles 
of spring tides and neap tides, weather, storm events, 
and the seasons. Thus, the correlations observed in 
this study, most notably between percent slope and 
sediment distribution and between seed deposition 
and beach characteristics, must be constantly re-
established and renewed over very short time 
periods.  
 The study was carried out during the rainy 
season, at a time with heavy surf and frequent 
storms.  The specific findings of the study, such as the 
slope of the beach, the frequency of sediment classes, 
and the species composition, density, and sizes of 
seeds, might differ if the work were done at a time of 
calm weather during the dry season.  However, the 
relationships between sediment size, slope, and seed 
size might still be present.  Studies of these variables 
at other times and seasons would be of interest.

Acknowledgments
 My gratitude must go to Diana and Milton 
Lieberman for providing the opportunity to carry out 
research in the Cabo Blanco Absolute Nature Reserve, 
and to Diana for her remarkable experimental design 
guidance and editorial support. I also thank Robert 
Tournay for his assistance in measuring beach slopes 
and kindly helping to process the large volume of 

collected seed samples. The National Park System 
of Costa Rica and the San Miguel Biological in the 
Cabo Blanco Absolute Reserve kindly provided 
access to the study site. Funding was provided by the 
Chancellor’s Office of California State University and 
the National Science Foundation through the CSU-
LSAMP Summer Research Program under the grant 
#0802628. 

References
Camacho Céspedes, F., & Lindquist, E. S. (2007).  

Common trees of Cabo Blanco Absolute Nature 
Reserve, Costa Rica. Santo Domingo de Heredia, 
Costa Rica: Instituto Nacional de Biodiversidad 
(INBio).  

Condit, R., Pérez, R. & Daguerre, N.  (2011).  Trees 
of Panama and Costa Rica.  Princeton, NJ: 
Princeton University Press.

Davidson, J. P., Reed, W. E. & Davis, P. M. (2002). 
Exploring Earth. Upper Saddle River, NJ: 
Prentice-Hall, Inc.

Greig-Smith, P. (1983). Quantitative Plant Ecology. 
Oxford, UK: Blackwell Scientific Publications. 

Invasive Species Specialist Group (ISSG) of 
the  Species Survival Commission of 
the  International Union for Conservation of 
Nature. (2018). Global Invasive Species Database.  
Retrieved from http://www.iucngisd.org/gisd/
search.php.  

Nathan, R., Schurr, F. M., Spiegel, O., Steinitz, O., 
Trakhtenbrot, A. & Tsoar, A.  (2008). Mechanisms 
of long-distance seed dispersal. Trends in Ecology 
and Evolution, 23, 638-647.

Smith, J. M. B. (1990).  Drift disseminules on Fijian 
beaches. New Zealand Journal of Botany, 28, 13-
20.

Zuchowski, W. (2005).  Tropical plants of Costa Rica: 
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