Please Click here to return to main page

 

Fruit Dispersal of Castilla elastica in  secondary forest and a developed area of the La Selva Biological Preserve, Costa Rica

 

By Wayne Cokeley, Gabriell DeBear Paye,

Catherine Roberts and Dean Birdsall.

Woodrow Wilson Foundation Fellows,

 

 

 

Introduction: Castilla elastica is a member of the Moraceae family, a very important group of trees in the Neotropics.  Many mammals and insects seem to make use of the tree and its fruits.  In this study we investigated the dispersal of Castilla elastica seeds and the role of this tree in the tropical forest community.  We hypothesized that there would be significant differences in the patterns of movement of fruit away from the parent trees and recruitment of new seedlings to the population.  Both the visiting wildlife and seed dispersion was compared at an internal secondary forest and a fringe secondary forest maintained by human activity as a lawn edge.       

The species of rubber tree found at La Selva Biological Station is characterized by a conical stipule covering the apical bud, a milky latex, smooth bark and tiny clusters of flowers. There are three species of Castilla. These trees are characterized by bark with narrow, horizontal, reddish, raised pustules and white latex. The leaves are hairy, finely serrate, and oblong. The twigs are also hairy. Fruits are orange and pulpy with numerous seeds inside. Fruiting trees often are the center of activity of numerous forms of frugivorous wildlife.

 

 

 

EXPERIMENTAL DESIGN:

            C. elastica trees were examined in a secondary forest section adjacent to the “Bike Trail” and behind the lawns of the research buildings on the grounds at La Selva. Observations consisted of three phases

1.      Direct Observation of Visitors to the Trees- Both tree stations were observed for a period of 4 hours from 5:30 AM- 9:30AM on July 18, 2000. All organisms visiting the trees and feeding on or moving fruits were recorded along with the time spent at the tree and brief notes of their activities. Additionally, observations were made regarding the passing of organisms feeding on the tree.

2.      Nearest Neighbor Analysis of Castilla elastica Trees- The locations of all C. elastica trees in the vicinity of the study tree were located by direct observation and the position and  diameter at breast height were recorded.

3.      Quadrat Studies of Seed Density- At the forest site a one square meter quadrant was set up in the seed shadow of the canopy. In that quadrat all Castilla seeds were recorded. Additionally the maximum distance of travel for the seeds from the tree and along the trail was recorded.

 

DATA :

            The tree was visited by the organisms in the table below;

 

Organism

Activity

Type of Fruit Use

Amazon Parrots,

Amazona farinosa and

Amazona autumnalis

Feeding on fruit in canopy

Crush fruits, eat pulp, expel seeds

Spider Monkey,

Ateles  geoffreyi   

 

Feeding on fruit in canopy

Crush fruits, eat pulp, expel seeds

Coati, Nasua narica

 

Feeding on fallen fruits

Consume fruits whole

Collared Peccary, Dicotyles tajacu

Feeding on fallen fruits

Consume fruits whole

Central American Agouti,

Dasyprocta punctata

Feeding on fallen fruits

Crush fruits, eat pulp, expell seeds

Tanager species and

other small birds

Feeding on fruits in canopy

Pick at and remove pulp, some removal of seeds, seeds expelled

 

Variegated Squirrel,

Sciurus variegatoides

Feeding on fruits in canopy

Crush fruit, eat pulp and drop seeds

 

Ants, two species

Feeding on fallen fruits

Consume and carry fruit, turn soil and bury seeds

 

            The quadrat study found that there were 371 seeds per square meter in the typical area of the seed shadow studied. The seed shadow area was estimated at 130 square meters. Seeds were observed on the hiking trail which passes beneath the seed shadow. This seeds were found 25 meters from the tree in the direction away from the developed area and 10 meters from the tree in the direction of the residential area.

            Data from the nearest neighbor analysis is also provided.

 

Discussion-

            The various facets of the study are discussed in the sections below.

 

Fruit Consumption and Seed Dispersal-

            Monitoring of the trees showed that the Castilla tree is an important species in the food web of the area. Many of the larger vertebrates, birds, and insects use the tree as a food source and apparently the trees are important centers of daily activity.  See the C. elastica photos.

            Based on our observations uses of the fruits can be separated into three categories;

 

I.                     Arboreal Fruit Feeders- These organisms harvest the ripe fruit, chew the pulp and expel the large seeds. We observed parrots, spider monkeys and squirrels doing this. Their primary interest appeared to be the bright orange juicy pulp of the ripe fruit.

 

II.                   Pulp Gleaners- Insects and small birds such as the Tanagers were observed to pick at the fruit and occasionally pull out a seed. Most of the effort was spent sucking the juices of the fruit or pulling away small bits of pulp. Seeds were dropped by the bird

 

III.                  Ground Foragers- these species rely on the rain of ripe fruit from above and pick up the material from the ground. Peccaries and Coatis were observed doing this and apparently swallowed the fruit whole. Agoutis separated the seeds from the pulp and spat them out. Scat from Coati found near the study site was packed with Castilla seeds.

 

 

 

 

 

 

 

Neighbor Distribution - In the secondary forest study the distribution of neighbor Castilla trees can be seen in the plot below

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

            Some trends in distribution of trees were noted. Most of the specimens seen were in close proximity to the walking trail. There appeared to be a line of large trees running at a diagonal across the trail and leading into the woods in two directions.

We can attribute the great number of trees at trailside to the open nature of the area adjacent to the trail. Castilla elastica is a light gap colonizing species and would be expected to rapidly colonize the trailside. Likewise, the use of the trail by the large mammals of the area such as Peccaries and Coatis would naturally result in distribution of rubber trees along the most frequented paths.

             The interpretation of the line of large trees transecting the woodlot was a bit more problematical. Two explanations of the pattern measured are the possible existence of game trails through the forest which would receive large doses of Castilla seeds and leave “lines of trees” or the existence of former roads and roadsides in historical times which once supported colonization by Castilla. Examination of the historical records for La Selva revealed that the study area was planted for rubber and cacao by the original owners of the land. The Castilla trees have traditionally been used as a shading overstory for the economically important cacao crop. Upon reexamination of the study area we discovered a good number of cacao trees in the forest with the Castilla . It is highly likely that our “linear trend” may have been human induced.  See the GIS land-use map from 1960 and 1998.


            The wooded population of Castilla trees appears to have fairly high seedling recruitment but high mortality among young trees. There also appears to be a relict group of large trees in the area( See figure). A variety of small plants can be seen in the brush of the forest floor from the base of the tree to distances in excess of 20 m. No intermediate size (10-29 cm DBH) trees were seen in the forest plot. Evidently the large recruitment of seedlings does not persist to become canopy trees and are eliminated while still small. The large trees of the stand maybe the original trees of the plantation or may represent occasional successful seedlings from a population which is largely eliminated as a young seedling. 

 

 



By comparison, in the site studied in the residential edge area only a few trees were found and there was a notable absence of seedlings. Their distribution can be seen in the diagram below. It is possible that there is far more intense seed predation on these tree’s seeds than in their forest counterparts  People and/or rodents may have selected for these seedlings, since half of the surrounding area of the tree is open and maintained. 

 


            The study area, despite having several relatively large trees which were bearing fruits, showed practically no seeds on the ground and very few small seedlings. The area around the base of the tree was thickly overgrown with brush and heliconians and it is very likely that this provides refuge for a number of seed eating rodents and finch like birds.


            The distribution of Castilla plants relative to the distance of the study tree is seen below.

            The absence of seedlings seems to indicate that the population in the edge areas is in decline and is no longer adding new trees despite being in a prime area for light.

 

 

History and Uses:

C. elastica is indigenous to Costa Rica and is found in Santa Rosa, Paloverde, Cabo Blanco, and La Selva among other places. In La Selva C. elastica was left for shade when the cocoa plantations were established in the secondary forest. It is common in wet tropical forests of Costa Rica. It  is an aggressive pioneer plant which grows only when large gaps appear in the forest. It was widely grown as an income generating tree among the early subsistence farmers in Costa Rica.

Of the three species of Castilla, C. elastica is the one typically used for rubber production. Its cell walls are impregnated with calcium carbonate and/or silica.

Although there are 20 botanical families and over 18,000 species of latex bearing plants, few are exploited. Castilla elastica is among 3 species that have been most extensively used for rubber production. Hevea brasiliensis of the Euphorbiaceae family and Ficus elastica are the other two. 

Precolombian mesoamerican societies have used rubber from C. elastica  for over 3,500 years. Pure rubber straight from the tree is unstable and will melt when it gets hot. Precolombian people solved this problem by combining the latex from C. elastica with that of Ipomoea alba (a vine in the morning glory family that happens to grow on C. elastica) to create a latex that was more stable. They made bouncing rubber balls and participated in elaborate and sometimes violent ball games.

When Colombus arrived in Central America he was impressed with the "rubber balls" he saw the native people using. A scientists named Joseph Priestly (also credited for discovering oxygen) named it "rubber" when he discovered that it could "rub out" pencil marks (as in an eraser.)  Later Charles Goodyear invented the process of vulcanization. By heating the latex and adding sulfur, it becomes stabilized and becomes stiff yet pliable under hot and cold conditions. Then he started his successful venture of creating the Goodyear tire company. Rubber can also be used to make gloves, condoms, soles of shoes and many other useful products.

Throughout history, wild (unmodified) genetic resources were common property to everyone. Seeds or plants were not  "patented" until recently. Because rubber seed was stolen and brought to Singapore so as to be grown there, Brazil lost its 98% share of the rubber industry sales to Malaysia in only 20 years.

C. elastica also supports a variety of tropical forest wildlife (Parrots, agouti, collared peccaries, spider monkeys, coatis and numerous small frugivorous birds eat its fruit). Lianas, orchids and bromeliads are supported by its branches.

In addition, C. elastica has been used for beekeeping, arts and crafts, industrial, medicinal and ornamental purposes.

 

Recent Research Findings:

C. elastica has been found to be an early successional tree in the rainforest. Despite its being indigenous to Costa Rica, it also tends to be invasive. Whenever there is a gap in the canopy, it is one of the first plants to establish itself.

There was a recent research project called "Some Studies of Effects of Elevated Carbon Dioxide on Tropical Forest Plants" by the Smithsonian Research Institute in Panama. During this study C. elastica and several other early successional species were found to produce 26% more biomass than normal when the level of carbon dioxide was doubled over a 25 to 39 day period. In contrast, the late succession trees showed little to no enhancement of dry matter accumulation over the same amount of time when given twice the normal background rate of carbon dioxide.

C. elastica is presently being considered for use and introduction in Australia because of all the above mentioned properties and its potential as an economic crop.

 

Ramifications of this Study- The two areas studied both showed that the Castilla rubber tree’s fruits are widely used by wildlife as a food source. The distribution of the trees in the secondary forest at La Selva are probably patterns of a historical cacao farming operation with some influence through seed dispersal by the local wildlife. It appears that the prime dispersers of the seeds of Castilla are the Collared Peccary and Coati, both of which swallow whole fruits and defecate the seeds elsewhere.

            Seed production at both sites was apparently high. Seed density within the seed shadow in the forest site was 371 seeds/sq. meter, a very high number. In the edge area the density was far lower- perhaps only 2-40 seeds per square yard observed. Both sites showed remains of fruit capsules on the ground. At the forest site there was a great pile of spoiling fruit under the tree and spattered on the leaves and branches of the shrubs. No doubt some seeds are spread through the spattering of the falling, spoiled fruits. It is also likely that the musty smell of the piled fruit under the tree is a signal for ground dewelling mammals to visit the plant and feed.

 

            At La Selva the trees in the secondary growth forest appear to be reproducing and are adding to the forest flora at some low rate. The trees located on the edges of the station grounds are not spreading, probably due to predation on their seeds by some unknown agent. A comparison of the survivorship curves for the two sites shows this trend clearly.

 


 

 


            One could speculate that in the development of homesteads from old rubber plantations or perhaps from the jungle itself, Castilla elastica will have a difficult time surviving. Since game animals such as Peccary and Coati are amongst the first species hunted out by settlers, the prime dispersal agents for the tree would be rapidly eliminated. La Selva is unusual in that these animals are protected and are free to feed where they would never be welcome in a village setting.The development of brushy areas around the fruiting trees seems to favor seed predators and discourages seedling production.

            In short, despite being a vigorous growing patch colonizing species, the Castilla rubber tree appears to survive best when growing inside the secondary forest with the appropriate mammalian and avian frugivores. 

 

 

CONCLUSION:

The data collected on neighbor distribution of the forest area studied reveals both a classic growth pattern and the aggressive nature of a gap species; however, our discovery of the prior land use as a cocoa plantation, helps to explain the abundance of this species as an adult. 

The patterns created by the nearest neighbors of Castilla elastica with in the secondary forest suggest that arboreal fruit eaters help carry the seeds outside of the mother trees shadow.  Their paths receive many seeds and when given the light opportunity, they have a fair chance at growth.  The human made trail also provides animal pathways and light that allows for the tree to survive. 

The forested site supported a wider diversity of tree sizes than did the fringed-residential edge area.  There are many possible reasons for this difference in neighboring trees.  These findings may indicate a need for managed planting of C. elastica in order for it to survive in populated places that are not frequented by the many species of wildllife that are necessary to it seed dispersal. 

This species of Rubber Tree may be considered as an economical asset in addition to being an important food source to wildlife. Castilla elastica is an important source of rubber for people. It may have additional medicinal and ornamental uses.  We would like to recommend that C. elastica be studied further for its potential to promote forest biodiversity while providing income and economic incentives to local people for preserving the forest in a sustainable agroforestry model.

 

 

REFERENCES:

 

Gentry, Alwyn. A Field Guide to the Families and Genera of Woody Plants on N.W. South America. Conservation International. Washington D.C., 1993. Pp. 627, 631-632.

 

Reid, Walter, et.al. Biodiversity Prospecting. World Resources Institute. USA. 1993. Pp. 20.

 

McDade, Lucinda, et. Al. Editors. La Selva. Ecology and Natural History of a Neotropical Rainforest. University of Chicago Press. Chicago, 1994. Pp. 77, 100, 299, 327, 372.

 

Web sites:

 

http://www.hear.org/pier/caela.htm

http://www.mobot.org/manual.plantas/025827/S025933.html

http://www.indb.org/othrub/castilla.htm

http://www.newcrops.uq.edu.au/listing/castillaelastica.htm

http://www.siu.edu/-leaflets/rubber.htm

http://www.mhhe.com/biosci/pae/botany/botany_map/articles/article_06.html

http://www.lifesci.ucla.edu/botgard/html/botanytextbooks/economicbotany/naturalrubber/

http://www.studyabroad.com/lasuerte/pruetz.html

 

 

Please Click here to return to main page