Photo by André Bärtschi, www.wildtropix.com
The Brazil Nut Tree: More than just nuts
Several interesting biological features characterize the Brazil nut tree (Bertholletia excelsa Humb. & Bonpl.: Lecythidaceae). Although a very important economic species in South America, as well as a hope for conservation of tropical forests, it is surprising how little attention has been given in to research of its basic biological characteristics and the complex ecological interactions with humans and other organisms in their natural habitat. In a study carried out in southeastern Peru and in other parts of Brazil, many of the mysterious interdependencies of the Brazil nut trees in their natural environment are being unraveled.
A Nuts Story
These 50 meter tall trees bear coconut-sized fruits that remain on the canopy for over 15 months. Their seeds (around 20 per fruit) may have different fates: grow into a seedling, be eaten by rodents, rot, or finally end in a party somewhere in the world. The Brazil nut seeds may well be in fact, the only tropical product widely consumed overseas that comes exclusively from wild populations, i.e. not from plantations. But, for these seeds to be potentially viable for regeneration or consumption, they depend on (and make dependent on them) a number of specialized organisms ranging from bees, moths, damselflies, mosquitoes, aguties, rats, pecaries, acouchis, macaws, to humans.
Euglossine bees ("orchid bees") are known to pollinate Brazil nut flowers. It is not clear which species are involved in a natural environment, and to what degree the trees are dependent on just a few species. Although, it is known that very few kinds are involved and mainly the females, while males are more specialized on orchids. These bees have also been described as negatively sensitive to disturbed forest, an increasingly common characteristic in most of Amazonia. Once the flower has been pollinated, it may develop into a full fruit, or it may be aborpted by the tree if an insufficient number of pollen ovules have been fertilized. What level the dependency bee-Brazil nut tree exists?, what are the consequences of forest disturbance on a "keystone species" as the Brazil nut tree?, how forest management may help to reconcile some degree of forest use? These questions are being answered in a research in the Peruvian Castañales. These interrogates are important not only in academic grounds, but also for another practical purpose: overall nut production may be increased through the identification of the requirements of their pollinators.
Predators and Fruit Production
While the fruits mature on the trees, macaws and other parrots attack them effecting an impact on fruit production. To what extent are pre-dispersal predators affecting seed availability for tree regeneration of commercial seed harvesting? Although parrots tend to concentrate in some individual trees, their impact was found to be minimal. Once the fruits fall to the ground, they may remain for several months until harversters collect them. Fruit collection is not made as soon as they fall, given the danger of these 2 kilo-fruits hitting a head. During that period, many fruits are taken and opened by only one animal with their teeth strong enough to gnaw them open: the agouti. It is an extraordinary case of dependence of a tree to a single seed-disperser.
The agouti, a 3 kilo-rodent, takes the fruits sometimes over 400 m. away from the parent tree. Although the whole fruit may be buried to avoid other agouti to still it, most of them are opened and their seeds taken out. Most of the seeds are eaten right away, although several are buried individually (scatterhoarded), to later be recovered. Such a basic aspect of the Brazil nut biology was only known from a 15-line report by Huber in 1910, and was never witnessed by any scientist. Only recently it is being investigated in Peru. The patterns of dispersal and predation, seed survival, as well as several other aspects related with seedling recruitment were investigated by marking thousands of seeds with magnets and then searched with the use of magnetic locators after dispersal. Several interesting results have come out from this research, such as the understanding of the strategies taken by individual agouties to avoid secondary predation of seeds (e.g. seeds and fruits buried underwater or on top of fallen logs) which may help to explain the regeneration patterns found in Brazil nut populations. Also, complex interactions with other vertebrate seed predators that are related to human activities have been discovered, such as hunting, abundance of big carnivores, and degrees of forest disturbance, all of them relevant for forest management.
Inhabitants of Empty Fruits
When an empty fruit is left on the ground by the agouti, after a rain it acts a container for a number of obligate interactions between specialized invertebrates. Mosquitoes, damselflies, a poison frog and a toad have been recently described and found to be using only Brazil nut fruits as breeding grounds, competing and racing against each other to avoid being eaten between themselves. Whoever gets there first will have a greater chance of surviving. It is a world by itself, the life inside a fruit.
Once a seed went through the obligate agouti-dispersal and was lucky to have been forgotten by the agouti, it may have an opportunity to germinate. But, for that to happen, the seed has to be buried in a site with the right environmental conditions (such as a gap), has to "escape" from other rodents and insects, and wait for over a year before the nut shell cracks open. It is still a mystery why there is so little natural regeneration in the wild, a common observation throughout the range of the Brazil nut tree, however this study is shedding some light into that question. The seed stage has been identified as a major bottleneck for recruitment. However, it is still under intensive study how fragile is the seedling stage. Nursery as well as field experiments are being carried out in order to answer this question.
Several Brazil nut plantations have been established in Brazil. Although trees have grown into reproductive age, their production is low, making them economical unprofitable. Why plantations do not produce as wild trees do? This research is investigating several possibilities, including pollination, given that, as it has been suggested, increasing forest disturbance and fires, may be affecting pollinator availability.
For Brazil nuts to be an effective sustainable resource that economically justifies forest protection, nut marketing has to benefit local peoples in a greater degree. The process that follows nut collection is a complex and fascinating one. Nuts go through local dealers, then to peeling and bagging factories, and then to exporting companies. Although profitable, most of the benefits go to the last link of the chain. Communal factories would help to locals to get a better share. This project is involved in supporting grassroot initiatives, and working closely with local peoples. This is another part of the story that this research is focusing as a major goal. Basic research is probing to serve more than the ideal of increasing the biological knowledge, but also having an impact in social as well as in economic aspects. Future work is pointing towards the implementation of forest enrichment plans which will help to keep biodiversity as well as helping rainforest peoples.
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Story by: Enrique G. Ortiz
Vice President, Amazon Conservation Association
Smithsonian NMNH Research Associate