Plant Diversity Improves Shea Tree Fruit Production in West Africa
This post is written by Aoife Delaney and Jane Stout, Trinity College Dublin. This work was also featured in The Conversation.
With demand on shea parklands increasing, Aoife Delaney and colleagues explore the pollination services to shea and how we can better support this resource of both ecological and economic importance. Their research was recently published in the Journal of Applied Ecology.
Shea trees grow across semi-arid parts of Africa and are prized for their fruit and seeds. From Senegal in the west to Uganda in the east, they are found in agroforestry systems called parklands — landscapes where trees are left to grow in cultivated and fallow fields.
These parklands occupy over a million square kilometres in a zone that’s home to 112 million people.
The dominance of shea trees in West African parklands reflects their value to society. The fruits ripen at a time when there are few food sources available, and the butter derived from shea nuts is the primary cooking oil for 88 percent of rural dwellers in Burkina Faso. It is estimated that about 10kg of shea butter is consumed per person every year in the shea zone, making shea an important source of dietary fat.
Shea is also an important source of income from local and global trade. Due to its utility in the production of soaps, cosmetics, and foods like chocolate, shea has become a globalized commodity in recent decades. For West African nations, international trade in shea is becoming an important source of revenue. In Ghana, for example, shea nuts were the fourth best performing agricultural export in 2018, with a value of over US$14 million.
Given that 20kg of fruit may be expected to yield 1.5kg of butter, the collection and processing of shea fruits requires considerable human effort, and this effort is almost exclusively furnished by women. In most households, the women who collect and process shea fruit own any related income; and since women are more likely than men to set aside money for educating children in the family, shea trees can represent an important resource to achieve development goals.
Today, shea parklands face unprecedented changes: population density in many parts of the shea zone has increased, and the expectation of financial return from farming has grown. Fallow periods have become shorter as there is more demand for cultivated land. With less time for regeneration, fewer saplings remain when fields are cleared for crops such as sesame, sorghum, soy, millet, hibiscus, and cotton. Simultaneously, the international market for shea butter is growing, prompting attempts to commercialize shea cultivation. Now more than ever, it is vital that we understand the ecological services that support fruiting of shea.
Shea benefits strongly from pollination by insects, primarily bees, to produce fruit. This link between shea trees and bees means that shea is connected with conditions in the wider environment because bees need a range of resources to survive. Working with local partners (Naturama, University of Ouagadougou) and the local community, as well as international organisations (BirdLife International, RSPB, Trinity College Dublin), Dr. Aoife Delaney and her colleagues investigated the relationship between pollination services to shea and the diversity of trees and shrub species in cultivated fields as well as the amount of uncultivated habitat near the fields.
By comparing the number of fruits on hand-pollinated with the fruit set on naturally pollinated blossoms, they found that fruit production of shea was limited by lack of pollination and that this limitation was greater at sites with less tree and shrub diversity. These findings show that more pollination occurs in fields with a greater range of trees and shrubs. This might be because a location that has a wide range of different species is likely to contain a variety of plant-based resources used by bees, like nesting sites, pollen, nectar, and resin, throughout the year.
Unexpectedly, given the role of local site-level diversity in driving pollination service, natural fruit set was lower at sites close to larger areas of uncultivated land. However, this may be because shea is a food source for fruit-eating wildlife including birds and mammals that might be more prevalent in larger, uncultivated areas. Thus, although local biodiversity promotes pollination, landscape biodiversity may promote natural levels of frugivory. Since only 42 percent of shea fruit is estimated to be harvested by people each year, this leaves a share for nature; but intensification of shea exploitation needs to consider both these biotic interactions.
Shea fruit represents an important ecological, societal, and economic resource; and if there were more pollinators in the landscape, more fruit would be produced. Conditions beneficial to both honey bees and other bee species should be fostered to maximize pollination. We recommend that pollination services to shea are supported by maintaining a diverse assemblage of woody species in parklands. These findings corroborate existing research, showing that the ecosystem services provided by tree and shrub diversity support the well-being of millions of people living in the Sudano-Sahelian zone of West Africa.
This work was funded by the Darwin Initiative (UK government) and made possible through the cooperation of BirdLife, the RSPB and Trinity College Dublin.