Fruit Trees Help Ensure Tropical Birds' Future on Coffee Plantations
By Tina Gheen
What relationship, if any, is there among the quality, size, and amount of fruit found in an area, and the number and variety of birds found in the same area? On coffee plantations in Latin America, the relationship between the 2 is unmistakable and compelling.
Coffee plantations make up 42% of all dedicated cropland in northern Latin America. Many coffee plantations are agricultural areas that combine trees with crops, also known as agroforestry systems.
Shade grown coffee farm
Because of the nature of these plantations, a larger variety and number of trees can be incorporated onto the sites. This diversity and quantity of trees, particularly fruit-bearing trees such as guava and banana trees, makes coffee plantations an excellent location to study the relationship between the abundance of fruit and the number of species of fruit-eating birds.
For farmers and other land management specialists concerned about the conservation of birds and other animals, figuring out this relationship can help them support local species.
For this study, scientists from the University of Georgia, Southeast Partners in Flight, and the Smithsonian Migratory Bird Center conducted research at the University of Georgia's San Luis Research Station in Northwestern Costa Rica. In 2008 the team surveyed birds and fruit trees for 10 months at 6 coffee plantations to determine if birds responded to the availability of fruit.
This study was different from previous surveys because the team used a different metric to determine the abundance of fruit resources in the area. This measurement is called Fruit Energy Availability, or FEA.
FEA is a new concept that incorporates 3 pieces of information:
- the number of ripe fruits on a tree,
- the calories contained in an individual fruit,
- and the fruit size.
This combined value is a better indicator of the value of the tree to birds that eat its fruit.
Of the 113 bird species the team recorded, 80 were observed eating fruit during the study. Calorie values were calculated for 27 plant species producing fruits consumed by the birds on the 6 coffee plantations. The abundance of fruit varied from location to location and changed over time due to seasonal temperatures and rainfall.
The team discovered a direct relationship between the average monthly FEA for each site and the number of different bird species found in each area, or species richness. The study clearly showed that plantations with higher average monthly FEA values also had more birds.
FEA was found to be an excellent tool for predicting bird species richness. For example, the types of birds observed on the 6 coffee plantations were very similar. However, bird species richness varied between plantations. During the study, 65% of the time there were only 3 or fewer tree species with fruits found on the plantations.
The FEA value for the tree species present explained 52% of the variation in bird species richness. Additionally, once the FEA value reached a certain threshold, in this case 12,000 kcal, birds such as parrots, pigeons, new world flycatchers, and thrushes would migrate into the habitat. Once the FEA dropped below 500 kcal, birds would leave.
Consistent, high quality sources of food lead to stable bird populations. Birds that eat fruit are an important part of the ecosystem, spreading the seeds of the plants they eat throughout the forest. Sadly, fruit-eating birds that spread seeds over long distances are predicted to have a much higher extinction rate over the next century than most other birds.
However, the incorporation of FEA into land management practices of agroforestry areas such as coffee plantations can help with bird conservation. By evaluating the amount of energy a tree's fruit can provide and including a variety of plants with high FEA values, agricultural areas such as the coffee plantations in this study can help ensure bird species survival.
This article summarizes the information in this publication:
Bird community response to fruit energy. 2010. Valerie E. Peters, Rua Mordecai, C. Ronald Carroll, Robert J. Cooper, Russell Greenberg. Journal of Animal Ecology 79(4): 824–835.
1. The abundance and predictability of food resources have been posited as explanations for the increase of animal species richness in tropical habitats. However, the heterogeneity of natural ecosystems makes it difficult to quantify a response of animal species richness to these qualities of food resources. 2. Fruit-frugivore studies are especially conducive for testing such ecological theories because fruit is conspicuous and easily counted. Fruit-frugivore research in some locations has demonstrated a relationship between animal abundance and fruit resource abundance, both spatially and temporally. These studies, which typically use fruit counts as the variable of fruit abundance, have never documented a response of species richness at the community level. Furthermore, these studies have not taken into account factors influencing the detection of an individual within surveys. 3. Using a combination of nonstandard approaches to fruit-frugivore research, we show a response of bird species richness to fruit resources. First, we use uniform and structurally similar, one-ha shade-grown coffee plots as replicated experimental units to reduce the influence of confounding variables. Secondly, we use multi-season occupancy modelling of a resident omnivorous bird assemblage in order to account for detection probability in our analysis of site occupancy, local immigration and local emigration. Thirdly, we expand our variable of fruit abundance, Fruit Energy Availability (FEA), to include not only fruit counts but also fruit size and fruit quality. 4. We found that a site’s average monthly FEA was highly correlated (0·90) with a site’s average bird species richness. In our multi-season occupancy model 92% of the weight of evidence supported a single model that included effects of FEA on initial occupancy, immigration, emigration and detection. 5. These results demonstrate that fruit calories can broadly influence the richness of a neotropical bird community, and that fluctuations of FEA explains much of the site occupancy patterns of component species. This study shows that in depauperate, managed landscapes fruit resource abundance supports more species and fruit constancy allows for higher levels of avian persistence, an important practical concept for conservation planning.
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