Jeremy Hance May 24, 2010
Tags: biodiversity, toucan, nutmeg, rainforest, Kimberly Holbrook, New Era Farms, paulownia
As we are leaving the week focusing on biodiversity and the rapidly increasing speed of the extinction of the species, it may be important to explore the role of diverse species to p`rotect the evironment. Scientists are just beginning to uncover the complex relationship between healthy biodiverse tropical forests and seed dispersers—species that spread seeds from a parent tree to other parts of the forest including birds, rodents, primates, and even elephants. By its very nature this relationship consists of an incredibly high number of variables: how abundant are seed dispersers, which animals spread seeds the furthest, what species spread which seeds, how are human impacts like hunting and deforestation impacting successful dispersal, as well as many others. Dr. Kimberly Holbrook has begun to answer some of these questions.
Performing her Masters works in Cameroon, Holbrook explained in an interview leading up to the 5th Frugivore and Seed Dispersal International Symposium, how she discovered that African forest hornbills spread seeds much further than expected.
"They have very large home ranges that varied between 925 hectares to more than 4,000 hectares. These home ranges are much larger than those reported for other African avian frugivores as well as all earlier estimates of African hornbill home ranges," Holbrook says, adding that "these two species of forest hornbill engage in landscape-scale movements travelling up to 290 kilometers over a two to three month period."
These findings are important because they show that hornbills are capable of carrying seeds incredible distances—i.e. long-distance dispersal—from parent trees, aiding forest diversity by carrying species to new areas. Think of the hornbills as the boats which carried the Polynesians from one far-flung island to another: without birds or other long distance dispersers a tree species could become stuck in an ecosystem 'island'.
"Long-distance dispersal is also fundamental to the spread and survival of plant populations and contributes to gene flow between populations," explains Holbrook. "Another advantage is that long-distance dispersers enable plant offspring to sample larger areas creating the opportunity to escape host-specific pathogens and reduce kin competition. Finally, long-distance dispersers may facilitate arrival of rare species and colonization to gaps, thus helping contribute to forest regeneration." Holbrook believes African hornbills travel such distances in order to follow fruiting trees.
On the other side of the world, Holbrook studied how human impacts affect seed dispersers and, in turn, the entire forest, including our impact on long-distance dispersals.
"In Ecuador I studied fruit removal and effective dispersal (or recruitment) distances at two sites that differed in hunting pressure. In the site with high hunting pressure, I found that significantly fewer numbers of large frugivores were visiting fruiting trees and that overall fewer seeds were removed from trees based upon seed trap data," Holbrook says.
She also found evidence that seeds traveled twice as far in the non-hunted sites, showing that intensive hunting targets the large fruit-eating species, such as big birds and primates, which are capable of carrying seeds the furthest.
"Seed dispersal is one of the many very important ecological processes that not only impact the ecology of forests and dispersers, but also the evolutionary relationships that exist there. For approximately 60-90 percent of tropical woody plant species, seed dispersal is a critical service essential for spread and survival," Holbrook says.
Despite the complexities of seed dispersal ecology, the lesson is quite simple: without seed dispersers, and especially long-distance marathon dispersers, the structure of the forest will change forever.
In a May 2010 interview with Kimberly Holbrook about the ecology of seed dispersal, the increasing importance of hornbills and toucans, the difficulties of estimating an economic value for seed dispersal, and human how hunting and deforestation impact successful dispersal.
How did you become interested in rainforest ecology?
Kimberly Holbrook: I first became interested in tropical forest ecology when I travelled for a year in Southeast Asia, India, and China. My first view of a rainforest was while travelling by ship up the west coast of Sumatra. I was completely captivated by the apparent diversity and complexity of the forest. Upon finishing my undergraduate degree, I took a volunteer position working with hornbills in Cameroon where I had my first opportunity to learn about the importance of ecological relationships in tropical forests. Given my interests in birds, plants, and seed dispersal, it made perfect sense to pursue plant-animal interactions in the tropics.
Any advice for students interested in studying rainforests?
Kimberly Holbrook: My first bit of advice is to go to the rainforest!! Get a job—even if it is to volunteer—or take a tropical ecology field course so you can have an idea of what the environment is like. It will also give you a chance to start thinking about the kinds of questions you might develop as a student. I think that the best way to find out what kinds of organisms and ecological interactions are most interesting is through direct experience.
Seed dispersal is one of the many very important ecological processes that not only impact the ecology of forests and dispersers, but also the evolutionary relationships that exist there. For approximately 60-90% of tropical woody plant species, seed dispersal is a critical service essential for spread and survival. For countless animal species, tropical tree fruits provide the necessary food and nutrition to survive and reproduce. Figs, for example, are especially important in providing food, often during non-fruiting seasons when other food sources are unavailable or much reduced. Other fruits such as those in the Myristicaceae (nutmegs) and Lauraceae (avocados) families provide high quality lipid-rich nutrition, crucial for many rainforest animals.
What is the 'seed shadow'? How do you determine this for fruit-eating species like hornbills?
Kimberly Holbrook: The seed shadow is the spatial distribution or pattern of seeds that are dispersed from fruiting plants. Determining where in the landscape that the next generation will establish, the seed shadow can vary greatly depending on the dispersal mechanism that a particular plant species relies upon. Technically, a seed shadow is a representation of the dispersal pattern generated by all visiting frugivores. To determine a specific frugivore-generated seed shadow, as I have done with hornbills and toucans, you would need to collect data on animal movements and on seed retention times. Animal movement patterns are usually estimated using radio telemetry and seed retention can be calculated with captive feeding trials where regurgitation and/or defecation times are measured. These two types of data are then combined to tell us the probability of the frugivore dispersing a seed to a particular distance. The result is a two dimensional curve showing probability of seed deposition plotted against distance dispersed.
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