Growing populations and resource demand are driving the continued destruction of tropical forests. Given that both populations and resource demand will continue to increase, it is important to try to do so as sustainably as possible.
Scientists in India and California have shown that agriculture and native biodiversity are compatible under certain conditions. By leaving native tropical vegetation within farmlands, native bird species biodiversity can be maintained over the long term.
Choosing the study area.
In order to test whether agriculture and native biodiversity can coexist over a long period of time, the scientists sought a tropical agricultural area that possessed three attributes. The study area must:
These requirements were fulfilled in a 20 kilometer long stretch of coastal land along the Western Ghats Range, in Karnataka, India. This region has been farmed continuously for over 2,000 years. Most of the current land uses in this region have been continuous for at least 200 years.
Five types of land use were studied: native, unused forest; native, used forest; shrubland; cashew, and arecanut palm plantations. Two types of land use, peanut fields and rice paddies, were not studied because, depending on the season, native vegetation and wildlife were essentially nonexistent.
Measuring the study area.
All 30 study sites (defined as a 200 meter line) within this large study area were separated from each other by at least 1 kilometer. All study sites were less than 300 meters in elevation.
Observers spent 30 minutes walking along each line, between 6 and 9 AM, in March and April 2004, noting all birds within 50 meters of the line. Canopy closure, plant composition, and plant structure were all measured along each line in 20 meter increments, for a total of 11 measurements per line.
For ease of comparison, plant height measurements were placed into one of four categories: 0-5 meters, 5-10 meters, 10-20 meters, or 20-30 meters. It turned out that bird species diversity was correlated with plant height measurements, and not the other measurements obtained by the scientists.
Overall bird species distribution and diversity.
How many bird species were observed in this study? The scientists observed 114 bird species, 109 of them (96%) outside of intact forest. This study is therefore representative of bird life in general, and is not limited to a particular species.
How were birds spread out across the study areas? Typically, each type of land use hosted a unique collection of bird species.
However, similar bird species distribution across families and overall species diversity were observed in all five types of land use, with the exception of shrubland. Shrubland possessed roughly half the bird species diversity of the other lands.
Fate of native habitat birds.
A particularly important question for conservationists is the distribution of birds which primarily reside in native habitat. If their habitat is lost, they will not survive. How does agriculture imapct their distribution across the study area?
The scientists identified 51 bird species that primarily reside in their native habitat. Happily, 46 of them (90%) were commonly observed outside of their native habitat.
They were most commonly observed in native, unused forest, but were also roughly 67% as likely to be present in native, used forest and arecanut palm plantations. They were were roughly 50% as likely to be present in peanut fields. However, the bird species which primarily reside in their native habitat were only roughly 10% as likely to be present in shrubland.
The news was positive overall for threatened bird species as well. Great Hornbills were observed within 33% of the arecanut palm plantation study sites, and Malabar Grey Hornbills were observed within 67% of them.
This demonstrates that native and threatened bird species make use of agricultural lands, and can survive long-term disruptions to their native habitat.
Where do birds congregate the most?
The above results demonstrate that, with the exception of shrubland, bird species distribution across families and overall species diversity was similar across all types of land use. Is there a physical characteristic of land use that encourages bird habitation?
The scientists found such a characteristic: tree height. Tree height was positively correlated with an increased level of bird species diversity. The greater the average height of the trees, the more birds tended to be spotted within the area.
What does this mean for bird conservation?
Several general conclusions, regarding birdlife in tropical agricultural lands, can be drawn from this study.
for more information:
Ranganathan, J.; Daniels, R. J. R.; Chandran, M. D. S.;
Ehrlich, P. R.; Daily, G. C.
Sustaining biodiversity in ancient tropical countryside.
Proc. Natl. Acad. Sci. USA 2008, 105, 17852-17854.