The Birds and the Trees: What the Relationship between Tropical Trees and Fruit-Eating Birds Can Tell Us about Conservation in Logged Tropical Forests


Most people have seen the effects of deforestation (personally or through pictures) and can lament how this human activity transforms miles upon miles of lush forest into a dilapidated wasteland. What if, in the near future, deforestation stopped? Do you think, like a rubber band, these forest ecosystems would “snap back” to the way they were before logging began? Nandini Velho and colleagues from the Indian National Center for Biological Sciences explored the ecological effects of past logging by examining the relationship between avian frugivores (“fruit-eaters”) and tropical trees in northeastern India. In their September 2012 article in Biological Conservation, Velho et al. concluded that the decline of large avian frugivores in previously logged areas has significantly decreased the number of new large-seeded fruit trees in these regions, a change that will likely lead to the domination of small-seeded trees.

In this study, Velho and his colleagues began by selecting four areas in or around the Pakke Wildlife Sanctuary and Tiger Reserve on which to collect data: two were unlogged protected areas and two were areas that were logged until the late 1970s. Through direct sightings and bird calls, the team estimated the number of avian frugivores in these regions and quantified how often the birds visited six species of fruiting trees (chosen randomly). To compare tree composition, small plots were defined in the studied areas and the number of saplings and adult fruit trees was calculated for each.

The collected data showed that the abundance and visitation of large birds markedly decreased in logged regions; conversely, the abundance and visitation of smaller avian frugivores did not decrease between regions and, in some cases, actually increased in logged regions. It was also found that bird-dispersed tree species, especially those with large seeds, had a lower number of saplings in the logged area. The authors concluded that because the larger birds tended to reside in the unlogged region, they were not dispersing as many large seeds to logged area. As effective seed dispersal is limited by beak width, the smaller avian frugivores were not able to compensate by dispersing large seeds, and the number of large-seeded fruit trees declined in the deforested area. The long-term result of this decline might be the domination of small-seeded trees in logged areas; this could lead to the decline or extinction of species which depend on the large-seeded fruit trees for food or shelter. Another problem that may arise is a feedback mechanism that could significantly lower large frugivore populations: As the number of large avian frugivores in logged areas decreases, the number of large-seeded fruit trees also decreases, leading, in turn, to the elimination of large avian frugivores in logged areas.

One of the challenges faced in this research was the difficulty in concluding why large avian frugivores decreased in logged regions. Velho et al. had stated that selective logging had occurred in the deforested area and that the large-seeded fruit trees were more likely to have been cut down than the small-seeded trees. The article implies that this might have produced the decline in large birds in the logged regions. However, the authors also acknowledge that this decline could be attributed to the inability of large avian frugivores to recover from hunting, which was prevalent in logged regions until the 1970s. This lack of definitive conclusion suggests that future researchers should strive to examine logged areas where significant hunting has not occurred in order to obtain more decisive data.

Despite this complication, this study is significant because it suggests that banning logging alone may not ensure the return of original flora and fauna to deforested areas. Active restoration, with a focus on large avian frugivores and large-seeded trees, will probably be essential. (Even if the decline of large birds in logged areas was due to past hunting, this conclusion is still valid because it is likely that hunting occurs in many deforested regions of the world.) In terms of future research, the authors suggest that the role of post-dispersal seed predators, such as rats, be examined in logged and unlogged areas to understand if these organisms are also contributing to the decline of certain tree species in logged areas.

Journal Reference:

1. Velho, N., Ratnam, J., Srinivasan, U., & Sankaran, M. 2012. Shifts in community structure of tropical trees and avian frugivores in forests recovering from past logging. Biological Conservation. 1 (153): 32-40.
http://www.sciencedirect.com/science/article/pii/S0006320712002108

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This entry was posted in Conservation Biology Posts, Conservation Blogs 2012-2013. Bookmark the permalink.

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