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Reforestation of Asian Tropical Rainforests with Avian Seed Dispersers and Natural Corridors
Department of Biology
Lake Forest College
Lake Forest, Illinois 60045
Over the last century, there has been a heightened awareness of the decline in density of tropical rainforests across the globe due to habitat fragmentation. This fragmentation has been attributed to the human interactions with tropical rainforests such as logging, road construction, and hunting (Naniwadekar et. al, 2015). This causes major global issues as this leads to the loss of biodiversity in both plants and animals in these ecosystems. As a way to combat these changes, studies on avian seed dispersal have risen in order to find new ways to reforest the tropics. One way to do this is through natural corridors to help lighten the stresses of habitat fragmentation and the subsequent edge effects that are harmful to the biodiversity of rainforests, which has been seen in studies in Central and South America (Sekercioglu et. al, 2015). However, Central and South America are not the only areas of tropical rainforest that are in danger, another area is Asia where markets for wood have caused a massive decrease in the density of tropical rainforests. Taking into account the studies of the use of natural corridors to aid avian seed dispersal agents, this paper will discuss how habitat fragmentation affects avian seed dispersal and the benefits of the natural corridors, as well as apply these concepts to the situation in Asian tropical rainforest.
Habitat fragmentation negatively affects avian seed dispersal in moving seeds to distant locations. Due to the edge effects that are implemented on fragmented forests, there is an increase in predation and other obstacles for birds to safely and efficiently travel to distant patches while carrying seeds. This leads to very distinct patches of genetically depressed forests due to the bird’s lack of ability to successfully disperse the seeds. In an Asian tropical forest of Western Ghats, the vast majority of parents of trees are found in the same patch, often at the rate of eighty percent. In stark contrast, only seventeen percent of seeds are dispersed to another patch, which is depicted in table one (Ismail et. al, 2015).
This shows that avian seed dispersers are unable to adequately take seeds from one fragmented patch to another. It can also lead to a situation similarly found in the Brazilian Atlantic forest where the low seed dispersal leads to extinction of these dispersal agents because of the overall lack of food. Due to the lack of food, generations of genetic depression will begin to set in, and plant species will begin to lose their reproductive ability and overall range of where they are located. Consequently, this leads to the lack of avian traits that could be attributed to forest fragments and leave them vulnerable to extinction with continuous changing of forests (Bovo et. al, 2018.). This a major concern because this is being accelerated by the global climate change that the planet is experiencing, and as a result, we could see an exponential increase in extinction due to changing habitat suitability and resources. Another problem with this is that the dispersal distances of these seeds by birds are very small. The average of the maximum distance for seed dispersal was 96m when the seeds were from the same patch and only about 55m when one seed came from another patch, as seen in table one (Ismail et. al, 2015). This is an issue since the seeds are being distributed to such small areas. The overall range will continue to shrink over generations until eventually the fragmented forest no longer exists and all the species inside it have either migrated out or died. Overall, we can see how the habitat fragmentation negatively affects avian seed dispersal by both narrowing the overall genetics of the birds as well as the distance and overall range of these species of seed-producing plants.
As a way to reverse these effects, the use of specifically placed natural corridors can help with avian seed dispersal by connecting fragmented forest patches and providing critical habitat areas. By connecting fragmented forest patches with planned reforestation, birds are able to travel easier because of the lack of predators that would be normally there without corridors. This can be seen in Costa Rica, where within these forests it can be seen that these birds travel in areas with a higher stem density, seen in figure three, as three out of four bird species traveled to these areas (Fagan et. al, 2016). This is important because it shows that avian dispersers would be more prone to traveling in areas with higher stem density and an overall connection between fragmented forest patches. This could allow for further connecting fragmented patches and make for easier avian seed dispersal.
It is critical to undergo reforestation because many bird species are sensitive to fragmentation of their habitats. One such bird is the Lepidothrix coronate (blue-crowned manakin) found in southern Costa Rica. These birds are highly sensitive because of their need for forest cover to protect themselves. This is useful because it is a general and common seed disperser, providing help with the expansion of the species range for organisms. With the introduction of natural corridors in the study area, the L. coronate was analyzed to use the corridors sixty-eight percent of the time when traveling from forest to forest (Sekercioglu et. al, 2015). This is important because it provides evidence that small fragmented forest should be conserved in order to connect them for these avian seed dispersers. In summary, the introduction of natural corridors can improve avian seed dispersal rates and ranges because it provides habitat for them to freely travel through. The corridors can also help with the reforestation of fragmented forest by placing them in areas of known seed dispersers because it will lead to an increase in plant density.
Finally, with information about how generalist avian seed dispersers can help reduce the effects of fragmentation in Central America, it can be applied to the situation in Asian tropical rainforests to help with reforestation by avian seed dispersal agents. The Asian rainforest has similar general avian seed dispersers as seen in central and South America. This is critical because the use of generalist seed dispersers for the reforestation of fragmented habitats can then be applied to Asian tropical forests. As demonstrated in Puerto Rico, generalist avian seed dispersers play a major role in the rate of reforestation. With these generalist seed dispersers, there was an overall increase in the growth of trees, shrubs, and vines after two years that was statistically significant (p-value = .0017) when compared to patches without the generalist birds, which is seen in Figure 2 (Carlo and Morales, 2016). This would then be applied to the generalist seed dispersal agent, Bucerotidae (Hornbill bird) in the Asian tropical forest. In its native habitat, Bucerotidae would be able to disperse seeds in a similar manner as the generalist birds of Puerto Rico. A display with the hornbill demonstrated that it is able to disperse seeds significantly more in areas that are undisturbed than in disturbed areas (p-value= .018) in figure 4c. This is useful because it shows that after the introduction of natural corridors and the regeneration of the tropical forest, the generalist seed disperser would be able to efficiently move seeds to different areas of the forest. These two pieces of information link the generalist birds from both central and South America to that of the Asian tropical forests.
In conclusion, the negative effects of habitat fragmentation could be reversed through natural corridors and be applied to the reforestation of Asian tropical rainforests. This is useful because habitat fragmentation is an issue due to the edge effects that fragmented forests are subjected to such as genetic depression, lowered mobility, and increased rates of extinction. However, this can be reversed through the use of natural corridors because of the increased connectivity of fragmented forests. When paired with the strategic placement of the natural corridors then the range of seed dispersal and its efficiency will be increased. Finally, this can be applied to the Asian tropical rainforest because of the use of generalist avian seed dispersal agents in the tropical forest in central and South America and the Asian tropical rainforest. Overall, with these steps, the Asian tropical rainforests could go into the process of reforestation much like the tropical forests of central and South America.
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Fagan, M. E., R. S. DeFries, S. E. Sesnie, J. P. Arroyo‐Mora, and R. L. Chazdon. 2016, July 5. Targeted reforestation could reverse declines in connectivity for understory birds in a tropical habitat corridor. Wiley-Blackwell. https://esajournals.onlinelibrary.wiley.com/doi/full/10.1890/14-2188.
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Ismail, S. A., J. Ghazoul, G. Ravikanth, C. G. Kushalappa, R. U. Shaanker, and C. J. Kettle. 2017, January 30. Evaluating realized seed dispersal across fragmented tropical landscapes: a two‐fold approach using parentage analysis and the neighbourhood model. Wiley/Blackwell (10.1111). https://nph.onlinelibrary.wiley.com/doi/epdf/10.1111/nph.14427.
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