Coevolution in the Tropics

Due to the warm, wet conditions, the tropics are home to a wide variety of tree species. Coevolutionary mechanisms are necessary in maintaining the diversity of this environment. One common form of coevolution found in the tropics is the method of biotic seed dispersal. The fleshy fruit of trees provide birds with high energy food sources while the birds provide seed dispersal away from directly below the parent tree. The Escape hypothesis states that this bird behavior allows the vulnerable seeds to escape from the high mortality of the ground below the parent tree. Furthermore, the Janzen-Connell hypothesis describes the relationship between this phenomenon and the spatial patterns of the forest. As tree species disperse their seeds, the forest becomes more diverse in areas and large areas dominated by one species become less frequent.

The Three-wattled bellbird is one such bird which disperses the seeds of Ocotea in the cloud forest (Nathan et al. 2000). The bellbird perches on a clear branch for most of the day singing in effort to attract a mate in the gaps of the forest (Wenny and Levey 1998). The observations that the bellbird sits in the gap of the forest shows that it is a prime source of seed dispersal in the proper area for the Ocotea tree. A challenge arises in the way of timing in this coevolutionary system, however. The Three-wattled bell bird typically only visits during mating season which lasts only two to five months (Powell and Bjork 2004). Because this time period is so short, it is imperative that the Ocotea tree is fruiting during the mating season of the bellbird. Also, the Ocotea only produces fruit during a very specific time frame. The bellbirds have to look elsewhere in between this window and therefore will benefit from greater diversity in the forest. Forest fragmentation restricts the biodiversity an area can achieve as it becomes more isolating for species and may result in greater edge effects. These consequences have led to a noticeable decline in the bellbird population. The decline is also due to the observation that the bellbirds do not always choose to go to the cloud forest. Instead, they sometimes choose to carry out their mating ritual at an unprotected, lowland site. These sites are being converted into plantations and are used for lumber (Powell and Bjork 2004). The clearing of lowlands may also be causing warming and consequently a lifting of the cloud forest which would most likely affect the timing of the fruiting season (Lawton et al.) A decrease in bellbirds means a more difficult time the tree will have in dispersing its seeds and therefore cause a decrease in the Ocotea.

Lawton, R. O., U. S. Nair, R. A. Pielke, and R. M. Welch. “Climatic Impact of Tropical Lowland Deforestation on Nearby Montane Cloud Forests.” Science 294, no. 5542 (October 19, 2001): 584–87.

Nathan, Ran, and Helene C. Muller-Landau. “Spatial Patterns of Seed Dispersal, Their Determinants and Consequences for Recruitment.” Trends in Ecology & Evolution 15, no. 7 (July 1, 2000): 278–85.

Powell, George V. N., and Robin D. Bjork. “Habitat Linkages and the Conservation of Tropical Biodiversity as Indicated by Seasonal Migrations of Three-Wattled Bellbirds.” Conservation Biology 18, no. 2 (April 1, 2004): 500–509.

Wenny, Daniel G., and Douglas J. Levey. “Directed Seed Dispersal by Bellbirds in a Tropical Cloud Forest.” Proceedings of the National Academy of Sciences 95, no. 11 (May 26, 1998): 6204–7.