Lessons from the 50-hectare plot

In the middle of the Panama Canal lays a 1,500-hectare island created by mankind during the formation of the canal. This island once was a mountain top but when its valleys flooded, all the animals on the mountain were faced with new challenges. Ten years after becoming an island in 1923, the United States Government deemed Barro Colorado Island (BCI) a nature reserve. It was one of the earliest of its kind in the Americas. An entomologist from Chicago, Dr. James Zetek, was appointed director of the small field station called Canal Zone Biological Area (CZBA) which later evolved into a world-renowned facility now known as the Smithsonian Tropical Research Institute. Researchers from around the world took advantage of the unusual set of circumstances that created BCI and started studying the biodiversity of the island as well as changes in such over time.

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BCI is an ideal location to test principles of island biogeography theory, such as colonization and extinction rates and long term effects of isolation on species richness and abundance. In the past decade, there has been a strong focus on the decline of plant and animal species on BCI and the underlying causes. Robinson (2001) reported a ≥50% decline in 37 species of birds on BCI in 25 years and Karr (1982) reported 50-60 species of birds disappeared from BCI since its separation from the mainland. Meanwhile Stapley et al. (2015) examined 40 years of data on tropical lizards on BCI and found a population decline correlating with the southern oscillation index of the previous year, suggesting that rainfall and temperature are driving factors of population fluctuation. Basset et al. (2015) hypothesized that plant-animal interactions were the cause of local extinctions (6%) and population declines in butterfly species (decrease of 211 species in 90 years) but many species disappeared without the loss of their host plant, bringing us back to the question of why.

Looking back at the ideas of Wilson and MacArthur, if there is decreased colonization then events such as drought, floods, etc. could bring the decline or extinction of a species on an island. Since the plants and animals on BCI had evolved to be mountain species where they are separated from other populations mostly by elevation, the lake surrounding the mountain (Gatun Lake) changed the dynamics and species flow. Moore et al. (2008) designed an experiment to look at the dispersal limitations of understory bird populations by capturing 10 different common understory specialists and releasing them from a boat at multiple distances (100, 200, and 300 meters) from BCI to see how far these birds could fly before crashing into the lake of exhaustion. Note: birds that failed to make it to the island were recovered, dried off, and returned to the site of capture. The long-billed hermit (Phaethornis longirostris), a hummingbird, was the only bird able to fly 300 meters back to BCI. The Checker-throated Antwren (Myrmotherula fulviventris) failed to make it even 100 meters. These birds would not need to fly far in the understory and have evolved under conditions of dense forest. While living on a mountain they could make their way to new regions through the forest but since 1913 these birds have been trapped, unable to leave the island.

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One of the most impressive features of BCI, from the scientific perspective, is the dedication of a 50-hectare plot of tropical forest in 1980. In the plot, every plant ≥1 cm in diameter has been numbered and is censused every 5 years. No manipulative research is allowed in the area, only observational so one study will not negatively impact another. This accompanied with climatic environmental data and arthropod abundance helps not just plant biologists, but also ornithologists, primatologists, herpetologists, etc. Several researchers have found El Niño events to cause fruit shortages on BCI leading to the famine of many different species (e.g. deer, peccaries, coatis, agoutis, howler monkeys, and capuchin monkeys) (Foster 1982, Wright et al. 1999, Milton et al. 2005). From December 2010 to February 2011 Milton and Giacalone (2013) found BCI capuchin monkey populations decreased 72-77% while the howler monkey population remained relatively stable. Typically, capuchin monkeys heavily rely upon arthropods in December because few ripe fruits are available. Milton and Giacalone hypothesized that the negative impacts of heavy rainfall on arthropod populations led to the great number of capuchin deaths. Howler monkey populations were believed to not be negatively impacted because during this time they primarily consume young leaves. This is a great example of how the multitude of research efforts being conducted at BCI can amalgamate.

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On the way to the 50-hectare plot seed traps begin to pop up around the base of trees, a little closer and neon flagging tape sets one tree apart from its brethren. Stepping into the plot, passing an invisible line, the forest is covered in aluminum tree tags, flagging tape everywhere, as far as the eye can see. Some trees have data loggers on them, metal bracelets hold equipment to the trunk with foil insulation covers, others have PVC pipe coming out of them. A living, outdoor laboratory.

On our journey, we had several researchers telling us of their own projects being conducted in the 50-hectare plot. They each knew every individual plant they worked with and formed a true connection with their study species. After hiking across the entire island to find a specific Zanthoxylum ekmanii tree that one of the researchers had been working with for 5 years, we found that tropical storm Otto had knocked it down as well as several other trees on that side of the island. We were all deeply saddened by the loss of this great tree but took comfort in knowing that even in death it would provide life to other organisms in the forest. Now, its thousands of seeds will be provided an opening in the canopy to obtain light, which is vastly limited in the rainforest, and perhaps one will take the place of its maternal source.

References

Basset Y, Barrios H, Segar S, Srygley RB, Aiello A, Warren AD, Delgado F, Coronado J, Lezcano J, Arizala S, Rivera M, Perez F, Bobadilla R, Lopez Y, Ramirez JA. 2015. The butterflies of Barro Colorado Island, Panama: local extinction since the 1930s. PLOS One 10(8): 1-22.

Foster RB. 1982. Famine on Barro Colorado Island. In: Leigh EG, Rand AS, Windsor DM, editors. The ecology of a tropical forest. Washington, DC: Smithsonian Press. p 201–212.

Karr JR. 1982. Avian extinction on Barro Colorado Island, Panama: a reassessment. The American Naturalist 119(2): 220-239.

Milton K, Giacalone J. 2013. Differential effects of unusual climatic stress on Capuchin (Cebus capucinus) and Howler monkey (Alouatta palliata) populations on Barro Colorado Island, Panama. American Journal of Primatology 76(3): 1-13.

Milton K, Giacalone J, Wright SJ, Stockmayer G. 2005. Do frugivore population fluctuations reflect fruit production? Evidence from Panama. In: Dew L, Boubli JP, editors. Tropical fruits and frugivores: the search for strong interactors. Dordrecht, the Netherlands: Springer. p 5–36.

Moore RP, Robinson WD, Lovette IJ, Robinson TR. 2008. Experimental evidence for extreme dispersal limitation in tropical forest birds. Ecology Letters 11: 960-968

Robinson WD. 2001. Changes in abundance of birds in a Neotropical forest fragment over 25 years: a review. Animal Biodiversity and Conservation 24(2): 51-65

Stapley J, Garcia M, Andrews RM. 2015. Long-term data reveal a population decline of the tropical lizard Anolis apletophallus, and a negative affect of El Nino years on population growth rate. PLOS One 10(2): 1-14

Wright SJ, Carrasco C, Calderon O, Paton S. 1999. The El Nino southern oscillation, variable fruit production and famine in a tropical forest. Ecology 80: 1632–1647.

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