 These photos are from Carara National Park. From top to bottom on this page: the unidentified tropical flower looks exotic. The lizard came to rest on my friend's backpack! The leaf cutter ants harvest leaves to make a compost underground. They tend to a kind of fungis that grows on their mulch and are somewhat like ant farmers. The bottom photo is the poison dart frog Dendrobates auratus. (Click on photos for larger images.)
Part 2 of 2
Costa Rica: rich coast
Last Thursday, I watched a soccer game here in Puntarenas that was real intense. Ticos take their "fútbol" seriously. It ended up in a tie, which dissapointed everyone, but there was plenty of action.
I'd like to continue my article from last week when I told you about the extraordinary number of unique life forms within Costa Rica's boundaries. How is such an incredibly rich diversity possible? What makes Costa Rica so unique? These are some of the questions that went through my mind when I was told some of the figures I mentioned to you last week, and they may be running through your mind, too.
During these first several weeks in my tropical conservation biology course, my professor, Ted Dearstyne, has helped answer these questions for me. Originally from New York, he has lived in Costa Rica for the last eight years, and says he isn't planning on going back anytime soon!. A biologist specializing in evolution, he is very knowledgable when it comes to the ecological history of this country. I'll do my best to pass on what I've learned . . . hopefully I don't lose you along the way!
The theory that is now widely accepted goes something like this. Based on the geological record, millions of years ago, there was one giant super-continent scientists have dubbed Pangea. Over a very long period of time, through the forces of continental drift and plate tectonics, the land mass split apart and formed the seven subcontinents that make up the present earth's surface. The plates are still moving today, as is evident in sporadic volcanic activity, earthquakes, and similar phenomena.
 Until about 5 million years ago, Costa Rica was an archipelago. North and South America had evolved its own set of parallel organisms. That is, different plants and animals that had basically the same roles in their ecosystems evolved simultaneously. For example, in S. America the dominant mammals were marsupials (think kangaroos), and had pouches to house the developing embryos. Meanwhile, the dominant mammals in N. America were placental with internal embryonic development. In the north there were placental wolves, saber-toothed tigers and sloths, while in the south there were marsupial versions of each.
There were also differences in the other forms of life besides mammals between the two Americas. Then, as a result of the plate tectonics, a land bridge emerged over a period of about two million years, creating a link between the once isolated continents. Organisms began to migrate across the new isthmus (in both directions), and interacted for the first time. In nature it's survival of the fittest, and, due to a number of advantages, placental mammals dominated over the marsupials. This is why only a few examples of marsupials exist today in the Americas, such as the opossum.
 As a result of the competition between species, the diversity in Costa Rica increased dramatically. What made the difference as far as the amount of diversity present in Costa Rica puzzled everyone at first, but a scientist named J.H. Connell came up with a hypothesis in 1978. His explanation, titled the intermediate disturbance hypothesis, has now been widely accepted by biologists (although, like most theories, there are some who disagree with certain parts).
The theory states that areas in the world that have relatively low disturbance will have a low diversity of organisms. By disturbance, I mean events that lead to partial or total destruction of an individual species or group of species, such as earthquakes or volcanic activity, periodic flooding or other occurrences. Why does low disturbance—think the Great Plains of the Midwest—result in low diversity? As a result of direct competition, specialists (like a rodent that eats only a certain kind of seed or insect) will have their niches taken by more generalist type species (like raccoons that can live in many different environments and survive by eating almost anything).
 On the flip side, areas with very frequent disturbance will also have low species diversity, because before new species can evolve to fill empty niches, another disturbance makes them extinct. An example of this would be a young island that has a lot of volcanic activity, or an area that has frequent flooding or fires.
So, why is Costa Rica so diverse? It falls into the area that has a balance between the extremes and, thus, the most biological diversity. The frequency of disturbance is low enough to allow life forms to evolve and fill the niches left open by recently wiped out species.
There are other processes at work that provide for such complex ecosystems, but I'll spare you these details. If you're really interested, I'm sure the ecology professors in the biology department at Tech could enlighten you further.
Well, that’s all for this week. I hope I haven't left you too puzzled. If you have any questions over the biodiversity subject, feel free to email me at dcfenlon@mtu.edu.¡Hasta luego!
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