Biodiversity in the Tropics

9 Feb

There are multiple factors coming into play when discussing why the tropics are so rich with biodiversity.  Most ecologists would agree that it is a combination of these factors acting together that allow for such a high number of different species in such a relatively small region of the world.  We summarized the four major causes of tropical biodiversity and detailed how each play a role in the unique ecological make up of the tropics, specifically Costa Rica

 

Climate

The position of the sun over the equator, the location of the tropics, creates consistent warm temperatures in tropical regions. Warm temperatures in the tropics have a significant effect on biodiversity.  An article posted by the  Smithsonian Institution claimed that during the period of rapidly warming climate change known as the ‘Paleocene-Eocene Thermal Maximum’ a significant amount of species evolved and extinctions were minimal. The world warmed by 3-5 degrees C and carbon dioxide levels doubled in only 10,000 years[1]. The main focus of this article was to show that increasing temperatures in the tropics might be beneficial for speciation to occur in the tropics.

Another reason why the tropics are so diverse is because of the high degree of geographical and climate variability within them.  The Tropics are continuously expanding, which provides more chance for evolution and diversification. As the Tropics grow and expand, habitat variation also increases. Habitat variation allows speciation as organisms evolve to survive in their specialized habitat.   There are a tremendous amount of “microclimates” in tropical regions.  One example of such variation can be from the giant buttress trees growing throughout the forests.  These trees create different “sections” around it, allowing for different organisms to make their own homes.  In one crevice a tarantula can make its nest, while on the other side, a snake can make a different home.   Mountains might separate lowland rainforests, or sections of the rainforest might be cleared out for agricultural use. This isolates segments of the population from one another, and allows them to become evolutionarily divergent.  In addition within a microclimate, plants, animals, and insects reach their peak amounts of variations, and as resources deplete, species begin to compete for these resources, and only those with dominant adaptations survive.

Evolutionary and Ecological Theories

 A different view is held by ecologists, who have suggested that the tropics could act as a cradle and/or a museum.  The cradle view holds that the tropics are uniquely suited to speciation:  species generation is high, and species tend to accumulate in tropical ecosystems far more than outside the tropics. This view reflects the ideas of Dobzhansky, who hypothesized that the equitable tropical climate—offering abundant rainfall, no frost or cessation of plant growth, warm and relatively constant temperatures, and few severe meteorological fluctuations overall—allows speciation to exceed extinction to a greater degree in the tropics than at higher latitudes.  In contrast to the cradle perspective, the museum view holds that speciation rates are not higher in the tropics, but that extinction rates are exceptionally low instead.  Consequently, the tropics maintain their older species while adding new ones through evolution, which leads to a high species richness[2].  Therefore, more diversity comes about when the rate of speciation is higher than that of extinction.  The tropics have the most favorable climate for survival with prime temperatures and constant rainfall.  This, along with the high number of niches, decreases the rate of extinction.

Another theory is the Janzen Connell hypothesis, which holds that disease prevents certain species from dominating an entire region, allowing multiple species to live in close vicinity.  Since specific pathogens kill certain species that are localized in certain regions, if the pathogens kill off dominating species, then the less aggressive species living in that area would be able to survive[3]. In 2005, researchers developed yet another theory to explain how different species share limited space.  According to this theory, variations in birth rates and mortality rates of different species determine community membership.  Because more abundant species have lower birth rates and higher death rates, while uncommon species have higher birth rates and lower death rates, the species regulate themselves.  Don’t be quick to confuse “diverse” with “abundant.” While the tropics are extremely diverse, some species are plentiful while others are rare[4].

Environmental Policy

The rainforest at one point took up approximately 12% of the world’s land, but has regressed today to approximately 5.6%.  While the amounts may seem small, 5.6% is around 2.6 million square miles[5].  One suggestion is that Costa Rica’s high biodiversity and species density has a lot to do with the country’s policies on conservation.

 

Education is key in conservation, and the government of Costa Rica promotes environmental education to both its citizens and tourists.
More than 26% of Costa Rica’s land is protected, and policies continue to be made in an effort to end deforestation and protect the beauty that is Costa Rica.  “Loss of area directly translates to loss of species.[6]”  When we allow deforestation and pollution to destroy ecosystems, species go extinct.  The high value that Costa Rica places on nature has helped shaped its national and international policies and has kept the number of species living in the small country of Costa Rica high.

Plant/Primary Production

 Costa Rica, because of its location is able to support the perfect environment for tropical rainforests.  Because of the high amount of sunlight, photosynthetic organisms can consistently find energy to grow, and because of the unique relationship between primary producers and decomposers, the tropics provide secondary consumers with plenty of resources. Fungi called mycorrhizae form a mutualistic relationship with the roots of the tree, allowing for more efficient nutrient exchange. However, in the tropics, this relationship is extremely prevalent, creating a much more effective cycle of nutrient flow from the soil into the roots of the plant.  Thus, though the total amount of nutrients within the soil of a tropical region is low compared to a temperate one, the amount of nutrient recycling within a tropical region is extremely high.  In other words, high biodiversity in the tropics is a result of efficient usage of available nutrients, not necessarily a large amount of nutrients[7].

This all contributes to the very high rate of nutrient flow from one organism to another which characterizes tropical environments, all making for a ton of organisms looking for constantly shifting nutrients.  Therefore, energy productivity is high all year round, and organisms are active all year as well so organisms can specialize in one food source with the expectation that it will be there all year.


[1] Dymala, Susan et al. (2009). Floral indicators of late Paleocene-Eocene thermal maximum climate change in the Bighorn Basin, Whyoming. GEOL 393/394.

[2] Kricher, John. Tropical Ecology. First Edition. Princeton University Press 2011

[5] Darwin, Origin of Species, 1859.

[6] Kricher, John. Tropical Ecology. First Edition. Princeton University Press 2011

[7] Vitousek, P.M. and Sanford Jr., J.R.  (1986). Nutrient cycling in moist tropical rainforest.  Ann. Rev. Ecol. Syst 17: 137-167

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