Archive | February, 2012

The Structure of the Tropical Rainforest

20 Feb

The structure of the tropical rainforest is very different from the temperate forests we’ve got up here in Rhode Island. There is a reason why people compare cities to jungles; the rainforest is every bit as architecturally diverse and lively as any city (but jungles are prettier and they smell better…) The structure of the rainforest is both vertical and horizontal, a three-dimensional panoply of microenvironments and niches. Here we are going to explore the nature of this structural diversity, and how niches make it possible. Animal species found in the same layer are said to feed in similar fashions as other animal species also found within the same layer. Plant species also follow a similar pattern of species convergence within the each layer. Many factors are responsible for the characteristics of the TRF. Those responsible include: the overall climate, sunlight, gaps, moisture, and nutrient. While not one factor  has been considered the limiting factor, many have commented on the availability of nutrients leading to the complexity of the TRF.

The structure of the rainforest is one of great complexity. The dense nature of the TRF illustrates evolution at its greatest challenge. With then number of plants and creatures encompassing these lands, much organization was done over time.

The TRF shows clear layers found in both its animal and plant grouping. According to P.W. Richards (1952), the vegetation of the TRF is divided into:

1)    Emergent Layers

2)    Main Story Canopy

3)    Understory

4)    Shrub Layer

5)    Herb Layer/ Forest Floor

These layers provide different microhabitats for animal and insect species which was defined by J.L. Harrison (1962) into six categories which are:

1)    Upper

2)    Canopy

3)    Middle- Flying animals

4)    Middle –Climbing animals

5)    Large Ground Animals

6)    Small Ground Animals

The strata of a TRF

Tropical rainforests are characterized by high temperatures, heavy rainfall and low seasonality. This creates a perfect greenhouse-like environment in which plants can grow, but also presents a large amount of competition between different plants struggling for limited light in the event of a tree-fall.

Less than a quarter of the sun rays reach the forest floor. All plants desire sunlight, it is especially apparent when a gap forms in the forest. The gap can be caused by fallen tree, storm damage, and the effects of humans clearing the forest. Once a gap is formed, competition for sunlight begins. The gap provides the opportunity for plants that have been dormant waiting for sunlight to become revitalized and create change. Beside the gaps formed by fallen trees, we must also include the gaps found within the canopy leading to the emergence of epiphytes.

A tree-fall lets light into the lower strata.

Each layer is home to millions of its own distinct species, in a vast cobweb of symbiotic relationships and other interactions.  Although there is a huge variety of trees in the forest, there is a “typical” rainforest tree structure; most trees have little branching, and wide roots to capitalize on as much of the poor soil as they can. They often have smooth bark to try to prevent epiphyte colonization. Buttress roots are common, too. Because some trees can have extremely widespread root systems (up to 3 acres in some paleotropical forests), most emergent trees are not that close to one another. The tallest and broadest trees are not found in the rainforest. The trees have their branches concentrated near the canopy, so as to not waste energy on leaves that are never going to see the sunlight. Most trees do not have large canopies or branches because of the lack of light in the undergrowth. Plants in the lower strata have “drip-tips” to prevent accumulation of water on their leaves, which would otherwise rot quickly.

Emergent trees exhibit the "typical" rainforest tree structure

       Drip-tips keep the leaf surface dry

Epiphytes are one of the elements of the rainforest’s architecture that is especially different from what we usually see in temperate forests. They are plants that grow on other plants. They usually do not parasitize the tree but simply use it as a “ledge to stand on” to get better access to sunlight. Epiphytes in the lower strata obtain their nutrients in other ways, by feeding on decaying matter, animal feces, and rainwater leftovers. Epiphytes have been found to live in greater abundance on trees with rougher bark. In the event of a tree-fall, the epiphyte population in that particular area will usually decrease.  They also receive nutrients from shallow roots and the vessel they have formed in their host plant.  Their location on the host plant occurs in gaps of leaves and branches found on the host. The emergence of epiphytes are one of the many adaptations plants have created to overcome competition further leading to the complexity of the TRF.

The soil of the TRF has often been called the “wet-desert” because of the lack of nutrients found within it. The nutrient cycle found within the tropical rain forest is very efficient but also in high demand. The majority of carbon is found from leaf litter, living vegetation, and dying wood trees. With the aide of fungi as recycling support any material that can be reused is converted to usable material and quickly taken in by neighboring plants. While nutrient necessary for plant growth, but so is the need for energy from sunlight.

Lianas help support tall rainforest trees

Liana plants are very characteristic of the structure of the rainforest; they are woody long-stemmed vines that grow in the soil and climb up trees to the top of the canopy for sunlight. They bridge many different trees together to help them withstand strong winds. The way the ubiquitous lianas function is a good metaphor for the rainforest as a whole, I think; if a liana is dragged down, it usually pulls several tall trees with it. In the same way,  the species of the rainforest are so intertwined that there’s no such thing as a solitary extinction, really; what would happen to Darwin’s magnificent orchid if its moth were gone?


Perhaps the most critical factor enabling the spectacular structural and biological diversity of the rainforest is the plethora of biological niches in which the species establish themselves. Niches allow a vast number of organisms to thrive in a relatively small area by minimizing the competition between species. The niches are established by microclimates that are created throughout the forest’s vertical column. The upper layers, for example, are dry and extremely hot; water is scarcer up here. The lower regions are much cooler and wetter. Species segregate according to what microclimate they’ve adapted to thrive in.

The adaptations that many plants are created often revolve around the competition for nutrients or sunlight. Those that revolve around nutrients include the formation of shallow roots, stilts, buttresses, and pneumatophores, just to name a few. Each allows the plant to quickly gather nutrients upon their availability in the poor soils of the TRF. Other adaptions can also include the emergence of carnivorous plants as a way to gather nutrients outside the normal plant nutrient cycle. Sunlight adaptations include brood leaves and buttresses to provide the plant with support to reach maximum sunlight available.

Tropical rainforests have an intricate structure that makes them truly unique among biomes. Popular culture makes us think of rainforests as being some Indiana Jones-esque jungle that is impermeable to all who don’t come slinging machetes. The reality is that the rainforest’s structure is extremely varied and the underbrush is not all that thick due to lack of light. If you want to stray from the established trail, you’re more than capable of doing so.

just don't step on me.



“The Distribution of Feeding Habits Among Animals in a Tropical Rain Forest”. J. L. Harrison Journal of Animal Ecology , Vol. 31, No. 1 (Feb., 1962), pp. 53-63

Richards, P. W. (1952). The Tropical Rain-forest, an Ecological Study. Cambridge.

Kricher John, Tropical Ecology. 1st edition, Princton university Press 2011

Schnitzer S.A., Bongers F. (2002) The ecology of lianas and their role in forests. Trends in Ecology and Evolution, 17 (5):223-230.

Addo-Fordjour P, Anning A, Addo M, Osei M. Composition and distribution of vascular epiphytes in a tropical semideciduous forest, Ghana. African Journal Of Ecology [serial online]. December 2009;47(4):767-773.

Kricher, John. Tropical Ecology. First Edition. Princeton University Press 2011. Pg 103-105.


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



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