The Tropical Rain Forest

 

Climate:

Warm and wet describes the tropical rain forest climate.  The average annual temperature is above 20° C; there is never a frost.  Rainfall varies widely from a low of about 250cm of rain per year to about 450 cm/year.  That means a range from about 8 to 14 feet of rain per year.

 

 

 

 

 

World Distribution:  

As you can see from the map to the right, the tropical rainforests are, indeed, located in the tropics, a band around the equator from 23.5° N (the Tropic of Cancer) to 23.5° S (the Tropic of Capricorn) (red lines on map, right).  Because the Earth tilts 23.5 degrees on its axis as it travels around the sun, at some point in the year (the solstices, June 22nd in the north, December 22nd in the south) the sun will be directly overhead on one of these lines.  At the equinoxes the sun is directly over the equator.

Within this band, solar radiation is most intense, and thus the surface of the planet warms the most.  The warmth leads to a lot of evaporation, and as warm, moist air rises, it cools, the water condenses, and the water falls back to the earth as rain.  Thus, the warmest areas of the planet also tend to be the wettest, and this sets the stage for the tropical rain forest.

Not all of the land in the tropics is tropical rainforest.  Some areas are too cold (mountaintops), or are too dry (the far side of a mountain range from the ocean gets less rain).  In some places there may be a lot of rain, but it falls seasonally and the long dry season prevents a tropical rainforest from developing.

Another biome similar to the tropical rain forest is the cloud forest.  These forests form on mountaintops in the tropics; I have been to such forests in Jamaica and Costa Rica, and they exist in other mountainous areas as well.  Because of their elevation, cloud forests are cooler than the tropical rain forests below them; much of the water there does not fall as rain but is instead wrested from the clouds by the plants living in the forest.  These forests are critically endangered by global warming; as the planet warms tropical rainforest is able to move up the mountainsides and the cloud forests are displaced into smaller and smaller regions at the tips of the mountains - and if these mountaintops get too warm the entire cloud forest will be replaced by tropical rainforest.  You can read more about cloud forests on this page from our trips to Costa Rica.

Tropical Rain Forest Distribution

      

 

Indicator Plant Species:

Because of the great diversity of plants in the tropics, naming indicator species is very difficult unless you are a botanist.  Certainly, a number of plant groups reach their greatest diversity here, but picking out individual species would be difficult.  I've put the chocolate tree here as a representative simply because just about everyone knows what chocolate is.  The chocolate is harvested from the pod you see growing here; the pods develop from flowers which are borne directly on the trunk.  This gives a good point of attachment for what will become a heavy pod.  Chocolate is native to the new world tropics, but has been transplanted to tropical regions around the world.

While it's hard to pick indicator plant species there is something that many of the tropical plants share.  That is a drip tip on the ends of the leaves (below).  This pointed tip promotes drainage from the surface of the leaf, and thus helps keep the leaf surface clean of epiphytes and fungi in the humid forest. 

How diverse are the rainforests?  A hectare in the Appalachians may have up to 30 species of trees; in the tropics a range of 40 to 100 is common and over 300 species is not unknown.  The entire La Selva site in Costa Rica has over 1,600 species of plants in 1,500 hectares; a station in the Amazon basin has over 1,800 species.  As many frogs have been collected from a single site in the rainforest as there are in all of North America.  One estimate of invertebrate diversity is for about 30 million species in tropical forests.  Read more about these estimates in this book.

One of the biggest challenges in biology is explaining the diversity of the rainforests.  Certainly the high productivity permitted by the ready availability of water combined with the warm temperatures is a factor.  The massive size of many of the trees provides a number of new habitats for animals to exploit. The extreme specialization to avoid competition is another factor.  Some scientists propose that human interference, in the guise of clearing small areas for agriculture, may play a key role as well.  And there is time; the rainforests are areas which have been relatively unaffected by the climactic shifts associated with the  glaciation which ended about 10,000 years ago. Which of these factors is most important?  Do all of them play a role? These are questions scientists are still grappling with.

Indicator Animal Species:

Gorilla - Gorilla gorilla

Gorilla - Gorilla gorilla

2-toed sloth (Cholepus hoffmanni)

2-toed sloth (Cholepus hoffmanni)

It's just as tough to select a few species of animals to represent the diversity of the tropical rainforest.  As an entomologist I'm tempted to turn to the insects, but I'll spare you that here.  Instead, we'll look at several groups that are found almost exclusively in the tropics.

The first of these are the great apes and monkeys; of the great apes only humans really get outside of the tropics to any great extent.  A few monkeys do as well, but for the most part these are species of the tropical forests.  The Gorilla is the largest of these, the Orangutan is today confined to the shrinking forests of Sumatra and Borneo.  The Spider Monkey represents the new world monkeys which are found in the tropical rain and deciduous forests of the western hemisphere.

Sloths are unique creatures of the rainforest; it is hard to imagine them living anywhere else.  We'll cover them in more detail below.

Toucans and their kin, like the Collared Aracari (below) are among the tropical birds adapted to feed on the profusion of fruits found in the forest.  The fruit is an energy rich diet and the birds repay the plants by passing the seeds out in another place.  Birds and monkeys both disperse seeds in this way and play a major role in maintaining the high diversity of the rainforest.

 

Orangutan (Pongo pygmaeus)

Orangutan (Pongo pygmaeus)

Collared Aracari - Pteroglossus torquatus

Collared Aracari - Pteroglossus torquatus

Spider Monkey - Ateles geoffreyi

Spider Monkey - Ateles geoffreyi

Ecological Notes:

Unlike a desert, water is not in short supply in the tropical rainforest.  Still, there is intense competition, only in this case the competition is for light and nutrients.

With many tall trees forming a multi-layer canopy, not much light gets to the forest floor, and competition for light is intense.  Sometimes the forest floor may be relatively bare because there just isn't enough light.  With this competition as a backdrop, the profusion of epiphytes and vines in the tropical rainforest makes sense.  Rather than grow huge trunks to hold their leaves up to the light, epiphytes and vines "cheat" by growing from the top down.  Here's how it works:  A monkey or bird eats the fruit of an epiphyte, and scurried or flies to another spot, where it defecates.  The seeds, embedded in a rich pile of feces, get caught up on the bark or in the crotch of a tree high up in the canopy where there is more light.  In the case of vines, they rapidly send roots down to the ground to get minerals (water is plentiful); epiphytes may grow against the tree or form a basin with their leaves.  The basin will fill with a mixture of water and feces from canopy dwelling animals and the epiphyte can extract minerals from this trapped water.  Perhaps no vine exhibits this strategy better than the Strangler Fig (right).  The seeds germinate in the crown of the canopy and a root slithers down to the forest floor.  Making contact, it immediately begins to send nutrients skyward, and the fig in the crown responds by dropping additional roots down the trunk.  Eventually the roots completely encircle the trunk and begin to fuse; above in the canopy the fig is shading out the host tree's leaves.  The host tree sickens and dies, and eventually decomposes.  This leaves the strangle fig standing tall, its fused roots forming a hollow trunk as tall as the original tree.

Getting light is only half the story.  Nutrients can be hard to come by as well.  The heavy rains dissolve nutrients from the forest soil and carry them away.  The remaining bedrock is slow to give up minerals, and these are again quickly washed away.  Plants, therefore, have to go to extreme lengths to retain the nutrients before they are washed away.

In a temperate forest, many of the nutrients are retained in the soil, and as leaves decompose additional nutrients are releases to the soil to be taken up almost at leisure by the trees.  When the forest is cut, the soil remaining is exceptionally rich and, with care, will sustain agriculture for a long time.  On the other hand, in the heat of the tropics leaves decompose quickly and the rain threatens to carry away the nutrients.  To stop this, a thick root mat develops in tropical forests, usually this root may is right on the surface, and leaves decompose and release their nutrients directly to the roots - the nutrients never enter the mineral soil and thus are not prone to being washed away.  This effect is particularly pronounced in poorer soils in the tropics.  The example to the left is from a volcanic slope in Costa Rica; volcanic soils are generally rich in nutrients so competition for nutrients - and development of a root mat - is not as extreme here as it is in some other places in the tropics.  It should also be mentioned that development of shallow roots is a sign of forest maturity outside the tropics as well; in the United States Beech trees are one of the trees characteristic of a mature forest and they typically have their roots right near the surface.

Another important aspect of the competition for nutrients is the role that fungi play.  Not only do fungi help decay the fallen leaves, but many of them form mycorrhizal associations with plant roots trading nutrients to the plants for sugars in return.  These mutualistic associations greatly increase the ability of forest plants to obtain nutrients.

Threats:

The usual suspects are at work again here.  First, let's consider human population growth.  Many of the fastest-growing human populations are located in the tropics, and as they clear land for sustenance farming there is a direct impact on the rainforest.  Unfortunately, once cleared, rainforest soils quickly lose their nutrients and then new areas have to be cut.  Population-driven conflicts also add to the problems; a series of wars in the Congo River basin have killed millions of people, and in the unrest forest protection (such as for the preserve where many of the world's remaining gorillas live) is impossible.  Industrialized agriculture is another threat which we will cover below.

Logging is a threat; many of the tropical trees are prized for their lumber (and some are just ground up to make toilet paper).  The pictures here are from Costa Rica, which has relatively good forest protection, but other areas of the world have little or no protection for their forests.  Often, logs are exported whole, denying the countries where the forests are even the jobs associated with turning the lumber into finished products.  The latter problem isn't limited to third world countries; the United States and Canada export a lot of logs this way (and in the meantime the corporations selling logs overseas protest government restrictions on logging by saying that it costs jobs).

Logging, Costa Rica

Logging, Costa Rica

Logging, Costa Rica

Logging, Costa Rica

Other problems face the rainforest.  Global and regional climate change are of particular concern.  You wouldn't expect global warming to have much of an effect in the tropics, and in fact the effects are less pronounced at the equator than they are at the poles.  However, in addition to warmer conditions, global climate change also means shifting rainfall patterns and that, of course, will affect rainforests.

In addition to the global threat there are regional ones as well.   As large areas of rainforest are cleared, they are no longer able to evaporate as much water through transpiration (the process by which water evaporates from the leaves and is replaced by water drawn up from the ground through the roots and the stem).  Less water evaporated means less water to fall elsewhere, and this means a drier rainforest downwind of the cut forest.  Fire is the traditional method used to clear rainforests; even in the wettest forest there may be one season a bit drier than the others and it is then that the fires are set.  In recent years, perhaps fed by drier conditions as result of the rainforest already cleared, some of these fires have reached epic proportions, particularly in Indonesia and in Brazil.

 

Hillside, Jamaica

Hillside, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

We'll start with a relatively simple rainforest on the island of Jamaica.  Parts of this island get enough rainfall to sustain rainforest, although it is not nearly as diverse as rainforests found on the mainland.  In the photo above you can see how some of this forest has been cleared to make way for farm fields and pasture.  In the upper right image a trail had recently been bulldozed up a hill; you can see the density of plant growth on the borders of this artificial light gap as the plants quickly respond to the increased sunlight.  The other two pictures to the right show the multilevel canopy of a rainforest and some of the vines there.  Below, ferns are very common in tropical forests, where the abundance of moisture makes it easy for these primitive pants to reproduce (they need water for the sperm to swim to the egg).  The waterfall below the waterfall is a reminder of the constant presence of water in the rainforest.

Rainforest, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

Rainforest, Jamaica

River Station, La Selva, Costa Rica

River Station, La Selva, Costa Rica

La Selva, Costa Rica

La Selva, Costa Rica

In 2005, the Marietta College Biology and Environmental Science Department sponsored a 3-week trip to Costa Rica.  This trip gave us an extended opportunity to visit an number of different biomes, and perhaps the highlight of the trip was the time we spent at the Organization for Tropical Research's field station at La Selva.  This is a good example of a lowland tropical rainforest.

We stayed at the River Station at La Selva (above left)  For a larger view of the map, try this link.  To control erosion, many of the trails at La Selva are paved (left) (otherwise, with the heavy rains the trail system would quickly turn into a system of gullies), there are also bridges over many of the streams.  This is a biological preserve, and walking the trails one encounters many plots where one or another scientific study is being conducted (below).

La Selva, Costa Rica

La Selva, Costa Rica

La Selva, Costa Rica

La Selva, Costa Rica

Whiptail Lizard, Ameiva festiva, La Selva, Costa Rica

Whiptail Lizard, Ameiva festiva, La Selva, Costa Rica

Spider Monkey - Ateles geoffreyi

Spider Monkey - Ateles geoffreyi

Chestnut-mandibled Toucan (Ramphastes swainsonii), La Selva, Costa Rica

Chestnut-mandibled Toucan (Ramphastes swainsonii), La Selva, Costa Rica

The wildlife at La Selva was diverse and omnipresent.  Our only look at Spider Monkeys - one of 4 species of monkey in Costa Rica - came there,  Lizards, including the Whiptail Lizard, were common on the forest floor and in the trees; in both places they were after insects.  We saw several species of Toucans; the large bills are used to remove fruits from trees.

The Rufous Motmot has a varied diet - it eats invertebrates, small vertebrates and fruits - and it has even been observed eating poison dart frogs in the wild.  The similar blue-crowned motmot was seen in the tropical dry forest at Santa Rosa.

Collared Aracari - Pteroglossus torquatus

Collared Aracari - Pteroglossus torquatus

Rufous Motmot (Baryphthengus ruficapillus) La Selva, Costa Rica

Rufous Motmot (Baryphthengus ruficapillus) La Selva, Costa Rica

Cicada - La Selva, Costa Rica

Cicada - La Selva, Costa Rica

Pleasing Fungus Beetle, La Selva, Costa Rica

Pleasing Fungus Beetle, La Selva, Costa Rica

  Wasp and Spider - La Selva, Costa Rica

Wasp and Spider 
 La Selva, Costa Rica

The insects at La Selva were extremely diverse.  Cicadas were discussed in the temperate deciduous forest section, we also found them at La Selva.  The Pleasing Fungus Beetle feeds on fungi, there were several large species in the forest at La Selva and also at Rincon de la Vieja, a rainforest on the slope of a volcano.

At night walkingsticks were fairly commonly seen.  Normally they perch motionless during the day, and they blend into the trees so well it is very hard to spot them.  At night however, with their main predators, birds, asleep, the walkingsticks begin to move about and feed on vegetation. 

There are a number of large ant species at La Selva, but none as large or as feared as the Bullet Ant (below).  These ants are about 1 inch (25mm) long and apparently have quite a nasty sting.  Reportedly, these ants' stings are much like getting shot with a bullet, which begs the question of how many people have experienced both to make the comparison.  In any event, we learned to watch carefully for these ants, whose small colonies were found at the bases of trees. Dr. Brown, who stayed on in Costa Rica for several weeks after the rest of the group had left, had told us that he was going to let one sting him, but he wimped out.  Of course, the rest of us were all willing to be stung for science's sake, but after he said he was going to do it we figured that one replication would suffice.

 

Pleasing Fungus Beetle, Rincon de la Vieja, Costa Rica

Pleasing Fungus Beetle, Rincon de la Vieja, Costa Rica

 

Bullet Ant (Paraponera clavata) - La Selva, Costa Rica

Bullet Ant (Paraponera clavata) - La Selva, Costa Rica

Walking Stick - La Selva, Costa Rica

Walking Stick - La Selva, Costa Rica

Blue-Jeans Dart Frog (Denrobates pumilio) La Selva, Costa Rica

Blue-Jeans Dart Frog (Dendrobates pumilio) La Selva, Costa Rica

Coati - (Nasua narica)  - La Selva, Costa Rica

Coati - (Nasua narica)  - La Selva, Costa Rica

Green Leaf Frog eggs (Agalychnis callidryas) - La Selva, Costa Rica

Gaudy Leaf Frog eggs (Agalychnis callidryas) - La Selva, Costa Rica

Coatis are common in Costa Rica; they are similar in many ways to a stretched-out raccoon (those live there as well).  In some places they are not afraid of people (and, of course, where they have been fed they will actually beg for food).  This particular individual maneuvered around a group of people to cross the long suspension bridge at La Selva, undeterred that a second group of people was mid-bridge at the time (he calmly walked past both groups).  They look to be well adapted to life in trees, but every specimen I saw was on the ground.  

We also saw the Blue-jeans Dart Frog on the ground - a lot.  These strawberry sized and colored frogs were quite common, particularly in the morning.  We saw them crawling about the leaf litter, on logs, on tree trunks and up in the vegetation.  One of us even saw a female transporting a tadpole (attached to her back) from its nest on the forest floor to a bromeliad high up in the canopy, where the tadpole would develop in the water retained at the center of the bromeliad (some of the large bromeliads can hold 10 gallons - 40 liters - of water).  The individual here is a male calling; that was the sound you heard when this page opened; you can check it out again here.

Near to the River Station was a swamp where Gaudy Leaf Frogs were mating.  We were able to observe them on several nights, and see them lay eggs.  Several times we found the adults elsewhere in  the forest sleeping during the day.  While asleep, they press themselves tightly to the leaf, both to blend in and to minimize evaporation.  In this sleeping position, all the "gaudy" parts (the bright red eyes, toepads, the blue striped sides, etc.) are well hidden.

Gaudy Leaf Frog (Agalychnis callidryas) - La Selva, Costa Rica

Gaudy Leaf Frog (Agalychnis callidryas) - La Selva, Costa Rica

Gaudy Leaf Frog (Agalychnis callidryas) - La Selva, Costa Rica

Gaudy Leaf Frog (Agalychnis callidryas) - La Selva, Costa Rica

Gaudy Leaf Frog (Agalychnis callidryas) - La Selva, Costa Rica

Gaudy Leaf Frog (Agalychnis callidryas) - La Selva, Costa Rica

Leaf-cutting Ant (Atta cephalotes) - La Selva

Leaf-cutting Ant (Atta cephalotes) - La Selva

Leaf-cutting Ant (Atta cephalotes) - La Selva

Leaf-cutting Ant (Atta cephalotes) - La Selva

Leaf-cutting ants were a primary feature of the forest at La Selva as they are at many other tropical forests.  Above, you can see one of the large workers using her mandibles to cut a piece of leaf (the cut is to the right of the ant).  Above right a large worker carries a piece of leaf back to the nest; smaller workers ride on the leaf fragment. They are there to discourage a species of parasitic fly from attacking the large worker and laying an egg on it (the small workers are too small for the fly larvae to develop).

Our students carried out an experiment while at La Selva, they counted the number of workers returning to the nest and the number of leaves they were carrying.  They also retrieved a number of the leaf pieces, which were then dried and weighed.  From this we could make rough estimates of how much leaf material was being carried underground by the ants (it's a lot!).  You can see the opening of a mound in the picture below; some ant nests are large enough that if they were excavated completely a person could stand in the resulting hole.  The leaves will serve as the substrate for a fungus to grow; the ants eventually consume the fungus as their food.  The fungus takes care of breaking down any chemicals that would be toxic to the ants (the ants are careful not to pick leaves with anti-fungal properties).  The ants move the fungus to new nests, remove competing fungi, and even use a bacterium to produce antibiotics to eliminate any fungal competitors that do get into the fungus farm.

Leaf-cutting Ant (Atta cephalotes) nest - La Selva

Leaf-cutting Ant (Atta cephalotes) nest - La Selva

Leaf-cutting Ant (Atta cephalotes) - La Selva

Leaf-cutting Ant (Atta cephalotes) - La Selva

Butterfly - La Selva, Costa Rica

Butterfly - La Selva, Costa Rica

Collared Peccary (Tayassu tajacu) - La Selva, Costa Rica

Collared Peccary (Tayassu tajacu) - La Selva, Costa Rica

Caterpillar - La Selva, Costa Rica

Caterpillar - La Selva, Costa Rica

Again, the wildlife was diverse and you never knew what you would see next - perhaps a Collared Peccary on your way to breakfast?  These pig relatives were relatively common, not just around the buildings, but also deep in the forest.  

There were also beautiful butterflies looking more like stained glass, caterpillars colored to blend in with epiphytes on the surface of leaves, and ground beetles hunting invertebrate prey on the forest floor.  

Another forest predator was the Bird-eating Snake.  Despite the name, this snake eats a variety of small animals in addition to birds; the one we encountered was on the forest floor in the arboretum.  It didn't seem too concerned with us and we were able to watch it closely for some time.

Carabid Beetle - La Selva, Costa Rica

Carabid Beetle - La Selva, Costa Rica

 

Bird-eating Snake (Pseustes poecilonotus), La Selva, Costa Rica

Bird-eating Snake (Pseustes poecilonotus), La Selva, Costa Rica

Butterfly - La Selva, Costa Rica

Butterfly - La Selva, Costa Rica

 

Dead-leaf Mimic Katydid, La Selva

Dead-leaf Mimic Katydid, La Selva

Working - even just walking - in the rainforest is a sweaty proposition and there are animals there to take advantage of every niche - even the licking salt off the wrist of biologists niche as the butterfly above is doing.  Salts are necessary for egg production and in some species the males make a significant contribution when they transfer salts to the female in the sperm package.  When you see butterflies "puddling" on the forest floor - or on a salted roadside - they are engaged in the same activity. 

Katydids make up a big proportion of the nighttime chorus at La Selva, and there are a number of different species.  What is amazing is the lengths they will go to camouflage  themselves in the daytime.  In Ohio, we have the common leaf mimic katydids, which look like a green leaf.  That doesn't cut it in the rainforest.  Katydids that forage on the forest floor with the dead leaves are brown, like the leaves they are walking on.  Most amazing are the damaged leaf mimics which look like a leaf which has been fed on by insects.  The level of detail is amazing, all the way down to the "veins" of the leaves (below right).

Bromeliads, as mentioned earlier, are epiphytes which avoid competition for light by growing on established trees.  Theoretically, since there is no direct impact on the tree we classify this type of symbiosis as commensalism, a relationship where one species gains without hurting the other.  You do have to wonder about the effect of this many bromeliads (below) on a tree - how much extra material had to go into the branches to allow them to support all that weight (a large bromeliad can hold 10 gallons of water, which would weigh 80 pounds (36+ kilos).  These aquaria in the sky are home to a number of animals including tadpoles of the poison dart frogs, mosquitoes, damselfly larvae and other organisms.

Damaged-leaf Mimic Katydid, La Selva

Damaged-leaf Mimic Katydid, La Selva

La Selva, Costa Rica

Bromeliads, La Selva, Costa Rica

Damaged-leaf Mimic Katydid, La Selva

Damaged-leaf Mimic Katydid, La Selva

Root Mat, La Selva, Costa Rica

Root Mat, La Selva, Costa Rica

Root Mat, La Selva, Costa Rica

Root Mat, La Selva, Costa Rica

Closed Canopy, La Selva, Costa Rica

Closed Canopy, La Selva, Costa Rica

As stated earlier, roots run shallow in mature forests, particularly rainforests.  In Venezuela, they can form a mat 45cm (almost 18") thick on top of the soil.  We didn't see anything that dramatic in La Selva, but in a few places the roots were exposed where a trail crossed them.

We did see evidence of the other type of competition, the competition for light.  The photo to the left shows a relatively intact canopy; light coming to the ground may pass through several leaves by the time it reaches the ground and thus most of the photosynthetically active wavelengths will already be absorbed.  On the other hand, when a tree falls (below), it opens up a significant gap in the canopy (falling trees often are interconnected to other trees by vines, and thus one falling tree might drag down several others with it).  This opening brings in light and promotes growth on the forest floor.  In the photo below, the treefall is fresh and the plants haven't really responded to the light yet.  In any event, the light-loving plants that develop in the light gaps increase the overall diversity of the forest.

Open Canopy, La Selva, Costa Rica

Open Canopy, La Selva, Costa Rica

Light Gap, La Selva, Costa Rica

Light Gap, La Selva, Costa Rica

Rio Puerto Viejo, La Selva, Costa Rica

Rio Puerto Viejo, La Selva, Costa Rica

La Selva, Costa Rica

La Selva, Costa Rica

With all the rain falling, there are sure to be rivers and streams to carry the water away.  Right outside our door at the River Station was the Rio Puerto Viejo.  We spent a fair amount of time in the water, exploring the banks and swimming (I found out after we returned that crocodiles had been spotted in the river there, but we didn't see any).  A number of smaller streams cross the station as well.  In the figure above right you can see one of these, running full with some silty water.  I imagine the source of the silt must be the trail system.  While many of the trails are paved (like the one to the right), others are not, and even the edges of the paved trails are subject to erosion.

It's hard to get a real sense of the size of the trees in the rainforest, since from below the multilevel canopy blocks a direct view of the treetops.  However, here and there along the rive or at the edge of a cleared area one can catch a glimpse of one of the larger trees such as the one below right.

Another hazard is the presence of planted specimens, particularly on the developed grounds of the station itself.  The breadfruit (below) looked right at home, but in fact was a planted cultivar.

La Selva, Costa Rica

 La Selva, Costa Rica

 

La Selva, Costa Rica

La Selva, Costa Rica

La Selva, Costa Rica

La Selva, Costa Rica

As in the tropical seasonal forest at Santa Rosa, we ran a transect through the forest to see how dense the plants were.  These pictures pretty much show the results.  We went to a part of the forest known to be primary forest - never logged - but the underbrush was incredibly thick anyway.  This contradicts much of the writing about the forest floor of tropical rainforests; they are often described as relatively open due to the lack of light.  For whatever reason, that was no the case here.  There are 6 students in the picture at the upper left - can you see them all?  The pictures above and left also give you a feel for the density of the plants in some areas.

The concept of nurse logs was covered more thoroughly in the section on temperate rainforests, but we'll mention it again here.  Sometimes a seedling sprouts on a fallen log.  This is an advantage to the seedling because it is above the litter and plants on the forest floor.  It gets more light, it gets nutrients out of the decaying log, and it gets a head start.  The situation is more common in the temperate rainforest because of the slow rates of decomposition there.  In the tropical rainforest, nurse logs are more rare, mainly because the trees rot so quickly after falling.  Still, some particularly rot-resistant trees can serve as nurse logs as is seen below left. 

Finally, from La Selva, we find that not everyone has gotten the word that sunbathing is bad for you.  Do you see the Iguana sunbathing in a tree 40 feet over the Rio Puerto Viejo?  Iguanas often bask over water; it gives them an added escape option. 

Nurse Log, La Selva, Costa Rica

Nurse Log, La Selva, Costa Rica

Iguana, La Selva, Costa Rica

Mountain Stream, Rincon de la Vieja, Costa Rica

Mountain Stream, Rincón de la Vieja, Costa Rica

Termite Nest,  Rincon de la Vieja, Costa Rica

Termite Nest, Rincón de la Vieja, Costa Rica

We also saw a rainforest on the slopes of the Rincón de la Vieja volcano in Costa Rica. Although located on the Pacific, or dry side of the country, it is at a high enough altitude that it gets enough rain during the dry season to avoid being a tropical seasonal forest like the forest downhill at Santa Rosa on the coast.  Not a lowland rainforest like the one at La Selva, it is nonetheless similar in many ways.  Above, you can see a clear mountain stream. 

Above left, an arboreal termite nest.  In Santa Rosa we found termites nesting on the ground, but in a rainforest that doesn't seem to be such a good idea, and instead the nests are built up in the trees (this one is on a vine).  The nests are made of carton, a material formed of termite saliva and chewed up wood pulp, similar to a very hard paper-mache.  Termites are one of the reasons trees disintegrate so quickly after they fall (and sometimes before they fall).  While the ants do much to recycle leaves, the termites handle the wood.

We also got a good look at strangler figs there (below right); there were several of these giants which had overtaken some really big forest trees.  A close-up of how the roots anastomose is shown in the image to the right, which is of a specimen in the cypress swamps of southern Florida. Learn more about Strangler Figs here.

Strangler Fig - Florida

Strangler Fig - Florida

Mountain Stream, Rincon de la Vieja, Costa Rica

Mountain Stream, Rincón de la Vieja, Costa Rica

Strangler Fig - Rincon de la Vieja, Costa Rica Strangler Fig - Rincon de la Vieja, Costa Rica

Strangler Figs - Rincón de la Vieja, Costa Rica

Capuchin Monkey (Cebus capucinus), Caño Negro, Costa Rica

Capuchin Monkey (Cebus capucinus), Caño Negro, Costa Rica

Mantled Howler Monkey (Allouata palliata), Caño Negro, Costa Rica

Mantled Howler Monkey (Allouata palliata), Caño Negro, Costa Rica

Spider Monkey - Ateles geoffreyi

Spider Monkey - Ateles geoffreyi

There are 4 species of monkeys in Costa Rica, and by the end of the trip at least one of us had seen all 4 types.  Everybody got to see the capuchins at Santa Rosa, Caño Negro and Manuel Antonio.  These monkeys were not too shy around people, and the open canopy at Santa Rosa gave us a really good look.  Likewise, we all saw howler monkeys, particularly at Santa Rosa where a troop moved into the tree over the dining hall for the duration of our stay.

There was a small troop of Spider Monkeys that a few of us got to see deep in the forest at La Selva (left)

Dr. Brown was the only one to see Squirrel Monkeys (below); he saw them on a return trip to Manuel Antonio after the rest of the class had left.  As you can see, they have adapted to humans pretty well, at least in the case of making use of our aerial highways.

If you want to know more about these monkeys, go here, we've got more pictures, larger pictures, and even some video of these 4 species.  You can see one of the videos here.

Squirrel Monkey  (Saimiri oersteddi), Manuel Antonio, Costa Rica

Squirrel Monkey  (Saimiri oersteddi), Manuel Antonio, Costa Rica

Squirrel Monkey  (Saimiri oersteddi), Manuel Antonio, Costa Rica

Squirrel Monkey  (Saimiri oersteddi), Manuel Antonio, Costa Rica

Army Ants - La Selva, Costa Rica

Army Ants - La Selva, Costa Rica

Army Ants - La Selva, Costa Rica

Army Ants - La Selva, Costa Rica

We saw army ants at two places in Costa Rica, La Selva and at Santa Rosa in the tropical seasonal forest.  The species at La Selva that we saw (above) was apparently one of the species that raids the nests of other ants.  We encountered them one evening as they were going out on a raid; they were moving along and across the concrete stairs leading up to the river station.  In the photo above left you can see one of the very large soldier ants next to two "regular" sized workers.  Above, the trail of ants itself; to the right of the image a number of ants have linked their bodies to form a living "bridge".  You can see a video of these ants  here; note that the video is big and you might have trouble downloading it.
Heart-of-Palm Plantation, Costa Rica

Heart-of-Palm Plantation, Costa Rica

Banana Plantation, Costa Rica

Banana Plantation, Costa Rica

Agriculture is a big threat to tropical rainforests.  The year round forest makes it possible to grow virtually any crop, and of course certain tropical food plants like bananas are restricted to tropical lands.  Costa Rica is not typical of the majority of tropical rainforest land when it comes to agriculture.  The volcanic soils are relatively young and rich, and continue to yield high harvests a number of seasons after the rainforest has been cut.  In many areas of rainforest, agriculture can be supported only for a few seasons after the forest is cut before the nutrients are depleted.  In addition, some tropical rainforest soils  undergo a physical transformation when exposed to heat and sun; they bake into an almost solid surface that is unsuitable for plant growth.  

That being said, let's take a brief look at some agricultural practices in Costa Rica.  Above, Heart-of-Palm plantations produce palms whose main product is oil.  Bananas, of course, are a plant many people think of when they think of tropical fruits; plantations in Costa Rica have tried to minimize a number of environmental problems, but still depend on large quantities of pesticides.  You can link to some movies of banana harvesting and processing from this page. Pineapples are actually a form of bromeliad (we met them above in their usual guise as epiphytes).   Again, this is a crop which depends on large inputs of pesticides.  Chocolate, below, is interesting because the trees can be crown in the shade of relatively intact rainforest.  This is a case where agriculture, carefully done, can help to preserve ecosystems.

Pineapple Plantation, Costa Rica

Pineapple Plantation, Costa Rica

Chocolate (Theobroma cacao), La Selva, Costa Rica

Pineapple Plantation, Costa Rica

Pineapple Plantation, Costa Rica

 

In many tropical areas the raising of livestock takes up considerable land.  The areas in the photos above were cleared to support dairy operations.  Clearing the forest in such areas, particularly in the mountains, can lead to erosion.  The erupting Volcán Arenal is visible in the background of the picture at the left, above. Below, an innovative Costa Rican solution to the problem of fencing in cattle.  Cuttings of a number of plant species are used as fence posts; the cuttings sprout and grow, providing shade and a windbreak. In Costa Rica this is called a living fence.

Bats, Caño Negro, Costa Rica

Bats, Caño Negro, Costa Rica

Banyan (Ficus bengalensis) - Cypress Gardens, Florida

Banyan (Ficus bengalensis) - Cypress Gardens, Florida

 

Bats, Caño Negro, Costa Rica

Bats, Caño Negro, Costa Rica

 

Above, the Banyan Tree, native to southeast asia, is grown as an ornamental in many places, including Florida.  This Fig tree, like the Strangler Fig, drops roots from above, the difference is it does grow its own central trunk.  The seeds are tiny and spread easily, the tree is considered to be an invasive species by some jurisdictions.

Right: Bats play a number of important roles in tropical forest ecosystems.  The bats to the left and above left were roosting during the daytime on a tree trunk leaning over a river.  Presumably, they would fly at night to feed on insects, but some bats specialize on fish or even frogs.  Note also the covered termite gallery on the tree trunk to the right of the bats in the picture to the immediate left. 

The Egyptian Fruit Bat (below left) feeds on fruits and is thus important as a dispersal agent of seeds; some bats also pollinate flowers.

The Green Vine Snake (below) is mildly venomous, an inhabitant of the rainforests of India and nearby areas.  It feeds on a variety of small vertebrates, especially frogs.

Egyptian Fruit Bat (Rousettus aegyptiacus) - Cleveland Metroparks Zoo

Egyptian Fruit Bat (Rousettus aegyptiacus) - Cleveland Metroparks Zoo

Green vine snake, Oxybelis fulgidus - Cleveland Metroparks Zoo

Green vine snake, Oxybelis fulgidus - Cleveland Metroparks Zoo

Central American Parrot Snake (Leptophis ahaetulla) Rincon de la Vieja, Costa Rica

Central American Parrot Snake (Leptophis ahaetulla)
 Rincón de la Vieja, Costa Rica

 

Central American Parrot Snake (Leptophis ahaetulla) Rincon de la Vieja, Costa Rica
Speaking of eating frogs, we saw this  Central American Parrot Snake eating a frog in the rainforest at Rincón de la Vieja.  It is really amazing what a snake can swallow - or thinks it can swallow!

Central American Parrot Snake (Leptophis ahaetulla) Rincon de la Vieja, Costa Rica

Central American Parrot Snake (Leptophis ahaetulla) Rincón de la Vieja, Costa Rica

Three-toed Sloth (Bradypus variegatus) with baby - Costa Rica

Three-toed Sloth (Bradypus variegatus) with baby - Costa Rica

Cecropia sp. - damaged leaves

Cecropia sp. - damaged leaves

2-toed sloth (Cholepus hoffmanni)

2-toed sloth (Cholepus hoffmanni)

Sloths are a unique member of the rainforest community.  These creatures are slow moving, in part because they have maximized their body size with a minimum increase in the size of the muscles.  The problem for the sloth is that you need a minimum body size in order to eat plant material (you basically have to let it rot in your gut to release nutrients, and that takes a lot of room).  If they get too heavy, however, they could be seriously injured falling from a tree, and such falls would be frequent as they moved onto small branches to get to leaves.  Therefore, they have very little muscle mass for their size, and can move very slowly.  They are even more helpless on the ground (they return to the ground once a week or so to urinate).  There are two species in Costa Rica, the 3-toed (above left) and 2-toed (below left).  You can see algae growing in the fur of the 3-toed sloth adult (the baby has brown fur); the algae help the sloth blend in with the lichen-covered tree bark. Eventually the baby's fur will be covered with algae as well.

One of the most likely places to see a sloth in Costa Rica is in a Cecropia tree.  These trees, with their distinctive umbrella-shaped leaves (above) are common both in the rainforests and even in cleared areas, where they either are not cut or are able to regrow quickly.  As you can see from the picture above, Cecropia trees are also targeted by herbivorous insects, which chew holes in the leaves.

The capybara (below left) is the largest rodent in the world, reaching the size of a medium dog.  It is found in the rainforests of South America, particularly in forests that flood seasonally.  Note the mouse, one of the smaller rodents, next to the capybara.

The iguana is a large, arboreal, herbivorous lizard of the American rainforests.  The young are more carnivorous in their tastes, but as they mature and grow they become large enough to digest plant material effectively and they begin to consume more plant material.  They can grow to a length of about 2 meters, with meat comparable to that of a chicken (in quantity, not necessarily taste).  In some places iguanas are "farmed"; gravid females are caught and their eggs and young raised by hand for a while.  Once the young are big enough to stand up to most predators, they are released to fatten up in the wild; at some point they are harvested.  These methods cause less damage to native habitats than raising poultry would, and provide some economic incentive to preserving rainforests.

 

Capybara (Hydrochaeris hydrochaeris) (and mouse) - Cypress Gardens, Florida

Capybara (Hydrochaeris hydrochaeris) (and mouse) - Cypress Gardens, Florida

Iguana (Iguana iguana) - Marietta College

Iguana (Iguana iguana) - Marietta College

Hog-nosed Pit Viper  - Porthidium nasutum Fer-de-Lance - Bothrops asper
Eyelash Viper - Bothriechis schlegelii - Costa Rica

Snakes are common in tropical rain forests, but they are not as ubiquitous as many people dread.  In 6 weeks spent trampling the countryside of Costa Rica we saw a total of 5 poisonous snakes - and we spent a lot of time looking specifically for them.  Venom enables the snake to more quickly overcome their prey - a handy tool for an organism that lacks legs to hold onto the prey (although the Parrot Snake above didn't seem to have any problem gripping a slippery frog high up in the trees).

 

My main problem with poisonous snakes in the tropics is that I always seem to find them when I have the shortest possible lens on my camera - meaning I have to get really close to get a picture!

See more poisonous snakes here.

Sulfur-crested Cockatoo (Cacatua galerita) - Captive Specimen

Sulfur-crested Cockatoo (Cacatua galerita) - Captive Specimen

 

Blue and Gold Macaw (Ara ararauna) - Captive Specimens

Blue and Gold Macaw (Ara ararauna) - Captive Specimens

Among the birds of the tropics - and birds in general - none are considered as intelligent as the various parrots, macaws, cockatoos and their kin.  The prevailing theory for this avian intelligence is the fact that they are frugivores, dependent on their intellect to navigate their way to a variety of trees that bear fruit at different times in very different places.  Finding such scattered resources and remembering where they are takes some brainpower; likewise birds like our blue and scrub jays, which must remember where they've cached their food, are smarter than the average pigeon.  The bright colors and varied vocalizations help these birds communicate in the rainforest.

It's not a bird, but it plays one on TV - the bird-of-paradise flower looks like its namesake but is actually a relative of the banana.

Finally, below, the rainforest meets the Pacific Ocean at Manuel Antonio, in Costa Rica.

Bird-of Paradise Plant - Strelitzia reginae (cultivated)

Bird-of Paradise Plant - Strelitzia reginae (cultivated)

Manuel Antonio, Costa Rica

Manuel Antonio, Costa Rica

You can read and see more about rainforests here:

Our trip to La Selva
Slide Show of La Selva
Slide Show from Rincon de la Vieja

Videos from Costa Rica

 

 

 

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