Primary- or Rainforest .
The forest floor
The primary forest is built up out of high trees with dense canopy and its soil is not rich of nutrients. Trees can be as high as 40 meters and only small amount of light can enter the forest floor. In the hilly areas rivers, small rivulets and creeks supply the drainage of the tropical rains that almost every day early mornings between 5 am and 9 am occurs. Most animal life can be found in the canopy where birds feed and apes seek their food as well. The undergrowth in a rainforest is restricted in many areas by the lack of sunlight at ground level. This makes it possible for people and other animals to walk through the forest. If the leaf canopy is destroyed or thinned for any reason, the ground beneath is soon colonized by a dense tangled growth of vines, shrubs and small trees called jungle. At the forest floor only birds like the Crax alector , in Guyana called "Powis" a black bird that only fly if hunted can be seen. Ants and many other insects, spiders, snakes and large mammals like big cats and tapir are living on the forest floor also.
The soil is low of nutrients and waters become dark brown colored by the tannin from fallen leaves. Although the color is brown the water is clear like thee is. These "blackwater" creeks are the ideal biotope for our Rivulus fishes. pH is very low and also very acid. Dissolved calcium is so low that a simple liquid test shows almost always no calcium at all. Only the use of a electronic microsiemens device can help to figure out what the value's are.
Rainforests are home to two thirds of all the living animal and plant species on the planet. It has been estimated that many hundreds of millions of new species of plants, insects and microorganisms are still undiscovered. Tropical rain forests are called the "jewels of the earth", and the "world's largest pharmacy" because of the large number of natural medicines discovered there. Tropical rain forests are also often called the "Earth's lungs", however there is no scientific basis for such a claim as tropical rainforests are known to be essentially oxygen neutral, with little or no net oxygen production.
The creeks and especially the swampy parts along the creeks give room for Rivulus and other killies like for instance Pterolebias and Moema. Do to the daily rains the creeks and swamps keep always water and if not occasionally the Rivulus can seek other niches by jumping over the forest floor and find a suitable pool or other creek. Often they live in small puddles that are not connected to a creek at all. In these micro habitats they live from tadpoles ants and mosquito larvae. No other fishes can enter these pools and predate on the Rivulus.
The rainforest is characterized by high rainfall, with definitions setting minimum normal annual rainfall between 1750 mm and 2000 mm (68 inches to 78 inches).
Tropical South American rainforests have some of the largest rivers in the world, like the Amazon, Negro, Orinoco, and Sao Francisco, because of the tremendous amount of precipitation their watersheds receive. These mega-rivers are fed by countless smaller tributaries, streams, and creeks. For example, the Amazon alone has some 1,100 tributaries, 17 of which are over 1,000 miles long. Although large tropical rivers are fairly uniform in appearance and water composition, their tributaries vary greatly. Many tropical rivers and streams have extreme high and low water levels that occur at different parts of the year.