Three Habitats. One Amazon Cruise.

March 24, 2014

Varzea. Igapo. Terra firma. These terms, while unfamiliar to most people, are extremely important in the types of habitats they describe. While on IE's Amazon River cruises, guides will occasionally use these terms and explain the meanings but rarely is there time to more fully describe how important and why these terms are meaningful.

Terra firma is upland habitat where the elevation does not allow water, even during high water season, to inundate the forest. The soils are often clay soils with very minimal top soil as the heat and humidity allows for amazing biological action and rather than building much soil, the dead leaves, trees and other organic matter is consumed. An intriguing feature of terra firma is the plants and animals that thrive in that habitat and many species are entirely dependent on terra firma. Because of my interest in amphibians, I am going to use a species of frog as an example. The beautiful reticulated poison-dart frog which is a diminutive creature is reddish with black spots on its sides. These bright little jewels of the forest are only found in terra firma habitats. As the elevation drops to the level of water inundation during high water, other poison-dart frogs may be found but not the reticulated species. This is just one of many hundreds if not thousands of species dependent on terra firma. There are countless insects as well as birds, primates, big cats, reptiles and amphibians that cannot exist in any other habitat.

Igapo — what a great word! I even enjoy saying the word as it sounds cool!  Igapo is a term that describes lowland forest that is seasonally flooded (high water) in a blackwater system. The blackwater systems of rivers, streams, lakes and oxbows are prevalent in sandy soil regions and the water takes on the color of tea. It is very clear but tannin rich from the decomposition of leaves and wood and is fairly acidic. These waterways are also fairly slow flowing and at times the flow rate may be unperceivable. It is in these habitats that the gorgeous reflections of trees and sky are mirrored on the surface of the water and in photographs it is often difficult to determine where the highly reflective water meets the surroundings. It is also in these areas where, during low water, tremendous numbers of fish eating birds (egrets, herons and cormorants) congregate in massive feeding flocks to fatten up on the bounty of fish entering the rivers as water recedes from the flooded igapo forest.
   
Varzea. Another wonderful word and it is similar in meaning to Igapo but varzea is seasonally flooded forest areas in whitewater habitats. The term whitewater in the Amazon does not have the same meaning that we are familiar with in the U.S. Whitewater, for many of us refers to high turbulence areas, such as rapids in our rivers. In the Amazon Basin, whitewater is actually best described as brown water. These waters are extremely turbid from sediment (especially from erosion) and typically fairly swift flowing which allows for particulate matter to remain suspended in the water column. In these habitats, many of the fishes have reduced vision but rely heavily on tactile senses, like the whiskers on cat fish. 

Of significance to mention, where blackwater and whitewater mix, such as where blackwater tributaries flow into the Amazon, there is a very distinctive demarcation where the waters flow along side of each other and then gradually downstream, the waters mix and the whitewater takes over.  As far as we know, where the two waters mix is some of the highest aquatic biodiversity in the Amazon. For that reason, local fisherman as well as birds, pink river dolphins and gray river dolphins converge on those “mixing” confluences as there is an abundance of food for all.

Naturalist Greg Greer is a favorite among IE travelers, and has gained a reputation for his friendliness and good humor, along with his incomparable knowledge of natural history. Greg's photos and articles have been widely published in books and magazines, including Georgia Outdoor News, Bird Watcher’s Digest, Alabama Outdoor News, Riversedge and Southern Wildlife.