... How Old are the Rainforests? ...
by Harry Pariser
Author of Explore Costa Rica

The rainforests were once believed to have been stable environments for 60 million years or more. However, thinking on this point has shifted owing to research conducted from the 1970s onward.

As the northern temperate zone underwent radical shifts in temperature during glacial and interglacial periods (with a resulting movement of ice sheets and shifts in vegetation and fauna), the tropics also underwent equally radical alterations known as pluvials (wet, warm periods) and interpluvials (dry, cold periods). As the glaciers would descend, the tropics would enter an interpluvial. Much of the Amazon had dry, scrub vegetation as recently as 6,000 years ago.

How old are the nation’s ecosystems? Quite young! The land mass was not even in place in Central America—along with the connections to Columbia and Mexico—until about three million years ago. This region is believed to have had its origins in the “Galapagos Hot Spot,” an active area of sea-floor spreading which drifted slowly until reaching its present position. This same process had happened previously, and the mass continued on through the Caribbean—forming Cuba and the Antilles as it went.

The rainforests, therefore, should not be viewed as timeless, stable relics but rather as dynamic, changing ecosystems whose vegetation shifts as climates change and new species colonize new areas as they become available and accessible to seeds. Rainforests are diverse because most times of the year are reasonably suitable for plant growth and animal activity.

In contrast, the temperate zone not only has a winter—which weeds out species through the environmental stresses it poses—but the northern hemisphere was also largely convered with glaciers (or affected by ice sheets) until relatively recently. Consequently vegetation is still recolonizing habitats there; recent studies of fossil pollen indicate that forest composition has undergone stiking changes in the past 5,000 years. This is another reason why the tropics boast more species. Also, as many species have had their origins here, the number of species dwindles as you leave the area.

— from information supplied by Drs. Diana and Milton Lieberman

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