From garbage tip to wildlife haven - new theory for Everglades tree islands
Today the tree-islands of Florida's Everglades are natural havens for the some of the most spectacular of the swampland's denizens - alligators, panthers and rare birds. But five millenia ago, they may have been home to a very different community, as sites where prehistoric humans settled - and dumped their garbage. That's the story being coaxed from the analysis of rocks and soil sampled from some of the tree-islands by a team from Montreal's McGill University.
Tree-islands have long been a curiosity of the Everglades, where they punctuate the flat marshy landscape as circular areas of raise ground. Here trees have taken root, alligators nest and a diverse range of other wildlife take refuge. Previous research has found that the islands owe much of their long-term resilience to a layer of hardened limy rock. It was initially thought that the islands simply formed on natural protrusions of this carbonate bedrock.
Now, excavation and analysis, by a team that included Gail Chmura, a paleoecologist, has shown that underneath the rock is more soil - and layers of human refuse, which are known as middens. These layers contained discarded food remains, including bones, charcoal fragments, and broken pieces of clay pots, as well as shell tools. It is now thought that the accumulation of this material over hundreds of years built up the land, and provided the high ground that the trees subsequently colonized.
Chmura interprets this garbage-tip evidence positively, saying ''This goes to show that human disturbance in the environment doesn't always have a negative consequence.'' The research also suggests how the hard carbonate cap, known as the 'perched carbonate layer', may have formed. It seems that the the probing roots of the trees drew up groundwaters from the bedrock, laden with dissolved carbonates. These precipitated to form the rocky layer, together with phosphates from discarded bones.
This layer not only provides a firm footing it protects the underlying soil from periodic fires, so that plants and trees can easily regrow afterwards. However, the ecology of these man-formed islands are now under threat from the actions of man once more. By cutting down trees, and drawing up water tables, the perched carbonate layer is at risk of being dissolved and loosened. This research may help Florida authorities to better manage these uniques features of the Everglades landscape.