Drainage is a big cause of the destruction of the Everglades; in the 1940's a large portion of the Everglades was drained for development purposes. The effects of this were devastating. There was a 90% reduction of bird populations. Some animals were at risk of being completely lost in the area. Many native grasses were choked out by exotic species.
Tilling for crop production has also greatly effected the Everglades. The sugarcane industry is one of the biggest offenders. Whats worse is the farmers are protected by the federal government and even recieve benefits to keep the industry alive. Farmers will block water ways and redirect the water the way they see fit disturbing the natural waterflow and effecting the ecosystem in the surrounding areas. Fertilizers are also becoming more prevelant in the water of the Everglades. The soil of the Everglades are not ideal for growing sugarcane. Phosphorus is a major component of many fertilizers. The effects are seen in algea blooms which leads to a depleted oxygen supply.

Heres a video from PBS discussing the destruction of the Everglades.
http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=video&cd=3&cad=rja&sqi=2&ved=0CEAQtwIwAg&url=http%3A%2F%2Fvideo.pbs.org%2Fvideo%2F1098938463%2F&ei=x1lcUMLHB6nDyQG3uIHACw&usg=AFQjCNHGsqTXgi5bFRe2CDmNPgaan9IRoQ&sig2=0vtDPqwHY4HRAAhuY6I70Q
Sources:
http://www.fff.org/freedom/fd0706f.asp
http://water.epa.gov/type/wetlands/vital_status.cfm
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