Thresholds Database > Vegetation shift; Florida Everglades, USA

Background

The 5000 year old Everglade wetland is primarily limited by phosphorus. In the late 1940's, the Everglades were divided into three designated land uses; agriculture, urban and conservation. Water levels in the channels changed and there was a large-scale shift in the vegetation in the 1970's and 1980's, from sawgrass to cattail-dominated marshes immediately adjacent to the agricultural land. A slow increase in soil phosphorus, has meant that disturbances such as fire, drought or frosts can no longer be absorbed and instead result in a shift in vegetation type.

Alternate Regimes

1. Sawgrass marshes (occur at <300ppm phosphorus)

2. Cattail, blue-green algae (occur at >300ppm phosphorus)

Mechanism

Soil phosphorus entered the marshes from the agricultural land. Above 300ppm phosphorus, disturbances such as fire, frost and drought allow cattails and blue-green algae to dominate. In addition, deeper water, as experienced in many channels, favours cattails over sawgrass.

Management Decisions in Each Regime

State 1: The designation of the northern third of The Everglades for agricultural land in the 1940's has been directly responsible for the addition of phosphorus into the water. In addition, in the early 1900's, the hydrology of The Everglades was altered with the construction of an extensive network of canals and levees. Some areas have been drained, whilst others experience increased water depths.



State 2: Efforts are being made to restore the hydroperiod of the managed Everglades.