Living Sanibel: Red Mangrove
It is impossible to underestimate the value of the red mangrove forests of Sanibel and Captiva. To start with, the early formation of both islands were in large part dependent on these trees catching and trapping the sand and sediments in their extensive root systems. Without red mangroves there might not be a “Living Sanibel” because there might not be a Sanibel or a Captiva to write about.
Beyond their island and land building capabilities, red mangroves offer an extensive list of other benefits to the estuarine and marine eco-systems. To begin with, every acre of mangroves produces 3.6 tons of leaf litter per year. This litter falls into the tidal waters and helps to feed a host of living organisms, from the micro-organisms that break down this detritus to oysters, barnacles, crabs, shrimps and the fishes that feed on them. An estimated 75 percent of all of Southwest Florida’s game fish and 90 percent of all commercial species depend on the mangrove system for cover and nutrients.
Their lush canopies are utilized by dozens of birds as roosting and nesting sites. These include almost all of the herons and egrets, the pelicans, cormorants, mangrove cuckoos, roseate spoonbills, and ibises, to mention but a few. Their trunks and tangled root systems harbor fiddler crabs, mangrove crabs, periwinkles, apple snails and scores of other spiders and insects. Their flowers feed butterflies, moths, hummingbirds and honey bees. In short, red mangroves are the cornerstones of our estuaries and without them, most, if not all of the back bay food chain would collapse.
Under ideal conditions red mangroves can grow to heights of eighty feet. Locally, the red mangroves seldom exceed thirty feet. They have the unique ability to survive and thrive in water that is so salty that 99 percent of the plants on earth, if exposed to these same levels of salinity, would die within weeks. They grow in soil that is anaerobic, which is the scientific term for soils holding little to no oxygen. To compensate for their harsh environment, the gnarled, exposed root systems of the red mangroves are covered in tiny holes, allowing the tree to take in the oxygen directly from the air.
Its seeds, called propagules, are fully matured when they drop from the tree and may last as long as a year floating in saltwater until they find a shallow mud flat or tide pool where they can take root. The red mangrove is a hermaphrodite, capable of self-pollinating. Another unique adaptation is that as older leaves accumulate excess salt, they are designed to drop, adding to the detritus below and ridding the tree of its unneeded sodium. Mangroves also vary the orientation of their leaves throughout the day to reduce evaporation from the tropical midday sun.
The saddest part of their story is our disregard and abuse of the mangrove eco-system. Studies indicate that more than 50 percent of the mangrove forests have been uprooted and destroyed due to coastal development in the past 100 years. Where mangrove forests once dominated the environment we now find only barren sea walls and development. In some coastal regions, such as in Louisiana and Mississippi, vast tracts of mangrove forests have been lost to agriculture and development, which allowed Hurricane Katrina’s massive storm surge to penetrate much further inland than it would have if these wave-reducing plants had been left in place.
Efforts are underway to replant and re-propagate the red mangrove all across South Florida. Special permits are required to trim mangroves and uprooting or killing them is strictly prohibited. Finding these trees is easy. They line both sides of Wildlife Drive, and inhabit almost all the back bay section of Captiva, including Roosevelt Channel and the bay side of South Seas Plantation. Worldwide there are more than 110 recognized mangrove species, with the largest diversity found in Indonesia and Malaysia. In Hawaii, where it is not a native species, red mangrove is considered an invasive plant and is eradicated accordingly.
This is an excerpt from Living Sanibel – A Nature Guide to Sanibel & Captiva Islands by Charles Sobczak. The book is available at all the Island bookstores, Baileys, Jerry’s and your favorite online sites.