CHAPTER 12. MANAGEMENT IMPLICATIONS 



12.1 INHERENT VULNERABILITY 



Mangroves have evolved remarkable 

 physiological and anatomical adaptations 

 enabling them to flourish under conditions 

 of high temperatures, widely fluctuating 

 salinities, high concentrations of heavy 

 metals (Walsh et al. 1979), and anaerobic 

 soils. Unfortunately, one of these adap- 

 tations, the aerial root system, is also 

 one of the plant's most vulnerable compo- 

 nents. Odum and Johannes (1975) have 

 referred to the aerial roots as the man- 

 grove's Achi 1 1 es' heel because of their 

 susceptibility to clogging, prolonged 

 flooding, and boring damage from isopods 

 and other invertebrates (see section 6 for 

 a discussion of the latter). This means 

 that any process, natural or man-induced, 

 which coats the aerial roots with fine 

 sediments or covers them with water for 

 extended periods has the potential for 

 mangrove destruction. Bacon (1970) men- 

 tions a case in Trinidad where the Caroni 

 River inundated the adjacent Caroni 

 Mangrove Swamp during a flood and 

 deposited a layer of fine red marl in a 

 large stand of black mangroves which sub- 

 sequently died. Many examples of damage 

 to mangrove swamps from human activities 

 have been documented (see section 12.2). 



One of the few natural processes that 

 causes periodic and extensive damage to 

 mangrove ecosystems is large hurricanes 

 (Figure 16). Craighead and Gilbert (1962) 

 and Tabb and Jones (1962) have documented 

 the impact of Hurricane Donna in 1960 on 

 parts of the mangrove zone of south 

 Florida. Craighead and Gilbert (1962) 

 found extensive damage over an area of 

 100,000 acres (40,000 ha). Loss of trees 

 ranged from 25% to 100%. Damage occurred 

 in three ways: (1) wind shearing of the 

 trunk 6 to 10 ft (2 to 3 m) above ground, 

 (2) overwash mangrove islands being swept 

 clean, and (3) trees dying months after 

 the storm, apparently in response to 

 damage to the prop roots from coatings by 

 marl and fine organic matter. The latter 

 type of damage was most widespread, but 

 rarely occurred in intertidal forests, 

 presumably because the aerial roots were 

 flushed and cleaned by tidal action. Fish 

 and invertebrates were adversely affected 



by oxygen depletion due to accumulations 

 of decomposing organic matter (Tabb and 

 Jones 1962). 



Hurricane Betsy in 1965 did little 

 damage to mangroves in south Florida; 

 there was also little deposition of silt 

 and marl within mangrove stands from this 

 minimal storm (Alexander 1 967). Lugo et 

 al. (1976) have hypothesized that severe 

 hurricanes occur in south Florida and 

 Puerto Rico on a time interval of 25 to 30 

 years and that mangrove ecosystems are 

 adapted to reach maximum biomass and pro- 

 ductivity on the same time cycle. 



12.2 MAN-INDUCED DESTRUCTION 



Destruction of mangrove forests in 

 Florida has occurred in various ways 

 including outright destruction and land 

 filling, diking and flooding (Figure 17), 

 through introduction of fine particulate 

 material, and pollution damage, par- 

 ticularly oil spills. To our knowledge 

 there are no complete, published docu- 

 mented estimates of the amount of mangrove 

 forests in Florida which have been 

 destroyed by man in this century. Our 

 conclusion is that total loss statewide is 

 not too great, probably in the range of 3 

 to 5% of the original area covered by 

 mangroves in the 19th century, but that 

 losses in specific areas, particularly 

 urban areas, are appreciable. This con- 

 clusion is based on four pieces of infor- 

 mation. (1) Lindall and Saloman (1977) 

 have estimated that the total loss of 

 vegetated intertidal marshes and mangrove 

 swamps in Florida due to dredge and fill 

 is 23,521 acres (9,522 ha); remember that 

 there are between 430,000 and 500,000 

 acres (174,000 to 202,000 ha) of mangroves 

 in Florida (see section 1.3). (2) 

 Birnhak and Crowder (1974) estimate a loss 

 of approximately 11,000 acres (4,453 ha) 

 of mangroves between 1943 and 1970 in 

 three counties (Collier, Monroe, and 

 Dade). (3) An obvious loss of mangrove 

 forests has occurred in Tampa Bay, around 

 Marco Island, in the Florida Keys, and 

 along the lower east coast of Florida. 

 For example, Lewis et al. (1979) estimated 

 that 44% of the intertidal vegetation 



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