6.2. ARBOREAL ARTHROPOD COMMUNITY 



A surprising variety of arthropods 

 inhabit the mangrove canopy. Because they 

 are frequently secretive or possess 

 camouflage coloration, their numerical 

 importance often has been overlooked. 

 Beever et al. (1979) pointed out that 

 arboreal arthropods have a variety of 

 ecological roles: (1) direct herbivory on 

 mangrove leaves, (2) predator-prey inter- 

 actions, and (3) biomass export through 

 frass production and leaf defoliation. 

 Direct grazing is typically patchy in 

 distribution. It is not unusual to find 

 extensive stretches of mangroves that have 

 scarcely been grazed. In nearby areas, as 

 much as 80% of the leaves may have some 

 damage (Beever et al. 1979). As a general 

 rule, it is probably safe to state that 

 healthy, unstressed mangrove stands nor- 

 mally have less than 10% of their total 

 leaf area grazed (Heald 1969). In many 

 locations, percent leaf area damaged is on 

 the order of 1% to 2% (Beever et al. 

 1979). There are exceptions. Onuf et al. 

 (1977) reported biomass loss to arthropod 

 grazers as high as 26% in a mangrove stand 

 where growth and nitrogen content of the 

 leaves had been enhanced by input of nu- 

 trients from a bird rookery. 



In terms of numbers of species, the 

 dominant group of arboreal arthropods is 

 insects. The most thorough inventory of 

 mangrove-associated insects was conducted 

 by Simberloff and Wilson to obtain the raw 

 data for their papers on island bio- 

 geography (Simberloff and Wilson 1969; 

 Simberloff 1976). These papers list over 

 200 species of insects associated with 

 overwash mangrove islands in the Florida 

 Keys. There is no reason to expect lesser 

 numbers in other types of mangrove com- 

 munities, except for the mangrove scrub 

 forests. The most thorough study of in- 

 sect grazing on mangrove leaves is that of 

 Onuf et al. (1977) (see section 2.6). 



Although not as numerically impres- 

 sive as the insects, the mangrove tree 

 crab, Aratus pisonii , appears to be poten- 

 tially as important in terms of grazing 

 impact (Beever et al. 1979). The life 

 history of this secretive little crab has 



been described by Warner (1967). In 

 Jamaica its numbers range from 11 to 16/m 

 at the edge of fringing swamps to 6/nr in 

 the center of large swamps. Beever et al. 

 (1979) reported typical densities for a 

 variety of sites in south Florida of 1 to 

 4 crabs/m . These same authors reported 

 some interesting details about the crab: 

 (1) the diet is omnivorous ranging from 

 fresh mangrove leaves to caterpillars, 

 beetles, and various insects; (2) the crab 

 suffers highest predation pressure while 

 in the planktonic larval stage; (3) preda- 

 tion on the crabs while in the arboreal 

 community is low and comes from birds such 

 as the white ibis, raccoons, other man- 

 grove tree crabs and, if the crabs fall in 

 the water, fishes such as the mangrove 

 snapper; and (4) in one location in south 

 Florida (Pine Island Sound) they found in 

 accordance with normal bi ogeographical 

 theory, the highest densities of crabs 

 associated with fringing forests and the 

 lowest densities on distant islands, but 

 at Sugar Loaf Key the unexpl ai nabl e 

 reverse distribution was found. 



Other invertebrates may visit the 

 canopy from below either for purposes of 

 feeding or for protection from high tides. 

 Included in this group are the pulmonate 

 gastropods, L i tt o r i na angul i f era , 

 Ceri thidea seal ari formi s , and Mel ampus 

 cof feus , the isopod, Li gea exoti ca , and a 

 host of small crabs. 



In summary, with the exception of a 

 half dozen key papers, the arboreal man- 

 grove community has been generally ig- 

 nored. Both insects and the mangrove tree 

 crab play significant ecological roles and 

 may affect mangrove productivity to a 

 greater extent than has been recognized. 



6.3 PROP ROOT AND ASSOCIATED MUD SURFACE 

 COMMUNITY 



These two somewhat distinct com- 

 munities have been lumped together because 

 of the large number of mobile organisms 

 which move back and forth between tidal 

 cycles. The aerial roots are used as 

 protective habitat and to some extent for 

 feeding while the nearby mud substrates 

 are used principally for feeding. 



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