rhizophorae , attack mangrove propagules 

 while still attached to the parent tree 

 (Onuf et al. 1977). 



2.7 WOOD BORERS 



Many people have the mistaken idea 

 that mangrove wood is highly resistant to 

 marine borers. While this may be true to 

 a limited extent for certain mangrove 

 species in other parts of the world, none 

 of the Florida mangroves have borer- 

 resistant wood. Southwell and Boltman 

 (1971) found that the wood of red, black, 

 and white mangroves has no resistance to 

 Teredo , Pholad and Simnorid borers; pieces 

 of red mangrove wood were completely de- 

 stroyed after immersion in ocean water for 

 14 months. 



An interesting controversy surrounds 

 the ability of the wood boring isopod, 

 Sphaeroma terebrans , to burrow into the 

 living prop roots of the red mangrove. 

 Rehm and Humm (1973) were the first to 

 attribute apparently extensive damage of 

 red mangroves stands within the Ten 

 Thousand Islands area of southwestern 

 Florida to an isopod, Sphaeroma . They 

 found extensive damage throughout 

 southwest Florida, some infestation north 

 to Tarpon Springs, and a total lack of 

 infestation in the Florida Keys from Key 

 Largo south to Key West. The destruction 

 process was described as follows: the 

 adult isopod bored into the prop roots (5- 

 mm diameter hole); this was followed by 

 reproduction within the hole and develop- 

 ment of juveniles within the root. This 

 process, combined with secondary decompo- 

 sition from fungi and bacteria, frequently 

 results in prop root severance near the 

 mean high tide mark. These authors 

 attributed loss of numerous prop roots 

 and, in some cases, loss of entire trees 

 during storms to isopod damage. 



The extent of damage in the Ten 

 Thousand Islands region led Rehm and Humm 

 (1973) to term the phenomenon an "eco- 

 catastrophe" of possibly great importance. 

 They further stated that shrinking of 

 mangrove areas appeared to be occurring as 



a result of Sphaeroma infestation; this 

 point was not documented. 



Enright (1974) produced a tongue-in- 

 cheek rebuttal, on behalf of Sphaeroma and 

 against the "terrestrial invader", red 

 mangroves. Snedaker (1974) contributed a 

 more substantial argument in which he 

 pointed out that the isopod infestation 

 might be an example of a long-term eco- 

 system control process. 



Further arguments against the "ecoca- 

 tastrophe" theory were advanced by Estevez 

 and Simon (1975) and Estevez (1978). They 

 provided more life history information for 

 Sphaeroma and suggested a possible ex- 

 planation for the apparently destructive 

 isopod infestations. They found two 

 species of isopods inhabiting red mangrove 

 prop roots, S^. terebrans and a sympatric 

 congener, S. quadridentatum . The latter 

 does not appear to be a wood borer but 

 utilizes S. terebrans burrows. Neither 

 species appeared to utilize mangrove wood 

 as a food source. Estevez and Simon 

 (1975) found extensive burrowing into 

 seedlings in addition to prop root damage. 

 In general, infestations appeared to be 

 patchy and limited to the periphery of 

 mangrove ecosystems. In areas with the 

 highest density of burrows, 23% of all 

 prop roots were infested. There appeared 

 to be more colonization by ^. terebrans in 

 regions with full strength sea water (30 

 to 35 ppt). 



The most important finding by Estevez 

 and Simon (1975) and Estevez (1978) was 

 that periods of accelerated activity by ^. 

 terebrans were related to periods of fluc- 

 tuating and slightly increased salinity. 

 This suggests that fluctuations in isopod 

 burrowing may be related to the magnitude 

 of freshwater runoff from the Everglades. 

 These authors agree with Snedaker (1974) 

 and suggest that root and tree loss due to 

 Sphaeroma activity may be beneficial to 

 mangrove ecosystems by accelerating pro- 

 duction and root germination. Simberloff 

 et al. (1978) amplified this last sugges- 

 tion by showing that root branching, which 

 is beneficial to individual trees, is 

 stimulated by isopod activity. 



24 



