Agricultural systems can be viewed 

 as highly managed ecosystems. These 

 agroecosys terns are maintained at low 

 species diversity by various human 

 activities, such as plowing, planting, 

 cultivation, fertilization, and chemical 

 pest control, to maximize net community 

 production (harvestable yield). Hop- 

 kinson (1978) studied the agroecosystems 

 of the Barataria basin from this per- 

 spective, and compared their structure 

 and function with the natural habitats 

 of the basin. For example, he compared 

 the energy cost of the production of 

 sugarcane with the production of 

 Spartina alterniflora in the salt marsh. 

 These energy budgets are shown in Table 

 6. They indicate that the total energy 

 costs of both sugarcane and marsh grass 

 are comparable, but that sugarcane 

 energy costs must be paid in fossil 

 fuel, while salt marsh costs are pri- 

 marily sunlight and other natural work 

 services that cost society nothing. 



Details on the computations in- 

 volved in this comparative budget can be 

 found in the original reference. The 

 absolute numbers are not as important as 

 are their relative similarity. Sugar- 

 cane production results in a human food- 

 stuff with high caloric content but 

 little nutritional value, while salt- 

 marsh production supports a detritus 

 food web culminating in protein-rich 

 fishery products. The implication of 

 this computation is that productive 

 habitats that receive large natural 

 energy subsidies should not be converted 

 to agricultural habitats in which fossil 

 fuel must be used to replace the lost 

 subsidies . 



BEACH AND DUNE (2) 



The beach and dune habitat repre- 

 sents a very small portion of the MDPR 

 (4,239 ha or 10,467 acres in 1978, or 

 0.12%, Table 5). This habitat serves as 

 a "sacrificial" barrier to storms. 

 During calm periods sand is stored in 

 beach dunes and then eroded during 

 storms, thus absorbing storm energy and 

 protecting the marshlands. 



The beach areas in Louisiana 

 (Figure 19) are characterized by gently 



sloping fine sand that extends several 

 meters from the shoreline and then rises 

 as dunes varying in height from 0.5 to 4 

 m (1.65 to 13 ft). 



Some beaches are located on barrier 

 islands and some line the mainland in 

 areas unprotected by islands. Barrier 

 islands in the MDPR are long and narrow, 

 with very low elevation. They are sepa- 

 rated from the mainland by shallow bays. 

 Many of these islands represent the 

 former margins of old delta lobes. The 

 geologic characteristics of barrier 

 islands have been examined more closely 

 than their ecological roles because most 

 of the barrier islands in Louisiana are 

 eroding and moving shoreward. In Mis- 

 sissippi the barrier islands are of 

 non-deltaic origin, but they are also 

 subject to erosion and migration, and 

 have tended to migrate westward (Otvos 

 1981). 



The vegetation characteristic of 

 the beach and dune habitat in the MDPR 

 is described in Bahr and Hebrard (1976). 

 The rooted vegetation closest to the 

 shoreline of the gulf is found in the 

 dunes. Plants here include beach 

 morning glory ( Ipomoea pes-caprae ) , 

 morning glory ( Ipomoea stolonifera ) , 

 frogbit ( Erigeron repens ) , Heterotheca 

 subaxilaris , evening primrose ( Oenothera 

 sp.), sandspur ( Cenchrus sp.), sea 

 rocket ( Cakile sp.), and sea oats 

 ( Uniola s p . ) 



Behind the foredunes there may be a 

 meadow zone inhabited by beardgrass 

 ( Andropogon sp.), fingergrass ( Chloris 

 petraea ) , saltmarsh fimbristylis ( Lippea 

 lanciolata ) , frogbit ( Limnobium spongia ) , 

 pennywort ( Hydrocoty 1 bonariensis ) , 

 black rush ( Juncus roemarianus ) , three 

 cornered grass ( Scirpus americanus ) , 

 softstem bulrush (S cirpus validus ) , 

 widgeon grass ( Ruppia maritima ) , sand- 

 spur, morning glory, Heterotheca, sab- 

 batia ( Sabbatia sp.), wiregrass, dog 

 tooth grass, and Bermuda grass ( Cynodon 

 dactylon) . 



Old dunes that have been stranded 

 inland from the meadow zone (chenieres) 

 and that have achieved sufficient eleva- 

 tion are typically colonized by trees, 

 including live oak ( Quercus virginiana ) , 



39 



