each basin is relatively autonomous from adjacent 

 basins in terms of water circulation. Six fairly dis- 

 tinct basins have been identified in the Chenier 

 Plain (fig. 4). Each basin has its own hydrodynamic 

 characteristics determined by such parameters as 

 size, drainage density, downstream flow, elevation 

 and slope of the basin, and extent of its connec- 

 tion with the Gulf via tidal passes. 



Most significant changes in a basin occur 

 through large-scale and cumulative effects over a 

 period of time measured in years, rather than in 

 hundreds of years. Examples include: effects of 

 deep shipping channels on saltwater intrusion; 

 changes in hydrology associated with stream chan- 

 nelization; canal dredging and associated spoil bank 

 formation; and cumulative wetland drainage for 

 urban and industrial development. 



HABITATS 



The habitat is the smallest ecological system 

 considered in our conceptual model. Wherever a 

 particular habitat occurs on the Chenier Plain it is 

 treated as the same basic functional unit, and can 

 therefore be treated as homogeneous, even though 

 we recognize the existence of gradients, specialized 

 niches, and discontinuities. Each habitat is a com- 

 plex ecological system characterized by its own 

 species, carrying capacities for those species, levels 

 of production, food web, nutrient cycles, and 

 physical inputs. The time scale of important events 

 is often seasonal, and short term impacts are 

 important at this level. 



Most habitats are intuitively distinct. For 

 example, aquatic systems are quite different from 

 upland forests; however, different kinds of natural 

 wetlands are not so clearly unique. For the Chenier 

 Plain we have identified and mapped 10 natural 

 habitats: nearshore Gulf; inland open water; salt, 

 brackish, intermediate, and fresh marsh; wetland 

 forest; upland forest; beaches; and cheniers and 

 ridges. Large areas have been modified by human 

 activity, which we have catalogued into four 

 additional habitats as impounded marshes, pastures, 

 rice and crop habitat, and urban habitat. 



Complex habitat level models have been con- 

 structed for each of the 14 habitats to give a quali- 

 tative functional understanding of each habitat, 

 and to guide the acquisition of data. As illustra- 

 tions of the habitat models, figure 5 shows the 

 aquatic inland open habitat model as it appears in 

 the conceptual model (Bahr et al. 1977). Figure 6 

 represents simplified version of the aquatic habitats 

 (inshore open water and nearshore Gulf of Mexico). 



In the conceptual model document, figure 5 is 

 accompanied by a detailed interaction matrix 

 keyed to each of the compartments. Figure 7 is the 

 generalized wedand habitat model, and figure 8 is 

 the agricultural model, both from the characteriza- 

 tion atlas. We are at present relatively ignorant of 

 the internal working of most habitats; thus, those 

 that are managed/exploited are manipulated at 

 some peril to the function of the whole system. A 

 better approach to management is to recognize 

 that certain renewable resources (or nonresources; 

 Ehrenfeld 1976) are associated with any habitat, 

 and in order to protect the resource, one must pro- 

 tect the habitat. 



POPULATIONS 



Habitats can be considered as ecological land- 

 scape units composed of many different popula- 

 tions interacting with each other and with their 

 physical surroundings. At the bottom of the con- 

 ceptual hierarchy of natural history, growth 

 dynamics and environmental limits are considered 

 for species of economic, recreational, or functional 

 importance in the Chenier Plain region. The carry- 

 ing capacity of a habitat for a particular species is 

 an important concept that relates the species to its 

 habitat. Major opportunities for management of a 

 single species or group of related species occur 

 through manipulation of habitat (for instance, by 

 impounding wetlands), or through direct control of 

 population size through harvesting (fig. 9). 



THE BASIN-LEVEL CONCEPTUAL MODEL 



The major kinds of manageable processes and 

 the time scales of manageable events appear to 

 occur at the basin level. For this reason, major em- 

 phasis in this discussion is placed on the basin-level 

 analysis. 



Figure 10 summarizes basin-level processes and 

 interactions. This model is the result of a series of 

 iterative changes and simplifications of earlier, 

 more detailed, models of basin function (Bahr et al. 

 1977). It is extremely aggregated and simpHfied in 

 order to include only the most critical components 

 and processes, and to show how water, wetlands, 

 and man interact in a hypothetical drainage basin. 



The basin model is divided into four linked 

 submodels (fig. 10) each representing a different 

 set of processes, and each in part responsible for 

 the present state of a basin, and for the rate at 

 which it is changing. The four submodels are: 



36 



