ecologically much differently from the adja- 

 cent wetlands. Large areas, wetland and 

 highland, have been modified for agricul- 

 ture or are managed for waterfowl or fur- 

 bearers. The region is far from homogeneous 

 and any model that ignores this will produce 

 information of limited management value. 



2. Ecological or functional complexity. Aside 

 from this spatial heterogeneity, within any 

 small, fairly homogeneous area, the ecologi- 

 cal food web is enormously complex and, 

 on the whole, poorly understood. 



3. Time scale of events. Events of ecological 

 interest in the Chenier Plain, which deter- 

 mine the physiography of the whole region, 

 occur on the scale of hours, days or seasons 

 for many biological processes; years or tens 

 of years for many cumulative impacts, such 

 as canal dredging and eulrophication; and 

 thousands of years for geological processes. 

 It is difficult to visualize a useful model 

 which can simultaneously simulate geologi- 

 cal processes and microbial kinetics in 

 terms usefvd to a manager. 



4. Management needs. In addition to the above 

 considerations the model must enable a 

 manager to evaluate the consequences of 

 alternate management strategies at appro- 

 priate levels of spatial, ecological and 

 temporal resolution. Existing models cover 

 a wide range of approaches, including 

 strategies to exploit or manage single com- 

 mercial species [such as fishery models 

 (Wagner 1969) or alligator models (Nichols 

 et al. 1976)]; models which treat ecosys- 

 tems as homogeneous in space in order to 

 elaborate the energetic interactions (Patten 

 et al. 1975; Wiegert et al. 1975); models 

 which treat spatial heterogeneity but con- 

 sider only a limited number of chemical or 

 biological parameters (Kremcr and Nixon 

 1975), and dramatically simplified, dynamic 

 world-view models (Forrester 1971). 



SOLUTION 



The problems of resolution, complexity, and 

 time frame were addressed by the construction of 

 nested hierarchical conceptual models at four levels 

 of resolution: region, drainage basin, habitat, and 

 population levels (fig. 2). Individual populations 



are components of habitats, the smallest ecological 

 units described in the Chenier Plain. Each habitat is 

 considered homogeneous in space. Each of the six 

 Chenier Plain basins is a spatially heterogeneous 

 area composed of a number of interacting habitats. 

 The time scale of events of interest increases from 

 habitat to region. 



THE CHENIER PLAIN REGION 



The Chenier Plain region is unified by a com- 

 mon geologic history; the sediments that underlie 

 this major coastal system originated primarily from 

 riverine sediments supplied by the Mississippi River. 

 The primary geophysical process responsible for the 

 unique physiography of the Chenier Plain has been 

 the periodic alteration in course of the main dis- 

 tributary of the Mississippi River. This switch has 

 occurred on the average about every 400 years over 

 the last 7,000 years, and has caused major changes 

 in sediment input to the Chenier Plain region. For 

 example, when the river is discharging on the eastern 

 side of its delta (as it is presently, see fig. 1), little 

 sediment reaches the Chenier Plain. But when the 

 discharge is on the western side much sediment 

 reaches the Chenier region. In the former case, ero- 

 sion dominates, and in the latter, deposition and 

 growth dominate. The Atchafalaya River, just east 

 of the Chenier Plain (fig. 1), is beginning the long 

 process of capturing the main channel flow of the 

 Mississippi River, and accretion is beginning to re- 

 verse the shoreline retreat measured over the past 



several decades. 



Change in sediment availability has in turn 



been reflected in the formation of the cheniers, 

 which are stranded dune ridges parallel to the 

 present shoreline. Man has had little effect on the 

 regional development of the Chenier Plain. 



The conceptual model of the Chenier Plain 

 region is primarily a model of geological processes 

 (fig. 3). The symbolic "energese" language (Odum 

 1972) is used in the models illustrated. It is dis- 

 cussed more fully in Bahr et al. (1977). Figure 

 legends are complete enough for readers to follow 

 the diagrams without full comprehension of the 

 symbols. These processes are not strongly influ- 

 enced by man, except as he controls the flow of 

 the Mississippi River. 



BASINS 



Drainage basins represent perhaps the most 

 natural category of ecological systems in the 

 Chenier Plain region, because each basin is integra- 

 ted by the flow of water over and through it; yet 



34 



