THE CONSTRUCTION OF A CONCEPTUAL MODEL OF 



THE CHENIER PLAIN COASTAL ECOSYSTEM 



IN LOUISIANA AND TEXAS 



L. M. Bahr, Jr.,1 J. W. Day, Jr.,i T. Gayle,2 

 J. G. Gosselink.i q s. Hopkinson.i ^nd D. Stellar^ 



INTRODUCTION 



Increasing interest in coastal areas on the part 

 of environmentalists, developers, and managers has 

 generated the need to understand the function of 

 these productive and fragile areas, and to predict 

 the effects of further alterations to them. The term 

 "function" as used throughout the following 

 description of the Chenier Plain conceptual model 

 is intended to describe the mechanics of the eco- 

 system, i.e., the pathways and processes by which 

 energy and matter are captured, transferred, par- 

 titioned, stored, cycled, and degraded by the 

 system. Examples of functional processes include 

 primary production, water flow, trophic exchanges, 

 and animal migrations. Functional understanding 

 of an ecosystem includes much more than an 

 inventory of important physical parameters and 

 organisms; it requires a holistic, systems-level 

 analysis which identifies important interactions 

 among biological and physical components of the 

 system, and all important control features and 

 feedback mechanisms. 



In late 1975, the Fish and Wildlife Service 

 (FWS), U.S. Department of the Interior, funded a 

 study of the Chenier Plain coastal ecosystem(s) of 

 southeastern Texas and southwestern Louisiana 

 (Galveston Bay, Tex., to Vermilion Bay, La.) in 

 which the area would be characterized ecologically 

 by the development of a conceptual model of the 

 system and a synthesis of all extant data. This char- 

 acterization was designed to serve as a pilot study 

 for similar projects which will eventually describe 

 all U.S. coastal ecosystems. The specific request was 

 for a "description of the important resources and 

 processes comprising the ecosystem and an under- 

 standing of their functional relationships." (FWS 

 Request for Proposal, 4 December 1975.) The 

 first requirement of this study (and the key to the 



^Center for Wetland Resources, Lxjuisiana State University, Baton 



Rouge, La. 70803 

 ^Center for Wetlands, University of Florida, Gainesville, Fla. 32601. 



entire project) was the formulation of a conceptual 

 model of the ecosystem(s). The model was to con- 

 sist of a schematic framework of ecosystem func- 

 tion in which all important processes and inter- 

 actions among components would be identified in 

 a qualitative manner. The completed model would 

 identify data requirements and gaps, and set the 

 stage for the two remaining portions of the study, 

 a characterization atlas, and a quantitative ecologi- 

 cal simulation model of the study area which could 

 be used to aid in making management decisions. 



The study area is called the Chenier Plain, so 

 named because of a series of prominent ridges 

 known as cheniers that transect the region from 

 east to west. "Chenier" is a French word meaning 

 "place of oaks;" the vegetation of undisturbed 

 chenier ridges is characteristically dominated by 

 live oak (Quercus virginiana) trees. / 



This report describes the structure of the con- 

 ceptual model developed for this study and dis- 

 cusses the technical and management problems it 

 was designed to solve. 



PROBLEM 



Any ecological model of the Louisiana-Texas 

 Chenier Plain must take into account the following 

 four factors: 



1 . Spatial heterogeneity. The area described as 

 the Chenier Plain (fig. 1) is highly variable 

 in space; from east to west it is broken up 

 by a series of rivers flowing southward into 

 the Gulf, through lakes of different sizes 

 and salinities, and over thousands of square 

 miles of wetland. The wetlands themselves 

 are not all homogeneous; vegetation ranges 

 from pure stands of saline oyster grass 

 (Spartina alterniflora) to fresh water bull- 

 tongue (Sagittaria falcata) and maidencane 

 (Panicum hemitomon). They are cut by ele- 

 vated cheniers or ridges which function 



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