4.0 c:hknii:r plain habitats 



4.1 INTRODUCTION 



Habitats are the key components of the Chenier 

 Plain ecological hierarchy. As used in this report, they 

 are components of basins and are also coimnunities 

 where individual species live and reproduce. 



The tenn "habitat" refers to the place occupied 

 by an entire community of organisms (Odum 1971). 

 A habitat can be described in terms of a range of 

 physical or abiotic parameters such as salinity and 

 temperature, water avaUabihty, soil type, and geo- 

 graphic reUef. It has geographic boundaries that can 

 be measured and mapped. 



Odum (1971) defines a community as "an or- 

 ganized unit that has characteristics additional to its 

 individual and population components and functions 

 as a unit through coupled metabolic transformations." 

 The terni "habitat," as used in this report, defines the 

 boundary of this community; it is not applied to indi- 

 vidual species, except to the extent that they belong 

 to defined communities. 



Any classification system is to some extent arbi- 

 trary. In this study, basins were subdivided into geo- 

 graphic units called habitats (table 3.53, and plates 

 3A and 3B). Because man is a significant influence on 

 the Chenier Plain, some of these habitats were also 

 land-use categories (e.g., rice field habitat). But they, 

 like the naturally occurring habitats, could also be 

 treated as functionally identifiable units. Fourteen 

 habitats were defined; nearshore Gulf, inland open 

 water, salt marsli, brackish marsh, intermediate marsh, 

 fresh marsh, swamp forest, impounded marsh, ridge, 

 beach, upland forest, rice field, pasture, and urban. 



Most of the land in the Chenier Plain basins is 

 wetland. Wetland habitats include the swamp forest, 

 impounded marsh, and four natural marsh types 

 identified by vegetation and salinity differences; 

 salt marsh, brackish marsh, intermediate marsh, and 

 fresh marsh (Penfound and Hathaway 1938, O'Neil 

 1949, Chabreck et al. 1968, Chabreck 1972). 



The plant community in a wetland area depends 

 upon the range of physical and chemical parameters 

 in that area. Generally speaking, as one moves inland 

 from the coast and salinities decrease, coastal salt 

 marshes grade into brackish, intermediate, and fresh 

 marshes. Figure 4-1 shows the vegetational trend in 

 southeastern Louisiana along an 80 km (50 mi) south 

 to north transect. The salt marsh habitat is in the 

 southernmost zone, where smooth cordgrass is domi- 

 nant (table 4.27 lists common and scientific names 

 of most vascular plants identified in this study). A 

 fairly distinct change occurs inland where saltmeadow 

 cordgrass and saltgrass become dominant in the 

 brackish marsh habitat. A third change in plant com- 

 position and diversity is observed in the Lntennediate 

 marsh habitat. This occurs with the appearance of 

 such plants as alligatorweed, maidencane, and Walter's 



millet. Halfway along the transect, the fresh marsh 

 habitat is distinguished by the presence of species 

 such as water hyssop, water hyacinth, and cattail. 



Vegetational transitions in the Chenier Plain are 

 generally similar to tliose shown in figure 4-1. How- 

 ever, they are less distinct, because ridge plants are 

 mixed with wetland species where cheniers modify 

 the natural salinity gradient. Figure 4-2 shows an 

 example of plant distribution in the region on a 

 south to north transect through the Calcasieu Basin. 

 Salt marsh species do not show up on this transect, 

 but three discernible groups of plants are evident. 

 With the exception of aster and seashore paspalum, 

 the southern group of brackish marsh species (salt- 

 meadow cordgrass, smooth cordgrass, and saltgrass) 

 are identical to those found in figure 4-1. The inter- 

 mediate marsh zone contains a variable group of 

 common freshwater species that can tolerate low 

 salt concentrations (alligatorweed, bulltongue,01ney's 

 three-comer grass). At the north end of the transect, 

 this group is augmented by strictly freshwater species 

 such as stonewort and yellow lotus. Even though the 

 zones are not distinct, the distribution of plant species 

 provides a plausible criterion for distinguishing the 

 four natural marsh habitat types. 



Vegetation differences also correlate with dif- 

 ferences in soil chemistry. Chabreck (1972) and 

 Brupbacher et al. (1973) reported that vegetation in 

 Louisiana marshes varied with the chemical charac- 

 teristics of soil sediments. As an example, figure 4-3 

 compares soil calcium and total salt concentrations 

 within four marsh types (Palmisano and Chabreck 

 1972). 



4.1.1 RELATION OF HABITATS TO 

 POPULATIONS 



Populations exist, grow, and interact within the 

 constraints of habitats. A habitat limits and molds a 

 population through external forces. In turn, as inter- 

 acting populations (the bio tic components of the com- 

 munity) change, they modify the habitat. Three habi- 

 tat characteristics are important for individual popula- 

 tions. First, the carrying capacity of a habitat depends 

 on the magnitude of primary production of the com- 

 munity and on the trophic position of the population 

 in question. Conceptually, a habitat has a carrying 

 capacity for every species population that it supports. 

 This carrying capacity can be managed by controlling 

 primary production (e.g., increasing the amount of 

 Olney's three-corner grass to aUow muskrat popula- 

 tions to increase), by manipulating the trophic struc- 

 ture of a habitat (e.g., reduction of predators in an 

 area often allows prey species to increase in numbers), 

 and by reducing limiting factors (e.g., providing nest- 

 ing boxes for wood ducks where natural cavities are 

 few). Since components of a community interact, 

 increasing the carrying capacity for one species 

 generally has repercussions for other species, and could 

 be detrimental to the community as a whole. 



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