WETLANDS. Vol. 17, No. 2, June 1997, pp. 275-283 © 1997, The Society of Wetland Scientists HABITAT STRUCTURE AND P L A N T C O M M U N I T Y C O M P O S I T I O N IN A NORTHERN EVERGLADES WETLAND LANDSCAPE Frank Jordan ~,3,4, H o w a r d L. Jelks 2,3, and Wiley M. Kitchens 2,3 1Department of Zoology 2 Department of Wildlife Ecology and Conservation -~Florida Cooperative Fish & Wildlife Research Un# University of Florida 117 Newins-Ziegter Hall Gainesville, FL 32611 4 Present address: Department of Biological Sciences Loyola University 6363 St. Charles St. New Orleans, LA Abstract: The structure of a priori-defined aquatic habitats was compared within the Arthur R. Marshall Loxahatchee National Wildlife Refuge, which comprises the northern remnant of the Everglades ecosystem. Total plant biomass, canopy height, water depth, and the relative abundance (percent cover) of plant species in adjacent sloughs (including alligator holes), wet prairies, and sawgrass stands were compared over a 30-month period. These habitats formed a mosaic of aquatic habitats of differing structure and plant community composition. Slough and alligator hole habitats were in deeper water with little or no canopy, and habitat structure was provided almost entirely by submersed bladderwort (Utricularia) and floating water lily (Nymphaea). Sawgrass stands were present in shallower water and had a well-developed canopy that was generated almost entirely by Cladium. Wet prairies were intermediate in water depth and canopy stature, with emergent sedges (i.e., Eleocharis, Rhynchospora) providing most habitat structure. Differences among adjacent habitats accounted for most of the observed variation in habitat structure and plant relative abundance, whereas differences among widely-separated areas in the Refuge (i.e, landscape-level differences) accounted for little of the observed variation. In contrast, water depth varied considerably among sampling areas, likely reflecting hydrologic gradients that occur across the Refuge landscape. Similarly, although differences among sampling months were minimal for most habitat features examined, most of the variation observed in water depth was accounted for by sampling rnonth, as would be expected in a seasonally dynamic wetland such as the Everglades. Discriminant function analyses using plant relative abundance data, habitat structure data, or both sets of data combined correctly classified most sites with respect to a priori-defined habitat type, despite similarities in plant community composition in sloughs and wet prairies. Overall, it appears that sloughs, wet prairies, and sawgrass stands are distinct with respect to habitat structure and plant corm'nunity composition and that differences among these habitats are persistent across the Refuge landscape. Key Words: alligator holes, disturbance, Everglades, habitat structure, landscape ecology, sawgrass, sloughs, vegetation, wet prairies INTRODUCTION banization resulted in a 50% areal reduction in the historic Everglades landscape during the last century (Kushlan 1990). Today, the Everglades ecosystem is a highly compartmentalized landscape that has suffered landscape fragmentation, alteration o f historical hydrologic conditions, and nutrient enrichment. Although The historic Everglades ecosystem stretched from the southern end of Lake Okeechobee to the mangrove forests o f Florida Bay and encompassed about 1 million ha of freshwater wetland habitat. Construction o f canals and levees to accommodate agriculture and at275 276 anthropogenic changes are pervasive, much of the remaining Everglades ecosystem is relatively intact and retains the mosaic nature of the historic marsh landscape. For example, sloughs and alligator holes, emergent wet prairies, sawgrass stands, and tree islands are interspersed with one another over much of the Everglades landscape and provide a diversity of aquatic habitats (Loveless 1959, Gunderson t994). Loveless (t959) provided a qualitative analysis of the structure and plant commumty composition of these habitats within the northern and central Everglades (Water Conservation Areas I and 2), along with an elegant description of the natural history of the Everglades ecosystem in general. The structure and plant community composition of aquatic habitats comprising the southern Everglades (Everglades National Park and Water Conservation Area 3) also have been well-studied (Gunderson 1994, David i996). However, little eftfort has focused on characterizing the aquatic habitat mosaic of the northern Everglades. Differences in hydrologic conditions, fire and nutrient regimes, and substrate conditions have led to the generation of a habitat mosaic that is distinct from other management compartments within the Everglades landscape (Goodrick 1974, Richardson et al. 1990, Pope 1991). The Arthur R, Marshall Loxahatchee National Wildlife Refuge (hereafter the Refuge) forms the northern remnant of the Everglades ecosystem. The Refuge is a 57,234-hectare impounded marsh system that is op-erated as Water Conservation Area 1 by the South Florida "Water Management District (see Figure 1). As part of its role in the vast south Florida water-routing system, the Refuge is encircled by a deep-water canal and dike system that drains an approximately equal area of the Everglades Agricultural Area (Richardson et aL 1990, Fermema et al. 1994). However, water derived from the Everglades Agricultural Area is enriched with phosphorus, nitrogen, and other contaminants that profoundly affect Refuge habitats near pumping stations and perimeter canals (Richardson et al. 1990). This water is not distributed evenly across the Refuge landscape because topographic variation in the underlying peat results in a gradient of increasing depth and prolonged hydroperiod along the northsouth axis of the Refuge (and, to a lesser degree, from the interior marshes to peripheral canals). Furthermore, the perimeter canal and dike system has intensified this hydrologic gradient through increased drainage in the northern end and increased "ponding" of water behind dikes in the southern end of the Refuge. Nutrient enrichment and alterations in hydrology have resulted in profound changes to the habitat mosaic comprising the Refuge (Richardson et al. 1990, Davis 1994, Davis et al. 1994; see Newman et al. 1996). The pre-drainage Refuge landscape was corn- WETLANDS, Volume 17, No. 2, 1997 \ I I F" I Everglades I't~ "l I BigCypress ~-. -LN~ional Preserve t 20 km 20 mi Ev~lades ! National "k Park I.~ 1 Figure 1. Map of lower Florida showing existing management compam-nents. The shaded area is the Arthur R. Marshall Loxahatchee National Wildlife Refuge (Water Conservation Area 1). prised of emergent wet prairies, sawgrass stands, sloughs, and elongate tree islands overlying a deep bed of peat. Recent studies, including spatially extensive ground surveys mad spectral analysis of satellite imagery, have resulted in the delineation of 18 habitat classes (Richardson et al. 1990, Pope 1991). This increased resolution of habitats does not simply reflect improvements in study design, censusing techniques, and statistical ordination procedures. Rather, new classes such as cat~ail, open water, and willow tree islands have become predominant in disturbed areas that are near canals and are therefore subject m increased nutrient loading and protracted hydroperiods. In comparison, interior marshes have seen an increase in sawgrass (Davis et al. 1994) and red bay-dahoon holly-wax my~le tree islands (Richardson et al. t990), likely due to altered hydrology. It is unknown how structure (e.g., plant biomass, canopy height) and plant cormnunity composition in sloughs, wet prairies, and sawgrass stands have responded to alterations in nutrient loading and modified hy-droperiod.