WETLANDS, Vol. 24, No. 3, September 2004, pp. 652±664 q 2004, The Society of Wetland Scientists SPATIAL SCALE AND ABUNDANCE PATTERNS OF LARGE FISH COMMUNITIES IN FRESHWATER MARSHES OF THE FLORIDA EVERGLADES John H. Chick1, Carl R. Ruetz III2, and Joel C. Trexler Department of Biological Sciences, Florida International University Miami, Florida, USA 33199 1 Present address: Great Rivers Field Station Illinois Natural History Survey 8450 Montclair Ave. Brighton, Illinois, USA 62012 E-mail: [email protected] 2 Present address: Annis Water Resource Institute Grand Valley State University 740 West Shoreline Drive Muskegon, Michigan, USA 49441 Abstract: Anthropogenic habitat alterations and water-management practices have imposed an arti®cial spatial scale onto the once contiguous freshwater marshes of the Florida Everglades. To gain insight into how these changes may affect biotic communities, we examined whether variation in the abundance and community structure of large ®shes (SL . 8 cm) in Everglades marshes varied more at regional or intra- regional scales, and whether this variation was related to hydroperiod, water depth, ¯oating mat volume, and vegetation density. From October 1997 to October 2002, we used an airboat electro®sher to sample large ®shes at sites within three regions of the Everglades. Each of these regions is subject to unique water- management schedules. Dry-down events (water depth , 10 cm) occurred at several sites during spring in 1999, 2000, 2001, and 2002. The 2001 dry-down event was the most severe and widespread. Abundance of several ®shes decreased signi®cantly through time, and the number of days post-dry-down covaried signif- icantly with abundance for several species. Processes operating at the regional scale appear to play important roles in regulating large ®shes. The most pronounced patterns in abundance and community structure oc- curred at the regional scale, and the effect size for region was greater than the effect size for sites nested within region for abundance of all species combined, all predators combined, and each of the seven most abundant species. Non-metric multi-dimensional scaling revealed distinct groupings of sites corresponding to the three regions. We also found signi®cant variation in community structure through time that correlated with the number of days post-dry-down. Our results suggest that hydroperiod and water management at the regional scale in¯uence large ®sh communities of Everglades marshes. Key Words: Everglades, large ®sh communities, hydroperiod, abundance patterns, spatial scale INTRODUCTION the ®eld of applied ecology because humans alter eco- Understanding the spatial and temporal scale of eco- systems at several temporal and spatial scales (Good- logical processes is fundamental to interpreting pat- win and Fahrig 1998, Hobbs 1998). Fragmentaton, iso- terns in the environment (Levin 1992). During the past lation of habitats, and other anthropogenic disturbanc- two decades, ecologists have gained greater apprecia- es to ecosystems and landscapes can be viewed as new tion for limitations imposed by scale on the generality scales imposed onto natural systems. Examining how of observed patterns, and they are gaining greater in- patterns vary across scales, including human imposed sights on how patterns observed at one scale are in¯u- scales, may be a useful tool to detect effects of, and/ enced by mechanisms and constraints operating at or establish hypotheses regarding, anthropogenic larger and smaller scales (Turner 1989, Wiens 1989, changes to ecosystems. Levin 1992). Scaling issues are especially important to The Florida Everglades is both one of the most 652 Chick et al., LARGE FISH COMMUNITIES IN THE FLORIDA EVERGLADES 653 unique and dramatically altered freshwater ecosystems glades marshes. Variation in small ®sh abundance oc- in the world. Once a contiguous wetland ecosystem curred mostly at local scales (i.e., 10 km), whereas extending from the southern shores of Lake Okeecho- limited data on large ®shes suggested that larger scale bee to Florida Bay, changes in land use have reduced variation (i.e., among management units) was more the spatial extent of Everglades marshes by more than important. These differences may re¯ect differences in 40% (Davis et al. 1994). Furthermore, the system has migrational ability between these two groups because been subdivided into a series of marsh reservoirs movement by ®shes tends to scale with body size (known as water conservation areas) separated by le- (Ware 1978). These differences may be particularly vees and canals. Whereas water once moved south- important in Everglades marshes, where the ability to ward in sheet ¯ow across the entire system, water locate deep-water refugia is critical for survival during movement through the Everglades today is disjunct seasonal dry downs. Differences in proximity to deep- and controlled by humans. By subdividing the Ever- water refugia and micro-topographic variation may glades, humans have imposed an arti®cial spatial scale structure populations and communities of small ®shes onto the system, with each water-management ``re- at local scales. If large ®shes are more successful at gion'' having its own hydroperiod (the number of days long distance dispersal and locating deep-water refu- inundated with water per year), nutrient loading rate, gia, proximity to deep-water refugia at local scales and level of connectivity with other regions. These may be less important to their population and com- changes are thought to have contributed to declines in munity structure. the abundance of wildlife (Ogden 1994). Large scale We conducted the ®rst large-scale study of the dis- restoration efforts are being planned and initiated with tribution and composition of large ®shes in the Ever- the goal of improving habitat quality of Everglades glades. Our objectives were to 1) examine patterns in marshes for ®sh and wildlife. However, our under- the abundance (all species, all predators, and individ- standing of how large-scale habitat alterations affect ual species) and community structure of large ®shes Everglades ®sh and wildlife communities is insuf®- across the central and southern portions of the Florida cient to develop sound restoration projects (Davis and Everglades in relation to spatial scale, time, and habitat Ogden 1994, Ogden 1994, Gaff et al. 2000). factors (hydroperiod, ¯oating mat volume, vegetation Fishes of the Everglades can be divided into two density); 2) determine which spatial scale, regional (an groups on the basis of size (Trexler et al. 2002, 2003). anthropogenic scale corresponding to water-manage- Small-bodied ®shes (SL , 8.0 cm as adults), mainly ment practices) or intra-regional (i.e., variation among composed of poeciliids and cyprinodontids, are the sites within a region), is most relevant to variation in dominant taxa both numerically and in standing crop large ®sh abundance and community structure; and 3) (Loftus and Kushlan 1987, Loftus and Eklund 1994, examine relationships between the community struc- Turner et al. 1999, Trexler et al. 2002). More than ture of large ®shes and hydroperiod, water depth, ¯oat- twenty years of density and compositional data are ing mat volume, and vegetation density. available for this group of ®shes, and these data have provided important insights into how the Everglades STUDY SITE ecosystem functions (Loftus and Eklund 1994, Turner et al. 1999, Trexler et al. 2002). The other group of From October 1997 to October 2002, we sampled ®shes inhabiting Everglades marshes is large-bodied ®shes at 11 sites in freshwater marshes from three re- ®shes (SL $ 8.0 cm), including several important gions of the Florida Everglades: Water Conservation predators such as largemouth bass Micropterus sal- Area-3A (WCA-3A) and both Shark River Slough and moides Lacepede and Florida gar Lepisosteus platyrhr- Taylor Slough in Everglades National Park, Florida, incus De Kay. Less is known about large ®shes in the USA (Figure 1). We limited our sampling to wet prai- Everglades because effective techniques for sampling ries and sloughs dominated by spikerush Eleocharis them in shallow, vegetated habitats have only recently spp., which are important habitats for ®shes and wad- been developed (Chick et al. 1999). The ecological ing birds (Loftus and Eklund 1994, Jordan et al. role of large ®shes in Everglades marshes is currently 1997a). Our sites were typical of many shallow, dense- a topic of debate. Some studies suggest that large ®sh- ly vegetated Everglades marshes that contain abundant es are important top-down regulators of small ®sh mats of ¯oating, benthic, or epiphytic periphyton abundance (Kushlan 1976), whereas others suggest (Gunderson 1994, Jordan et al. 1997b, Turner et al. that large ®shes are too rare to affect small ®sh abun- 1999, Trexler et al. 2002). Everglades marshes are typ- dance (Loftus and Eklund 1994, Jordan 1996). ically mosaics of spikerush-dominated wet prairies and Trexler et al. (2002) provided evidence that the sloughs, intermixed with dense stands of sawgrass abundance and community composition of large and Cladium jamaicense Crantz. Compared to adjacent small ®shes varied at different spatial scales in Ever- spikerush habitats, sawgrass stands are typically 10 to 654 WETLANDS, Volume 24, No. 3, 2004 version projects in the 1960s combined with drainage and development of the headwaters of this region (Light and Dineen 1994). The longest hydroperiod marshes occur in WCA-3A, where water ¯ow out of this region