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Ecological impacts of ridge and slough degradation

Summary
Background
Introduction
Causes of change
Evidence of change
Formation & Maintenance
Ecological impacts
Recommendations
Performance measure
Acknowledgements
References

Even though the mechanisms of ridge and slough landscape degradation are not fully known, barriers to flow are resulting in the conversion of the ridge and slough wetland mosaic to more uniform stands of sawgrass. It is likely that this conversion is related to the elimination of flow across the broad expanse in which it once existed. Even though culverts were constructed as part of Alligator Alley, Tamiami Trail, U.S. 27, and other levees to convey water past these barriers, landscape degradation has occurred probably due to the elimination of flow, changes in quantity and distribution of water conveyed, and associated physical and biological components. Conversion of the ridge and slough landscape pattern to uniform sawgrass stands has had, and will continue to have, deleterious impacts on Everglades plants and animals.

Flow may be essential for tree island formation and survival

In addition to the disappearance of the ridge and slough landscape, barriers to flow may have a negative impact on tree islands – an important component of the Everglades landscape. Tree islands now are known to be biodiversity hotspots and critical nesting and foraging habitats for numerous species. Tree islands are tear-shaped islands in the ridge and slough landscape whose long axis normally runs more or less north-south (i.e., upstream-downstream). Both their shape and orientation suggest that water flow has played a major role in their development. The two basic kinds of tree islands in the Everglades are floating tree islands and fixed tree islands. Floating tree islands primarily are found in the Arthur R. Marshall Loxahatchee National Wildlife Refuge and occasionally in other areas with deep peats. Fixed tree islands are found south of the Refuge in Water Conservation Areas 2A, 3A, 3B, and Everglades National Park.

Floating tree islands originate when a large piece of peat, known as a battery, detaches from the bottom during a period of high water and floats to the surface. This new island eventually becomes colonized by a variety of shrubs and trees. The role of flow in this process is completely unknown.

Although there are known exceptions (Loveless 1959), the majority of fixed tree islands are believed to develop because of topographic highs in the limestone bedrock underlying the Everglades. The small bedrock pinnacles or platforms, associated with the heads of the islands, are typically the highest part of a fixed tree island. The tallest trees and shrubs are found on the heads of tree islands. Tails are long, linear mounds of peat that form behind the heads. The elevation of a tail gradually drops from that of the head to that of the surrounding wetland. The vegetation of the tail ranges from tall trees and shrubs immediately downstream of the head, through ever shorter and sparser shrub communities, to a mix of tall sawgrass, ferns, and cattail just before it becomes indistinguishable from the surrounding vegetation. This combination of head and tail gives fixed tree islands a characteristic, elongated tear shape.

Two hypotheses have been proposed to explain the formation of tails – the hydrodynamic and chemo-hydrodynamic hypotheses (Sklar and van der Valk in press). According to the hydrodynamic hypothesis, the tail develops due to litter from the head (or other sources) being deposited in its lee by water currents. According to the chemo-hydrodynamic hypothesis, the tail develops due to the release of nutrients from the head. These nutrients are leached from the head by surface water or shallow groundwater. This leaching creates a plume of nutrients behind the head that increases plant growth and a differential build-up of peat in the plume area when compared to nearby areas outside the plume. Both hypotheses require water flow.

In a study of the size, shape, orientation, and distribution of tree islands in the Arthur R. Marshall Loxahatchee National Wildlife Refuge, Brandt et al. (2000) found that tree islands decreased in size, and that the overall area of tree islands decreased between 1950 and 1991. Photo plots in the interior of the wetland demonstrate a tendency for tree islands to become irregularly shaped, possibly due to the loss of water flow, reduction of pulse magnitude, and ponding of water along the wetland perimeter levees. Pristine tree islands have been described as being small and circular (Loveless 1959) or large and orientated in the direction of water flow (Gleason et al. 1984). It has been hypothesized that the elongated tree islands are found in areas with higher flow rates, and that reductions in these flows due to compartmentalization have resulted in changes in tree island shape and reduction in size.

When viewed from space, the Everglades is clearly a landscape of large-scale hydrologic gradients. Fixed tree islands are not distributed randomly across the landscape. They seem to be associated with the ridge and slough topography of the Everglades. Although flows are hypothesized to be critical to tree island formation and sustainability (Stone et al. in press, Wetzel in press), the nature of the relationship between the ridge and slough pattern and tree islands is not clear.

The alterations described above in the ridge and slough landscape and in tree islands already have caused major ecological impacts to Everglades ecosystem structure and function. The importance of major landscape features to Everglades ecology and the ecological impacts that result from changes to these features, particularly the loss of open water habitat such as sloughs and wet prairies, are described below.

Sloughs and wet prairies are species rich

Although sawgrass marsh is the predominant wetland plant association in the Everglades, sawgrass marshes in unimpacted regions are interspersed with open water sloughs and wet prairies. An Everglades landscape increasingly dominated by sawgrass, which may exceed 10 feet in height and form an impenetrable mass (Kushlan 1990), will support fewer numbers of animals and a lower diversity of animals. Sawgrass habitats in the southern Everglades are the poorest habitat in both the number of fish species inhabiting them (Loftus and Kushlan 1987) and in the numbers of individuals of those species (Bill Loftus, unpublished data). In contrast, adjacent wet prairie/slough habitats have twice the number of species (Loftus and Kushlan 1987) and much higher densities of organisms (Trexler et al. 2002).

Wet prairies and wet meadows, such as those found in the ridge and slough environment, are wetland types that are associated with high biological diversity. Keddy (2000) notes that in shoreline communities, wet meadows are the least studied community type and most at risk from changes in hydrology. Also, there seems to be a general lack of appreciation among wetland ecologists for how much higher plant diversity is in infertile (nutrient limited) wet prairies and wet meadows than in other wetland habitats. Moore et al. (1989) demonstrate that infertile wet meadows had vastly more species, and more rare species, than other wetland types.

A diverse landscape mosaic is important to the Everglades animal community. Trexler et al. (2002) find that wet prairies and sloughs interspersed among sawgrass ridges are important sources of refugia, particularly for small fishes, during periods of dry down. Periods of dry down are normal features of Everglades hydrology, and Trexler et al. (2002) note that hydrology is a major factor in shaping fish communities. Benefits to the fish community from restoration of more natural hydrology by CERP implementation may be offset by the lack of availability of suitable refugia during dry periods if there are fewer sloughs.

In a study of prawn, crayfish, and small fish distribution in the Arthur R. Marshall Loxahatchee National Wildlife Refuge, Jordan (1996) shows that the wetland is composed of a mosaic of habitat structure, and that decapod crustaceans and fishes respond to this mosaic. These organisms are important prey of higher trophic levels, and his research indicates that vegetation structure, along with hydropattern and other factors, govern the availability of prey. With reference to sawgrass stands, Jordan (1996) found fewer prawns and fishes than adjacent wet prairies or sloughs.

Chick and Trexler (in review) find that the large-fish community of Water Conservation Area 3A is dominated by large-bodied predators, such as Florida gar and largemouth bass, whereas the Shark River Slough community consists primarily of less piscivorous species such as lake chubsucker and spotted sunfish. The density of smaller fishes, of the size consumed by many wading bird species, is greater in Shark River Slough than in Water Conservation Area 3A, possibly because of the larger community of predatory fishes in Water Conservation Area 3A. These data suggest that the longer hydroperiod marshes of Water Conservation Area 3A, particularly at its southern end just north of Tamiami Trail, seemed to promote growth of large predatory fish, which, in turn, limit the abundance of small fishes. They suggest that restoration may be best achieved by increasing the spatial extent of long hydroperiod marshes, such as Shark River Slough, to improve marsh quality for wading bird prey.

In summary, the importance of elevation changes in the ridge and slough habitat to the biological community is very important. One of the most dramatic features of wetlands is the degree to which small changes in elevation can allow large numbers of species to coexist (Paul Keddy, personal communication). In one study of coastal plain vegetation in Nova Scotia (vegetation that shares many genera with the Everglades), the distribution of each of more than 30 wetland species was centered upon slightly different elevations along a gradient of less than 5 feet (Keddy 1984). If the topographical variation of the Everglades landscape was lost, there is no doubt that biological diversity of both plants and animals would dramatically decline.

Wading birds need a mosaic of wetland habitats

Wading birds are an important component of the Everglades ecosystem. Their foraging and nesting success often are used as indicators of the overall health of the system. Wading birds also are one of the most visible and highly regarded parts of the Everglades. In fact, the overall decline of wading birds over the last century – to less than one-fifth of their abundance during the 1930s (Davis and Ogden 1994) – has been one of the major factors galvanizing public support for Everglades restoration. Scientists understand the central role that wading birds play in Everglades restoration – so much so that wading bird ecology is being used to define hydrologic targets for restoration, and their population responses to hydrologic restoration will be used as measures of restoration success.

The conversion of ridge and slough landscape to dense sawgrass stands will have a negative impact on wading birds and other important birds of the Everglades. In a study of the factors affecting foraging behavior of wading birds in the Everglades, Peter Frederick (personal communication) states that his researchers did not even attempt to include moderately dense to dense sawgrass habitat in their sampling scheme because they never saw birds in this habitat. The control of vegetation over wading bird ecology is strong enough that Kushlan (1989) states, “Whatever determines vegetation patterns will also, to a large degree, determine bird use of wetlands.”

Wet prairie habitat is dominated by spikerush, although other plant species include maidencane, beakrush, and arrowheads, with bladderworts and periphyton mats interspersed among the emergent species (Loftus and Eklund 1994). Although these open water habitats do not occupy as much of the unimpacted Everglades as sawgrass marshes, sloughs and wet prairies are more heavily utilized by fish and wading birds (Hoffman et al. 1994).

In a review of systematic reconnaissance flight data collected in the late 1980s, Hoffman et al. (1994) demonstrate that Great Egrets, Great Blue Herons, and White Ibises almost never were associated with dense grass habitats. These types of dense grass habitats only tended to support wading birds when water levels were high, and then only in relatively small numbers. Hoffman et al. (1994) go on to recommend that to improve wading bird habitat, particularly in the Water Conservation Areas, managers should promote the interspersion of slough and wet prairie habitats into areas of dense grass. In addition, Bancroft et al. (1994) suggest that loss of natural connectivity from compartmentalization might have resulted in a reduction of fish prey for wading birds. They hypothesize that levee construction increased the duration of flooding caused by dry season rainfall events, which ultimately hindered the concentration of prey items that occurs during the dry season.

Kushlan (1989) shows that vegetation structure affects wading bird and waterfowl use of wetlands, because different species have different habitat requirements. For example, long-legged species such as Great Blue Herons and Great Egrets feed by wading in shallow open water marsh. Waterfowl that swim require deeper zones without emergent marsh vegetation. Coots, Gallinules, and Grebes feed in open water but remain near the cover of emergent vegetation which they use for nesting (Kushlan 1989). Resident water bird species that nest in swamp forest vegetation depend on nearby marshes for foraging (Kushlan 1990).

The mechanisms by which vegetation structure affects wading birds are the subject of ongoing research. There is strong evidence to suggest that nesting wading birds in the Everglades are limited by prey availability (Gawlik 2002). This hypothesis, termed the Everglades prey-availability hypothesis (Figure 17), was first proposed as an explanation for the decline of the Wood Stork population in south Florida (Kahl 1964), but it has wide support today for several other species of wading birds (Frederick and Spalding 1994, Gawlik 2002). Based on experimental studies, this hypothesis was modified to focus specifically on the occurrence of high-quality landscape patches, or patches with a high availability of prey (Gawlik 2002).

flow chart of Everglades prey-availability hypothesis
Figure 17. Everglades prey-availability hypothesis (Gawlik in press). [larger image]

Although there have been no studies examining the influence of flow rates on wading birds in the Everglades, it is hypothesized that water flow may affect the occurrence of high-quality landscape patches indirectly by affecting hydroperiod and characteristics of the environment such as topography and vegetation structure (Figure 17). If water flow is necessary for maintaining the ridge and slough mosaic, then flow could affect high-quality feeding patches via effects on vegetation structure and topography. Fish are most vulnerable to capture by birds where vegetation has an open structure (Surdick 1998). During the wet season, dense sawgrass ridges in the Arthur R. Marshall Loxahatchee National Wildlife Refuge provide habitat for many fish (Frank Jordan, unpublished data), but they are protected from wading bird predation, which is probably why wading birds avoid that habitat (Hoffman et al. 1994). However, as the dry down progresses and water levels decline, fish on sawgrass ridges are forced to move into more open sloughs where they are vulnerable to capture by wading birds. Also, the concentration of fish from higher elevation ridges to lower elevation sloughs during the drydown increases fish density 20-150 times in the remaining pools of water (Carter et al. 1973, Kushlan 1974, Kushlan 1976, Loftus and Eklund 1994, Howard et al. 1995). Not surprisingly, densities of some wading bird species are highest where slough habitat is most abundant, even after accounting for the effect of water depth (Bancroft et al. 2002). Species that seem to require both high prey densities and vulnerable prey (i.e., Wood Stork, White Ibis, and Snowy Egret) have declined the most since the 1930s (Ogden 1994).

Collectively, these patterns suggest that the interaction of topography, vegetation structure, hydroperiod, water depth, and draw-down rate can determine the quality of feeding patches for wading birds. Changes to this interaction, from flow or other processes, could alter the timing and magnitude of high-quality prey patches resulting from the seasonal drydown. Coincidentally, two major changes in wading bird populations since the 1930s have been the timing and magnitude of nesting effort (Ogden 1994). Frederick et al. (2001) note considerable degradation of active or former wading bird colony substrate in Water Conservation Areas 2 and 3, and hypothesize that this trend imposes limits on available nesting substrates for wading birds. It is known that compartmentalization has resulted in an increase in ponding in the Water Conservation Areas, especially just upstream of levees.

A diverse wetland mosaic is important to birds other than wading birds. The endangered Snail Kite is a highly specialized bird of prey that depends almost exclusively on one species of aquatic snail (apple snail). The presence of relatively sparse emergent vegetation in open-water areas is an important component of Snail Kite foraging habitat, and kites are unable to forage effectively in dense emergent vegetation (Bennetts et al. 1994). Open water habitats also may be important habitat for foraging by the apple snail population. Therefore, suitable Snail Kite foraging habitat is best represented by a mix of emergent vegetation with open-water communities, which includes the ridge and slough habitat type.

Barriers to fish migration

In addition to altering flow patterns and wetland landscape patterns, barriers to flow serve as barriers to movement of aquatic animals. Joel Trexler (unpublished data) conducted analyses of the population structure of three aquatic species (mosquitofish, spotted sunfish, and grass shrimp). These analyses were conducted by examining relative gene flow, which is a measure of the average number of fish in a generation that move from one area to another and successfully reproduce. Relative gene flow was estimated by comparing gene frequencies from fishes collected in different areas of the Everglades and applying standard population genetics theory to interpret the differences observed (Slatkin 1985, Beerli and Felsenstein 1999, Beerli and Felsenstein 2001). Trexler found that the Tamiami Trail limits migration by mosquitofish between Water Conservation Area 3A and Shark River Slough. A comparison of migration within Water Conservation Area 3A and Shark River Slough and between these two regions suggests that migration within these areas is greater than migration between them (Fig. 18).

graph of estimates of migration rate within Shark River Slough and Water Conservation Area 3A, and between the two areas
Figure 18. Estimates of migration rate within Shark River Slough and Water Conservation Area 3A, and between the two areas. Sites were chosen to have separations of approximately equal distances within each slough, and between the two sloughs. [larger image]

An ongoing study of aquatic animal use of shallow, peripheral Everglades marshes is beginning to demonstrate how those animals move over the landscape in response to flow direction and rate (Loftus et al. 2001). Even at very slow flows, it is evident that fishes and decapod crustaceans can detect and use flow in their dispersal, and that their orientation to flow appears to be species specific.



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