We described fish-diet differences across habitats and seasons by analyzing stomach contents of > 4,000 fishes of 32 native and introduced species. Major foods included periphyton, detritus/algae mix, small invertebrates, aquatic insects, decapods, and fishes. Florida gar, largemouth bass, pike killifish, and bowfin were at the top of the piscine food web. Using prey volumes, we grouped the fishes as herbivores, omnivores, or carnivores. Stable-isotope analysis gave an independent and similar assessment of trophic placement. Community trophic patterns were similar to those from tropical communities.

The trophic ecology of Everglades fishes is poorly documented. In tropical-fish communities, hydrologic seasonality greatly affects patterns of food-resource use, such as local and long-range movements among habitats, seasonal changes in diets, diet specializations, the presence and seasonality of diet overlaps, etc. (Lowe-McConnell 1975; Winemiller 1989, 1990).

We analyzed Everglades native and introduced fish diets to identify the most important invertebrate and plant prey, and to compare trophic positions of species within and among three habitats and two seasons (Fig. 1). We used the complementary techniques of stomach and stable isotope analysis for both instantaneous and time-integrated views of animal diets (Vander Zanden and Rasmussen 1996).

We then compared diet patterns in the temperate Everglades fish community with those from seasonal aquatic systems in the tropics. We posed the following hypotheses in examining those patterns:

H_o1: Herbivory and omnivory are common trophic states in the Everglades.

H_o2: Seasonal changes in diets within a habitat are usual .

H_o3: Specialization in diet is common; generalists are rare.

H_o4: Diet overlap is highest during high-water periods when food is plentiful.

This study is a first step in using data from across habitats and seasons for use in constructing a empirical food web for the Everglades marsh.

• ~ 4,000 specimens of 26 native fishes collected by rotenone, nets, angling, and electrofishing from 1977-1981, supplemented by 6 introduced species during 1995-1997.
• Each species separated into arbitrarily chosen size-groups of ~ 25 fish; length and wet mass recorded.
• Volumes measured to 0.001ml using a blood-sedimentation tube; volume of digested prey not reconstructed; Small size of food items required the pooling of like items from each size-class of fish for volumetric analysis.

• We grouped fishes into 5 Trophic Classes using continuous scores (_i ) based on sums of prey scores, multiplied by the proportion of diet comprised by each prey type (Adams et al. 1983; Winemiller 1990).

  _i = 1.0 + _j( F_ij ), where _i = of species i, _j = of species j , and F_ij = proportion of the food volume of species i comprised by prey j.

  1 = Mainly Herbivorous, >50% plant material (Score = 1.0 -1.74);

  2 = Omnivorous, 25-50% plant material (Score = 1.75-2.19);

  3 = Omnivorous, <25% Plant material (Score = 2.20-2.74);

  4 = Omnivorous, mostly animal prey (Score = 2.75-2.99);

  5 = Predominantly carnivorous, fish and decapods (Score = >3.0).

H_o1: Only a few species were strict herbivores, detritivores, or piscivores. Most were omnivores, and these included the most numerous species in the community (Fig. 2).

H_o2: Seasonal changes in diets within habitats occurred, as seen in the NonmetricMultiDimensional Scaling plots (Fig. 3), although membership in trophic classes did not shift much with season (Fig. 4). At low water, volumes of detritus increased in pond fish (Wilcoxon Z=0.283, p<0.05), and fish prey increased in spikerush fish (Z=2.24, p<0.05).

Total food volume decreased in pond fish at low water (Z=2.31, p<0.025), but increased in sawgrass (Z=2.35, p<0.02).

H_o3: Everglades fishes focused on a few major prey, shown by low diet breadth (Fig. 5) (Levins B). B decreased in ponds between high and low water (Wilcoxon Z=2.55, p<0.01), but not in other habitats.

H_o4: Pianka’s Dietary overlap (a_ij ) within habitats between high- and low-water periods was generally low and communities did not differ significantly (Mantel test, p<0.001). However, some species /size classes within habitats had diets that were similar:

Does the Everglades system behave more like a temperate or a tropical wetland, based on its faunal and abiotic characteristics? Environmental periodicity driven by seasonal rainfall causes the Everglades to resemble tropical wetlands.

• At High Water:
  • Habitat is non-limiting; fishes move into marshes.
    
  • Stomachs are full with diverse prey.
    
  • Diet overlap is high in only a few species pairs.
• At Low Water:
  • Large-bodied fishes move into ponds; smaller ones stay mainly in marshes.
    
  • Abiotic stresses and predation affect pond fishes.
    
  • Lower food consumption and more empty stomachs.

• Most Everglades fishes are generalistic omnivores. Specializations (mud, scale, fin, and vascular-plant feeding) seen in tropical communities is absent.
• The seasonality of flooding affects habitat use and diet. Heaviest feeding at high- vs. low-water times, although some prey are more prevalent in diets at low water.
• Everglades fauna responds to hydrology similarly to tropical faunas, but with less marked seasonal diet shifts. This may reflect temperate origin of native fauna. In temperate waters, specialized herbivores and detritivores are uncommon. The introduced species included three mainly herbivore/detritivores, two omnivores, and a piscivore.

Adams, S. M., B. L. Kimmel, and G. R. Plosky. 1983. Sources of organic matter for reservoir fish production: a trophic-dynamics analysis. Canadian Journal of Fisheries and Aquatic Sciences 40, 1480-1495.

Lowe-McConnell, R. H. 1975. Fish communities in tropical freshwaters: their distribution, ecology, and evolution. Longman, Inc., New York.

Vander Zanden, M. J., and J. B. Rasmussen. 1996. A trophic position model of pelagic food webs: impact on contaminant bioaccumulation in lake trout. Ecological Monographs 66: 451-477.

Winemiller, K. O. 1989. Ontogenetic diet shifts and resource partitioning among piscivorous fishes in the Venezuelan llanos. Environmental Biology of Fishes 26: 177-199.

Winemiller, K. O. 1990. Spatial and temporal variation on tropical fish trophic networks. Ecological Monographs 60: 331-367.

Winemiller, K. O. 1996. Factors driving temporal and spatial variation in aquatic floodplain food webs, pages 298-312 IN G. A. Polis and K. O. Winemiller (Eds.). Food Webs: integration of patterns and dynamics. Chapman and Hall, New York. 472 p.

We could not have completed this study without the efforts of Victoria Foster and Jennifer Rehage, who assisted in summarizing the raw data for analysis and in preparing the report and poster. Tom Atkeson, FL DEP, provided funds that helped to complete the analysis.

Support for the study was provided by: USGS-Biological Resources Division, U. S. National Park Service, Florida Department of Environmental Protection, Florida International University

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For more information contact:

1 USGS Biological Resources Division Everglades NP Field Station 40001 State Road 9336 Homestead, Florida 33034
2 Dept of Biology Florida International Univ. University Park Miami, Florida 33199

Related information:

SOFIA Projects: Influence of Hydrology on Life-History Parameters of Common Freshwater Fishes from Southern Florida

Population Structure and Spatial Delineation of Consumer Communities in the Everglades National Park

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