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Impacts of Waterfowl and Water Levels on Wetland VegetationJay Frentress, Adrienne Froelich, David Lodge, Department of Biological Sciences, University of Notre Dame

Field Experiment Methods Eight paired exclosure cages and reference plots were installed in Snow Goose Bay, a natural wetland at Lake Mattamuskeet, NC.

Above and belowground biomass was sampled every three months from each plot.

Introduction Wetlands are vital habitat for waterfowl, but the number and quality of wetlands continent-wide have been greatly diminished.

Loss of winter wetland habitat may be an important cause for declines in some species of waterfowl, while other species, like the snow geese focused on here, have increased because agricultural land provides rich food. Herbivory by waterfowl can change wetland vegetation composition and reduce productivity. Increasing populations of snow geese may deplete already dwindling wetland habitat. In a field experiment we tested this hypothesis and the impact of fluctuating water level on vegetation.

Conclusion Exclusion of waterfowl had a significant effect on vegetation composition and productivity. See Figs. 1,3.

There was very little S. americanus belowgound biomass in September ‘96 to account for the large amount of aboveground biomass the following season (Fig. 1). It is likely that these stands are the result of growth from seed, which germinates better under drought rather than high water conditions.

In the greenhouse experiment the exclusion of waterfowl led to significant higher levels in aboveground and belowground biomass, species richness and seed abundance (Figures 4, 5).

While S. americanus tubers are mainly consumed by Snow Geese, their seeds are commonly eaten by dabbling ducks. Given the almost complete lack of S. americanus in reference plots, it is unlikely that S. americanus could persist in the presence of abundant snow geese and dabbling ducks.

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-5 cm 0 cm 10 cm

Fig. 3 Exclosure in the summer of 1997 showing a dense stand of S. americanus. (left)

Greenhouse Experiment MethodsA greenhouse experiment was conducted to determine the impact of seed bank and fluctuating water levels on vegetative productivity and composition.

Sediment cores (15cm depth) were taken from exclosure and reference areas at Snow Goose Bay. Cores were grown under greenhouse conditions, maintaining water levels at –5, 0 and 10 cm.

Each container was destructively sampled for above and belowground vegetative biomass before the plants could set seed.

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Field Experiment Results

Dramatic increase in S. americanus aboveground biomass in exclosures (no waterfowl) relative to references (waterfowl) (Figure 1, A).

Relatively low amount of S. americanus belowground biomass the season (Sep 96) preceding dramatic S. americanus growth (Fig. 1, B)

In the last year when water levels were high, S. americanus above and belowground biomass fell.

Greenhouse Experiment Results

Total aboveground biomass was more than twice as great in exclosures at all depths (p< 0.001).

There was not a significant effect of water depth unless Chara spp. is excluded, limiting the analysis to vascular plants (p = 0.009).

Seed abundance was more than twice as high in soil from exclosures than from references.

Snow Goose (left). More than 100,000 waterfowl (comprising 20 species) overwinter at Lake Mattamuskeet.

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-0.3 -0.2 -0.1 0 0.1 0.2 0.3 0.4 0.5 0.6

Emergent vegetation: Scirpus, Carex, Cyperus, Pluchea

Semi-emergent vegetation: Sagittaria, Eleocharis spp.

Submergent vegetation: Vallisneria, Najas, Charophytes

Water level was a good indicator of species richness and vegetative composition (Fig. 2). In particular, the three-square bulrush (S. americanus) thrived during low water periods.

Sci

rpus

bio

mas

s (g

m-2

dry

wei

ght +

/- 9

5% C

I)

B) Belowground

A) Aboveground

Fig. 1. Biomass of three-square bulrush (Scirpus americanus)

Water level (m)

Spe

cies

ric

hnes

s

Fig. 2 Species richness as a function of water levelFig. 4 Total vascular aboveground biomass produced from seed from exclosures and references at different

water levels

ANOVA resultsBird p<0.001depth p=0.009bird*depth p=0.177

Fig. 5 Seed bank abundance

Num

ber

of s

eeds

Scirpus Other All intact seeds

No Waterfowl

Waterfowl

No Waterfowl

Watefowl

Water level in plots (m

)

No Waterfowl Waterfowl

Water level

Water level

To

tal a

bo

veg

rou

nd

bio

mas

s (g

)