Impacts of Waterfowl and Water Levels on Wetland Vegetation

1
Impacts of Waterfowl and Water Levels on Wetland
Vegetation
Jay Frentress, Adrienne Froelich, David Lodge,
Department of Biological Sciences, University of
Notre Dame

• 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.

Greenhouse Experiment Results
Fig. 2 Species richness as a function of water
level
Fig. 4 Total vascular aboveground biomass
produced from seed from exclosures and references
at different water levels
No Waterfowl
ANOVA results Bird plt0.001 depth
p0.009 birddepth p0.177
Waterfowl
Total aboveground biomass (g)
Species richness

• Total aboveground biomass was more than twice as
  great in exclosures at all depths (plt 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).

Water level (m)

• 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.

• 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.

Fig. 3 Exclosure in the summer of 1997 showing a
dense stand of S. americanus. (left)
Fig. 5 Seed bank abundance
No Waterfowl
Watefowl
Field Experiment Results
Number of seeds
Fig. 1. Biomass of three-square bulrush (Scirpus
americanus)
Scirpus
Other
All intact seeds
A) Aboveground

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

Water level

• 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.

Water level in plots (m)
Scirpus biomass (g m-2 dryweight /- 95 CI)
B) Belowground
Snow Goose (left). More than 100,000 waterfowl
(comprising 20 species) overwinter at Lake
Mattamuskeet.
Water level

• Greenhouse Experiment Methods
• A 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.

No Waterfowl
Waterfowl

• 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.