Northern Prairie Wildlife Research Center
Environmental Characteristics Associated with the Occurrence of Avian
Botulism in Wetlands of a Northern California Refuge
Management Implications
The number of permanent wetlands (4) used in this study was small and all were
located within a narrow geographic area (SNWR); therefore, the range of environmental
conditions we measured was not very broad. We were able to analyze the data
for simple associations between environmental parameters and botulism outbreaks,
but not for complex associations that might be expected with variables like
pH and redox potential, for which optimal ranges are likely to occur. For example,
botulism outbreaks occurred in wetland enclosures with a sediment pH (pH S1)
that ranged between 7.0 and 7.5 (Table 1), whereas
a much broader range of pH was evident in enclosures where outbreaks did not
occur (6.0–8.3). In spite of its limitations, the results of our study
convey a preliminary indication of wetland characteristics that are associated
with botulism outbreaks and provide a basis for future research and development
of management strategies to reduce botulism losses. Most notably, we identified
several environmental conditions associated with the occurrence of botulism
outbreaks in high risk wetlands at SNWR, including higher temperature and higher
invertebrate abundance or biomass. However, because these factors were not consistently
higher in outbreak wetlands compared to nonoutbreak wetlands, they may have
a more proximate effect in initiating an outbreak. Other wetland characteristics,
such as redox potential, appear to play a greater role in determining whether
a wetland is at high or low risk for botulism outbreaks, and these risk factors
apparently vary annually.
Because wetlands tend to be seasonally dynamic, evaluation of environmental
conditions that confer risk is complicated. Our results suggest several factors
warrant further examination, including temperature, redox potential, pH, and
conductivity. Future investigations should consider if characteristics of
individual wetlands predetermine their risk for botulism outbreaks, and if
other proximate conditions are required to initiate mortality. Also, more
work is needed to determine how mortality rates from botulism are related
to environmental conditions. Knowledge of these factors could be used to develop
wetland management strategies that reduce the risk of botulism outbreaks and
reduce mortality of waterbirds.
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U.S. Geological Survey