Snail v. Snail v. Rat: The enemy of my enemy is my friend. That may be a concise description of the findings of two University of Hawai`i researchers into the predation of black rats on two species of invasive snails, the giant African snail, Achatina fulica, and the predatory snail Euglandina rosea. The researchers, Wallace Meyer III, of the university’s Department of Zoology, and Aaron Shiels, of the Department of Botany, wanted to know what might happen if rats, whose predation on native endangered snails is well documented, were eradicated from the snails’ habitat. Would the populations of non-native snails, uncontrolled by the rats, explode to the point they’d pose a threat to the natives?
Sadly, the answer seems to be yes. As Meyer and Shiels report in the July 2009 issue of Pacific Science, the “reduction or eradication of R. rattus [the black rat] populations may cause an ecological release of some nonindigenous snail species where these groups coexist. As such, effective restoration for native snails and plants may not be realized after R. rattus removal in forest ecosystems as a consequence of the complex interactions that currently exist among rats, nonindigenous snails, and the remaining food web.”
With rat control efforts in conservation areas on the increase, Meyer and Shiels warn that “the complex interactions among R. rattus and nonindigenous snail species … suggest that managers should proceed cautiously with management and control efforts that involve these species.” Removing rats from the Wai`anae mountains, where resource managers are trying to protect native snail habitat, “may result in E. rosea population increases,” the authors write. “This may have negative effects on native snail populations, which may be irreversible…” (See “Black Rat (Rattus rattus) Predation on Nonindigenous Snails in Hawai`i: Complex Management Implications,” Pacific Science, 63: 339-347.)
Kaukonahua Comments: The state Department of Health has completed its assessment of sources of pollution in the Upper Kaukonahua Stream (North Fork and South Fork), in central O`ahu. A public hearing on the proposed Total Daily Maximum Load (TMDL) for nitrogen and turbidity in these stream sections was scheduled for August 28; the deadline for comments is September 8.
Close readers of last month’s Environment Hawai`i may recall that a TMDL study for Kaukonahua Stream was said by the DOH to be necessary before the agency could issue a National Pollutant Discharge Elimination System permit to the City and County of Honolulu to cover releases from its wastewater treatment plant that discharges into Lake Wilson, which also receives flows from the Upper Kaukonahua Stream.
Necessary, perhaps, but not sufficient. Apparently more studies are needed before the DOH has the information required to set discharge requirements for the city’s wastewater plant.
If the upper reaches of Kaukonahua Stream are cleaned up enough to meet the TMDLs proposed, the DOH says, this should lead to improved water quality in Lake Wilson (also known as Wahiawa Reservoir), as well as the lower reaches of the stream and, eventually, Kaiaka Bay on O`ahu’s North Shore. “However, the extent to which water quality standards will be attained in Wahiawa Reservoir and further downstream largely depends upon the manner in which Wahiawa Reservoir, its other pollutant sources, and its irrigation uses are operated and managed in the future,” the report says.
To review the document or learn more about how to submit comments, see: [url=http://hawaii.gov/health/environmental/env-planning/index.html]http://hawaii.gov/health/environmental/env-planning/index.html[/url]
Volume 20, Number 3 September 2009
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