An irruption interrupted: eradication of wild turkeys Meleagris gallopavo from Santa Cruz Island, California

Eradication of introduced species is often necessary to conserve native biota on islands. Seven wild turkeys Meleagris gallopavo were introduced to Santa Cruz Island, California, in 1975 and the population began to irrupt in the early 2000s. Turkeys posed a variety of threats to native species, including that they could replace the previously eradicated population of feral pigs Sus scrofa as a prey subsidy for golden eagles Aquila chrysaetos, which through incidental predation had driven three subspecies of island fox Urocyon littoralis to near extinction. We implemented a four-phase programme to eradicate the turkey population, based on general principles for eradication efforts. For example, we focused on preventing individual turkeys from becoming aware of our methods, which increased the likelihood we would be able to detect and dispatch all of the birds. Leveraging the tendency of turkeys to aggregate during winter, we used baited drop nets, precision shooting, and monitoring of surgically sterilized, radio-telemetered ‘Judas turkeys’ to eliminate the population. We estimate the population comprised 310 individuals when the project began in 2006 and that the last bird died in December 2012. Methods used in this project could be applied to other alien bird eradication programmes, of which there are few examples in the scientific literature.

Is the decline of Burrowing OwlsSpeotyto cuniculariain prairie Canada linked to changes in Great Plains ecosystems?

SummaryWe examined population and ecosystem patterns hoping to inform conservation strategies for Burrowing OwlsSpeotyto cuniculariain Canada. The owls chose moderately to heavily grazed grasslands for nesting and roosting, and avoided cultivated fields. Where grassland patches were isolated in 90% cultivation, owls dispersed later, for shorter distances and less often. Mortality rate during the 5-month study was high (adult ratio 0.45, juveniles 0.55), which may contribute to local declines. Additional mortality may occur on migration and during winter. We extrapolate from local effects (loss of grassland habitat with burrows for roosting, and ~2% “incidental” predation) to suggest that such changes detrimental to the owls occur throughout the central Great Plains ecosystem which the owls occupy year-round. The changes include ecosystem processes such as plant succession, owl dispersal and predation. These changes are likely to be irreversible on a scale that would be required to help this owl. The species may face extinction in Canada, at the northern limit of its range.

Ingestion of Small-Bodied Zooplankton by Zebra Mussels (Dreissena polymorpha): Can Cannibalism on Larvae Influence Population Dynamics?

The zebra mussel Dreissena polymorpha established populations in western Lake Erie in 1986 and achieved densities exceeding 3.4 × 105 individuals∙m−2 during 1990. We assessed apparently incidental predation on Lake Erie and Erindale Pond zooplankton by adult mussels. Dreissena larvae and small rotifers (Polyarthra spp., Keratella spp., Trichocerca) sustained moderate to high predatory mortality whereas larger taxa (Bosmina, Scapholeberis) were invulnerable to predation. Larval Dreissena almost always sustain > 99% mortality in European lakes. While mortality has been ascribed primarily to lack of suitable settling substrate and unfavourable environmental conditions, it may be confounded by larval predation by adults. We demonstrate using STELLA™-modelling that with a larval mortality rate of 99%, settled mussel densities observed in western Lake Erie during 1990 would not be achieved until at least 1994. A model that combines a lower rate (70%) of abiotic mortality with larval predation by adult mussels correctly predicts 1990 mussel densities. This model of Dreissena population growth is also consistent with larval settlement patterns in Lakes Erie and St. Clair and some European systems.