Analyses of two mute swan populations and the effects of clutch reduction: implications for population management

Abstract

Summary The accidental or deliberate release of alien species may be very disruptive to native biota, principally through competition or predation. Naturalized populations of mute swans Cygnus olor in western Europe and North America have overgrazed native aquatic vegetation, competed with other waterbirds, and damaged arable and fodder crops. Numbers may be controlled by destroying or oiling a proportion of eggs in each clutch to prevent hatching (clutch reduction). A difference equation model was used to examine the effectiveness of clutch reduction on a mute swan population in the Wylye Valley, Wiltshire, UK. Model parameters were derived mainly from a long‐term study of individually marked birds. Survival and emigration were parameterized as a combined function. The model focused on the non‐breeding subpopulation, considered to have a negative impact on local fisheries by overgrazing aquatic macrophytes. The model was also parameterized for another swan population in the West Midlands, UK. This population was characterized by rapid growth in a much larger area, compared with the smaller, comparatively stable population in the Wylye Valley. There were insufficient data available to parameterize accurately the model for other areas. Annual survival rates were high in both populations, ranging between 68% and 73% for juveniles, first‐years and non‐breeding adults, and between 72% and 90% for breeding adults. Immigration was an important factor in the dynamics of both populations. The effects of different levels of clutch reduction on the Wylye Valley mute swan population were simulated. Reducing clutches to two eggs per clutch lowered non‐breeding numbers by 30% over a 10‐year period. Total destruction of all eggs in each clutch stabilized the non‐breeding subpopulation but did not eradicate it. The effects of clutch reduction were offset by high survival rates and immigration. In the West Midlands, the simulated restriction of clutches to two eggs stabilized the non‐breeding subpopulation but did not affect breeding numbers. Total destruction of all eggs in each clutch markedly reduced the non‐breeding subpopulation. However, immigration was underestimated in this model and may have further diluted the effects of total egg destruction. Synthesis and applications. Clutch reduction is labour intensive, requires persistence to be effective, and its effects may vary between populations depending on immigration rates. An evaluation of the consequences of clutch reduction in advance of implementation in the field is therefore highly desirable. The population model described in this study provides the means to do this. This study has demonstrated that control of breeding output did not alleviate a localized conflict, in this case overgrazing by swans in rivers. Deterrent measures and habitat management at the site of conflict may be more effective. The model permits the exploration of a range of demographic manipulations to determine optimum population management regimes before they are implemented.