Range expansion of raptors in Britain and the Netherlands since the 1960s: testing an individual-based diffusion model

1. Legal protection since the 1950s, and the reductions in the use of DDT and other organochlorine compounds from around 1970, made recovery and resettlement of raptor species possible in both Britain (GB) and the Netherlands (NL). Data on the distribution of various species between 1960 and 1994, are used for calculating the velocity of range expansion in both countries. 2. The relationship of the square root of the area occupied as a function of time is linear, which implies a constant rate of range reoccupation. The sequence in observed velocities of reoccupation from slowest to fastest is as follows: red kite, Milvus milvus L. (GB); buzzard, Buteo buteo L. (GB); osprey, Pandion haliaetus L. (GB); buzzard (NL), marsh harrier, Circus aeruginosus L. (GB); sparrowhawk, Accipiter nisus L. (NL); sparrowhawk (GB); goshawk, Accipiter gentilis L. (Wales); peregrine, Falco peregrinus Tunst. (GB); and goshawk (NL). 3. Expansion of the buzzard in Britain 1972-92 was lower than during the period 1915-54. This species is reported to be heavily persecuted, especially along the edge of the distribution range. For the same reason the goshawk in Britain expanded only in some areas, and disappeared in others. 4. Using the model of Van den Bosch et al. (1990), an expected velocity of range expansion was calculated based on published data on reproduction, survival and dispersal of new breeding birds. These three parameters sufficed to obtain a reliable fit to the observed velocities of range expansion for a part of the distribution range of a species. The expected velocities are about the same as the observed ones. 5. In the species under study, different types of range expansion are distinguished: (i) colonization of an island (an invasion sensu stricto; osprey, goshawk GB); (ii) colonization from a very small starting area (an invasion sensu lato; marsh harrier, red kite); and (iii) reoccupation of former breeding grounds in a part of the distribution range (recovery; goshawk NL, buzzard NL GB, sparrowhawk GB NL). In all three cases, the processes underlying the expansion as defined in the Van den Bosch model, proved to be sufficient to explain the observations. 6. Based on simulations with the model, the present study concludes that at levels in lifetime reproduction (R-o) just above 1.0 fledglings per female, minor increases in survival or reproduction can accelerate the velocity of range expansion substantially. Moreover, the more a species shows long-distance dispersal, the faster its range will expand from levels of R-o just above 1.0. Minor changes in reproduction and/or survival can lift a stable population (R-o = 1.0) over the threshold towards expansion.