R. Lensink, RANGE EXPANSION OF RAPTORS IN BRITAIN AND THE NETHERLANDS SINCE THE 1960S - TESTING AN INDIVIDUAL-BASED DIFFUSION-MODEL, Journal of Animal Ecology, 66(6), 1997, pp. 811-826
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 betw
een 1960 and 1994, are used for calculating the velocity of range expa
nsion in both countries. 2. The relationship of the square root of the
area occupied as a function of time is linear, which implies a consta
nt rate of range reoccupation. The sequence in observed velocities of
reoccupation from slowest to fastest is as follows: red kite, Milvus m
ilvus L. (GB); buzzard, Buteo buteo L. (GB); osprey, Pandion haliaetus
L. (GB); buzzard (NL), marsh harrier, Circus aeruginosus L. (GB); spa
rrowhawk, Accipiter nisus L. (NL); sparrowhawk (GB); goshawk, Accipite
r gentilis L. (Wales); peregrine, Falco peregrinus Tunst. (GB); and go
shawk (NL).3. Expansion of the buzzard in Britain 1972-92 was lower th
an during the period 1915-54. This species is reported to be heavily p
ersecuted, especially along the edge of the distribution range. For th
e 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. (199
0), an expected velocity of range expansion was calculated based on pu
blished data on reproduction, survival and dispersal of new breeding b
irds. 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 invasi
on sensu stricto; osprey, goshawk GB); (ii) colonization from a very s
mall starting area (an invasion sensu lato; marsh harrier, red kite);
and (iii) reoccupation of former breeding grounds in a part of the dis
tribution range (recovery; goshawk NL, buzzard NL GB, sparrowhawk GB N
L). In all three cases, the processes underlying the expansion as defi
ned in the Van den Bosch model, proved to be sufficient to explain the
observations. 6. Based on simulations with the model, the present stu
dy 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. Moreove
r, the more a species shows long-distance dispersal, the faster its ra
nge will expand from levels of R-o just above 1.0. Minor changes in re
production and/or survival can lift a stable population (R-o = 1.0) ov
er the threshold towards expansion.