Desante D.F., Nott M.P. & O'Grady D.R. (2001) Identifying the proximate demographic cause(s) of population change by modelling spatial variation in productivity, survivorship, and population trends. ARDEA 89 (1): 185-208

A technique for identifying the proximate demographic cause(s) of population decline at two spatial scales is proposed and evaluated. The approach involves modelling spatial variation in vital rates (productivity and survivorship) as a function of spatial variation in population trends. Productivity indices and time-constant annual adult survival-rate estimates were modelled from the Monitoring Avian Productivity and Survivorship (MAPS) Program. For the larger scale, productivity and survivorship of Gray Catbird Dumetella carolinensis during 1992-98 were modelled from stations in areas comprised of physiographic strata where the breeding bird survey (BBS) population trend was either significantly positive or negative. Adult survival-rate estimates were area-dependent while productivity indices were independent of area. Differences in modelled population changes agreed well with differences in BBS population trends, although the modelled population changes for both areas were substantially more negative than BBS trends. Although MAPS productivity indices appear to be biased low, these results suggest that low survival of adults was the proximate demographic cause of population decline in catbirds in physiographic strata where they were declining, and that management strategies to reverse the declines in catbirds by increasing their productivity will not be successful. At the smaller scale, productivity and survivorship were modelled during 1994-99 for Carolina Chickadee Poecile carolinensis, Gray Catbird, Ovenbird Seiurus aurocapillus, Yellow-breasted Chat Icteria virens, and Field Sparrow Spizella pusilla from stations on military installations in Kansas and Missouri (western Midwest) and Indiana and Kentucky (eastern Midwest). Selected species were those whose trend in adult captures over the six years 1994-99 was significantly positive or negative in the eastern or western Midwest and of the opposite sign (but not necessarily significant) in the other area. We were able to identify the proximate demographic cause(s) of population decline for each species. Moreover, the regression of modelled population change on trend in adult captures showed a significant positive relationship, although the gamma-intercept was negative (-0.418), again suggesting that MAPS productivity indices are biased low, but that the biases are relatively constant between areas and among species. The short-comings of this approach are discussed. We conclude that the approach is indeed useful for identifying the proximate demographic cause of population change, but that an optimal approach would include consideration of both spatial and temporal variation in vital rates and population trends