Identifying the proximate demographic cause(s) of population change by modelling spatial variation in productivity, survivorship, and population trends

A technique for identifying the proximate demographic cause(s) of populatio n decline at two spatial scales is proposed and evaluated. The approach inv olves modelling spatial variation in vital rates (productivity and survivor ship) as a function of spatial variation in population trends. Productivity indices and time-constant annual adult survival-rats estimates were modell ed from the Monitoring Avian Productivity and Survivorship (MAPS) Program. For the larger scale, productivity and survivorship of Gray Catbird Dumetel la carolinensis during 1992-98 were modelled from stations in areas compris ed of physiographic strata where the breeding bird survey (BBS) population trend was either significantly positive or negative. Adult survival-rate es timates were area-dependent while productivity indices were independent of area. Differences in modelled population changes agreed well with differenc es in BBS population trends, although the modelled population changes for b oth areas were substantially more negative than BBS trends. Although MAPS p roductivity indices appear to be biased low, these results suggest that low survival of adults was the proximate demographic cause of population decli ne in catbirds in physiographic strata where they were declining, and that management strategies to reverse the declines in catbirds by increasing the ir productivity will not be successful. At the smaller scale, productivity and survivorship were modelled during 1994-99 for Carolina Chickadee Poecil e carolinensis, Gray Catbird, Ovenbird Sciurus aurocapillus, Yellow-breaste d 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 ne gative 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 pr oximate dempographic cause(s) of population decline for each species. Moreo ver, the regression of modelled population change on trend in adult capture s showed a significant positive relationship, although the y-intercept was negative (-0.418), again suggesting that MAPS productivity indices are bias ed low, but that the biases are relatively constant between areas and among species. The short-comings of this approach are discussed. We conclude tha t the approach is in deed useful for identifying the proximate demographic cause of population change, but that an optimal approach would include cons ideration of both spatial and temporal variation in vital rates and populat ion treads.