6. RESEARCH ON POPULATION PROCESSES

6.1 Population Dynamics of Declining Species

Population monitoring by the BTO has revealed severe population declines for a wide range of species. This has led to the inclusion of relatively common, but severely declining species, on lists of species of conservation concern drawn up by government and non-governmental conservation organisations. The identification of population declines is the starting point for research into the population processes that may be driving the declines and to identify any changes in the environment that may have caused the declines.

The BTO has undertaken a number of analyses of population dynamics of declining species, drawing on productivity data from the NRS and survival rates data from the Ringing Scheme, in addition to data from environmental databases such as the ITE’s Land Cover and Land Classification databases, government agricultural statistics and Meteorological Office weather data. Examples include:

6.2 Impact of Avian Predators on Songbirds

During the last 30 years the marked declines in the populations of many British songbirds have coincided with population increases and range expansion of two of their main predators, the Sparrowhawk and Magpie. Using large-scale, long-term census data from the CBC, BTO has shown that Magpies and Sparrowhawks are unlikely to have caused songbird declines because patterns of year-to-year population change did not differ between sites with and without these predators. Other large-scale changes in the environment, such as the intensification of farming practices, appear to be responsible for songbird declines.

6.3 Effects of Spatial Scale on Population Processes

A NERC-funded project, in collaboration with Professor W J Sutherland at the University of East Anglia, has explored the influence of spatial scale on the population dynamics of British birds using BTO data.
Patterns of breeding and natal dispersal have been assessed from national ringing data, spatial synchrony in population level has been explored using CBC data, and spatial variation in reproductive success using NRS data. Further work has examined the links between dispersal and spatial synchrony, and density dependence in demographic parameters.

On the basis of this information we plan to develop and explore scale-dependent models of population processes.

6.4 Local Population Dynamics and Regional Distribution

In a NERC-funded project in collaboration with the Centre for Population Biology at Silwood Park, University of Sheffield, ITE and University of Leeds, we have tested and extended theoretical models of relationships between local population abundance and geographic range size for British flora and fauna. Special emphasis has been given to breeding bird populations using CBC data and both breeding and wintering bird populations using the BTO atlases.

The project has produced the most extensive set of linked empirical analyses of patterns in both interspecific and intraspecific abundance-distribution relationships. We understand how intraspecific relationships are generated and have tested eight hypotheses to explain interspecific relationships. These findings are important because the abundance-distribution relationship is one of the most general ecological patterns, with significant implications for conservation.

Key References

Baillie, S.R. & Peach, W.J. (1992). Population limitation in Palaearctic-African migrant passerines. Ibis, 134 (Suppl.1): 120-132.

Blackburn, T.M., Gaston, K.J. & Gregory, R.D. (1997). Abundance-range size relationships in British birds: is unexplained variation a product of life history? Ecography, 20: 466-474.

Blackburn, T.M., Gaston, K.J., Quinn, R.M., Arnold, H. & Gregory, R.D. (1997). Of mice and wrens: the relationship between abundance and geographical range size in British mammals and birds. Phil. Trans R. Soc., 352: 419-427.

Gaston, K.J., Blackburn, T.M. & Gregory, R.D. (1997). Abundance-range size relationships of breeding and wintering birds in Britain: a comparative analysis. Ecography, 20: 569-579.

Gaston, K.J., Blackburn, T.M. & Gregory, R.D. (1997). Interspecific abundance-range size relationships: range position and phylogeny. Ecography, 20: 390-399.

Gaston, K.J., Blackburn, T.M. & Gregory, R.D. (1998). Interspecific differences in intraspecific abundance-range size relationships of British breeding birds. Ecography, 21: 149-158.

Gregory, R.D. (1998). An intraspecific model of species’ expansion, linking abundance and distribution. Ecography, 21: 92-96.

Krebs, J.R., Wilson, J.D., Bradbury, R.B. & Siriwardena, G.M. (1999). The second silent spring? Nature, 400: 611.

Paradis, E., Baillie, S.R., Sutherland, W.J. & Gregory, R.D. (1998). Patterns of natal and breeding dispersal in birds. Journal of Animal Ecology, 67: 518-536.

Peach, W.J., Crick, H.Q.P. & Marchant, J.H. (1995). The demography of the decline in the British Willow Warbler population. Journal of Applied Statistics, 22: 905-922.

Peach, W.J., Thompson, P.S. & Coulson, J.C. (1994). Annual and long-term variation in the survival rates of British Lapwings Vanellus vanellus. Journal of Animal Ecology, 63: 60-70.

Peach, W.J., Siriwardena, G.M. & Gregory, R.D. (1999). Long-term changes in over-winter survival rates explain the decline of Reed Buntings Emberiza schoeniclus in Britain. Journal of Applied Ecology, 36: 798-811.

Siriwardena, G.M., Baillie, S.R., Buckland, S.T., Fewster, R.M., Marchant, J.H. & Wilson, J.D. (1988). Trends in the abundance of farmland birds: a quantitative comparison of smoothed Common Birds Census indices. Journal of Applied Ecology, 35: 24-43.

Siriwardena, G.M., Baillie, S.R. & Wilson, J.D. (1998). Variation in the survival rates of some British passerines with respect to their population trends on farmland. Bird Study, 45: 276-292.

Siriwardena, G.M., Baillie, S.R. & Wilson, J.D. (1999). Temporal variation in the annual survival rates of six granivorous birds with contrasting population trends. Ibis, 141: 621-636.

Thomson, D.L., Baillie, S.R. & Peach, W.J. (1997). The demography and age-specific annual survival of song thrushes during periods of population stability and decline. Journal of Animal Ecology, 66: 414-424.

Thomson, D.L., Green, R.E., Gregory, R.D. & Baillie, S.R. (1998). The widespread declines of songbirds in rural Britain do not correlate with the spread of their avian predators. Proc Roy Soc. B,.265: 2057-2062.

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