Print PageOne of the few quantitative relationships in population ecology is the linear
relationship between the variance of population fluctuations and mean population
size, known as Taylor’s power law. The presence of such a general functional
relationship will be of large importance in developing population ecology as a
predictive science because it provides a tool to derive population characteristics
from some simple set of variables and to explain general macroecological patterns
in species abundances both in space and time. Although the underlying ecological
mechanisms are only poorly understood, especially for temporal data, the slope for
the increase in variance with mean population size seem in general to be between
1 and 2 on a logarithmic scale. Taylor and coworkers suggested that these slopes
represent species-specific characteristics, related to the movement behaviour and
the ecology of the species.
Results/Conclusions
We tested the presence of Taylor’s power law on time series in 45 British bird
species. This study show that the combined effects of demographic and environmental
stochasticity produce slopes close to 1.5 for the relationship at a logarithmic
scale between the variance and the average population size within each
species. Since the influence of demographic stochasticity decrease at larger population
sizes it follows that the variance of the annual changes in log population
size of most species decrease with increasing population size. For the bird species
we investigated the populations sizes were in general small. Thus, in smaller population
sizes the increased influence of demographic stocasticity results in slopes
between 1 and 2 for the temporal (log-) variance-mean relationship. This slopes
were also influenced by interspecific differences in life history. These analyses
explicitly show that a macroecological pattern is generated from the interplay between
stochastic and density dependent factors, modulated by life history.
