Posted by: oikosasa | March 12, 2013

The little black dress of ecology

Taylor´s power law and bird populations are studied with in the new Early View paper “Interspecific differences in stochastic population dynamics explains variation in Taylor’s temporal power law”, by Marit Linnerud and her coworkers. Here’s Marit’s summary of the study:

Taylor’s power law – an oldie but goodie!

Taylor’s power law is like the little black dress of ecology, a general law that fits every species regardless of size or other personal characteristics.  According to the law the variance of population abundance in either time or space can be described by a function of the mean. A reasonable null expectation following from the definition of the variance is that as the abundance of a population increase by one unit on a logarithmic scale the variance is expected to increase by two logarithmic units, resulting in a slope of two. However, empirically the slope is often less than two, thus revealing some interesting ecological dynamics. Although the causal mechanism behind the law is not agreed upon, it seems likely that several factors are at play. A theoretical framework based on stochastic population dynamics provides testable predictions of what causes the deviations from the expected slope of two.

In our recent study we estimate the temporal mean-variance relationship for a large number of British bird populations. There were two significant challenges. Firstly, we could not ignore that sampling in itself could bias the estimates of the variance and secondly the estimated of variance increases with the number of years the population have been studied. Taking this into account we evaluated how the different deterministic and stochastic factors known to affect temporal population dynamic influenced the slope of the power law. It turns out that differences in demographic stochasticity among species were the main explanation of the variation in the slopes of Taylor’s power law.

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