Population dynamics across species range: Among-tidal level difference in population dynamics of intertidal barnacle Chthamalus dalli

Background/Question/Methods

How population dynamics vary across species range? Two alternative predictions exist for this question: (1) Population variability is larger in center of species range compared to marginal area because strong density effect destabilizes population dynamics. (2) Population variability is larger in margin than center because individuals are more susceptible to environmental fluctuation in marginal populations. These two will effectively be tested by statistical fitting of population dynamics model such as logistic model to long term observation data of populations. In addition, based on the fact that population variability is often related to mean population size, it will be possible that spatial difference in population size affect spatial pattern of population variability. If such a mean-dependent spatial structure of population variability exists, it will be captured by using a macroscopic law of population variability such as Taylor's power law (hereafter TPL). In order to explore spatial pattern of population dynamics across a species range, we focused on intertidal barnacle Chthamalus dalli and examined differences in population dynamics along their vertical range (i.e., along tidal level). Changes in population coverage were monitored for 10 years at 25 plots in northeastern Hokkaido, Japan and analyzed by fitting of logistic growth model and TPL.

Results/Conclusions

Results of statistical fitting of the logistic model supported the prediction (2) above. Population variability was higher in range margins compared to center, reflecting increased impact of stochasticity on population growth rate. Although density dependence in range center was stronger than that in margins, it was not strong enough to destabilize population dynamics. Analysis of TPL, on the other hand, highlighted importance of spatial difference in mean population size on population variability. Spatial variation in population variability was tightly related to mean population size. Estimated scaling exponent of TPL suggested that relative variability of population size was negatively correlated with mean population size. This mean-variance relationship of population size was consistent across different tidal levels. We conclude that in general, there is a tendency for variability of C. dalli populations to increase from center of their vertical range toward range margins, while taking spatial difference in mean population size into account is also important for understanding spatial pattern of population variability. Our results suggest that combining two different models of population variability, the logistic model and TPL, will effective for revealing patterns and processes of population dynamics across a species range.