The spectral colour of population dynamics and its underlying causes have received a fair amount of attention in the literature (e.g. García-Carreras & Reuman 2011; Inchausti & Halley 2002, 2003; Laakso et al. 2003). Most of the studies have demonstrated that the population dynamics of most species is reddened (Inchausti & Halley 2002; Laakso et al. 2003 but see Erb et al. 2001). Yet, no clear agreement exists about the drivers of this phenomenon. According to “reddened forcing hypothesis”, the population dynamics is following the colour of its environment, but there is a support both for (García-Carreras & Reuman 2011; Laakso et al. 2003) and against this explanation (Inchausti & Halley 2002). Alternative explanations have been proposed, just to name some of them: overlapping generations, consideration of spatial processes, etc. Erb et al. (2001) distinguished extrinsic (e.g. diseases, trophic-level interactions) and intrinsic (behavioural, such as dispersal or sociality) forces that are responsible for the “colour pattern” observed in population dynamics. However, stating that intrinsic factors are leading to direct density-dependent effects whereas extrinsic factors underlie the lagged effects is a bit of a simplification, as underlined by the authors themselves. Indeed, intrinsic processes, such as dispersal can also lead to lagged population responses.
It seems that after many years of studying the colouring of population dynamics, the analysis of long-term population time-series has so far uncovered the pattern, but the mechanisms leading to the observed colour of population dynamics remains unrevealed.
I can see two alternative ways to try to understand the mechanisms and distinct processes leading to specific colours of population dynamics:
– Mechanistic modelling of each process explicitly which would allow comparison of the issuing population dynamics with the pattern observed in the long-term time-series data. However, this approach is not free of drawbacks: the results of any model will largely depend on its structure and chosen parameters…
– Experimental testing of the alternative hypotheses about diverse forces driving the population dynamics and leading to its colour. Such designed experiments are not likely to be possible with large animals as study species, and here the long-established laboratory species models (such as daphnia, drosophila and tetrahymena, see about studies on this species in our lab on our blog) would be of great help! (see more on the choice of the study system here).
It’s hard to imagine that there is a clear-cut between intrinsic and extrinsic factors, and it is rather unlikely that some of them are predominantly driving the population dynamics of certain species. More likely, they are acting together, and, possibly, even an interaction among certain population processes gives a rise to a specific population dynamics with its spectral colour. It would therefore be an exciting research adventure: to use established laboratory systems to try with the well-designed experiments to disentangle the role of intrinsic and extrinsic factors, and in a second step, contrast the results of those experiments with the analyses of long-time population series collected for large mammals and birds.
I am looking forward to your critics and opinions about how it would be possible to reveal what processes are leading to the observed spectral colour of population dynamics!
Erb, J., M. S. Boyce, and N. C. Stenseth. 2001. Population dynamics of large and small mammals. Oikos 92:3-12.
García-Carreras, B., and D. C. Reuman. 2011. An empirical link between the spectral colour of climate and the spectral colour of field populations in the context of climate change. Journal of Animal Ecology 80:1042-1048.
Inchausti, P., and J. Halley. 2002. The long-term temporal variability and spectral colour of animal populations. Evolutionary Ecology Research 4:1033-1048.
Inchausti, P., and J. Halley. 2003. On the relation between temporal variability and persistence time in animal populations. Journal of Animal Ecology 72:899-908.
Laakso, J., K. Löytynoja, and V. Kaitala. 2003. Environmental noise and population dynamics of the ciliated protozoa Tetrahymena thermophila in aquatic microcosms. Oikos 102:663-671.