Modelling Density Dependence, Environmental Variability, and Demographic Stochasticity from Population Counts: An Example Using Wytham Wood Great Tits

The relative role of density-dependence and of environmental variables in the fluctuations of animal populations was the subject of a well-known debate in the 1950’s (Lack, 1954; Andrewartha and Birch, 1954). The present consensus (e.g., Royama, 1977), which applies also to plant populations (e.g., Harper, 1967, p.21), can be summarized roughly as follows : —(1) long term stability in numbers can only result from densitydependence, which acts as a negative feedback (in cyberneticians’ language); —(2) large short term variations can be induced by environmental variability, which thus may mask the effects of density-dependence; —(3) population numbers result from the interplay of these two mechanisms in a process which is intrinsically multiplicative over time. As a consequence population numbers rarely follow simple patterns. Great Tit Parus major counts in Wytham wood (Oxfordshire) provide a striking example of this complexity (Fig.1).

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