Population Growth and Urban Management in Metropolitan Regions: the Contribution of Natural Balance and Migration to Polycentric Development in Barcelona

The present study tests if the spatial variability in endogenous and exogenous components of population growth (natural balance and migration balance) reflects the transition from a mono-centric (compact-dense) settlement structure towards a more polycentric agglomeration based on sub-centers. The spatial distribution of population growth rates across municipal units in Barcelona province (Spain) was analyzed over a sufficiently long time period (44 years between 1975 and 2018, partitioned into four intervals of equal length, 1975-1985, 1986-1996, 1997-2007, 2008-2018) at five concentric rings around Barcelona. Natural balance and migration rates were investigated vis a vis selected territorial indicators using descriptive, inferential and multivariate statistical techniques. The contribution of natural balance to overall population growth decreased with the distance from downtown Barcelona; the contribution of migration balance increased with population density. In the first period (1975-1985), natural balance was higher in peri-urban locations and sub-central municipalities. In the following two periods (1986-1996 and 1997-2007), the contribution of natural balance to total population growth decreased, showing a greater spatial concordance with migration rate. In the last period (2008-2018), natural population growth decelerated further, and the impact of migration balance was extremely variable across space. The empirical results of this study shed light on the (apparent or latent) demographic processes underlying Barcelona's long-term growth, and provide evidence in favor of a relationship between polycentric development and the shift from natural growth to immigration as the main engine of urban expansion in recent times.