Assessing the 'Small Population' Paradigm: The Effects of Stochasticity on Evolutionary Change and Population Growth in a Bird Metapopulation

Habitat loss is leading to smaller fragmented populations, increasing their susceptibility to stochasticity. Quantifying the effects of demographic and environmental stochasticity on population dynamics and the contribution of selection versus drift to phenotypic change is essential to assess the potential consequences of environmental change. We examined how stochasticity influenced population growth and body mass changes over 22 years in 11 insular house sparrow (Passer domesticus) populations. Environmental stochasticity induced synchrony in growth rates across populations while also causing substantial island-specific fluctuations. Additionally, demographic stochasticity led to larger annual growth rate fluctuations in smaller populations. Although heavier individuals generally had higher fitness, we detected non-directional evolutionary change in body mass, driven by drift rather than selection. Our study provides a unique quantitative assessment of the ‘small population’ paradigm, highlighting the importance of theoretically driven analyses of long-term individual-based data to understand the drivers of phenotypic evolution and a population's long-term viability.