Geometric growth rate
According to the extinction vortex, as a consequence of declining individual fitness due to genetic deterioration and Allee effects, the year-to-year rate of population change (geometric growth rate) is expected to become increasingly negative as population size diminishes. We calculate geometric growth rate (λ) as: λ = ln (Nt / Nt+1), where Nt is the population abundance in a given year and Nt+1 is the population size one year further away from extinction. To permit finite estimates of a populations’ final growth rate before extirpation, we add a constant [1] to each population abundance measurement. We were not able to calculate geometric growth rate in years where there were gaps of more than one year until the next abundance count. We fit LMMs with the structure λ ~ log10(population size) + log10(BM) + log10(population size):log10(BM) . A positive coefficient for population size in these models would support the hypothesis that per capita growth rate decreases with population size.