Mortality estimates from age distributions: Critique of a method used to study seagrass dynamics

Abstract

Age structure of seagrass samples has been used to estimate survival and recruitment and then used to estimate population growth rate. Survival rate can be estimated from age structure only if the population is neither growing nor declining (r = 0), so the age distribution is both stable and has stationary structure. If survival is estimated from age structure, it must be assumed that r = 0 or additional information about the population must be known. If a decaying exponential model is used for number (N) in each age class, In N versus age has a slope of -(M + r), and so an incorrect survival rate, exp (-M), would be estimated if r ≠ 0. Simulations show that when r > 0, the slope of the regression of In N versus age is too steep and hence mortality rate would be overestimated, and the reverse when r < 0. Ignoring the assumption of r = 0 is a fundamental flaw in many seagrass studies and calls into question the mortality and population growth rates that have been published.