Abstract
Information about the survival of species is important in many ecological applications. Yet, the estimation of a species’ natural mortality rate M remains a major problem in the management and conservation of wild populations, often circumvented by applying empirical equations that relate mortality to other traits that are more easily observed. We show that mean adult M can be approximated from the general law of decay if the average maximum age reached by individuals in a cohort is known. This is possible because the proportion P of individuals surviving to the average maximum age in a cohort is surprisingly similar across a wide range of examined species at 1.5%. The likely reason for the narrow range of P is a universal increase in the rate of mortality near the end of life, providing strong evidence that the evolutionary theories of ageing are the norm in natural populations.
Highlights
Information about the survival of species is important in many ecological applications
We propose to estimate the average natural mortality rate during the main reproductive phase from the observed maximum age, tmax, taken ideally as the mean across maximum ages observed over a number of years or the mean age of the most long-lived 10% in a cohort[17], i.e., within a group of individuals all born in the same year, and here approximated for natural populations by the mean age of the eldest individuals reported for a species
We found a surprisingly narrow range for the proportion of individuals surviving from birth to the average maximum age in a cohort, P, across 202 species from 6 vertebrate classes (Supplementary Table S1; Fig. 1), with a median P at 0.015 and 90% of the estimates falling between 0.0004 and 0.11
Summary
Information about the survival of species is important in many ecological applications. We show that mean adult M can be approximated from the general law of decay if the average maximum age reached by individuals in a cohort is known. From Eq (4) it follows that if tmax and M are known P can be calculated from P 1⁄4 eðÀM*tmaxÞ It has been proposed for various species that the average maximum age is reached by about 1−5% of individuals in a cohort, including fish[18,19,20,21,22], reptiles[23], birds[24] and trees[25]. If the proportion of individuals surviving from birth to the average maximum age in a cohort, P, would be the same for different species within different taxonomic groups, and if mortality follows an exponential decay, knowledge of P would make Eq (4) a direct and universal natural mortality estimator, requiring only observations on maximum age. The purpose of this study is to estimate P from wild and natural, not hunted or harvested, populations for which tmax and M are known and to draw conclusions about its variability and its general applicability
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