Intra-population interactions between stages counteracts positive population-level effects of mortality

MAXWELL JULIAN; CONSTANZA SANCHEZ DE LA VEGA; ALEJANDRA C VENTURA

Rosario

Congreso; L Reunión Anual de la Sociedad Argentina de Biofísica; 2022

Sociedad Argentina de Biofísica

For an unstructured population model, meaning a model for thedynamics of indistinguishable individuals that accounts for populationabundances only, conditions can be selected for the decreasingdensity-mortality rule to hold. This rule indicates that an increase inper-capita mortality leads to both a decrease in population density atequilibrium. Under the same conditions, except now structure consideredinto the model, taking into account differences between individuals in age orsize, this rule loses generality. The phenomenon of a population increasing inresponse to an increase in its per-capita mortality rate has been termed the"hydra effect".We evaluated the hydra effect in a resource-consumer structuredpopulation model (system of ODEs). Resources were kept unstructured. Themodel includes foraging, assimilation (including production of new juvenileoffspring and maturation) and aging, and is stage-structured based on age:juveniles, adults, and post-reproductive adults, or seniors (J, A, S). An increasedmortality rate in post-reproductive individuals is considered, and itsimpact over the total population size in equilibrium is calculated. We foundconditions leading to hydra effect and to anti-hydra effect. Interestingly, wefound that adding a cooperation interaction (analog to commensalism betweenspecies) between post-reproductive (S) and juvenile (J) individuals counteractsthe hydra effect, removing positive population-level effects of mortality.“Social populations”, whose individuals interact cooperatively (inthis case, senior helping juvenile individuals), respond negatively to anincrease in old-age-related diseases, decreasing the steady state totalpopulation density. Since an empirical correlation has been found betweensocial mammal populations and lifespan of its individuals, these results mayhave implications in evolutionary biology.