A 'cohort effect' is a phenomenon where cohorts of a population differ from each other in some key attribute(s), such body mass. Birth mass, birth dates, rate of growth of juveniles, survival over the first winter, age of first reproduction, and adult survival rates are often related to the influence of the time of birth, which itself is a product of the environmental conditions cohorts face at birth as well as the nutritional condition of their mothers (itself a reflection of environmental conditions faced at or prior to birth). Environmental conditions that can influence birth and early growth attributes include droughts, abnormally high rainfall, late snowfall, and high population density. These and other factors influence birth attributes and early development of juveniles, and these early effects may persist and affect phenotypic quality throughout the lifetime of the cohort. Because many of these cohort effects - body size, body mass, etc. - are tied to survival and reproductive fitness, cohort effects can influence population dynamics above and beyond the effects associated with typical age-specific reproductive potential or survival patterns.
Cohort effects are most often expressed as lowered reproductive output in the population as a whole as well as lowered lifetime reproductive success of individuals in that cohort. Differences among cohorts in lifetime reproductive performance have been demonstrated in a diverse array of taxa, including large mammals, marine mammals, birds, fish, insects, and plants. Although populations need not have complex age-class structure to show cohort effects (e.g., annual plants), cohort effects most commonly occur because of environmental or density stress, which most commonly affects relatively ^-selected species that also tend to have a complex age-class structure.
Cohort effects can be either short term or long term. Short-term cohort effects influence age structure and population dynamics by affecting the numbers of individuals in the cohort that live to be recruited. Long-term cohort effects affect overall reproductive success of cohorts over time through their effects on phenotypic quality. Cohort effects can also vary by sex; for example, cohort may affect growth of males but not females due to the greater reproductive activities of males in polygynous species.
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