Drivers of a temporal change in the adult sex ratio of a population of Roosevelt elk (Cervus elaphus roosevelti)
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Ecological processes driving female-biased adult sex ratios (ASR, males:female) in ungulate populations have been addressed theoretically but empirical study is lacking. The female-substitution hypothesis asserts that a female-biased ASR reflects an overall fitness benefit to females and also males competitive in access to reproductive females. The hypothesis predicts that as female abundance increases females should acquire forage in a given area in lieu of males, thereby resulting in a declining ASR via scramble competition. My study examined a population of Roosevelt elk (Cervus elaphus roosevelti) inhabiting the Redwood National and State Parks, California. I sought to discern which of two potential ecological mechanisms could explain the female-biased ASR. The first mechanism was that increasing female abundance associated with a decline in forage abundance led to the passive displacement of males into the study periphery, and the second was that a decline in ASR was precipitated by a lack of males in the area. Systematic population surveys across 24-years were done by driving along a predetermined route within meadow complexes to estimate abundance and ASR, and in nearby areas to assess male abundance. Forage biomass was estimated from vegetation height and cover measurements in quarter-m2 plots randomly placed in meadows inhabited by elk. My multiple regression model detected an inverse relationship between abundance and ASR indicating it was density dependent. Males were in the study periphery when female abundance increased, and male abundance declined in the study area. A generalized least squares model indicated declining food supplies across years when female abundance increased. My empirical findings were consistent with the female-substitution hypothesis.