Comparative changes in adult vs. juvenile survival affecting population trends of African ungulates

Author(s)Norman Owen-Smith and Darryl R. Mason
Year Published2005
JournalJournal of Animal Ecology
Page Numbers762–773
Size395.14 KB
Abstract:
  1. Among large mammalian herbivores, juvenile survival tends to vary widely and may thus have a greater influence on population dynamics than the relatively constant survival rates typical of adults. However, previous studies yielding stage-specific survival rates have been mostly on temperate zone ungulates and in environments lacking large predators.
  2. Annual censuses coupled with assessments of population structure enabled annual survival rates to be estimated for the juvenile, yearling and adult segments of nine ungulate species in South Africa’s Kruger National Park. Four of these populations persisted at high abundance after initial increases (zebra, wildebeest, impala and giraffe), while five showed progressive declines during the latter part of the study period (kudu, waterbuck, warthog, sable antelope and tsessebe).
  3. The magnitude of the reduction in adult survival between periods showing contrasting population trends was similar to or greater than the corresponding change in juvenile survival for five of the nine species. Accordingly alterations in population phase, from increasing to stable or stable to declining, were brought about mostly through reduced survival within the adult segment. Elevated predation risk may have been responsible.
  4. Estimates were derived of the relative survival rates of juveniles, yearlings and adult segments associated with zero population growth, and the survival differential between adult males and females, for all nine species. Stage-specific survival rates appeared dependent on body mass, but with some anomalies. The sex difference in adult survival showed no obvious relation with sexual size dimorphism.
  5. For large mammalian herbivores, assessments of relative elasticities of stage-specific survival rates on population growth are problematic for several reasons. Sensitivity to corresponding increments in either survival or mortality rates provides a better basis for ecological or adaptive interpretation. Survival rates of adults seem to vary more over multiyear periods compared with mainly annual fluctuations in juvenile survival. More studies are needed on tropical species and in environments retaining large predators to support generalizations about factors influencing ungulate life-history patterns.

Keywords: Life history, Population Dynamics, Predation, Temporal variability, Vital rates

Authors: Norman Owen-Smith and Darryl R. Mason

Journal: Journal of Animal Ecology


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