 Reference Base  Reproduction and population density: trade-offs for the c... |
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Rachlow, J.L.; Berger, J., 1998. Reproduction and population density: trade-offs for the conservation of rhinos in situ. Animal Conservation 1 (2): 101-106, figs. 1-3, table 1
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| Location: |
World |
| Subject: |
Ecology - Population |
| Species: |
White Rhino |
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Population growth after reintroduction. The population of white rhinos in the fenced WGP has undergone two periods of growth during which translocations did not occur: 1967-1974 (early period) and 1987-1994 (late period). Rates of annual increase calculated for the early and late periods were 10.4 and 6.6 %, respectively. The ecological density in the WGP increased by >200% during this time, from 0.23 rhino/ km? in 1967 to 0.83 rhino/ km? in 1994.
We examined data on recruitment rate of the WGP population as a function of population density. A simple linear regression revealed that recruitment rate of calves (calves that survived >2 years) decreased significantly (P = 0.007) with the density of adult and subadult rhinos (>2 years of age). Because rhinos are non-seasonal breeders and reproduce at intervals of 2-4 years, annual recruitment rates vary markedly, however, the decreasing trend in recruitment with population density is clear.
Discussion
Numerous factors probably interact to affect reproduction in female mammals. Several studies have identified a threshold body mass for reproduction among young female ungulates, and the relationship between reproduction and body mass may be influenced by population density. Other studies have distinguished between total body mass and body fat, demonstrating that fat reserves independent of body size can positively influence reproduction in females Although the index of body condition we used for the white rhinos is crude, it revealed seasonal declines in body condition related to both population density and reproductive status of females. This suggests that competition among individuals occurred in the high-density population and affected body condition during the dry season when forage availability is lower.
Population growth - Rapid rates of increase have been reported in large herbivore populations following introduction into unoccupied habitats. The rate of growth of the white rhino population in the WGP following introduction (10.4% per annum) is among the highest rates documented for free-ranging populations of either species of African rhino. A maximum rate of 9.6 % per annum was calculated for white rhinos in Umfolozi Game Reserve, South Africa, and 10.5% was estimated to be a theoretical maximum for the species (Owen-Smith, 1988). A maximum rate of increase was estimated to be 6.0% in Ndumu Game Reserve, a small South African reserve. However, a growth rate of 9.7% was reported for a small, low-density population of northern white rhinos in Garamba National Park, Zaire. The white rhino population in the WGP appears to have achieved a near-maximal rate of growth following introduction into this unoccupied habitat.
Recruitment rate in a closed system is a population-level index of reproduction. The decrease in the recruitmcnt of calves with population size in the WGP indicates that reproduction declined as the population density increased. Densities varied markedly between the early and late periods, and differences in recruitment during these two periods probably contributed to the observed differences in population growth rates.
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