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  • 1
    Electronic Resource
    Electronic Resource
    Crotofolane diterpenoids from the African shrub Croton dichogamus Pax (1989)
    s.l. : American Chemical Society
    The @journal of organic chemistry 54 (1989), S. 1654-1657 
    Details
    ISSN: 1520-6904
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/jo00268a029
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  • 2
    Electronic Resource
    Electronic Resource
    Lunar cycle and timing of mating season in Serengeti wildebeest (1977)
    [s.l.] : Nature Publishing Group
    Nature 267 (1977), S. 832-833 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] I have calculated individual conception dates from records of (1) foetus weights collected randomly and (2) birth dates. Conception date can be calculated from foetus weight (ref. 9) (Fig. 1), and from birth date with a known gestation length. This lies between 8 and 9 months (refs 10, 11) and I ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/267832a0
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  • 3
    Electronic Resource
    Electronic Resource
    Patterns of predation in a diverse predator–prey system (2003)
    [s.l.] : Macmillian Magazines Ltd.
    Nature 425 (2003), S. 288-290 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] There are many cases where animal populations are affected by predators and resources in terrestrial ecosystems, but the factors that determine when one or the other predominates remain poorly understood. Here we show, using 40 years of data from the highly diverse mammal community of the ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/nature01934
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  • 4
    Electronic Resource
    Electronic Resource
    Does competition or facilitation regulate migrant ungulate populations in the Serengeti? A test of hypotheses (1982)
    Springer
    Oecologia 53 (1982), S. 364-369 
    Details
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Interspecific competition and facilitation have both been proposed as processes promoting species separation and co-existence in African ungulates. In one group of grazers on the Serengeti plains, comprising wildebeest (Connochaetes taurinus), zebra (Equus burchelli), and Thomson's gazelle (Gazella thomsoni), these processes have also been suggested to regulate the populations. Censuses of these populations over 20 years have shown changes that allow a test of which, if either, process regulates population numbers. Wildebeest numbers have levelled off as a result of intraspecific competition for food following a five-fold increase due to release from disease and an increase in food supply. Zebra numbers have remained stationary throughout the same period. Gazelle numbers have declined in the last 10 years. These results are contrary to the facilitation hypothesis, which predicts that wildebeest numbers should not have increased if those of zebra did not, and that gazelle numbers should have increased since the wildebeest population increased. The gazelle results are consistent with the interspecific competition hypothesis, but the zebra results are contrary to it. We propose an alternative hypothesis that predation regulates the zebra population, and we suggest ways of testing this.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00389015
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  • 5
    Electronic Resource
    Electronic Resource
    Problems of development aid (1990)
    [s.l.] : Nature Publishing Group
    Nature 343 (1990), S. 587-587 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] SIR-The world is experiencing more frequent collapses of natural resource systems leading to disasters that require international relief. But many of these disasters are themselves the ecological consequences of previous 'development' aid1. The reasons are illustrated by a recent ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/343587a0
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  • 6
    Electronic Resource
    Electronic Resource
    Can predation cause the 10-year hare cycle? (1987)
    Springer
    Oecologia 74 (1987), S. 185-192 
    Details
    ISSN: 1432-1939
    Keywords: Snowshoe hare ; Predation ; Population cycle
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary We relate causes of mortality of snowshoe hares to density of hares over an 8-year period that included a peak in numbers. We then use simulation modeling to examine whether these density-dependent relationships could produce changes in hare density similar to those observed in our study are in Yukon, Canada. Predation during winter was the largest source of mortality for snowshoe hares at Kluane, Yukon during 1978–84. There was a one-year lag in the response of winter predation mortality rate to hare density. There was a two-year lag in the response of winter mortality not caused by predators to hare density. A simple simulation model with density-dependent predation produced 8–11 year cycles only within a narrow range of parameters that are inconsistent with data from the Kluane region. However, a simulation model that predicted winter mortality rates using a delayed density-dependent numerical response and a Type II functional response by predators, produced 8–11 year cycles within the range of parameter values measured in our study. Yet another simulation model that predicted both summer and winter mortality rates using a delayed density-dependent numerical response and a Type II functional response by predators, did not produce 8–11 year cycles within the range of parameter values measured in our study. Lack of data on juvenile mortality may be one reason for this result.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00379358
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  • 7
    Electronic Resource
    Electronic Resource
    Limits to predator regulation of rabbits in Australia: evidence from predator-removal experiments (1992)
    Springer
    Oecologia 89 (1992), S. 102-112 
    Details
    ISSN: 1432-1939
    Keywords: Predator removal ; Predator regulation ; Functional response ; Numerical response
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Predator-prey studies in semi-arid eastern Australia demonstrated that populations of rabbits (Oryctolagus cuniculus) could be regulated by predators. The functional, numerical and total responses of foxes (Vulpes vulpes) to rabbits and the numerical response of feral cats (Felis catus) to rabbits, are described. Measurement of the rabbit component of foxes' stomach contents indicates a Type III functional response. The size of the fox population in summer was dependent on the availability of rabbits over the immediately preceding rabbit breeding season but there appeared to be no density-dependent aggregation of young foxes in areas of surplus food. The total response of foxes, estimated using the short-term numerical response of dispersing foxes, was directly density-dependent for low rabbit densities and inversely density-dependent for high rabbit densities. Two states are possible with this form of total response: a state with low rabbit densities regulated by predators and a state with high rabbit densities which occurs when rabbits escape predator regulation. The boundary between regulation and non-regulation by predators was demonstrated by a predator-removal experiment. In the treated areas, predators were initially culled and rabbits increased to higher densities than in an untreated area where predators were always present. When predators were allowed back into the treated areas, rabbit populations continued to increase and did not decline to the density in the untreated area. This is the critical evidence for a two-state system. When predators were present, rabbits could be maintained at low densities which were in the density-dependent part of the total response curve for foxes. Exceptionally high rabbit recruitment, or artificially reduced predation, could result in rabbits escaping predator-regulation. Under these circumstances, rabbits could move into the inversely density-dependent region of the total response curve for foxes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00319021
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  • 8
    Electronic Resource
    Electronic Resource
    Population regulation of Serengeti Wildebeeest: a test of the food hypothesis (1985)
    Springer
    Oecologia 65 (1985), S. 266-268 
    Details
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The food hypothesis proposes that density dependent mortality regulates populations through food shortage. For Serengeti wildebeest, we found an empirical relationship between dry season adult mortality rate, density and food supply. This relationship predicted that: (1) the population would stabilize between 1.0 and 1.5 million animals, (2) dry season mortality would be density dependent and sufficient to account for the levelling off of this large ungulate population. Recent observations have tested and confirmed these predictions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00379227
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  • 9
    Electronic Resource
    Electronic Resource
    An antifeedant in balsam poplar inhibits browsing by snowshoe hares (1989)
    Springer
    Oecologia 79 (1989), S. 189-192 
    Details
    ISSN: 1432-1939
    Keywords: Trihydroxydihydrochalcone ; Balsam poplar ; Snowshoe hare ; Herbivore ; Plant defense
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary The ‘plant defense guild’ hypothesis for the evolution of plant secondary chemicals predicts that plant species defend themselves against generalist herbivores such as the snowshoe hare (Lepus americanus) in the Canadian boreal forest by evolving unique antifeedant chemicals. Plant species may coevolve in an ecosystem by presenting an array of chemicals to herbivores. We report further evidence for this idea from the presence of 2,4,6-trihydroxydihydrochalcone in the CH2Cl2 extracts of Populus balsamifera juvenile twigs. These extracts, added to rabbit chow, were offered to hares in choice tests. The bioassay established that the chemical acted as an antifeedant for hares.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00388477
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  • 10
    Electronic Resource
    Electronic Resource
    Does competition regulate ungulate populations? Further evidence from Serengeti, Tanzania (1990)
    Springer
    Oecologia 82 (1990), S. 283-288 
    Details
    ISSN: 1432-1939
    Keywords: Competition ; Regulation ; Ungulates ; Disease ; Serengeti
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Changes in populations of several ungulate species in the Serengeti-Mara region of East Africa over the past 30 years suggest several hypotheses for their regulation and coexistence. Recent censuses in the 1980s have allowed us to test the hypotheses that: (1) there was competition between wildebeest (Connochaetes taurinus) and Thomson's gazelle (Gazella thomsoni). This predicted that gazelle numbers should have declined in the 1980s when wildebeest were food limited. Census figures show no change in gazelle numbers between 1978 and 1986, a result contrary to the interspecific competition hypothesis; (2) wildebeest and African buffalo (Syncerus caffer) populations were regulated by intraspecific competition for food. Since both populations reached food limitation in the 1970s, the hypothesis predicted that the populations should have been stable in the 1980s. The results confirm these predictions for wildebeest and the buffalo population in the Mara reserve. In the Serengeti the buffalo population declined 41% over the period 1976–1984. The decline was not evenly distributed over the park, some areas showing an 80–90% decline, others no change or an increase in numbers. The decline was associated with proximity to human habitation; (3) an outbreak of the viral disease, rinderpest, in 1982 may have been the cause of the drop in buffalo population. Blood serum samples to measure the prevalence of antibodies were collected from areas of decreasing, stable and increasing populations. If rinderpest was the cause of decrease there should be a negative relationship between the prevalence of rinderpest and the instantaneous rate of increase (r). The results showed no relationship. We conclude that rinderpest was not the major cause of the drop in buffalo numbers. Elephant (Loxodonta africana) numbers dropped 81% in Serengeti in the period 1977–1986. In the Mara there was little change. The evidence suggests that extensive poaching in northern and western Serengeti during 1979–1984 accounted for the drop in both elephant and buffalo numbers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00323546
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