Techniques for the Study of Primate Population Ecology (1981) / Chapter Skim
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DETERMINANTS OF POPULATION DENSITY AND GROWTH
DETERMINANTS OF POPULATION DENSITY AND GROWTH
Pages 176-196
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From page 176... ...
MEASURES OF POPULATION SIZE IN RELATION TO THE ENVIRONMENT ESTIMATES OF DENSITY AND BIOMASS In estimating the vital statistics of a population, knowledge of only the numbers of animals is relevant. Often, however, population size is expressed as a density, which is defined as the number of animals per unit area.
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From page 177... ...
Thus, expressed mathematically, £ nxWx Biomass = -- where «x and Wx are the number and average weight, respectively, of individuals in age class x, there being a total of7 age classes, occupying an area of size A The weight of inert gut contents may be subtracted in estimates of mean weights.
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From page 178... ...
MEAN EQUILIBRIUM DENSITY (jc) Figure 8-1 illustrates the pattern of changes in a hypothetical population having an annual birth pulse.
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From page 179... ...
THE CONCEPT OF CARRYING CAPACITY The mean equilibrium density, K, varies both with the features of the species and of the environment. The fact that different values
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From page 180... ...
In such assessments, carrying capacity is represented by the actual mean equilibrium density, K, rather than by the temporary maximal or minimal fluctuations away from the mean, particularly if such fluctuations are short term, as result, for example, from an annual birth pulse (Figure 8-1)
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From page 181... ...
. ENVIRONMENTAL FACTORS We have noted that many environmental factors influence population size and that the observable mean equilibrium density (AT)
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From page 182... ...
secondary influence FIGURE 8-2 Interrelationships among physical and biological factors that influence the population growth rate and mean equilibrium density of a species. Physical Factors Basically, the physical environment determines the diversity and abundance of plant life, on which all animals ultimately depend.
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From page 183... ...
The type of vegetation will in large part determine its dependent animal populations. For example, stable savanna grasslands support large and diverse populations of grazers and predators that prey on these grazers, whereas tall rain forests support fewer terrestrial animals.
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From page 184... ...
. The food supply is a major factor influencing the size of primate populations (Dittus, 1977a, 1980)
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From page 185... ...
The proportion of mortality in natural populations that is attributable to pathogens can be assessed only through close studies directly addressing this problem. Morbidity and mortality statistics are unavailable for primate populations that may be affected by parasites and diseases such as yellow fever, malaria, tuberculosis, Lassa fever, and Marburg virus.
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From page 186... ...
RELATIONSHIP BETWEEN THE DIVERSITY AND PRODUCTIVITY OF FOOD PLANTS AND POPULATION SIZE The diversity of primate species correlated positively with the diversity of tree species in the rain forests at Kibale, Uganda (Struhsaker, 1975) , and New Indenau, Cameroun (Gartlan and Struhsaker, 1972)
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From page 187... ...
months drought per annum 4-5 2-4 none Forest Type arid semimontane scrub evergreen cloud Forest productivity in metric evergreen tons/ha/yr° 2 4.5 5 Tree species diversity* low moderate moderately Primate Biomass (kg/km2)
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From page 188... ...
It is obvious from this example that a great deal of ecological information is required to draw conclusions concerning ecological relationships. EFFECTS OF FOOD SUPPLY ON THE VITAL STATISTICS OF A POPULATION The relationship between food supply and the demographic parameters that determine K density are illustrated by the following examples: • Natural populations of vervet monkeys, Cercopithecus aethiops, and of yellow baboons, Papio cynocephalus, decreased in size by over 40% and 90%, respectively, as a result of the destruction of their natural food plants through edaphic changes in the Masai-Amboseli Game Reserve in Kenya (Hausfater, 1975; Struhsaker, 1973, 1976)
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From page 189... ...
In short, social behavior regulates the size and age-sex composition of many primate societies in relation to the food supply, in a fashion that maximizes the reproductive success of some of its members. POPULATION REGULATION BY BEHAVIORAL AND PHYSIOLOGICAL MECHANISMS This section deals with the manner in which environmental factors can influence the vital statistics in a primate population -- its rates of mortality, fecundity, and maturation.
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From page 190... ...
utilize common resources that are in short supply or, regardless of whether the resources are in short supply, when animals (or groups) harm one another in utilizing the same resources (modified after Krebs, 1972)
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From page 191... ...
More subtle competitive behaviors involve the gradual encroachment and displacement of another individual from the vicinity of a desired resource. Displacements need not involve only food items or water; they may involve, for example, priority of access to a safe perch when scrambling to avoid a predator or an aggressive conspecific or when selecting secure sleeping perches.
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From page 192... ...
. Most primates appear to adhere to this pattern, and it is likely that food competition is a major factor in adjusting primate population density to the food supply (Dittus, 1980)
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From page 193... ...
Primate studies that have investigated mate competition (Dittus, 1977a; Lindburg, 1971; Vandenbergh and Vessey, 1968; Wilson and Boelkins, 1970) indicate that males are frequently wounded and die as a result of wounds incurred in fights with other males.
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From page 194... ...
Although food competition is likely to affect all age and sex classes in most primate species, mate competition, as outlined above, appears to be most prevalent among sexually dimorphic species where males are larger than females. With the exception of the study of the toque macaque that directly addressed the question of population regulation (Dittus, 1977a)
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From page 195... ...
Access to such refuges is likely to be determined by differences in social dominance relations so that the effect of predation on vital statistics would be determined at least in part by social behavior. THE INFLUENCE OF BEHAVIOR ON PHYSIOLOGY THAT MEDIATES SURVIVORSHIP AND REPRODUCTIVE PERFORMANCE Except for the outright killing of individuals, behavior exerts its influence on survivorship and fecundity through physiological changes.
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From page 196... ...
Thus, death or lowered reproductive performance may be mediated by a variety of endocrine changes that are brought about by behavior (see reviews by Christian, 1963, 1970)
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