Showing papers in "Journal of Animal Ecology in 1981"

Population Ecology of Raptors

Abstract: Relationship between the sexes dispersion breeding density winter density problems concerning nest-sites breeding strategies breeding rates behaviour in the breeding season fidelity to breeding areas movements mortality human persecution DDT and other organo-chlorines other pollutants and pesticides conservation management breeding from captive birds scientific names of raptors.

Competition on a Divided and Ephemeral Resource: A Simulation Model

Abstract: (2) Coexistence between two species can be extended by dividing the resource into more and smaller breeding sites. (3) Aggregation of the superior competitor also promotes coexistence, and can lead to an equilibrium between the two species if contagion is strong enough. (4) If the degree of aggregation is allowed to vary with density in a realistic way equilibrium is nearly always obtained. (5) These results may explain the high species diversity commonly observed on divided resources.

The Descriptive Properties of Some Models for Density Dependence

Abstract: (1) The descriptive abilities of several difference equation models for density dependence have been examined, first by a qualitative comparison of their flexibility and secondly by considering their ability to describe thirty sets of data on density dependent mortality. (2) The qualitative comparison demonstrated that two of the models have forms which are particularly flexible. The quantitative comparison indicated that of these two, that due to Maynard Smith & Slatkin (1973) is better able to describe a wide range of data.

Feeding specialization of the european badger meles meles in scotland

Abstract: SUMMARY (1) The food of European badgers Meles meles L. is described from six areas in Scotland, using faecal analysis. The estimated relative volume of various foods in the diet is compared with the frequency of their occurrence. Variation between areas and differences between seasons are related to food availability, and the food diversity and variance of different foods with time are presented. (2) The dominant food everywhere was earthworms, Lumbricus terrestris L. and L. rubellus Hoffmeister but their importance in the diet varied little in time and between areas, and there was no correlation with availability. Other less important foods included rabbits, cereals, insects and tubers, and several minor food categories; they were consumed relative to availability in those cases where this availability could be measured. (3) It is suggested that badgers change their foraging effort to compensate for fluctuations in earthworm availability, consuming a range of secondary foods

The Peregrine Falcon

Abstract: (Falco peregrinus) The peregrine falcon is one of nature's swiftest and most beautiful birds of prey. Its name comes from the Latin word peregrinus, meaning " foreigner " or " traveler. " This impressive bird has long been noted for its speed, grace, and aerial skills. Now, it is also a symbol of America's recovering threatened and endangered species. Three subspecies of the peregrine falcon inhabit North America: the American (Falco peregrinus anatum), Arctic (Falco peregrinus tundrius), and Peale's (Falco peregrinus pealei). Peregrine falcons are roughly crow-sized—about 15 to 21 inches long—with a wingspan of about 40 inches. As with many raptors, or birds of prey, females are larger than males. Adults have slate blue-gray wings and backs barred with black; pale undersides; white faces with a black stripe on each cheek; and large, dark eyes. Younger birds are darker below and browner. Peregrine falcons live mostly along mountain ranges, river valleys, and coastlines. Historically, they were most common in parts of the Appalachian Mountains and nearby valleys from New England south to Georgia, the upper Mississippi River Valley, and the Rocky Mountains. Peregrines also inhabited Pacific Coast from Mexico north to Alaska and in the Arctic tundra. The peregrine falcon also is found in other parts of the world. Most peregrines from northern Alaska, Canada, and Greenland migrate in the fall to Central and South America. On the way, they often hunt along the barrier islands on the Atlantic and Gulf of

Feeding behaviour of vervet monkeys on acacia tortilis and acacia xanthophloea: with special reference to reproductive strategies and tannin production

Abstract: SUMMARY spp., A. tortilis and A. xanthophloea, were found to account for over 50% of all feeding time. (2) Food preferences were determined from data on time spent eating different parts of the Acadia spp. in relation to their availability, the length of feeding bouts, and the difference in feeding time of high- and low-ranking animals. Measures of food preference were generally consistent and suggested that A. tortilis was preferred for immature leaves, seeds, fruits and flowers, and A. xanthophloea for gum. However, A. tortilis products were less often available than those of A. xanthophloea. (3) Both Acacia spp. were found to produce hydrolysable and condensed tannins. Different parts of the two Acacia spp. and several other food plants were assayed for their content of phenolic compounds and specifically for condensed tannins. Analysis of qualitative and quantitative data suggested that condensed tannins formed the majority of total phenolics in the Acacia spp. None of the other food plants appeared to produce tannins.

Effect of additional food on reproductive success in the magpie (pica pica)

Abstract: SUMMARY (1) Magpie Pica pica pairs were given extra food (fish) before and during the breeding season. Reproductive success was compared with that of the same pairs in other seasons, when no food was offered, and with that of a control group, without extra food. Food was also offered in empty territories, occupied in earlier seasons, in the hope of inducing reoccupation. (2) Experimental pairs started egg-laying earlier, had larger clutches, heavier eggs, and greater hatching success, and produced more fledglings than control pairs. This was found also when comparing the same females under experimental and control conditions. Food did not, however, attract new settlers to empty territories. (3) Experimental pairs not only laid earlier, but they also started their laying within a significantly shorter period of time than did controls. The relatively small-scale effects of extra food on timing of breeding suggest that food alone does not determine start of laying, but that it is also stimulated by some other factors. Shortly after the additional factors were perceived by the birds, laying started. As experimental birds were in a better nutritional state than controls, they responded faster, laid larger clutches and heavier eggs. (4) The difference in reproductive success between the two groups was established during the nestling phase. Predation, mainly by hooded crows Corvus cornix L., and starvation, especially during adverse weather, caused the mortality. As invertebrate activity decreases in such periods, food scarcity is suggested to be the ultimate mortality factor. Extra food buffers adverse periods, giving a more stable temporal distribution of food as compared with natural conditions.

Density dependence in host-parasitoid models

Abstract: SUMMARY (1) Insect populations frequently suffer some density dependent effect in addition to mortality from insect parasitoids. This paper follows Wang & Gutierrez (1981) in examining the question: 'Does the ordering of density dependence and parasitism in the host life cycle have a significant effect on the dynamics of the interaction?' (2) The properties of three models in which density dependence either acts before or after parasitism are displayed in terms of the hosts' rate of increase and the extent to which parasitism depresses the host equilibrium below its carrying capacity. (3) From this analysis, we conclude that the sequence of these events in the host life cycle can indeed have a marked affect on the population dynamics. In particular, the model with parasitism acting before density dependence stands apart in permitting a stable host-parasitoid interaction with the host population above its parasite-free carrying capacity. (4) These conclusions have implications for the use of parasitoids as biological control agents.

Spatial and Temporal Components of Resource Assessment by Flower-Feeding Insects

Abstract: (1) Per-flower insect visitation rates on two plant species within Rocky Mountain subalpine meadows were measured using fluorescent powders and found to be positively correlated with local flower density, suggesting that insects concentrate their foraging in dense patches of flowers. (2) Visitation rates on both species are also correlated with the presence of other plant species which share the same visitors; these correlations are positive, suggesting cooperative rather than competitive plant-plant relationships, at least insofar as visitation is concerned.

The effect of extra food on small rodent populations: ii. voles (microtus townsendii)

Abstract: SUMMARY (1) Voles on three areas were provided with different levels of extra food in the form of laboratory chow and oats for approximately one year. (2) On areas with intermediate and high extra food, population density increased to twice the control density. (3) Extra food increased immigration and reproduction, and shortened the winter non-breeding season. (4) Males and females had smaller home ranges on areas with extra food. This may have facilitated immigration to these populations. (5) Voles were removed from three other areas and re-colonization was measured. Immigration was related to the density of extra food; three times as many voles colonized a high food area compared with a control. (6) These results, combined with those of previous research which has shown that a viable surplus of voles exists in natural populations, suggest that the vole's population size is limited by both behaviour and food.

An experimental investigation of inter-relationships between the wood-ant (formica rufa) and some tree-canopy herbivores

Abstract: SUMMARY (1) Previous work on interactions between F. rufa and some canopy herbivores is briefly reviewed. (2) In limestone woodland in N. England, populations of defoliators (illustrated by the Winter Moth, Operophtera brumata L.), a non-tended aphid species (Drepanosiphum platanoidis Schr.) and a tended aphid species (Periphyllus testudinaceus Fernie), are all affected by the presence of the ant. (3) Predation of D. platanoidis by F. rufa results in significant depression of the population compared with experimental conditions where ants were excluded from shoots. (4) Trees without ants have significantly higher populations of Lepidoptera larvae than do foraged trees. (5) On trees with ants an average of 1% of the leaf area had been removed by defoliators at the end of the season; without ants the total was nearly 8%. (6) Formica rufa tended eight aphid species and one psyllid in the woodland. The association of F. rufa with one of the aphids (P. testudinaceus) was studied. (7) Periphyllus testudinaceus populations were significantly increased in the presence of ants. When ants were excluded from colonies on shoots a significant fall in numbers occurred. There was probably also a reduction in the proportion of alatae. (8) The results are discussed in relation to previous work on aphid predation by ants and to the practice of introducing ants into woodland to protect trees from the effects of defoliators. It is concluded that this practice may be ill-founded.

Individual variation and competition in the everglades pygmy sunfish

Abstract: SUMMARY (1) Populations of Everglades pygmy sunfish (Elassoma evergladei, family Centrachidae) were manipulated in both the laboratory and the field to determine how individuals responded to changes in the intensity of competition. (2) In laboratory experiments, individually identifiable fish were raised at different densities (sixteen, eight, four, and one fish per tank) and each individual's competitive ability was measured by its growth rate and reproductive condition. For females, estimates of reproductive potential were ovary weight and egg number. For males, reproductive ability was estimated by recording the frequency of occurrence of bobbing, a sexual behaviour. In field experiments fish were raised at the same densities but in screened enclosures to evaluate the realism of the laboratory experiments. (3) Individual fish differed in their growth rates, and these differences were magnified under competitive conditions. Increased density also increased the variation among females within a population with respect to ovary size and egg number. (4) Increased population density markedly decreased the growth rate of the 'average' fish in the population, but only slightly the growth of the 'best' competitors. In each density there were always some fish who grew as quickly as the 'average' fish of a population half as large. (5) Increases in population size also produced non-linear reductions in estimates of both male and female potential reproductive success. For females these reductions could be attributed in part to density-induced reductions in growth rate. But density also had a direct effect on female reproductive success: for females of similar size or growing at a similar rate, those raised at higher densities had larger ovaries and more eggs.

A mathematical model of the functional relationship between density and spatial distribution of a population

Abstract: SUMMARY (1) A model of the functional relationship between the mean population density and the proportion of unoccupied patches in a patchy environment is proposed. (2) Provided a population is distributed according to the negative binomial distribution, the zero-term of this distribution can be set equal to the model predicted proportion of empty patches. The parameter k of the negative binomial will, for a given mean density u, be a root in this equation. (3) The model is fitted to the data of the two-spotted spider mite (Tetranychus urticae) and its phytoseiid predator Phytoseiulus persimilis, and the density behaviour of 1/k is predicted. The agreement between observed and expected values of 1/k is relatively poor due to the large scatter of the empirical values of 1/k obtained by the method of maximum likelihood. A test for goodness of fit, however, reveals that the model provides better fit to the data than an alternative model of Taylor, Woiwod & Perry (1979) does, although the two models are qualitatively very similar.