Showing papers in "Journal of Animal Ecology in 1983"

Aggregation and the `ideal free ` distribution

Abstract: Presentation d'un modele mathematique permettant de decrire la densite spatiale des predateurs et simplifiant l'analyse de la stabilite des populations

The relationships between beech (nothofagus sp.) seedfall and populations of mice (mus musculus), and the demographic and dietary responses of stoats (mustela erminea), in three new zealand forests

Abstract: SUMMARY (1) There was a significant relationship between seedfall and populations of mice in all forests studied. Good seedfalls were followed by increases in density of mice and changes in population structure not seen in years when the seedfall failed. Predictability of these changes decreased with increasing diversity of forest composition. (2) Stoats responded both numerically and functionally to increases in mice. There was a significant relationship between the densities of mice and of stoats in summer in two forests. (In a third, this relationship was modified by the presence nearby of many lagomorphs, an important alternative prey for stoats.) The numerical response was due to increased survival, in the uterus or the nest, of young stoats born in spring when mice were abundant, not to increased fecundity of adult female stoats. In all three forests, stoats ate significantly more mice in the summers that mice were abundant. (3) Ship rats (Rattus rattus (L.)) were most abundant in the most diverse of the three forests. The density index for R. rattus increased there after the 1976 seedfall, but not after that of 1979. A few kiore (R. exulans (Peale)) coexisted with R. rattus in this forest. (4) Stoats did not eat significantly fewer birds when there were plenty of mice, either in summer or autumn, but nevertheless there was a significant negative correlation between the proportions of birds and mice in the diet in autumn. In summer this correlation was very weak and probably non-existent. (5) A 'bird predation index', which takes into account both the numbers of stoats present and what they ate, suggests that more birds are eaten in summers of peak numbers of mice and stoats, because of the strong numerical response by stoats to mice and lack of 'buffering' of birds by mice. In autumn this effect is weakened by a regular seasonal switch by stoats from birds to mice, especially in the autumn of a mouse peak year. (6) The indirect effects of mouse irruptions on populations of forest birds in New Zealand should be further examined. Control of stoats in mouse peak summers could prevent a temporary increase in predation on birds by stoats during the highest risk period, though whether this would benefit the birds is unknown.

The systematic formulation of tractable single-species population models incorporating age structure

Abstract: SUMMARY (3) We illustrate the use of the formalism in the construction and analysis of two models of laboratory insect populations: a detailed model of Nicholson's blowflies and a 'strategic' model of larval competition intended to refine the design of experiments in progress on the dynamics of the Indian meal-moth Plodia interpunctella (Hiibner). (4) The model of Nicholson's blowflies is dynamically identical to a model previously derived heuristically. We have fitted the parameters using a more comprehensive range of data, and have illustrated explicitly the passage of large, quasi-cyclic fluctuations through the age structure. (5) We use the larval competition model to distinguish the effects of 'uniform competition' (all larvae competing) and 'cohort competition' (larvae of a given age competing). The former can produce quasi-cycles of a type characteristic of delayed regulation, while the latter causes quasi-cycles consisting of 'bursts' of population propagating through the age structure. (6) We also use the larval competition model to demonstrate that apparently minor changes in the description of adult survival can induce dramatic alterations in model behaviour. This supports our emphasis on rigour in the formulation of the model, so as to distinguish genuinely interesting dynamics from mathematical artifacts. SUMMARY (3) We illustrate the use of the formalism in the construction and analysis of two models of laboratory insect populations: a detailed model of Nicholson's blowflies and a 'strategic' model of larval competition intended to refine the design of experiments in progress on the dynamics of the Indian meal-moth Plodia interpunctella (Hiibner). (4) The model of Nicholson's blowflies is dynamically identical to a model previously derived heuristically. We have fitted the parameters using a more comprehensive range of data, and have illustrated explicitly the passage of large, quasi-cyclic fluctuations through the age structure. (5) We use the larval competition model to distinguish the effects of 'uniform competition' (all larvae competing) and 'cohort competition' (larvae of a given age competing). The former can produce quasi-cycles of a type characteristic of delayed regulation, while the latter causes quasi-cycles consisting of 'bursts' of population propagating through the age structure. (6) We also use the larval competition model to demonstrate that apparently minor changes in the description of adult survival can induce dramatic alterations in model behaviour. This supports our emphasis on rigour in the formulation of the model, so as to distinguish genuinely interesting dynamics from mathematical artifacts. SUMMARY (3) We illustrate the use of the formalism in the construction and analysis of two models of laboratory insect populations: a detailed model of Nicholson's blowflies and a 'strategic' model of larval competition intended to refine the design of experiments in progress on the dynamics of the Indian meal-moth Plodia interpunctella (Hiibner). (4) The model of Nicholson's blowflies is dynamically identical to a model previously derived heuristically. We have fitted the parameters using a more comprehensive range of data, and have illustrated explicitly the passage of large, quasi-cyclic fluctuations through the age structure. (5) We use the larval competition model to distinguish the effects of 'uniform competition' (all larvae competing) and 'cohort competition' (larvae of a given age competing). The former can produce quasi-cycles of a type characteristic of delayed regulation, while the latter causes quasi-cycles consisting of 'bursts' of population propagating through the age structure. (6) We also use the larval competition model to demonstrate that apparently minor changes in the description of adult survival can induce dramatic alterations in model behaviour. This supports our emphasis on rigour in the formulation of the model, so as to distinguish genuinely interesting dynamics from mathematical artifacts. SUMMARY (3) We illustrate the use of the formalism in the construction and analysis of two models of laboratory insect populations: a detailed model of Nicholson's blowflies and a 'strategic' model of larval competition intended to refine the design of experiments in progress on the dynamics of the Indian meal-moth Plodia interpunctella (Hiibner). (4) The model of Nicholson's blowflies is dynamically identical to a model previously derived heuristically. We have fitted the parameters using a more comprehensive range of data, and have illustrated explicitly the passage of large, quasi-cyclic fluctuations through the age structure. (5) We use the larval competition model to distinguish the effects of 'uniform competition' (all larvae competing) and 'cohort competition' (larvae of a given age competing). The former can produce quasi-cycles of a type characteristic of delayed regulation, while the latter causes quasi-cycles consisting of 'bursts' of population propagating through the age structure. (6) We also use the larval competition model to demonstrate that apparently minor changes in the description of adult survival can induce dramatic alterations in model behaviour. This supports our emphasis on rigour in the formulation of the model, so as to distinguish genuinely interesting dynamics from mathematical artifacts.

Thermoregulation, habitat quality and the behavioural ecology of gelada baboons ( Theropithecus gelada).

Abstract: SUMMARY SUMMARY (1) Gelada baboon (Theropithecus gelada) populations living in three habitats that differed markedly in vegetation and climate are compared in order to assess the influence of temperature and habitat quality on their behavioural ecology. (2) Habitat and seasonal differences in diet are attributed to the local availability of preferred food types (in particular, grass). (3) Time spent feeding increased with altitude due to a combination of increasing temperature-dependent energy requirements and declining habitat quality. Resting time was used as a reserve store from which additional feeding time could be drawn; social time was conserved. (4) Day journey length increased as a linear function of band size because the increase in food requirement with altitude was exactly offset by a corresponding increase in food density. (5) Although range size was also a linear function of band size, the amount of graze required to support one animal increased with declining ambient temperatures. SUMMARY SUMMARY

Breeding and natal dispersal of the goldeneye, bucephala clangula

Abstract: SUMMARY (1) Breeding and natal dispersal were examined for female goldeneye breeding in nest boxes in Sweden. Females changing nest box between years dispersed a median distance of 0 75 km to the new nest box (or, to the tenth nearest nest box). Individuals (N = 17) which returned to the natal area to breed, nested in the vicinity (median = 0 73 km) of the box in which they had hatched. (2) Neither the proportion of females changing nest box, nor the distances moved between nest boxes, varied among years. (3) Forty-two percent of females reoccupied the same nest box in successive years. Females which failed to breed successfully were less likely to return to the same nest box than females which bred successfully. (4) Females which moved to another nest box were less likely to hatch their broods successfully than females which returned to the same nest box. When they moved they produced smaller clutches and broods, and bred later in the season than when they returned to the same nest box. (5) Among females which moved, those which had failed to breed successfully moved further to another nest box than those which had bred successfully. In the case of females which lost clutches to predators, moving away from the vicinity of the previous nest box may have been advantageous, since boxes in which clutches were preyed-upon tended to be preyed-upon in the following year and also tended to occur close together. (6) The greater the distance from the previous nest box at which females bred, the less likely they were to start egglaying early in the season. (7) Females whose previous nest box was occupied by another female moved further to new nest boxes than females whose previous nest box was left empty.

The systematic formulation of tractable single-species models incorporating age structure

Abstract: (1) We develop a mathematically rigorous approach to modelling the effects of age structure, in which the life-history of a species is divided into age classes of arbitrary duration and attention is focused on the sub-populations of the various classes. (2) By assuming that all individuals in a particular age class have the same birth and death rates (which may be time- and density-dependent), we reduce the normal integro-differential equations describing an age-structured population with overlapping generations to a set of coupled ordinary delay-differential equations which are readily integrated numerically. (3) We illustrate the use of the formalism in the construction and analysis of two models of laboratory insect populations: a detailed model of Nicholson's blowflies and a `strategic' model of larval competition intended to refine the design of experiments in progress on the dynamics of the Indian meal-moth Plodia interpunctella (Hubner). (4) The model of Nicholson's blowflies is dynamically identical to a model previously derived heuristically. We have fitted the parameters using a more comprehensive range of data, and have illustrated explicitly the passage of large, quasi-cyclic fluctuations through the age structure. (5) We use the larval competition model to distinguish the effects of `uniform competition' (all larvae competing) and `cohort competition' (larvae of a given age competing). The former can produce quasi-cycles of a type characteristic of delayed regulation, while the latter causes quasi-cycles consisting of `bursts' of population propagating through the age structure. (6) We also use the larval competition model to demonstrate that apparently minor changes in the description of adult survival can induce dramatic alterations in model behaviour. This supports our emphasis on rigour in the formulation of the model, so as to distinguish genuinely interesting dynamics from mathematical artifacts.

The role of alternative host-plant species in the life of a polyphagous moth, operophtera brumata (lepidoptera: geometridae)

Abstract: SUMMARY (1) As a major determinant of larval winter moth population levels is known to be the degree of synchrony between larval eclosion and the bud burst of the primary host-plant, the oak, it is argued that two hitherto unstudied factors have a considerable effect on the insects' population dynamics. These are: (a) the events occurring immediately after eclosion and the effect of the period of food deprivation which occurs should no food be available locally; (b) the degree to which larvae can utilize food-plant species other than the primary host. (2) Experimental evidence is presented which shows that first instar larvae can discriminate between host-plant species, and that this is unaffected by the degree of food deprivation relevant to field conditions. Also, it is shown that early deprivation has little if any adverse affects on the subsequent growth and development of larvae, and that it may confer mild advantages. (3) A comparison is made of the development of larvae reared on primary and alternative hosts before, during and after the period of peak level eclosion and oak bud burst in the field. Four of the five alternative food-plant species tested are found to support satisfactory larval growth. Further, it is only during the period of maximum oak bud burst that an oak diet results in a considerably greater reproductive success than that supported by a diet of alternative hosts. (4) It is argued that the polyphagy of winter moth larvae is a mechanism which promotes synchrony of larval eclosion with the primary host plant. It is also associated with several unusual features of winter moth biology; in particular the larval, as opposed to adult host selection and dispersal, as well as increased adult fecundity. SUMMARY (1) As a major determinant of larval winter moth population levels is known to be the degree of synchrony between larval eclosion and the bud burst of the primary host-plant, the oak, it is argued that two hitherto unstudied factors have a considerable effect on the insects' population dynamics. These are: (a) the events occurring immediately after eclosion and the effect of the period of food deprivation which occurs should no food be available locally; (b) the degree to which larvae can utilize food-plant species other than the primary host. (2) Experimental evidence is presented which shows that first instar larvae can discriminate between host-plant species, and that this is unaffected by the degree of food deprivation relevant to field conditions. Also, it is shown that early deprivation has little if any adverse affects on the subsequent growth and development of larvae, and that it may confer mild advantages. (3) A comparison is made of the development of larvae reared on primary and alternative hosts before, during and after the period of peak level eclosion and oak bud burst in the field. Four of the five alternative food-plant species tested are found to support satisfactory larval growth. Further, it is only during the period of maximum oak bud burst that an oak diet results in a considerably greater reproductive success than that supported by a diet of alternative hosts. (4) It is argued that the polyphagy of winter moth larvae is a mechanism which promotes synchrony of larval eclosion with the primary host plant. It is also associated with several unusual features of winter moth biology; in particular the larval, as opposed to adult host selection and dispersal, as well as increased adult fecundity. SUMMARY (1) As a major determinant of larval winter moth population levels is known to be the degree of synchrony between larval eclosion and the bud burst of the primary host-plant, the oak, it is argued that two hitherto unstudied factors have a considerable effect on the insects' population dynamics. These are: (a) the events occurring immediately after eclosion and the effect of the period of food deprivation which occurs should no food be available locally; (b) the degree to which larvae can utilize food-plant species other than the primary host. (2) Experimental evidence is presented which shows that first instar larvae can discriminate between host-plant species, and that this is unaffected by the degree of food deprivation relevant to field conditions. Also, it is shown that early deprivation has little if any adverse affects on the subsequent growth and development of larvae, and that it may confer mild advantages. (3) A comparison is made of the development of larvae reared on primary and alternative hosts before, during and after the period of peak level eclosion and oak bud burst in the field. Four of the five alternative food-plant species tested are found to support satisfactory larval growth. Further, it is only during the period of maximum oak bud burst that an oak diet results in a considerably greater reproductive success than that supported by a diet of alternative hosts. (4) It is argued that the polyphagy of winter moth larvae is a mechanism which promotes synchrony of larval eclosion with the primary host plant. It is also associated with several unusual features of winter moth biology; in particular the larval, as opposed to adult host selection and dispersal, as well as increased adult fecundity. SUMMARY (1) As a major determinant of larval winter moth population levels is known to be the degree of synchrony between larval eclosion and the bud burst of the primary host-plant, the oak, it is argued that two hitherto unstudied factors have a considerable effect on the insects' population dynamics. These are: (a) the events occurring immediately after eclosion and the effect of the period of food deprivation which occurs should no food be available locally; (b) the degree to which larvae can utilize food-plant species other than the primary host. (2) Experimental evidence is presented which shows that first instar larvae can discriminate between host-plant species, and that this is unaffected by the degree of food deprivation relevant to field conditions. Also, it is shown that early deprivation has little if any adverse affects on the subsequent growth and development of larvae, and that it may confer mild advantages. (3) A comparison is made of the development of larvae reared on primary and alternative hosts before, during and after the period of peak level eclosion and oak bud burst in the field. Four of the five alternative food-plant species tested are found to support satisfactory larval growth. Further, it is only during the period of maximum oak bud burst that an oak diet results in a considerably greater reproductive success than that supported by a diet of alternative hosts. (4) It is argued that the polyphagy of winter moth larvae is a mechanism which promotes synchrony of larval eclosion with the primary host plant. It is also associated with several unusual features of winter moth biology; in particular the larval, as opposed to adult host selection and dispersal, as well as increased adult fecundity.

Regulation of breeding density in Microtus pennsylvanicus

Abstract: Les M.p. sont observes dans une prairie de 10 ha voisine de l'aeroport de Toronto. On confirme l'hypothese selon laquelle les femelles reproductrices controlent la densite de population reproductrice. On suggere que les mâles forment des hierarchies de dominance pour obtenir l'acces aux femelles

Population ecology of the light brown apple moth, Epiphyas postvittana (Lepidoptera: Tortricidae).

Abstract: (1) The population ecology of the light brown apple moth, Epiphyas postvittana (Walker) (Tortricidae) was studied in Victoria for sixteen generations at one site and nine generations at another between 1971 and 1977. Life-tables are given for the two populations. Key factor analyses were carried out using data from one site which provided information on six summer generations, five autumn-winter generations and five spring generations. (2) Mortality of eggs has been identified as the key factor in spring and autumn-winter generations with mortality of first instar larvae and of eggs as the key factors in the summer generation and these two stages therefore determine trends from one season to the next. Mortality of other stages, particularly of larval instars II-V was found to be unimportant. Mortality of instar VI and pupal stages ranked next to that of eggs and/or first instar larvae in key factor analyses. (3) Predation by arthropods was identified as the key factor in determinations of sub-mortalities of the egg, instars II-V, instar VI and pupal stages. Other causes of mortality such as infertility, a Nuclear Polyhedrosis Virus disease, and a complex of parasitoids of the egg, larvae and pupae were found to be unimportant in determining population abundance, although some of these factors caused death of large numbers of individuals. (4) Failure to attain maximum potential natality or losses in fecundity ranked next to predation as a key 'mortality' factor of the egg stage. Only about 14-30% of the maximum egg-laying potential was achieved. Fecundity was dependent on the weather, quality and variety of food plants available and the succession of these plants. Summer generation moths laid fewer eggs for this reason, in contrast to those of autumn-winter and spring generations. (5) Population fluctuations of the moth are determined by climate and food acting in a density-independent manner, and by egg and/or first instar larval predation acting in a density-independent manner. Exceptionally dry and warm seasons, such as the 1973 summer, can greatly accentuate the effects of weather, almost eliminating summer generations. Milne's concept of population regulation appears to hold well for E. postvittana.

Predation, Cover, and Food Manipulations During a Spring Decline of Microtus townsendii

Abstract: Piegeages effectues au printemps 1981 dans des prairies du Delta de la riviere Fraser (Canada). Pendant les 6 premieres semaines de cette epoque, le declin etait du a la predation et pendant les 4 dernieres semaines il etait le resultat d'une dispersion liee a la densite

Assembly of Mangrove Ant Communities: Patterns of Geographical Distribution

Abstract: (1) The mechanisms by which communities of mangrove ants develop are examined. (2) Eighty-one small mangrove islands in the Florida Keys were surveyed for ant species. Islands varied four orders of magnitude in size. (3) Each of the five major species was found only on islands of a certain minimum size (MSR) or larger. (4) For two species, termed Primary species, experimental introductions showed that the MSR was due to island unsuitability. For two other species, termed Secondary species, the MSR was shown to be the result of competitive interactions with the Primary species. (5) Experiments involving the two Primary species showed that either was capable of preventing the invasion of the other species. Simultaneous introduction experiments showed that one species invariably invaded while the other invariably became extinct. (6) Behavioural interactions between all pairs of the species were tested in arena experiments. The patterns of aggression and avoidance were consistent with, and presumed to be the cause of, the experimental results and patterns of geographical distribution.

Dispersal of sparrowhawks between birthplace and breeding place

Abstract: SUMMARY (1) Most sparrowhawks settled to breed in the general area where they were born, and recoveries fell off rapidly with increasing distance from the birthplace. Dispersal occurred in all directions, and no evidence was found for return migration. (2) In general, cocks moved less far from birthplace than did hens, with median (and maximum) distances of 14 (and 185) km for cocks and 27 (and 265) km for hens. These distances could be compared with a minimum between territories of 0.4 km. (3) Most sparrowhawks made the main movements of their lives in August/September, within a few weeks (in some cases days) of leaving their parents' territories, and probably stayed in the same general area thereafter. Very few individuals made long movements in adult life. (4) Changes in the median dispersal distances from year to year did not correlate with changes in population densities. Years when many cocks were recovered at long distances were not always the same as years when many hens were recovered at long distances. (5) Within each sex, young born in upland habitats tended to disperse further than young born in lowland. However, in neither sex were the distances moved by individuals significantly related to the grade of territory where they were born, to brood-size, or to fledging date in the season. (6) In both sexes, young which moved long distances seemed to perform less well in later life than young which moved short distancwes. Th se tendencies were significant only in hens, and only in quality of breeding habitat, layir g date and clutch size, which were themselves probably interrelated. (7) In distance, but not in direction, movements by brood-mates were highly correlated with one another, the young from some nests making short movements and from other nests long movements (the sex difference was still maintained). Distances moved by young from the same mother in different yea rs were also correlated, but not the distances moved by parents and offspring. Experience :n the first few weeks of life was probably important in influencing the distance moved, but we could not rule out inheritance too. SUMMARY (1) Most sparrowhawks settled to breed in the general area where they were born, and recoveries fell off rapidly with increasing distance from the birthplace. Dispersal occurred in all directions, and no evidence was found for return migration. (2) In general, cocks moved less far from birthplace than did hens, with median (and maximum) distances of 14 (and 185) km for cocks and 27 (and 265) km for hens. These distances could be compared with a minimum between territories of 0.4 km. (3) Most sparrowhawks made the main movements of their lives in August/September, within a few weeks (in some cases days) of leaving their parents' territories, and probably stayed in the same general area thereafter. Very few individuals made long movements in adult life. (4) Changes in the median dispersal distances from year to year did not correlate with changes in population densities. Years when many cocks were recovered at long distances were not always the same as years when many hens were recovered at long distances. (5) Within each sex, young born in upland habitats tended to disperse further than young born in lowland. However, in neither sex were the distances moved by individuals significantly related to the grade of territory where they were born, to brood-size, or to fledging date in the season. (6) In both sexes, young which moved long distances seemed to perform less well in later life than young which moved short distancwes. Th se tendencies were significant only in hens, and only in quality of breeding habitat, layir g date and clutch size, which were themselves probably interrelated. (7) In distance, but not in direction, movements by brood-mates were highly correlated with one another, the young from some nests making short movements and from other nests long movements (the sex difference was still maintained). Distances moved by young from the same mother in different yea rs were also correlated, but not the distances moved by parents and offspring. Experience :n the first few weeks of life was probably important in influencing the distance moved, but we could not rule out inheritance too. SUMMARY (1) Most sparrowhawks settled to breed in the general area where they were born, and recoveries fell off rapidly with increasing distance from the birthplace. Dispersal occurred in all directions, and no evidence was found for return migration. (2) In general, cocks moved less far from birthplace than did hens, with median (and maximum) distances of 14 (and 185) km for cocks and 27 (and 265) km for hens. These distances could be compared with a minimum between territories of 0.4 km. (3) Most sparrowhawks made the main movements of their lives in August/September, within a few weeks (in some cases days) of leaving their parents' territories, and probably stayed in the same general area thereafter. Very few individuals made long movements in adult life. (4) Changes in the median dispersal distances from year to year did not correlate with changes in population densities. Years when many cocks were recovered at long distances were not always the same as years when many hens were recovered at long distances. (5) Within each sex, young born in upland habitats tended to disperse further than young born in lowland. However, in neither sex were the distances moved by individuals significantly related to the grade of territory where they were born, to brood-size, or to fledging date in the season. (6) In both sexes, young which moved long distances seemed to perform less well in later life than young which moved short distancwes. Th se tendencies were significant only in hens, and only in quality of breeding habitat, layir g date and clutch size, which were themselves probably interrelated. (7) In distance, but not in direction, movements by brood-mates were highly correlated with one another, the young from some nests making short movements and from other nests long movements (the sex difference was still maintained). Distances moved by young from the same mother in different yea rs were also correlated, but not the distances moved by parents and offspring. Experience :n the first few weeks of life was probably important in influencing the distance moved, but we could not rule out inheritance too. SUMMARY (1) Most sparrowhawks settled to breed in the general area where they were born, and recoveries fell off rapidly with increasing distance from the birthplace. Dispersal occurred in all directions, and no evidence was found for return migration. (2) In general, cocks moved less far from birthplace than did hens, with median (and maximum) distances of 14 (and 185) km for cocks and 27 (and 265) km for hens. These distances could be compared with a minimum between territories of 0.4 km. (3) Most sparrowhawks made the main movements of their lives in August/September, within a few weeks (in some cases days) of leaving their parents' territories, and probably stayed in the same general area thereafter. Very few individuals made long movements in adult life. (4) Changes in the median dispersal distances from year to year did not correlate with changes in population densities. Years when many cocks were recovered at long distances were not always the same as years when many hens were recovered at long distances. (5) Within each sex, young born in upland habitats tended to disperse further than young born in lowland. However, in neither sex were the distances moved by individuals significantly related to the grade of territory where they were born, to brood-size, or to fledging date in the season. (6) In both sexes, young which moved long distances seemed to perform less well in later life than young which moved short distancwes. Th se tendencies were significant only in hens, and only in quality of breeding habitat, layir g date and clutch size, which were themselves probably interrelated. (7) In distance, but not in direction, movements by brood-mates were highly correlated with one another, the young from some nests making short movements and from other nests long movements (the sex difference was still maintained). Distances moved by young from the same mother in different yea rs were also correlated, but not the distances moved by parents and offspring. Experience :n the first few weeks of life was probably important in influencing the distance moved, but we could not rule out inheritance too.

Territory size and population density in relation to food supply in the nuthatch Sitta europaea (Aves)

Abstract: Sites observes: deux parcelles experimentales a Stenbrohult, Suede. La taille optimale du territoire des individus varie avec la densite des ressources alimentaires, qui conditionne la densite des oiseaux

Honey bee foraging ecology: optimal diet, minimal uncertainty or individual constancy?

Abstract: SUMMARY (1) Experiments using honey bees and artificial flower patches were designed to test three alternative foraging ecology models: optimal diet, minimal uncertainty, and individual constancy. Honey bee responses to a mixed colour flower patch and to flower morph associated differences in reward quantity, quality, and frequency were measured. (2) Each honey bee visiting a patch of randomly distributed blue and yellow flowers was constant to one colour, even though that behaviour was suboptimal. (3) When reward quantity was unequal between the two flower morphs each bee was constant to one colour, even though that behaviour often resulted in suboptimal reward. (4) When reward quality was unequal between the two colour morphs each bee was constant to one colour, even though that behaviour often resulted in suboptimal reward. (5) When reward frequency was higher in one flower morph than in the other each bee was constant to one colour, even though that behaviour often failed to maximize reward or minimize uncertainty.

The role of nest-site availability and territorial behaviour in limiting the breeding density of Kestrels

Abstract: (1) During 1976-9 European kestrels, Falco tinnunculus, in an upland area of young conifer plantation in south Scotland bred mainly in disused crow Corvus corone nests, usually in small woods in the valleys, and fed largely on voles. (2) In every year there was an excess of usable nests in the area. Usable nests were clumped with several nests of different ages in each small wood. In general only one pair of kestrels bred in each wood, but in the good vole years of 1978 and 1979 some pairs nested within 250 m of one another in the same woods. (3) During the breeding season, kestrels defended parts of their range around the nest, but overlap of male ranges increased with increasing distance from the nest. Territory size varied considerably within years, and, on average, was larger in the poor vole year of 1977 than in the better year of 1978. (4) The erection of artificial nests, which were subsequently occupied by kestrels, showed that lack of suitable nest sites had previously prevented breeding in some parts of the area.

Variable parasitoid sex ratios and their effect on host-parasitoid dynamics

Abstract: SUMMARY (1) Three forms of density-dependent parasitoid sex ratios are discussed: (a) where sex ratio is a function of the size of the adult female population (P), (b) where sex ratio depends on the ratio of females to hosts (P/N) and (c) the case of heteronomous hyperparasitoids, where sex ratio is related to the frequency of encounters with parasitized and unparasitized hosts (yielding male and female progeny, respectively). (2) The dynamic properties of difference equation models including each of these mechanisms in turn are displayed. Sex ratios as a function of P or P/N are considered within the framework of conventional host-parasitoid models. Heteronomous hyperparasitoids, however, require a new model structure. In each case, models are explored in which encounters with hosts are either assumed to be random (Poisson) or contagiously distributed with the probability of parasitism given by the negative binomial distribution. (3) With random host encounters, sex ratios as a function of P or P/N can be sufficient to be the sole cause of stability of the interaction, but only with 'fine tuning' of the relevant parameters. These critical parameters values, however, broaden considerably when some additional stability is added via contagion in the distribution of parasitism. (4) In the special case of randomly acting heteronomous hyperparasitoids, the unusual result occurs of a neutrally stable interaction over a wide range of host rates of increase. The slightest additional stability added to the interaction (e.g. by some contagion in the host attacks) is sufficient to 'tip the balance' and convert the neutral stability to a locally stable equilibrium. We conclude that such interactions should be highly stable in the real world.

The distribution between feeding sites of herring gulls breeding at walney island, u.k.

Abstract: (1) The major foods of herring gulls breeding at Walney were earthworms and terrestrial invertebrates obtained mainly on pasture fields within an hour of sunrise, domestic waste obtained mainly on refuse tips from 08.30 to 16.30 h on weekdays and 08.30 to 11.30 h on Saturday mornings, Mytilus edulis and Carcinus maenas obtained mainly when the height of water was below 1.9 m below O.D., and Asterias rubens obtained mainly when the height of water was below 3.1 m below O.D. (2) The minor foods included Cancerpagurus, Macoma balthica, marine fish, fish offal discarded by fishermen, eggs and chicks obtained by cannibalism, sheep and cattle food obtained from fields, and waste food scraps obtained from litter bins, streets and gardens. (3) Earthworm availability was in part determined by the wetness of the ground. (4) More gulls (herring and lesser black-backed) were seen at refuse tips which received more domestic waste. At refuse tips further from the colony fewer of the adult gulls present were herring gulls, and more were lesser black-backed gulls. (5) More gulls were seen at Walney refuse tip if more domestic waste had been unloaded in the previous 1 5 h. (6) The number of herring gulls feeding at mussel skears fluctuated with the tidal cycle. Herring gulls ate mainly 1st year mussels >3 mm which were ground into small fragments in the gizzard, the residue being evacuated in the faeces. (7) The ideal free distribution predicts the distribution of gulls between refuse tips but leaves as a paradox the low energy returns obtained by herring gulls feeding at other sites when the tip was open.

Patch time allocation and parasitization efficiency of asobara tabida, a larval parasitoid of drosophila

Abstract: SUMMARY (1) Patch time allocation by the parasitoid Asobara tabida Nees was studied on patches with different host densities. (2) The parasitoid increased its searching time and giving up time with increasing host density. These increases were caused by a response of the parasitoid to the number of encounters with unparasitized hosts; the amount of kairomone in a patch may have had an additional incremental effect on patch time. (3) Encounters with parasitized hosts had no effect on searching time and giving up time. (4) The percentage of hosts parasitized first increased with host density and then levelled off at densities of more than four host larvae per patch, thus producing an accelerating functional response. (5) The foraging behaviour of A. tabida is compared to that predicted by optimal foraging models.

Spacing and breeding density of willow ptarmigan in response to an experimental alteration of sex ratio

Abstract: SUMMARY (1) Willow ptarmigan usually pair monogamously and each member of the pair defends the territory against intruders of the same sex during the breeding season. (2) I determined the effect of reducing numbers of one sex on the density of the same and opposite sex to test the hypothesis that spacing behaviour limits breeding density of both sexes independently. (3) When removed, territorial females were largely replaced by yearlings, of unknown origin, able to breed. (4) A large reduction in the density of females did not result in a similar reduction in the density of males.