Resource Dispersion Research Articles

Dispersion in the Wood Mouse, Apodemus sylvaticus: Variable Resources in Time and Space

(1) The abundance and dispersion of a population of Apodemus sylvaticus was investigated with respect to tree seed availability and vegetative structure over three harvests. (2) Tree seed production varied considerably between years and from place to place, being overdispersed for all species during all harvests. Greatest seed production was associated with increased reproduction in winter but depressed levels of reproduction in the enhanced densities of the following summer. (3) The distributions of seed supply and the A. sylvaticus population were correlated positively in all winters, particularly in the year of highest overall population density and production of deciduous seeds. The influence of seed on population dispersion declined progressively from winter to summer. (4) In 1984-85, when numbers of A. sylvaticus were greatest, the relationship between seed supply and dispersion was weaker among adult males, particularly reproductively active males, than among adult females. Such sex differences did not occur at other times. (5) The spatial distribution of the A. sylvaticus population as a whole was also related to density of high level vegetation. Cover was less important than seed availability only in December 1984 and September 1985. There were no consistent differences between male and female A. sylvaticus with respect to habitat use. (6) The distribution of males and females was also related to that of the opposite sex. The relative importance of sex, food and vegetation variables varied over time, social factors being of little importance in early autumn while exerting considerable influence on dispersion of either sex at other times. (7) It is concluded that the influence of habitat patch quality on the distribution of A. sylvaticus varies considerably and may be dependent on the overall quality of the habitat. Further, sex differences in population response to resource dispersion occur infrequently and inconsistently. Female A. sylvaticus aggregate in areas of high resource value where they may breed in winter but are subject to a density-dependent inhibition of reproduction limiting recruitment and population growth during the following summer. High resource availability in winter and spring may permit individual females to indulge in selectively advantageous breeding at a young age.

A model for territory and group formation in a heterogeneous habitat

We present a simple model for the formation of territories in habitats containing discrete food patches and for the formation of social groups based on these territories. Territories are determined by the distribution of food resources; they are assumed to be as small as is possible while providing adequate food resources for their primary occupants, allowing for the discrete distribution of food, and its variability over time. We show that such territories may often contain sufficient resources to support additional animals at no cost to the primary occupants, especially if individual food patches have a high yield, the variation between patches is large, or the requirements of secondary animals are smaller than those of primary animals. The model thus gives a rigorous basis for the resource dispersion hypothesis of Macdonald (1983, Nature, Lond. 301 , 379–384.) The model predicts that, provided the variation between patches is not too small, the correlation between group size and territory size within such a habitat is likely to be low. The correlation between the amount of resources a territory contains and the size of the group it can support, on the other hand, is likely to be high. In addition, the model predicts that, counter-intuitively, group size does not necessarily increase as the variability of yield over time increases: in some cases, group size may decrease with increasing variability of yield. The main predictions of the model are shown to be unaffected by small changes in parameters, and by changes in some of the model's assumptions.

Deterministic and Stochastic Models of Social Behaviour based on the Resource Dispersion Hypothesis

Deterministic and Stochastic Models of Social Behaviour based on the Resource Dispersion Hypothesis

Resource quality, dispersion and mating prospects for crabs occupying bryozoan colonies

Responses by the xanthid crab Pilumnus sayi Rathbun to varied resource conditions were tested in a series of laboratory and field experiments and from those results aspects of mating strategies were inferred. Crabs discerned differences in both quality and dispersion of their critical resource, the bryozoan Schizoporella pungens Canu et Bassler. Crabs preferred bryozoan heads > 17-ml volume displacement, heads attached to the substratum and heads with cavities. Crabs moved quickly to heads along straight paths and occupied heads alone. These results, combined with previously published data, implicated refuge from predation as the primary resource value of heads. A preference for clumped rather than dispersed heads was sex-dependent and males moved between heads more frequently than did females. Movement by males was inhibited by the presence of a larger male, with or without females present. When resource quality was controlled in the field and only head dispersion varied, clumped heads favored a spatial pattern among crabs consistent with large males amidst several females. Overdispersed heads resulted in that pattern plus individual females amidst several males. Smaller males on clumped heads also had apparently greater access to prospective mates than did their counterparts on overdispersed heads. A working model of flexible mating strategies for P. sayi is proposed.

Satellites and Transients: Ecological Constraints On Alternative Mating Tactics in Male Grasshoppers

Abstract Alternative tactics of male mating behaviour, broadly classifiable as "dominant/ territorial" versus "subordinate/non-territorial", have now been described for numerous species. Furthermore, across diverse taxa the mating tactics of subordinate/non-territorial males often appear as one of two distinct types, satellite or transient behaviour. Despite general recognition of this dichotomy, though, little effort has been made to identify the circumstances under which one of these behaviours is adopted over the other. We compared the mating systems of two congeneric species of desert grasshoppers (genus Ligurotettix) to investigate specifically the role of resource dispersion in shaping the behaviour of subordinate males. The utility of the comparative approach derives from two basic similarities between the species: both Ligurotettix coquilletti and Ligurotettix planum feed almost exclusively on a single host plant species, and the majority of males in both species defend individual host plants to gain access to females. However, the two species are associated with host plants that are dispersed very differently; i.e., L. coquilletti encounter a small number of large plants and L. planum a large number of small ones. In L. coquilletti, subordinate males, individuals noted by their lack of success in aggression and in obtaining matings, were characterized as satellites that remained silent on host plants defended by territorial males. Subordinate males in L. planum, however, were transients that sang regularly but moved frequently among different host plants. We propose that this behavioural discrepancy results from (1) the large difference between the number of potential female encounter sites (i.e., individual host plants) available to the males of the two species and (2) the large difference between the sizes of resource patches defended by the two species, which influences the ability of dominant males to eject subordinates.

Search orientation in adultDrosophila melanogaster: Responses of rovers and sitters to resource dispersion in a food patch

Drosophila melanogasteradults were employed in single resource patches of varying density and size and in a multiple-patch array to determine the degree to which resource dispersion influences searching success. Individuals from rover and sitter selected lines, with extreme genotypes for local search duration, are not as successful as control-line (wild-type) flies in locating sucrose drops in single patches varying in size and density. The number of new drops located differed significantly between fly lines in all patch types, except in a high-density patch, and within each fly line over the different patch sizes and densities. The similarities in number of drops found by rovers and sitters in all patch types are not reflected in the time periods spent searching. In the multiple-patch array sitters never left the central patch, whereas most rovers and con-trol-line flies found additional patches. The proximate explanations for the success or failure of the three fly lines in different patch sizes and densities relate to the looping locomotor pattern characterizing local search in D. melanogaster.The reactivation of searching each time a drop is ingested or revisited keeps an individual in the immediate vicinity of the last encountered resource. Flies from the selected lines, each exhibiting extreme types of locomotor patterns, leave patches relatively unexploited because local search consists either of rapid, nearly linear movement away from a drop in rovers or of relatively long bouts of local search in sitters, which promotes revisiting rather than locating new drops. Control-line flies locate more drops than either rovers or sitters and in less time than sitters, suggesting that their intermediate phenotype for search behavior allows for more flexibility in searching in various patch sizes and resource densities. The results are discussed with reference to environmental and physiological factors that may modify searching behavior and, possibly, enhance the survival of individuals with extreme genotypes.

Sociality in molerats : Metabolic scaling and the role of risk sensitivity.

The foraging behaviour and diets of the various bathyergid molerat species are reviewed briefly, and inferences are drawn concerning their dietary specialisation. A simple model has been constructed which investigates the risks of unproductive foraging by specialist feeders as a function of resource dispersion characteristics and group size. The model suggests that the principal benefit of group foraging is a reduction in foraging risk, rather than increased resource procurement per se. Meeting the energetic costs of non-workers in social groups necessitates a reduction of the total energetic expenditure of the colony. This is achieved by reducing body size, huddling in the nest, and scaling mass-specific resting metabolic rate virtually independent of mass.

Variations in the Foraging Behaviour and Burrow Structures of the Damara Molerat <i>Cryptomys damarensis</i> in the Kalahari Gemsbok National Park

Aspects of two habitat-specific foraging behaviours of the social subterranean rodent Cryptomys damarensis, are discussed in terms of burrow structure, resource dispersion patterns, sand moisture content, burrow temperature regimes, and predatory pressures, in the Kalahari Gemsbok National Park, South Africa.

Risk-averse foraging by bananaquits on negative energy budgets

Adult bananaquits on negative energy budgets were presented with a patch containing two flower types with identical mean rewards, but different variances. The flower patch contained a random array of 85 yellow and 85 red artificial flowers. Flowers of one color were filled with the same quantity of nectar (constant flowers); flowers of the other color were filled with variable quantities of nectar (variable flowers). In the first series of experiments the birds were given three presentations, followed by three more presentations with the flower colors reversed, to control for color preferences. Some individuals were occasionally indifferent during a presentation, but overall the birds significantly preferred the constant flowers. In the second series of experiments two birds were give five presentations of the floral patch during a day at a rate less than minimally required to meet all 24-h energy costs. In all experiments, bananaquits on negative energy budgets were either indifferent or risk-averse, but never risk-prone. The absence of risk-prone foraging might be attributed to resource dispersion pattern, reward skew, or a species characteristic.

Territory inheritance and the evolution of group-living in carnivores

The Territory Inheritance Hypothesis (TIH) on the evolution of group-living in carnivores considers (1) the increased fitness of the original territory holders when they ensure that the territory will be inherited by a carrier of their own genes, and (2) the increased fitness of a subadult that remains at home and survives in an otherwise saturated environment. A comparison of the fitness of both adults and subadults (at different rates of population growth and adult survival) who either do or do not choose the strategy of building up a group, indicates that groups are favoured in decreasing to slightly increasing populations at moderate to high (>0.5) annual rates of adult survival. Furthermore, an analysis of maximum group size under the TIH suggests that groups of five or more adults should split up. The TIH as an explanation for group-living in the red fox, Vulpes vulpes L., and the badger, Meles meles L., is compared with the Resource Dispersion Hypothesis (RDH, Macdonald 1983) and the Constant Territory Size Hypothesis (CTSH, von Schantz 1984a), which consider spatio-temporal fluctuations in food supply. The RDH is the best explanation of social organization in the badger; the RDH and the CTSH both explain red fox data under special conditions, but are not satisfactory as general explanations of group-living in this species; the TIH may explain why group-living, at least once it is established, is a stable strategy.

The sociality of solitary foragers: a model based on resource dispersion

Under versions of what may broadly be called the Resource Dispersion Hypothesis (RDH) several authors have concluded that territory size and group size are limited respectively, and independently, by the dispersion and richness of patches of food. This paper presents a model that shows how the frequency distribution of resources available per unit time within a territory may permit the formation of groups even in the absence of any functional advantage to any individual from the presence of another. In this model, animals (called primary occupants) occupy territories containing sufficient resources to meet or exceed their requirements for a critical proportion of feeding periods. The availability of these resources is described in terms of their mean richness and their heterogeneity, and plots of these parameters indicate the circumstances within which individuals may share the minimum territory with the primary occupants. The model shows how, under plausible conditions of resource dispersion, a territory that provides almost total food security for two occupants could also provide, at no cost to the original occupants, substantial food security for an additional group member, even if it never used the same food patches as the originals. It is not therefore, as is sometimes supposed, a necessary condition for the RDH that members of a group often forage simultaneously in the same patch. Thus the model describes ecological circumstances whereby groups could evolve amongst species whose members neither forage communally, nor even meet frequently.

The ecology of carnivore social behaviour

Diverse selective pressures have contributed to the evolution of the varied social groups of carnivores: the benefits of strength of numbers for defence of kills and territory, and in the hunting and killing of large prey; the ability to intimidate predators and to be vigilant against their approaches; the potential for information transfer and social learning, and a suite of alloparental behaviour patterns. Each of these may operate within the constraints upon group size and home range size set by patterns of resource dispersion. Between and within species, the magnitudes of costs and benefits attendant upon group life vary with circumstances and between individuals

Territorial responses to energy manipulations in the Anna hummingbird.

Territorial activity in the Anna hummingbird (Calypte anna) was measured while energy availability on the territory was varied. On days when energy availability was unlimited, residents defended highly exclusive territories primarily by energetically expensive defense behaviors. As energy availability decreased, exclusiveness declined gradually, relative use of energetically inexpensive defense increased, and owners spent less time on the territory.Territorial behavior also varied with short term depressions in energy availability: A lower percentage of intruders was chased and departures of an owner from its territory were more frequent shortly after feeding.When resource dispersion was increased without changing substantially total rewards per territory, chasing by owners increased.

Social Systems in a Tropical Forest Avifauna

An analysis of correlations between social systems, food disperson, and behavioral and morphological attributes related to predation vulnerability in a tropical avifauna indicates the following results and conclusions: (1) Solitary foraging is typical of species whose characteristics of activity, watchfulness, size, and habitat intuitively indicate reduced conspicuousness and vulnerability to relevant predators. (2) Active arboreal birds frequently forage in flocks. Their active search for food in the more open strata of the forest indicates greater vulnerability to predation in comparison to the rest of the avifauna. (3) Flocking is more probably an adaptation reducing susceptibility to predation than a development primarily determined by characterstics of food dispersion. Relative support is based on the number of exceptions to predicted correlations. (4) Among flocking species, flock composition is based to a large extent upon resource dispersion. Species with clumped resources flock intraspecifically;...

Ecological Determinants of Group Sizes of Foraging Lions

Ecological Determinants of Group Sizes of Foraging Lions