Free Distribution (free + distribution)

Distribution by Scientific Domains

Kinds of Free Distribution

  • ideal free distribution

  • Selected Abstracts

    Resolving the departures of observed results from the Ideal Free Distribution with simple random movements

    A. L. Jackson
    Summary 1The Ideal Free Distribution (IFD) is one of the most widely applied theoretical concepts in ecology. Many experimental tests of the IFD incorporating unequal competitors have revealed three consistent departures of observed results from those predicted by the theory. Specifically these are: (1) the full range of predicted equilibria are rarely realized; (2) under-matching of individuals to resource inputs; and (3) continued movement of individuals after equilibrium is reached. 2It has been suggested previously that a model including simple random movements can explain simultaneously all three of these departures. The acceptance of this theory has been hindered by arguments about model construction relating to the timing of events and whether an analytical or individual-based model is more appropriate. 3Here we present the first internally consistent and biologically realistic model of the IFD incorporating random non-IFD movements (movements made at a rate independent of intake rates). We use this model to confirm that random movements alone can explain all three of the observed departures from the IFD. Furthermore, it is not necessary to have any detailed a priori knowledge about the motivation behind the non-IFD movements, in order to predict their consequences. We expect non-IFD movements to be widespread in animal systems, and so warrant further experimental consideration and investigation. [source]

    Habitat selection by juvenile Atlantic salmon: the interaction between physical habitat and abundance

    R. D. Hedger
    The effect of physical river habitat variables on the distribution of juvenile Atlantic salmon Salmo salar L. in the Rivière de la Trinité, Québec, Canada, was examined using generalized additive modelling. A survey of Atlantic salmon fry and parr densities and habitat variables (flow velocity, water column depth and substratum size) was conducted in the summer months from 1984 to 1992. Clear patterns of habitat use existed: specific ranges of habitat variables were selected, with parr preferring greater velocities, depths and substratum sizes than fry. There was a large variation, however, in juvenile densities for given velocities, depths or substratum sizes, with this variation being greatest in optimal habitats. On examination of an individual year, interaction between the variables was found to explain some of the variation. On a year-to-year basis the juvenile Atlantic salmon population was found to exhibit an ,Ideal Free Distribution', which resulted in greatest variation in optimal habitats with year-to-year changes in population abundance. [source]

    The Need to Look Beyond the Production and Provision of Relief Seed: Experiences from Southern Sudan

    DISASTERS, Issue 4 2002
    Richard B. Jones
    Free distribution of seeds in selected areas of southern Sudan has been widespread as a way of increasing food security. Field research in areas targeted for seed relief found that farmer seed systems continue to meet the crop and varietal needs of farmers even following the 1998 famine. Donor investments in seed multiplication of improved sorghum have not been sustained due to a lack of effective demand for the improved seed beyond that created by the relief agencies. The article argues that rather than imposing outside solutions, whether through seed provisioning or seed production enterprises, greater attention needs to be given to building on the strengths of existing farmer systems and designing interventions to alleviate the weaknesses. The case is made to support dynamically the process of farmer experimentation through the informed introduction of new crops and varieties that can potentially reinforce the strength and diversity of local cropping systems. [source]

    The role of resources and natural enemies in determining the distribution of an insect herbivore population

    Iain S. Williams
    Summary 1. Both resources and natural enemies can influence the distribution of a herbivore. The ideal free distribution predicts that herbivores distribute themselves to optimise utilisation of resources. There is also evidence of herbivores seeking out refuges that reduce natural enemy attack (enemy-free space). Which of these theories predominates in a thistle,tephritid Terellia ruficauda (Diptera: Tephritidae),parasitoid interaction is examined. 2. The plant, Cirsium palustre, had a contagious distribution approximated by the negative binomial distribution. Terellia ruficauda foraged preferentially and oviposited on isolated plants although its larvae gained neither nutritional benefit nor reduced natural enemy pressure from such behaviour. 3. Parasitoids of T. ruficauda foraged and oviposited more frequently on isolated than on crowded T. ruficauda, resulting in inverse density-dependent parasitoid attack at all spatial scales examined. Neither the herbivore nor natural enemies distributed themselves according to the predictions of the ideal free distribution and the herbivore did not oviposit to reduce natural enemy attack. 4. Extrapolating from the theoretical predictions of the ideal free distribution and enemy-free space to the field requires considerable caution. Terellia ruficauda and its parasitoids appear to select their oviposition sites to spread the risk of losses through factors (e.g. mammal herbivory) that may damage dense clusters of C. palustre. [source]

    Foraging on spatially distributed resources with sub-optimal movement, imperfect information, and travelling costs: departures from the ideal free distribution

    OIKOS, Issue 9 2010
    Shuichi Matsumura
    Ideal free distribution (IFD) theory offers an important baseline for predicting the distribution of foragers across resource patches. Yet it is well known that IFD theory relies on several over-simplifying assumptions that are unlikely to be met in reality. Here we relax three of the most critical assumptions: (1) optimal foraging moves among patches, (2) omniscience about the utility of resource patches, and (3) cost-free travelling between patches. Based on these generalizations, we investigate the distributions of a constant number of foragers in models with explicit resource dynamics of logistic type. We find that, first, when foragers do not always move to the patch offering maximum intake rate (optimal foraging), but instead move probabilistically according to differences in resource intake rates between patches (sub-optimal foraging), the distribution of foragers becomes less skewed than the IFD, so that high-quality patches attract fewer foragers. Second, this homogenization is strengthened when foragers have less than perfect knowledge about the utility of resource patches. Third, and perhaps most surprisingly, the introduction of travelling costs causes departures in the opposite direction: the distribution of sub-optimal foragers approaches the IFD as travelling costs increase. We demonstrate that these three findings are robust when considering patches that differ in the resource's carrying capacity or intrinsic growth rate, and when considering simple two-patch and more complex multiple-patch models. By overcoming three major over-simplifications of IFD theory, our analyses contribute to the systematic investigation of ecological factors influencing the spatial distribution of foragers, and thus help in deriving new hypotheses that are testable in empirical systems. A confluence of theoretical and empirical studies that go beyond classical IFD theory is essential for improving insights into how animal distributions across resource patches are determined in nature. [source]

    Search and navigation in dynamic environments , from individual behaviors to population distributions

    OIKOS, Issue 5 2008
    Thomas Mueller
    Animal movement receives widespread attention within ecology and behavior. However, much research is restricted within isolated sub-disciplines focusing on single phenomena such as navigation (e.g. homing behavior), search strategies (e.g. Levy flights) or theoretical considerations of optimal population dispersion (e.g. ideal free distribution). To help synthesize existing research, we outline a unifying conceptual framework that integrates individual-level behaviors and population-level spatial distributions with respect to spatio-temporal resource dynamics. We distinguish among (1) non-oriented movements based on diffusion and kinesis in response to proximate stimuli, (2) oriented movements utilizing perceptual cues of distant targets, and (3) memory mechanisms that assume prior knowledge of a target's location. Species' use of these mechanisms depends on life-history traits and resource dynamics, which together shape population-level patterns. Resources with little spatial variability should facilitate sedentary ranges, whereas resources with predictable seasonal variation in spatial distributions should generate migratory patterns. A third pattern, ,nomadism', should emerge when resource distributions are unpredictable in both space and time. We summarize recent advances in analyses of animal trajectories and outline three major components on which future studies should focus: (1) integration across alternative movement mechanisms involving links between state variables and specific mechanisms, (2) consideration of dynamics in resource landscapes or environments that include resource gradients in predictability, variability, scale, and abundance, and finally (3) quantitative methods to distinguish among population distributions. We suggest that combining techniques such as evolutionary programming and pattern oriented modeling will help to build strong links between underlying movement mechanisms and broad-scale population distributions. [source]

    Size of environmental grain and resource matching

    OIKOS, Issue 3 2000
    Esa Ranta
    For most animals their foraging environment consists of a patch network. In random environments there are no spatial autocorrelation at all, while in fine-grained systems positive autocorrelations flip to negative ones and back again against distance. With increasing grain size the turnover rate of spatial autocorrelation slows down. Using a cellular automaton with foragers having limited information about their feeding environment we examined how well consumer numbers matched resource availability, also known as the ideal free distribution. The match is the better the smaller the size of the environmental grain. This is somewhat contrary to the observation that in large-grained environments the spatial autocorrelation is high and positive over long distances. In such an environment foragers, by knowing a limited surrounding, should in fact know a much larger area because of the spatially autocorrelated resource pattern. Yet, when foragers have limited knowledge, we observed that the degree of undermatching (i.e., more individuals in less productive patches than expected) increases with increasing grain size. [source]

    Behavior of queen bumblebee pollinators on Primula sieboldii (Primulaceae) in response to different patch sizes and spacing

    Abstract Pollinator visitation patterns are strongly affected by the spatial structure of a plant population. We assessed the effects of the size and spacing of patches on the foraging behavior of queens of Bombus diversus tersatus on a heterostylous perennial species, Primula sieboldii, using an experimental population. We compared Akaike's information criterion (AIC) among candidate statistical models, and models with patch size had the lowest AIC in various aspects of visitation behavior. Visitation frequency per patch increased with increasing patch size, but the proportion of visited flowers decreased. As a result of this combination of visitation patterns, visitation frequency per flower was only slightly affected by patch size. This result indicates that the bumblebees came close to an ideal free distribution for the use of resources in terms of the patch size within a population. From the plant's point of view, increasing the number, but decreasing the proportion, of flowers visited in larger patches would maximize reproductive success by minimizing inbreeding. Although patch size strongly affected visitation frequencies, patch spacing had a moderate effect only on the visitation frequency per patch. Visitation frequency per patch was higher with closer spacing between patches. No evidence for an interaction between patch size and spacing was detected. [source]

    Matching and ideal free distributions

    OIKOS, Issue 7 2008
    Alasdair I. Houston
    Ideal free distributions characterise how a group of animals should be distributed between sources of food. In a simple case, the ratio of the number of animals matches the ratio of input rates, a result known as input matching. The matching law characterises how an animal should allocate responses to sources of food. If matching holds then the ratio of behaviour allocated to the sources matches the ratio of rewards obtained. Several authors have drawn attention to the analogy between input matching and the matching law. I present a critical review of this topic, and go on to investigate the claim that the distribution of animals should be analyzed in the same way that the generalized matching law is analyzed. This involves assuming that the relationship between the ratio of animals and the ratio of resources obeys a power law with two parameters, one corresponding to bias and the other to sensitivity. On this view, a plot of the log of the ratio of animals against the log of the ratio of input rates will be a straight line and its intercept and slope will provide estimates of bias and sensitivity. I show that this approach can give a good fit even when the underlying model does not result in a power law. A consequence is that the parameters estimated cannot be interpreted as bias and sensitivity. I conclude that there are no compelling reasons for analyzing the distribution of animals using log-log plots, and the method has the disadvantage of encouraging the mistaken view that deviations from input matching follow a power law. [source]