Ideal Free Model Research Articles

Governing open access: livestock distributions and institutional control in the Karakum Desert of Turkmenistan

On the edge of the Karakum Desert in Turkmenistan, the distribution of livestock in space and time can with reasonable accuracy be interpreted in terms of the model of the ideal free distribution: The number of livestock supported by desert settlements varies with the level of resources in a settlement; the propensity of herds to migrate seasonally is a density-dependent function of grazing pressure; migratory cycles exploit temporal fluctuations in feed and water quality between regions. Ideal free models are premised on the assumption that resource consumers have unrestricted access to resources. In conventional economic analyses, open or unrestricted access is equivalent to the absence of land tenure. A formal tenure system, involving the state ownership of land and the management of state lands by collective farms, nonetheless operated in the study area, and was referred to by farm managers and by pastoralists when making decisions about herd movement and resource use. The operation of this tenure system was also demonstrated by the restrictions that it imposed on communities with insufficient resource entitlements. In the great majority of cases, however, and in terms of the land use system as a whole, the tenure system facilitated a process of orderly and free distribution of livestock relative to resources. This paper examines the reasons for and functioning of this theoretical oxymoron—a regulated system of open access. The study contributes to a growing body of literature on the non-exclusive nature of pastoral tenure systems in Africa and Asia.

Spin resolved bandstructure imaging with a high resolution momentum microscope

We present a spin resolving “momentum microscope” for the high resolution imaging of the momentum distribution of photoelectrons. Measurements of the band structure of a Au(111) single crystal surface demonstrate an energy resolution of ΔE=12meV and a momentum resolution of Δk∥=0.0049A˚−1, measured at the line-width of the spin–orbit split Shockley surface state. The relative accuracy of the k∥ measurement in the order of 10−4A˚−1 reveals a deviation from the ideal two-dimensional free electron gas model of the Shockley surface state, manifested in a threefold radial symmetry. Spin resolution in the full momentum image is obtained by an imaging spin-filter based on low-energy electron diffraction at a Au passivated Ir(100) single crystal. Using working points at 10.5eV and 11.5eV scattering energy with a completely reversed asymmetry of ±60% we demonstrate the efficient mapping of the spin texture of the Au(111) surface state.

Computational modelling based wear resistance analysis of thick composite coatings

A computational modelling and simulation approach was developed and applied for wear resistance analysis of composite coatings. Three new numerical finite element models were developed to include microstructural properties of typical thick thermal spray and laser cladded metal matrix coatings. The first was an ideal synthetic defect free material model, the second an advanced synthetic model containing defects and the third an image based real model. A thermal spray WC-CoCr coating and a laser cladded WC-NiCrBSi coating were characterised and the information obtained of their microstructure and properties was used for computational stress and strain simulations. The simulations were carried out for a set of indentation and scratch test contact conditions. Wear related features were validated empirically by abrasive rubber wheel testing and sliding contact pin-on-disk testing in dry conditions. Features like high local curvature, notches, abnormally large particles, thin ligament or throat-like structures of a specific material phase, clusters of interlinked carbides or high local fraction of a specific material phase had a great impact on the resulting stress state and wear resistance of the coating. The composite structures of the coatings offered a 2 to 50 times lower abrasive wear and more than four orders of magnitude lower sliding wear rate compared to the reference steel surface.

Can settlement in natal-like habitat explain maladaptive habitat selection?

The study of habitat selection has long been influenced by the ideal free model, which maintains that young adults settle in habitat according to its inherent quality and the density of conspecifics within it. The model has gained support in recent years from the finding that conspecifics produce cues inadvertently that help prebreeders locate good habitat. Yet abundant evidence shows that animals often fail to occupy habitats that ecologists have identified as those of highest quality, leading to the conclusion that young animals settle on breeding spaces by means not widely understood. Here, we report that a phenomenon virtually unknown in nature, natal habitat preference induction (NHPI), is a strong predictor of territory settlement in both male and female common loons (Gavia immer). NHPI causes young animals to settle on natal-like breeding spaces, but not necessarily those that maximize reproductive success. If widespread, NHPI might explain apparently maladaptive habitat settlement.

Making habitat selection more “familiar”: a review

Behavioral ecologists generally agree that animals derive benefits from familiarity with spaces that they inhabit or visit, yet site familiarity is rudimentary or lacking in most models of habitat selection. In this review, I examine evidence for the occurrence of site familiarity and its fitness benefits, describe the difficulty of measuring site familiarity, note its omission from the influential ideal free and ideal despotic models, and use a literature search to test an assumption of the ideal models that has become widespread in habitat selection theory: that animals behave without regard for site familiarity. I find little support for such “familiarity blindness” in vertebrates. Next I discuss how the study of public information has drawn attention away from site familiarity and point out that both kinds of information are likely to be important in habitat selection. I proceed to examine current models of initial settlement (exploration and settlement of prebreeders on first territories) and optional resettlement (site fidelity or dispersal by established breeders following a period of prospecting) and find that the latter include only basic forms of site familiarity. Hence, I develop the concept that an inhabited space holds a unique “private value” to an animal based on its familiarity with the space and offer a simple model for optional resettlement based on private value that generates several novel predictions, including site fidelity based on cumulative breeding site familiarity and high site fidelity among species with complex territories.

Sequential settlement and site dependence in a migratory raptor

Habitat selection models, such as the ideal free, ideal despotic, site-dependent, and conspecific cuing models, are of great importance to behavioral ecologists given their capability to predict habitat distributions and to link individual behavior to population processes. However, there have been relatively few field tests of their predictions. We tested the 4 models by studying the process of sequential settlement on territory in 2 distant populations of a migratory raptor, the black kite Milvus migrans. Results were mainly consistent with the site-dependent model: on arrival, kites settled on progressively lower quality territories, and earlier arriving individuals were older, larger, and in better body condition than later arriving ones, leading to a state-dependent arrival sequence also predicted by a previous theoretical model of settlement pattern. Occupation of superior territories by superior phenotypes resulted in cascading advantages for earlier arriving individuals in terms of subsequent reproductive performance. At the population level, the populations expanded/retracted from lower quality sites during population increases/declines. The above scenario was consistent across the 2 populations, and a review of the literature uncovered a remarkably consistent picture of state-dependent arrival, progressive monopolization of best quality sites, and cascading effects on subsequent breeding performance. We propose as a general paradigm of sequential settlement the following process: 1) arrival date is a reliable surrogate of phenotype quality, 2) early-arriving individuals have preferential access to the best quality sites and partners, 3) the above conditions cascade into a number of benefits ultimately related to higher fitness for earlier arriving individuals.

Dynamics of habitat distribution in breeding mallards: assessing the applicability of current habitat selection models

The applicability of ideal free, ideal despotic and ideal preemptive habitat selection models to explain dynamics of habitat distribution of breeding mallards (Anas platyrhynchos) was explored. Data from 35 lakes studied between 1985 and 2000 were used to examine overall habitat distribution of breeding pairs, breeding success in different habitats, within‐year order of habitat occupation, and density dependence of habitat distribution and breeding success. Two habitat types, rich and poor, were defined based on the structure and luxuriance of shore vegetation; each lake belonged to one or the other of the habitat types. Breeding pairs used the rich habitat more than expected, breeding density also being higher there than in the poor habitat. Both average brood density and breeding success were higher in the rich habitat than in the poor. Breeding success was not density dependent, neither when analysed separately for the habitat types, nor in the study area in general. Within season, arriving mallard pairs did not occupy rich lakes earlier than poor lakes. An isodar analysis based on between‐year variation of the breeding density in rich and poor habitats revealed that habitat distribution of breeding pairs was not density dependent. By contrast, density in the rich habitat increased and proportional use of the poor habitat decreased with increasing overall population density, i.e. the rich habitat got increasingly crowded. None of the habitat selection models considered was applicable to explain the dynamics of habitat distribution of breeding mallards.

The Influence of Habitat Quality on Dispersal, Demography, and Population Dynamics of Voles

We compared the effects of habitat quality on dispersal, demography, dynamics, and fitness of prairie vole (Microtus ochrogaster) and meadow vole (M. pennsylvanicus) populations by manipulating habitat patches in experimental landscapes. Four habitat patches in each of four replicate landscapes differed in availability of high-quality food and amount of vegetative cover in a 2 × 2 factorial design. High cover had a strong positive effect on basic habitat quality, as reflected by the performance of founders early in the season, but supplemental food had only a small effect. Population growth ceased for prairie voles after week 18 (mid-October) when densities had reached much higher levels in habitats with high cover (260 ± 27 voles/ha in high cover with either high or low food; mean ± 1 se) than in habitats with low cover (115 ± 38 voles/ha in high food, low cover and 60 ± 15 voles/ha in low food, low cover). Population growth had ceased in three habitat types for meadow voles by week 20 at much higher densities than for prairie voles in high cover (636 ± 101 voles/ha with high food, high cover; 556 ± 117 voles/ha with low food, high cover; 110 ± 74 voles/ha with high food, low cover; and 51 ± 16 voles/ha with low food, low cover). Correlates of fitness for prairie voles, particularly the number of young surviving to adulthood per female, indicated greater individual fitness in high cover than in low cover, which suggests ideal despotic habitat selection. The proportion of the total population found in habitat patches with high cover continued to increase until prairie vole populations stabilized late in the season. This result did not agree with expectations of either the ideal free model or the ideal despotic model of habitat selection. Fitness differed little for meadow voles in different habitat types, and the proportions of the populations in different habitats remained constant after the founders had settled and population growth began. Both of these patterns supported the ideal free model of habitat selection for meadow voles. Because positive net recruitment occurred in all habitats for both species of voles, source–sink dynamics could not occur in our experimental system. Two subpopulations of meadow voles in low-food, low-cover habitats did go extinct temporarily, but this habitat type did not appear to be a population sink because the losses occurred primarily from emigration rather than mortality. Emigration rates for both species of voles were inversely related to carrying capacity of the habitat (estimated as the density at which populations stabilized). This relationship and ideal free habitat selection are required by the balanced dispersal model, which produces equal numbers of dispersers between a pair of habitat types. Dispersal of meadow voles was balanced throughout the growing season, but dispersal of prairie voles was unbalanced, with net movement of individuals from low-quality to high-quality habitats until late in the season when populations stabilized. Unbalanced dispersal early in the season may have reflected a delayed response to habitat quality by prairie voles. Populations in high cover approached stable densities largely as a result of density-dependent in situ net recruitment, whereas populations in low cover stabilized more as a result of density-dependent net movement for both species of voles. Per capita emigration did not show a consistent positive relation to population growth in high cover, as would be expected if dispersal contributed to the tendency for prairie voles to display multi-annual population cycles in such habitats. In fact, emigration generally declined as density increased (inversely density dependent), which should have fostered greater population growth as densities increased. Nevertheless, population growth declined late in the season, which implied that emigration (dispersal) did not strongly affect the pattern of population dynamics in high-quality habitats. The overall results for the two species were remarkably similar, although prairie voles did differ from meadow voles in several respects. Meadow voles performed less well in low-cover habitats than did prairie voles, which probably reflected their greater sensitivity to predation risk. Prairie voles tended to move into habitats with supplemental food, and young prairie voles had greater body growth in habitats with supplemental food, whereas meadow voles showed neither response. Finally, although the results for meadow voles consistently indicated ideal free habitat selection throughout the growing season, those for prairie voles did not until late in the season. This difference between species likely reflected a more rapid response to habitat quality by the more vagile meadow voles.

Relative competitive ability does not change over time in blackbirds

Summary Individual variation in feeding rate in the absence of competitors (absolute feeding rate) and decline in feeding rate in the presence of competitors (susceptibility to interference) are frequently used as measures of competitive ability or fitness in short‐term foraging studies. They are also crucial parameters of individual‐based population models that predict the distribution of animals across patches. There are, however, few data on whether absolute feeding rate and susceptibility to interference competition remain constant for individuals between years. The hypothesis that absolute feeding rate and susceptibility to interference were similar between years was tested by observing 25 European blackbirdsTurdus merulafeeding in seminatural experimental patches, during January to March over a 2–4‐year period 1995–98. Absolute feeding rate was measured as the feeding rates of solitary blackbirds. Susceptibility to interference was measured as the change in feeding rate of a focal bird when it fed in the presence of different numbers of competitors. Individuals changed significantly in their feeding rate between years, but most individuals did not change in their feeding rate relative to others in the population. The absolute feeding rate of an individual was significantly positively correlated with its feeding rate in the subsequent year. There was no significant variation in susceptibility to interference for individuals between years. Only two of 27 blackbirds showed a significant change in susceptibility to interference between years. Relative absolute foraging rate and susceptibility to interference competition were reasonably similar between years in blackbirds. Relative fitness measures derived from short‐term measures of foraging ability may therefore be valid over long periods. In ideal free models that incorporate individual competitive ability, rather than population averages, temporal changes in competitive ability can possibly be ignored.

The Dispersal System of a Butterfly: A Test of Source-Sink Theory Suggests the Intermediate-Scale Hypothesis.

Theory predicts source-sink dynamics can occur in species with the ideal preemptive distribution but not with the ideal free distribution. Source-sink dynamics can also occur in species with passive dispersal, in which a fixed fraction of the population disperses each generation. However, in nature, dispersal often approximates random diffusion rather than ideal choices or fixed probabilities. Here, I ask which dispersal system occurred in a butterfly (Euphydryas editha) known to have source-sink dynamics. The study used 13 experimental sites, where vacant and occupied habitat patches were juxtaposed. I estimated movement during the flight season and tested hypotheses about the type of dispersal system. Ideal free and ideal preemptive models were rejected because per capita movement rates were density independent. Passive dispersal was rejected because per capita rates were related to patch area and habitat preference. The diffusion model best explained the data because it predicted both the area relationship and an odd feature of the habitat preference: immigration was not higher in preferred habitat; rather, emigration was lower. The diffusion model implied that source-sink dynamics were driven by diffusion from areas of high to low population density. Existing source-sink theory assumes fine-scale patchiness, in which animals have perfect knowledge and ease of mobility. The results from the butterfly suggest that source-sink dynamics arise at coarser spatial scales, where diffusion models apply.

The influence of sediment type on the aggregative response of oystercatchers, Haematopus ostralegus, searching for cockles, Cerastoderma edule

Models that describe the dispersion patterns of predators between a series of patches that vary in prey density frequently assume that predators, in the absence of interference, will aggregate in patches with the highest prey density, at any point in time. This assumption has important implications for patterns of prey mortality, and the extent to which prey mortality is density dependent. In natural predator-prey systems, it is likely that environmental factors interact with spatial variation in prey density to influence the aggregative response of predators. We show data consistent with this idea on a population of overwintering oystercatchers foraging on cockles. There was no evidence that birds aggregated in patches with the highest biomass density of cockles. The biomass density of cockles was highest in muddy patches at the start of winter, and birds aggregated in patches that switched from being muddy at the start of winter to being sandy at some point during the winter. We argue that sediment type influences foraging costs experienced by the birds, so birds avoid feeding in muddy patches unless the fine sediment is removed from a patch, as happens during winter storms. When this happens a high biomass density of cockles suddenly becomes available and the birds aggregate in such patches. The rate of biomass loss was greatest in patches used intensively by birds for feeding, suggesting that the birds’ aggregative response influences cockle mortality. We discuss the implications of our results for ideal free models.

The Dispersal System of a Butterfly: A Test of Source-Sink Theory Suggests the Intermediate-Scale Hypothesis

Theory predicts source‐sink dynamics can occur in species with the ideal preemptive distribution but not with the ideal free distribution. Source‐sink dynamics can also occur in species with passive dispersal, in which a fixed fraction of the population disperses each generation. However, in nature, dispersal often approximates random diffusion rather than ideal choices or fixed probabilities. Here, I ask which dispersal system occurred in a butterfly (Euphydryas editha) known to have source‐sink dynamics. The study used 13 experimental sites, where vacant and occupied habitat patches were juxtaposed. I estimated movement during the flight season and tested hypotheses about the type of dispersal system. Ideal free and ideal preemptive models were rejected because per capita movement rates were density independent. Passive dispersal was rejected because per capita rates were related to patch area and habitat preference. The diffusion model best explained the data because it predicted both the area relationship and an odd feature of the habitat preference: immigration was not higher in preferred habitat; rather, emigration was lower. The diffusion model implied that source‐sink dynamics were driven by diffusion from areas of high to low population density. Existing source‐sink theory assumes fine‐scale patchiness, in which animals have perfect knowledge and ease of mobility. The results from the butterfly suggest that source‐sink dynamics arise at coarser spatial scales, where diffusion models apply.

The ideal free distribution in humans: an experimental test.

How the distribution of resources affects the spatial distribution of animals is a central concern of behavioral ecology. One influential model relating population dynamics to individual foraging behavior is that of ideal free distribution (Fretwell & Lucas, 1970). This model assumes foragers of equal competitive abilities, moving freely from one habitat to another, the choices made by each individual subject are supposed to equalize gains across habitats. The resulting distribution at the group level, or ideal free distribution, has been tested with various animal species. Here we report an experimental test with human subjects competing for money. The results approximate those predicted by the ideal free model, the degree of approximation being consistent with that obtained in other species. This similarity of results supports the application of behavioral ecology models to humanperformance.

Interference with ideal free models

Interference with ideal free models

Unequal competitor ideal free distribution in fish?

Key predictions of unequal competitor ideal free distribution models were tested using a continuous input situation. Ten individually identifiable cichlid fish competed for food items at either end of their tank. Their distribution fitted the predictions of the equal competitor, continuous input ideal free model almost perfectly. However, examination of individual intakes revealed significant variation in individual success and relative competitive ability between patches. Contrary to expectations, fish did not exclusively use the patch where their intake was higher, although individuals experiencing greater differences in intake rate between patches were more selective. We found no evidence for a truncated distribution or even a correlation between competitive ability and patch quality. Changing the input regime to reduce competition did not produce a decrease in the range of intake rates between individuals. This study indicates the value of future empirical and theoretical work on how relative competitive ability varies with the nature of the foraging environment.

Habitat selection and breeding success in Yellow‐legged Gulls Larus cachinnans

The habitat selection and breeding performance of Yellow‐legged Gulls Larus cachinnans were studied in the Medes Islands colony, northeastern Spain, during 1995 and 1996. Of the three main habitats on the islands (shrubs, grass and bare areas), gulls first occupied those with the highest percentage of tall vegetation. Gulls tended to select nest sites with 20–75% cover despite great differences in the cover in the habitats and territories, suggesting that the presence of a suitable nest site may play a major role in the choice of breeding habitat. Nest‐site tenacity did not influence the preferences of gulls at any level since the same pattern of choice was observed in an area subjected to annual culls (i.e. where most of the breeding pairs were culled annually and replaced by naive birds). In spite of great differences in the physical characteristics of the habitats, little difference was found in breeding performance of the gulls between habitats. Gulls nesting in the least preferred habitat (i.e. mainly bare) had smaller clutches than those nesting in the other two habitats, possibly as a result of their later seasonal laying. Despite the similar breeding success in different habitats, gulls did not seem to distribute according to the ideal free model reported for Herring Gulls Larus argentatus since the density in the preferred habitat (i.e. shrubs) was never higher than in the other two. We suggest that the habitat selection by Yellow‐legged Gulls within the colony could follow an ideal despotic distribution.

The Ideal Free Distribution When Predators Differ in Competitive Abilities

The basic ideal free model (all individuals are alike, omniscient, and free to move between patches) predicts the spatial distribution of animals in a patchy environment. The notion that ideal free models for individuals that differ in competitive ability, predict a distribution that deviates basically from the ideal free distribution of equal competitors is widespread. In this paper I present a distribution model in which differences in competitive ability among individuals are assumed. The model. however, predicts a distribution equal to the ideal free distribution of competitors that are all alike. The model belongs to a class of models in which an individual's payoff depends on the sum of the amount of interference it experiences from each of the competitors in the patch. The amount of interference that a competitor experiences from another competitor can vary. The interference relationship between all pairs of individuals is presented in the form of an 'interference matrix'. Several possible interference matrices are presented. All models assume that an individual's payoff will never increase with increasing numbers of competitors in the patch, and that an animal will experience more interference when surrounded with relatively strong competitors than when surrounded with weaklings. I show that models which have previously predicted distributions radically different from the ideal free distribution of equal competitors, i.e. the truncated phenotype distribution, have made the same claims (i.e. that an individual's payoff will never increase with increasing numbers of competitors) for the wrong reasons.

The Demography of Blackbirds Turdus merula in Rural Habitats: Is Farmland a Sub-Optimal Habitat?

1. A population of 54-76 pairs of European blackbirds Turdus merula L. encompassing woodland, farmland and edge (wood/farm boundary) habitats was studied to investigate whether farmland is a sub-optimal habitat for a bird species whose ancestral habitat was woodland. 2. Woodland was occupied at a higher density than farmland, but there was also variation within habitats: 'hot-spot' areas of woodland were occupied at high density, 'sparse' areas at low density, while other areas were completely unoccupied. 3. There was no effect of habitat or territory type on survival, although movements between years were more likely to be from farmland to woodland, and from sparse to hot-spot territories than vice versa. 4. There was a higher proportion of young males in farmland than in woodland, and in sparse territories compared to hot-spot territories; but there was no difference in the age structure of females across habitats. 5. Timing of breeding and clutch size did not differ between habitats or territory type, but farmland females laid smaller eggs than other females. 6. Reproductive success was low in all habitats (only 19% of attempts produced any fledglings), but there were significant differences between habitats in various measures of reproductive success. Success in farmland and woodland was similar, but hot-spot territories tended to be more successful than sparse territories. Predation was the major cause of failure. 7. Our results indicate that farmland blackbirds exhibit certain characteristics of populations in sub-optimal habitat, but the effect of habitat on reproductive success probably operated at a finer scale than this relatively crude habitat classification. 8. The distribution of breeding blackbirds is discussed in relation to ideal free and source-sink models.

Competition for Food in Swans: An Experimental Test of the Truncated Phenotype Distribution

Ideal free models for unequal competitors predict a truncated competitor phenotype distribution among patches when relative payoffs of phenotypes vary across patches. Partial truncation is expected under field conditions. This prediction was tested in a field experiment with an overwintering population of mute swans (Cygnus olor). Two food patches were generated in which adult and subadult swans were expected to have different relative success. In one patch ('clumped'), pieces of bread were thrown over a small area on the water surface. In the other patch, pieces of bread were scattered over a larger area. When only one patch was offered at a time (no choice situation), adult swans were more successful than subadult swans in the 'clumped' patch, but were similarly successful in the 'scattered' patch. Relative payoffs of adult and subadult swans differed significantly between patches. When the two patches were offered simultaneously, black-headed gulls (Larus ridibundus) competed with the swans to a considerable extent in some replicates. The gulls appeared to be the poorest competitors. They snatched more bread in the scattered than in the clumped patch. Both classes of swans avoided the scattered patch but not the clumped patch with increasing competition from gulls. Gulls preferred the scattered patch and swans increasingly preferred the clumped patch under gull competition, creating a partially truncated distribution. Without the four replicates in which the gulls had consumed more than 25% of the bread, the percentage of adult swans that chose the clumped patch was significantly higher than the percentage of subadult swans that chose that patch. This is the first experimental verification of a partially truncated phenotype distribution. Subadult swans significantly preferred the scattered patch whereas adults tended to prefer the clumped patch. This distribution was predicted from the 'no choice' experiment, where competition by gulls had been similarly weak.

Effects of Food Density on the Behavior and Distribution of Nonbreeding American Flamingos in Yucatan, Mexico

We sampled potential food items of American Flamingos (Phoenicopterus ruber ruber) in the Celestun Estuary, Yucatan, Mexico, from October 1992 to January 1993, to explore the influence of food density and distribution on flamingo behavior in relation to the ideal free distribution model. Food items were found in the substrate and vegetation and were dominated by four groups: gastropods (40%), muskgrass bulbils (26%), crustaceans (11%), and chironomids (10%). Overall, food was patchily distributed and food abundance decreased over time. Flocks initially concentrated in areas where food was most abundant, but appeared to deplete food resources in these areas after which they dispersed throughout the Celestun Estuary. Time spent feeding also was lower in the first two time blocks (χ = 26%) when food was most abundant, and then increased and remained constant (56%) in the remaining five time blocks. Mean flock size was 457, and did not differ among locations through time, except in the last time block when flocks tended to be larger (χ = 821). We concluded that the distribution of flamingo flocks was consistent with the ideal free model based on food density and distribution, but that major perturbations (storms) can dramatically affect flamingo distribution. However, relative payoff for individuals within flocks has yet to be determined