No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Travel Cost and the Ideal Free Distribution
... In particular, many of these models intend to relax the ideal and free assumptions of the original IFD by taking into account individual and environmental characteristics, 5 which are essential in understanding the underlying dynamics of the entire group [13]. For instance, in [14,15], the authors discuss the concept of travel constraints in IFD models. Likewise, the authors of [16] consider how the cost of traveling between resource sites might diminish the expected benefits of moving to another site. ...
... "Free" indicates that the nodes of the network can connect instantaneously and at no cost to any other node. Although our framework assumes that both assumptions will hold, we have seen that there is an extensive literature that shows how an IFD may still be achived even when one or both assumptions are relaxed (e.g., see [13,14,16,57,58,59,60,61]). Our work here focuses on understanding the general idea behind the emergence of IFD distributions and scale-free networks. ...
... Motivated by this particular scenario we will then introduce a generic discrete model for the study of the behavior of a spatially distributed group of heterogeneous agents. We will assume that they have only noisy perceptions about the suitability levels they can sense, and rely on them to untangle the "multi-agent to multi-area" coupling described above (thereby addressing key challenges in studies like [23,65,66,67] for vehicles and [14,15,16] for animals). Our theoretical results will prove that despite having noisy perceptions an approximate IFD pattern will emerge (i.e., where all nodes have approximately equal suitability). ...
... The key assumptions of the IFD are that the cost of movement is negligible, and foragers are omniscient about resource distributions. If these assumptions are violated, distribution of pollinators is predicted to deviate from the IFD 20,29,30 . The cost of movement and the perception of resource distribution may depend on the distance between clusters of resources 20,30 . ...
Flowers cluster at various spatial scales, so pollinators use information from multiple scales when foraging in natural plant populations. Little is known about the effects of interactions between scales or their relative strength. We examined bumblebee foraging behaviour in a natural population of Salvia nipponica in 10 and 7 patches in 2019 and 2020, respectively. We recorded within-patch factors (display size of racemes and local open raceme densities) and patch-level factors (patch size and distance from the nearest patch) and analysed their relationships with pollinator behaviour. The numbers of visits per raceme and flower were mainly affected by the interaction of patch size and raceme density; they were higher in locations with lower raceme density in larger patches. The ratio of flowers visited to all open flowers in a raceme during a raceme visit, which relates to a bumblebee’s choice to leave a raceme, was mainly affected by the interaction of display size and local open raceme density; in 2019 it was higher in racemes with smaller display sizes, while in 2020 the strength and direction of the relationship depended on the open raceme density. These results suggest that pollinators relied on the sizes of flower clusters at different spatial scales when visiting and leaving racemes and adjusted their responses to the sizes of flower clusters depending on the distances between clusters. Therefore, it is important to evaluate factors at various spatial scales and their interactions to fully understand pollinator behaviour in natural plant populations.
... [57]) or cause cyclical dynamics [58]. We anticipate that these effects would reduce the overall movement of consumers (increasing their retention in resident patch(es)) but would not qualitatively alter the relative distribution of consumers among patches (see [59]). Given that consumer demography itself can produce IFD-like patterns (figure 5), we suspect that the incorporation of consumer demography should tend to lessen any additional departures from IFD arising from travel costs or delayed movement responses. ...
An animal's movement rate (mobility) and its ability to perceive fitness gradients (fitness sensitivity) determine how well it can exploit resources. Previous models have examined mobility and fitness sensitivity separately and found that mobility, modelled as random movement, prevents animals from staying in high-quality patches, leading to a departure from an ideal free distribution (IFD). However, empirical work shows that animals with higher mobility can more effectively collect environmental information and better sense patch quality, especially when the environment is frequently changed by human activities. Here, we model, for the first time, this positive correlation between mobility and fitness sensitivity and measure its consequences for the populations of a consumer and its resource. In the absence of consumer demography, mobility alone had no effect on system equilibria, but a positive correlation between mobility and fitness sensitivity could produce an IFD. In the presence of consumer demography, lower levels of mobility prevented the system from approaching an IFD due to the mixing of consumers between patches. However, when positively correlated with fitness sensitivity, high mobility led to an IFD. Our study demonstrates that the expected covariation of animal movement attributes can drive broadly theorized consumer-resource patterns across space and time and could underlie the role of consumers in driving spatial heterogeneity in resource abundance.
... Although HBE was initially applied primarily to mobile hunter-gatherer populations, more complex societies have increasingly become the subject of similar models in the past several decades (e.g., Smith and Winterhalder, 1992;Winterhalder and Smith, 2000;Kennett and Winterhalder, 2006;Codding and Bird, 2015). One HBE model that has received increasing attention by archaeologists has been the ideal free distribution or IFD (Åström, 1994;Fretwell and Lucas, 1969;Fretwell, 1972;Sutherland, 1983Sutherland, , 1996Treganza, 1995), which is well-suited for understanding the effects of a range of environmental and socioeconomic factors on human settlement and broader patterns of decision-making and culture change (e.g., Kennett, 2005;Kennett et al., 2006Kennett et al., , 2009Kennett and Winterhalder, 2008;McClure et al., 2009;Winterhalder et al., 2010;Culleton, 2012;O'Connell and Allen, 2012;Jazwa et al., 2013Jazwa et al., , 2016bCodding and Jones, 2013;Giovas and Fitzpatrick, 2014;Moritz et al., 2014;Codding and Bird, 2015;Jazwa, 2015). In this paper, we expand the reach of this model to test predictions about patterns in the expansion and contraction in the number and distribution of settlement sites in Bronze Age (ca. ...
In this paper, we model patterns of expansion and contraction of settlement in Bronze Age Messenia (ca. 3100–1050 BCE) using the ideal free distribution (IFD). We rank potential settlement locations on the landscape using environmental and cultural variables, including watershed size, which is a proxy for fresh water availability, net primary productivity, and the distance to the location of the Palace of Nestor at Pylos, a central site of political importance throughout most of the Bronze Age. The settlement chronology for the region is derived from an existing database of survey data and conforms to the predictions of the IFD. The highest-ranked habitats were settled first and the population expanded to lower-ranked habitats as population density increased. The data also conforms to the IFD prediction that important political centers should follow the same pattern, expanding from higher- to lower-ranked habitats. This model helps to provide an understanding of the effects of the unique environmental and cultural landscape of the region on settlement prior to the rise of the major Mycenaean centers in mainland Greece. The application of the IFD to a sociopolitically complex case demonstrates its broad potential for understanding historical trajectories in settlement patterns throughout the world.
... The IFD has been tested extensively in birds (e.g., Shochat et al. 2002;Jensen and Cully 2005;Zimmerman et al. 2009), mammals (e.g., Morris 1988;Ovadia and Abramsky 1995;Lin and Batzli 2002;Tadesse and Kotler 2010), fish (e.g., Rodríguez 1995;Morita et al. 2004;Haugen et al. 2006;Knight et al. 2008), and invertebrates (e.g., Krasnov et al. 2003Krasnov et al. , 2004Lerner et al. 2011) and has also been used to examine optimal foraging behavior (see Kennedy and Gray 1993 for a review). Despite the unrealistic 'free' and 'ideal' assumptions of the IFD (see review in Kennedy and Gray 1993; but see also Åström 1994 andMilinksi 1994), the IFD remains an important and powerful theoretical framework to predict the habitat selection of organisms. ...
The ideal free distribution concept predicts that organisms will distribute themselves between habitats in a density-dependent manner so that individuals, on average, achieve the same fitness in each habitat. In ectotherms, environmental temperature has a strong impact on fitness, but temperature is not depletable and thus not density dependent. Can density-dependent habitat selection occur in ectotherms when habitats differ in thermal quality? We used an observational study of habitat selection by small snakes in field and forest, followed by manipulative habitat selection and fitness experiments with common gartersnakes in enclosures in field and forest to test this hypothesis. Snakes were much more abundant in the field, the habitat with superior thermal quality, than in the forest. Gartersnakes in our controlled experiment only used the forest habitat when snake density was highest and when food was more abundant in the forest; habitat selection was largely density independent, although there was weak evidence of density dependence. No female gartersnake gave birth in the forest enclosures, whereas half of the females gave birth in the field enclosures. Growth rates of females were higher in field than in forest enclosures. Overall, our data indicate that temperature appears to be the most important factor driving the habitat selection of gartersnakes, likely because temperature was more limiting than food in our study system. Snakes, or at least temperate snakes, may naturally exist at population densities low enough that they do not exhibit density-dependent habitat selection.
... The global stability was studied by Lou and Wu [28]. We also refer to [1,7,18,22,30,33,36,37] and references therein for studies including travel loss. ...
The dispersal of organisms plays an important role in determining the dynamics of ecological models. Ecologically, it is of interest in understanding how dispersal strategy influences the distribution of populations. An ideal free distribution (IFD) of populations has been used to predict the distribution of organisms among patches, where a key assumption is to assume that species can move freely between patches without paying any cost. If instead one assumes that there are losses when species moves from one patch to another, then ideal free distributions may not appear. In this note, we examine a two-patch resident-mutant model with travel loss and predict the optimal dispersal strategy for resident and mutant. Moreover, such strategy which produces a non-IFD is evolutionarily stable. Some same and different features of patch models with travel loss are discussed.
... The location of larger sites with permanent or semipermanent occupation usually takes into account a suite of environmental and socioeconomic factors. The ideal free distribution or IFD (Åström 1994;Fretwell and Lucas 1969;Fretwell 1972;Sutherland 1983Sutherland , 1996Treganza 1995) is used in behavioral ecology and increasingly in anthropology and archaeology to understand how such factors affect human settlement and broader patterns of decision-making and culture change (e.g., Kennett 2005;Kennett et al. 2006Kennett et al. , 2009Kennett and Winterhalder 2008;McClure et al. 2009;Winterhalder et al. 2010;Culleton 2012;O'Connell and Allen 2012;Codding and Jones 2013;Giovas and Fitzpatrick 2014;Moritz et al. 2014;Codding and Bird 2015). ...
Using targeted survey, excavation, and radiocarbon dating, we assess the extent to which human settlement patterns on California's northern Channel Islands fit predictions arising from the ideal free distribution (IFD): (1) people first established and expanded permanent settlements in the regions ranked high for environmental resource suitability; (2) as population grew, they settled in progressively lower ranked habitats; and (3) changes in the archaeological record associated with high population levels such as increases in faunal diversity and evenness in high-ranked habitats are coinci-dent with the expansion to other areas. On Santa Rosa Island, the early permanent settlements were located in both high-and middle-ranked locations, with the most extensive settlement at the highest ranked locations and only isolated sites elsewhere. Settlement at a low-ranked habitat was confined to the late Holocene (after 3600 cal BP). Drought influenced the relative rank of different locations, which is an example of climate adding a temporal dimension to the model that episodically stimulated population movement and habitat abandonment. Because the IFD includes a wide range of cultural and environmental variables, it has the potential to be a central model for guiding archaeological analysis and targeted field research.
... Using analytical models, both Kennedy and Gray (1993) and Morris (1987) predicted that with increasing travel costs the distribution of foragers among spatially structured resources becomes more extreme relative to the IFD (with richer patches attracting more foragers). However, the model by Kennedy and Gray (1993) was criticised, mainly because they extended a model based on individual behaviours to a group (Milinski 1994, Åström 1994, Lessells 1995. Also, conclusions reported by Morris (1987) were based on the assumption of unidirectional migration from a richer habitat to a poorer one; a simple extension of his model by Åström (1994) later showed that eff ects of travelling costs on bidirectional movements could potentially cancel out. ...
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.
... We agree that this model is flawed but not for the reasons outlined by Milinski. Astrom (1994) has pointed out that, in our enthusiasm for analogies between the IFD and the matching law we inappropriately extended a model based on the behaviour of an individual animal to a group. In our analysis of travel costs we deducted the costs of switching from a site from the foragers that remained in that site. ...
... " Free " indicates that the nodes of the network can connect instantaneously and at no cost to any other node. Although our framework assumes that both assumptions will hold, there is an extensive literature that shows how an IFD may still be achived even when one or both assumptions are relaxed (e.g., see [15][21]). It is, however, the general idea behind the IFD concept and the emergence of scale-free networks that is the most important and novel contribution of this paper. ...
This letter presents a class of network optimization processes that account for the emergence of scale-free network structures. We introduce a mathematical framework that captures the connectivity and growth dynamics of a network with an arbitrary initial topology. We show how selection via differential node fitness affects the proportion of connections a node makes to other nodes, and how a heavy-tailed connectivity behavior manifests itself from consecutive achievements of ideal free distributions (IFDs). Finally, we present simulation results that show how this class of networks may emerge even when consecutive IFDs are not perfectly reached.
... These violations include unequal competitive abilities of the foragers (Parker and Sutherland, 1986;Houston and McNamara, 1988), interference (Sutherland, 1983), and imperfect knowledge of resource distribution (Abrahams, 1986). Kennedy and Gray (1993) also suggested that travel costs may cause deviations from the IFD (but see Astrom, 1994). ...
We studied the distribution of migratory warblers (genus: Sylvia) in poor and high quality habitat patches at a stopover site in the northern Negev, Israel. The purpose of our study was to test predictions based on the ideal free distribution (IFD) model by using a natural ecosystem which has a high turnover of individuals moving between unfamiliar foraging patches. We trapped birds in two groves of Pistacia atlantica embedded within a coniferous forest. The fruit-density ratio between these groves was 45:1. We compared bird density, body condition and habitat matching (the ratio between bird density and resource density) at the two sites. To analyse the data we integrated two approaches to density-dependent habitat selection: the isodar method and the habitat matching rule. As predicted by the IFD model, we found that habitat suitability decreased with bird density with a high correlation between warbler densities in the two habitat patches. Contrary to IFD predictions, warbler density in the poor patch was higher than expected by the habitat-matching rule. This habitat under-matching, had a cost: in the rich habitat the average energy gain per individual bird was higher than in the poor habitat. Further analysis suggests that the apparent habitat under-matching is not due to interference or differences in warbler competitive abilities. Therefore, we suggest that this migratory bird community is not at equilibrium because the birds possess imperfect knowledge of resource distribution. We propose that this lack of knowledge leads to free, but not ideal distributions of migrant birds in unfamiliar stop over sites.
... In particular, many of these models try to relax the ideal and free assumptions of the IFD by taking into account individual and environmental characteristics, which are essential in understanding the underlying dynamics of the entire group. For instance, in [3] the authors discuss the concept of travel cost and constraints in IFD models (e.g., they consider how the cost of traveling between habitats might diminish the expected benefits of moving to another habitat). Here, the IFD model we introduce extends the one in [4], [5]. ...
A mathematical model is introduced for the study of the behavior of a spatially distributed group of heterogenous agents which possess noisy assessments of the state of their immediate surroundings. We define general sensing and motion conditions on the agents that guarantee the emergence of a type of "ideal free distribution" (IFD) across the environment, and focus on how individual and environmental characteristics affect this distribution. In particular, we show the impact of the agents' maneuvering and sensing abilities for different classes of environments, and how spatial constraints of the environment affect the rate at which the distribution is achieved. Finally, we apply this model to a cooperative vehicle control problem and present simulation results that show the benefits of an IFD-based distributed decision-making strategy
... That is to say, at an IFD, there may exist a fitness difference between patches that is less than or equal to the fitness cost one would pay to travel between patches (Morris 1987; DeAngelis et al. 2011 ). Several studies of the effects of travel costs on movement (Rosenzweig 1974; Charnov 1976; Rosenzweig 1981) between patches have relied on this definition of the IFD (Morris 1987; Kennedy and Gray 1993; Astrom 1994; Kirkland et al. 2006; Matsumura et al. 2010; DeAngelis et al. 2011). This differs from the second notion of " equal fitness between patches " where, although an organism will certainly stop once the IFD is reached, it may in fact stop before the IFD is reached if travel costs are sufficiently high. ...
Abstract Movements made by real organisms--such as movements involved in dispersal, migration, and habitat selection--are expected to occasionally be suboptimal because of realistic constraints imposed by incomplete information, perceptual limitations, and stochasticity. Previous theory considering such constraints has shown that movements appropriately conditioned on habitat or resource characteristics can balance out suboptimal components of movement and thereby lead organisms to ideal free distributions and fitness maxima, whereas movements conditioned on fitness differentials cannot. These findings suggest a somewhat paradoxical hypothesis: even if organisms have information about their fitness, movement strategies that maximize fitness may be conditioned on something other than fitness per se. We test this hypothesis by investigating the evolutionary stability of generalized, conditional movement strategies that vary in their use of information on fitness versus information on habitat characteristics. We show that when costs of sensory machinery are included, natural selection should favor movement strategies that completely ignore fitness information. Finally, we synthesize previous work by showing how several previous important theoretical results for adaptive movement strategies are united under our one general model.
... IFD assumes that animals are free to move between habitats. However, dispersal has costs and constraints (Bernstein et al. 1991b;Åström 1994;Kennedy and Gray 1997), so distant habitat patches are harder to reach than close ones. In addition, the random probability of Wnding a distant patch is lower. ...
The ideal free distribution model incorporating the Allee effect was published by Fretwell and Lucas (1970), but went almost unnoticed within the ecological literature. The model is relevant to populations distributed among patchy habitats. It predicts a sporadic but substantial decline in populations at high densities, which in turn induces the rapid growth of new populations. In this paper, I show that the simple process explained by this model can be used to change our view of several phenomena within the field of population ecology, behavioural ecology and conservation. The ecological consequences of the model are well known. A key feature of Fretwell and Lucas's model is what I call the "Allee paradox:" there is a range of local population densities at which local individual fitness is less than the potential mean gain that could be obtained in the environment; however, individuals cannot disperse. This paradox can be used to explain why helping appears before suitable breeding areas are fully occupied, and why breeding females aggregate when male coercion is a reproductive cost. The model also predicts high clustering between related populations, and, in conservation biology, it can identify unfounded concerns about the dangers of extinction, delays in recolonisation processes after human-induced population decline, and latency periods in the initial phases of expansion of invasive species.
... We have chosen to model the simplest system that allows us to examine the links between unequal competitor and perceptual constraint approaches. There are many complications that have been added to the IFD (for reviews, see Tregenza 1995;Giraldeau and Caraco 2000), including interference competition (Sutherland 1983), travel costs (Korona 1990;Å ström 1994;Kennedy and Gray 1997), resource dynamics (Lessells 1995), predation risk (Giske et al. 1997), and spatially explicit resource dynamics (Stephens and Stevens 2001;Kshatriya and Cosner 2002). These are all worthy complications to be added once a simpler approach has been explored. ...
Understanding and predicting the spatial distribution of social foragers among patchily distributed resources is a problem that has been addressed with numerous approaches over the 30 yr since the ideal free distribution (IFD) was first introduced. The two main approaches involve perceptual constraints and unequal competitors. Here we present a model of social foragers choosing among resource patches. Each forager makes a probabilistic choice on the basis of the information acquired through past foraging experiences. Food acquisition is determined by the forager's competitive ability. This model predicts that perceptual constraints have a greater influence on the spatial distribution of foragers than unequal competitive abilities but that competitive ability plays an important role in determining an individual's information state and behavior. Better competitors have access to more information; consequently, we find that competitive abilities and perceptual constraints are integrated through the social environment occupied by individual foragers. Relative competitive abilities influence the forager's information state, and the ability to use information determines the resulting spatial distribution.
... " Free " indicates that the nodes of the network can connect instantaneously and at no cost to any other node. Although our framework assumes that both assumptions will hold, there is an extensive literature that shows how an IFD may still be achieved even when one or both assumptions are relaxed (e.g., see [16], [11], [17]). It is, however, the general idea behind the IFD concept and the emergence of scale-free networks that is the most important and novel contribution of this paper. ...
This paper presents a class of network optimization processes that account for the emergence of scale-free network structures. We introduce a mathematical framework that captures the connectivity and growth dynamics of a network with an arbitrary initial topology. We show how selection via differential node fitness affects the proportion of connections a node makes to other nodes, and how a heavy-tailed connectivity behavior manifests itself from consecutive achievements of ideal free distributions (IFDs). Finally, we present simulation results that show how this class of networks may emerge even when consecutive IFDs are not perfectly reached.
This study is an application of the ideal free distribution (IFD) model to understand settlement patterns during the Mauretanian and Roman periods in northwest Morocco. We use proxies for potential agricultural productivity and distance from a viable harbor for each of the locations of known settlement sites. There is a clear relationship between the distance from harbors and habitat suitability, with sites that were established earliest all on or close to a point of access to the ocean. This is consistent with a focus on maritime trade and a connection with the Mediterranean Sea. Agricultural potential is also an important factor influencing settlement, although to a lesser extent, with earlier, low-density settlement more focused on trade potential. Consistent with predictions of the IFD, there is an expansion through time to progressively lower ranked habitats, with persistent settlement at the highest ranked habitats, including at Lixus, which is generally considered to be the earliest city in northwest Africa. During the Roman period, there is a focus of interior settlement near Volubilis. Subsidiary sites likely benefit from their proximity to this important political site, despite their relatively low rank within the IFD, perhaps reflecting patterns consistent with an ideal despotic distribution. This study demonstrates how the IFD can be used in a simple context with limited available survey data to assess the important environmental variables influencing settlement decisions. It has broad implications for understanding the formation of urban centers prior to and during the Roman period.
The ideal free distribution (IFD) theory, which predicts that a population of individuals will match the distribution of a patchily distributed resource, is widely used in ecology to describe the spatial distribution of animals. While many studies have shown general support of its habitat matching prediction, others have described a systematic pattern of undermatching, where too many animals feed at patches with fewer resources, and too few animals feed in richer patches. These results have been attributed to deviations from several of the assumptions of the IFD. One possible variable, the cost of travelling between patches, has received little attention. Here, we investigated the impact on resource matching when travel costs were manipulated in a simple laboratory experiment involving two continuous input patches. This experiment allowed us to control for extraneous variables and decouple time costs from energetic costs of travel. Two experiments examined the impact of varying travel costs on movement rates between foraging patches and how these travel costs impact conformity to the IFD. Our data demonstrated that there was less movement between patches and greater discrepancies from the IFD predictions as the cost of travel increased.
Changes in social organization accelerated on California's northern Channel Islands beginning around 1300 cal BP. These changes were associated with shifts in settlement and subsistence patterns related in part to drought conditions during the Medieval Climatic Anomaly (MCA; 1150-600 cal BP). By the end of the MCA, settlement patterns demonstrate evidence for territoriality and can be described by the ideal despotic distribution. The occupants of the most productive habitats prevented new settlers from moving in and accessing the available resources. We use faunal data from five sites on western Santa Rosa Island (CA-SRI-15, -31, -97, -313, and -333) to trace changes in settlement and population aggregation through this period. Fishing, which can support higher population densities than harvesting shellfish, increased overall from the Middle (2550-800 cal BP) to Late Period (650-168 cal BP), but there were fewer settlement sites on western Santa Rosa Island. In the centuries before the Middle to Late Period Transition (MLT; 800-650 cal BP), people occupied sites geographically dispersed along the west coast of the island. After the MLT, fishing was restricted to fewer large coastal villages. We argue that environmental stress and an increase in warfare on the northern Channel Islands drove the growth of more permanent consolidated villages, the development of territoriality, and settlement patterns consistent with greater resource defense and therefore a more despotic distribution.
The ideal free distribution (IFD) predicts that optimal foragers will select foraging patches to maximize food rewards and that groups of foragers should thus be distributed between food patches in proportion to the availability of food in those patches. Because many of the underlying mechanisms of foraging are temperature dependent in ectotherms, the distribution of ectothermic foragers between food patches may similarly depend on temperature because the difference in fitness rewards between these patches may change with temperature. We tested the hypothesis that the distribution of Common Gartersnakes (Thamnophis sirtalis) between food patches can be explained by an IFD, but that conformance to an IFD weakens as temperature departs from the optimal temperature because fitness rewards, interference competition and the number of individuals foraging are highest at the optimal temperature. First, we determined the optimal temperature for foraging. Second, we examined group foraging at three temperatures and three density treatments. Search time was optimized at 27°C, handling time at 29°C and digestion time at 32°C. Gartersnakes did not match an IFD at any temperature, but their distribution did change with temperature: snakes at 20°C and at 30°C selected both food patches equally, while snakes at 25°C selected the low food patch more at low density and the high food patch more at high density. Food consumption and competition increased with temperature, and handling time decreased with temperature. Temperature therefore had a strong impact on foraging, but did not affect the IFD. Future work should examine temperature-dependent foraging in ectotherms that are known to match an IFD.
The ideal free distribution (IFD) predicts that individuals should be distributed between habitats in proportion to habitat suitability such that mean fitness is equal in each habitat. The IFD is useful in studies examining habitat selection, yet its key assumptions are often violated and the expected IFDs are not consistently detected. While the use of aggregation pheromones by insects is expected to evolve in systems that experience positive density dependence (Allee effect), through a series of experiments we test the hypothesis that aggregation pheromones may limit the ability of individuals to achieve an IFD. Using red flour beetles (Tribolium castaneum), we specifically test the prediction that beetles in groups with an equal sex ratio or in the presence of an artificial aggregation pheromone will deviate from an IFD, whereas female-only groups will achieve an IFD. We also test the hypothesis that aggregation pheromones evolved to promote Allee effects by testing the prediction that beetle fitness will show positive density dependence at low densities. Consistent with our first hypothesis, female groups achieved an IFD, while mixed sex groups and females in the presence of an aggregation pheromone in the low food habitat under-matched the IFD. We found no evidence of Allee effects at low density, but we did find evidence of strong negative density dependence. We demonstrate that the use of aggregation pheromones may negatively impact a population’s ability to achieve an IFD and cast doubt on the hypothesis that male aggregation pheromones evolve to promote Allee effects.
Long-distance migration is a widespread process evolved independently in several animal groups in terrestrial and marine ecosystems. Many factors contribute to the migration process and of primary importance are intra-specific competition and seasonality in the resource distribution. Adaptive migration in direction of increasing fitness should lead to the ideal free distribution (IFD) which is the evolutionary stable strategy of the habitat selection game. We introduce a migration game which focuses on migrating dynamics leading to the IFD for age-structured populations and in time varying habitats, where dispersal is costly. The model predicts migration dynamics between these habitats and the corresponding population distribution. When applied to Atlantic bluefin tunas, it predicts their migration routes and their seasonal distribution. The largest biomass is located in the spawning areas which have also the largest diversity in the age-structure. Distant feeding areas are occupied on a seasonal base and often by larger individuals, in agreement with empirical observations. Moreover, we show that only a selected number of migratory routes emerge as those effectively used by tunas.
Variability in the spatial and temporal patterns of stream biota has been well documented, yet the causes and consequences of this variability are poorly understood. Most early work focused on how changes in mean environmental parameters over space or time influence the abundance or distribution patterns of biota. We argue for an increased focus on the study of variability in these parameters. To date, only a few lotic studies have used variance as a dependent variable i.e., showing that changes in the variance in some parameter can be related to changes in the mean of that or another parameter. These studies have suggested interesting links between ecological patterns and processes at a variety of scales. The use of variance as an independent variable and its experimental manipulation (i.e., hold the mean of some parameter constant while altering its variance) may also lead to new understandings of how pattern and process are linked. We begin by outlining when and why the study of variance may be important to lotic ecology. Next, we suggest a 3-step process in the use of variance in stream studies: 1) documentation of a pattern with respect to variance; 2) examination of descriptive data to explore potential causes of the pattern; and 3) direct manipulation of variance in experiments. We provide an example of this 3-step process using data from preliminary experiments in a warmwater stream that suggested spatial variability in the abundance of chironomids and copepods could be related to flow variability. We close by suggesting that future directions for the use of variance in stream ecology should attempt to couple scale-dependent physical patterns with biological patterns or processes, develop experimental frameworks to determine if species interactions or other biotic processes vary with scale, embrace statistical methods for determining the relative importance of variance-generating processes, develop methods for the quantification of variance in biologically meaningful ways, and focus on the identification and interpretation of domains of homogeneous variance in streams.
Resumen La fauna silvestre es un recurso natural de gran importancia en las Provincias de Salta y Jujuy, en donde se desenvuelven actividades comerciales, recreativas, educacionales y hasta religiosas relacionadas con ella. La amplia variedad de ambientes con que cuentan estas provincias hacen que posean una riquísima diversidad de especies que es imprescindible mantener y conservar. Entre las actividades de mayor impacto sobre la fauna se puede mencionar la caza recreacional o deportiva, practicada principalmente por habitantes de ciudades y pueblos. La actividad de cacería se evaluó a través del método de encuestas por correo realizadas durante 1997 y 1998. La nómina de encuestados se elaboró a partir de un padrón de 251 cazadores registrados en las Direcciones de Recursos Naturales de Salta y de 430 de la Provincia de Jujuy. La tasa efectiva de respuesta fue de 21,8 %. Se evaluaron las características socioeconómicas de los cazadores, como edad, profesión, nivel de instrucción y nivel de ingresos. Se presentan como resultados el listado de animales que se cazan regularmente (mamíferos, aves y reptiles), el número promedio de animales abatidos por año, las áreas preferidas de caza, la frecuencia y duración de las excursiones, las distancias recorridas y el medio de transporte utilizado. En la región del Bosque Chaqueño se cazan 22 especies, 8 de mamíferos y 14 de aves; en la Selva de Transición se cazan 11 especies de mamíferos, 13 de aves y 2 de reptiles; en la región de la Selva Montana se cazan 8 especies de mamíferos y 6 de aves y en la región de la Puna se cazan solamente 3 especies de mamíferos. Con el fin de evaluar el impacto económico de la cacería recreacional en las economías provinciales, se utilizó la metodología de valoración de bienes ambientales denominado método de Costo de Viaje (CV). El total de cazadores registrados realizan gastos anuales directos de la actividad como combustibles, amortización de vehículos, equipos de caza, perros y tiempo libre, la suma de 137,311 dólares y gastos indirectos por 155,612 dólares. El gasto total que incurre cada cazador por temporada es de 430,14 dólares. Se discute la diferencia entre estos resultados y las reglamentaciones regulatorias dictadas por las autoridades de las Provincias. Introducción La fauna silvestre es un recurso natural de gran importancia en las Provincias de Salta y Jujuy, situadas en extremo noroeste de Argentina, en donde se desenvuelven actividades comerciales, recreativas, educacionales y hasta religiosas relacionadas con ella. La amplia variedad de ambientes con que cuentan estas provincias hacen que posean una riquísima diversidad de especies que es imprescindible mantener y conservar. Entre las actividades de mayor impacto sobre la fauna se puede mencionar la caza, la cual presenta diversas modalidades: Caza comercial: Tiene como fin la actividad lucrativa, y se desarrolla capturando animales vivos para la venta, o extrayendo productos y subproductos de animales abatidos (pieles, cueros, plumas, etc.) para su venta directa o bien, para su elaboración y posterior industrialización. Cuando se habla de caza comercial es muy importante diferenciar y excluir de esta categorización a la comercialización de elementos de la fauna silvestre que provienen, como subproductos, de la cacería de subsistencia.
Resumen La fauna silvestre es un recurso natural de gran importancia en las Provincias de Salta y Jujuy, en donde se desenvuelven actividades comerciales, recreativas, educacionales y hasta religiosas relacionadas con ella. La amplia variedad de ambientes con que cuentan estas provincias hacen que posean una riquísima diversidad de especies que es imprescindible mantener y conservar. Entre las actividades de mayor impacto sobre la fauna se puede mencionar la caza recreacional o deportiva, practicada principalmente por habitantes de ciudades y pueblos. La actividad de cacería se evaluó a través del método de encuestas por correo realizadas durante 1997 y 1998. La nómina de encuestados se elaboró a partir de un padrón de 251 cazadores registrados en las Direcciones de Recursos Naturales de Salta y de 430 de la Provincia de Jujuy. La tasa efectiva de respuesta fue de 21,8 %. Se evaluaron las características socioeconómicas de los cazadores, como edad, profesión, nivel de instrucción y nivel de ingresos. Se presentan como resultados el listado de animales que se cazan regularmente (mamíferos, aves y reptiles), el número promedio de animales abatidos por año, las áreas preferidas de caza, la frecuencia y duración de las excursiones, las distancias recorridas y el medio de transporte utilizado. En la región del Bosque Chaqueño se cazan 22 especies, 8 de mamíferos y 14 de aves; en la Selva de Transición se cazan 11 especies de mamíferos, 13 de aves y 2 de reptiles; en la región de la Selva Montana se cazan 8 especies de mamíferos y 6 de aves y en la región de la Puna se cazan solamente 3 especies de mamíferos. Con el fin de evaluar el impacto económico de la cacería recreacional en las economías provinciales, se utilizó la metodología de valoración de bienes ambientales denominado método de Costo de Viaje (CV). El total de cazadores registrados realizan gastos anuales directos de la actividad como combustibles, amortización de vehículos, equipos de caza, perros y tiempo libre, la suma de 137,311 dólares y gastos indirectos por 155,612 dólares. El gasto total que incurre cada cazador por temporada es de 430,14 dólares. Se discute la diferencia entre estos resultados y las reglamentaciones regulatorias dictadas por las autoridades de las Provincias. Introducción La fauna silvestre es un recurso natural de gran importancia en las Provincias de Salta y Jujuy, situadas en extremo noroeste de Argentina, en donde se desenvuelven actividades comerciales, recreativas, educacionales y hasta religiosas relacionadas con ella. La amplia variedad de ambientes con que cuentan estas provincias hacen que posean una riquísima diversidad de especies que es imprescindible mantener y conservar. Entre las actividades de mayor impacto sobre la fauna se puede mencionar la caza, la cual presenta diversas modalidades: Caza comercial: Tiene como fin la actividad lucrativa, y se desarrolla capturando animales vivos para la venta, o extrayendo productos y subproductos de animales abatidos (pieles, cueros, plumas, etc.) para su venta directa o bien, para su elaboración y posterior industrialización. Cuando se habla de caza comercial es muy importante diferenciar y excluir de esta categorización a la comercialización de elementos de la fauna silvestre que provienen, como subproductos, de la cacería de subsistencia.
1. A two-stage observational study of sandfly populations in chicken sheds was conducted on Marajo Island, northern Brazil, to identify determinants of feeding success within populations of female Lutzomyia longipalpis Lutz and Neiva during the dry and wet seasons. 2. We show, for the first time for a sandfly population, that per capita feeding success, measured as bloodmeal size, decreases with increasing density of other females at the feeding site, and increases with host density. 3. Interference with sandfly feeding is evidently host-mediated, as with some other bloodsucking insects. 4. The fact that female feeding success varies between sheds suggests that, with respect to bloodfeeding, female sandflies are not distributed according to the Ideal Free Distribution (IFD), i.e. they do not maximize individual resource gains. Probable costs of reduced bloodmeal size are discussed in terms of female fecundity anti mortality. 5. By fitting a generalized version of Sutherland's interference model, which allows patch quality as well as female interference to vary non-linearly, we infer that blood-feeding within sheds is predominantly on a subset of the available fowl. This too is consistent with the view that female flies disobey IFD. 6. We also demonstrate that increasing densities of female mosquitoes are associated with smaller bloodmeals in female Lu. longipalpis, suggesting that competition for bloodmeals can also occur between families of bloodsacking insects.
The Ideal Free Distribution was developed to predict the distribution of organisms at a habitat level. The theory of the Ideal Free Distribution assumes that travel between resource sites has a negligible affect on the distribution of organisms. In this experiment we tested whether the Ideal Free Distribution, and in particular its prediction of habitat matching, is robust to violations of this assumption. In an experiment with free-living ducks we manipulated the distance between two food sites. We used two conditions, one with 16 m between the two food sites and another with 45 m between the resource sites. We found that the distribution of organisms became less extreme with increased travel distance. This result is probably due to two effects: that travel distance caused a decrease in the ducks' ability to discriminate between the sites' profitabilities and by a decrease in the number of ducks travelling between the resource sites with increased distance. The decreased number of ducks travelling alone can explain only a relatively small amount of the change in the distribution. The decrease in discriminability may be due to either (or both) the increased distance causing a decrease in the foragers' ability to visually judge the relative profitabilities of the sites or by a decrease in switching rate associated with travel distance (if physical sampling of a site is needed to gather information). Because even a minor change in travel distance can cause a significant change in the distribution of foraging organisms, caution is urged about making extrapolations from experiments at a small spatial scale to the habitat level.
In this paper we first establish an analogy where we view both animals and vehicles as generic agents. We introduce a model of the ecological behavior of a group of agents and establish sufficient conditions for the group to achieve an ideal free distribution (IFD), even when we lift some of the "ideal" and "free" assumptions. Finally, we apply this model to cooperative vehicle control problems and present simulation results that show the benefits of an IFD-based distributed decision-making strategy.
The classical theory of the ideal free distribution (IFD) predicts that the spatial distribution of consumers should follow the distribution of the resources they depend on. Here, we study consumer–resource matching in a community context. Our model for the community is a food chain with three levels. We study whether the primary consumers are able to match resources both under predation risk and in its absence. Both prey and predators have varying degrees of knowledge of the global and local resource distribution. We present two versions of the model. In the “resource maximising” model, the consumers consider the availability of their resource only. In the “balancing” model, individual consumers minimise predation risk per unit of resource that they can gain access to. We show that both models can lead to perfect matching of consumers on resources and predators on consumers, assuming that individuals have full knowledge of the whole environment. However, when the consumers’ information and freedom of movement are greater than those of the predators, then the predators generally undermatch the consumers. In the opposite case, we observe overmatching and high consumer movement rates. Furthermore, undermatching of predators on consumers tends to induce overmatching of consumers on resources.
Ideal Free Distribution (IFD) theory predicts the number of animals choosing habitats of differing quality. Most experimental tests of the IFD have been conducted at small spatial scales (i.e. smaller than maximum daily movement of animals) by comparing the number of animals foraging at adjacent food patches of different quality. Urban pigeons (Columba livia) feed in large, open aggregations, and can distribute according to predictions of the IFD at alternative food patches. In this study, we test IFD predictions over a much larger spatial scale by comparing the abundance of urban feral pigeons at four sites spread over the city centre of Montréal, Québec, Canada, to the amount of anthropogenically provided food in each site. We found that the pigeons’ distribution among the four sites qualitatively matched that of resources available at these sites. After controlling for the effect of stochastic variation in food resources, two pair-wise comparisons between sites indicated undermatching, one indicated matching and three indicated overmatching of consumers to resources. These results suggest that the pigeons inhabiting the downtown area of Montréal may behave as a single population that distributes qualitatively among foraging sites in proportion to the quantity of food offered, and that deviations from expectations cannot be attributed simply to stochastic variation in the food levels at the sites.
The ideal free distribution assumes that habitat selection is without cost and predicts that fitness should be equal in different habitats. If habitat selection has a cost, then individuals should only move to another habitat when potential fitness in the new habitat exceeds that in the source habitat by an amount greater than the cost of habitat selection. We used isodar techniques to assess the cost of habitat selection. In an experimental landscape, we monitored density, movement, and reproductive success of adult female prairie voles, Microtus ochrogaster, in adjacent paired habitats with low and high cover. We tested the following hypotheses: (1) adult female prairie voles exhibited density-dependent habitat selection; (2) the cost of habitat selection was density-independent. Habitat quality based on population density and fitness of adult females was higher in high cover habitats. Net movement was from low cover to high cover habitats. The results indicated that adult female prairie voles exhibited density-dependent habitat selection. Furthermore, there was a significant cost of habitat selection, and the cost was density-independent.
To understand how cost of dispersal affects population dynamics, we study a three-trophic level food chain model, proposed by DeAngelis et al. [D. DeAngelis, G. S. K. Wolkowicz, Y. Lou, Y. X. Jiang, M. Novak, R. Svanback, M. Araujo, Y. S. Jo, and E. A. Cleary, Am. Nat., 178 (2011), pp. 15–29], in two patches. The system consists of one resource species, two consumers, and a top predator. The top predator feeds on two consumers and both consumers feed on the resource. Only consumers move between the patches, possibly with a fraction of loss in population during the movement. The two competing consumers are identical in every aspect except their dispersal rates between two patches. If two consumers have the same dispersal rate from patch 1 to patch 2, we completely determine the global dynamics of the model and show that there exists an "optimal" dispersal rate from patch 2 to patch 1 for the consumer such that, in terms of the theory of adaptive dynamics, it is a globally evolutionarily stable strategy and also a convergent stable strategy. If there is a minimum dispersal speed from patch 1 to patch 2, we are able to completely determine the evolutionarily stable strategy for dispersal between two patches. Our results offer insights into the evolution of dispersal in multitrophic level food chains, e.g., how the evolution of fast or slow dispersal for the consumer species depends upon the variation of the predation risk in the habitat. Our result suggests that even if most individuals die during the movement, a positive dispersal rate can still evolve.
A key assumption of the ideal free distribution (IFD) is that there are no costs in moving between habitat patches. However, because many populations exhibit more or less continuous population movement between patches and traveling cost is a frequent factor, it is important to determine the effects of costs on expected population movement patterns and spatial distributions. We consider a food chain (tritrophic or bitrophic) in which one species moves between patches, with energy cost or mortality risk in movement. In the two-patch case, assuming forced movement in one direction, an evolutionarily stable strategy requires bidirectional movement, even if costs during movement are high. In the N-patch case, assuming that at least one patch is linked bidirectionally to all other patches, optimal movement rates can lead to source-sink dynamics where patches with negative growth rates are maintained by other patches with positive growth rates. As well, dispersal between patches is not balanced (even in the two-patch case), leading to a deviation from the IFD. Our results indicate that cost-associated forced movement can have important consequences for spatial metapopulation dynamics. Relevance to marine reserve design and the study of stream communities subject to drift is discussed.
Thesis (PH.D.) -- Syracuse University, 2002. "Publication number AAT 3065200" Microfilm of typescript.
In a concurrent-chains procedure, pigeons chose between outcomes that differed in the rate of response-independent delivery of food and electric shocks. The application of functional measurement techniques confirmed the matching relation—between choice and rate of reinforcement value—for two of three pigeons. Scale values of the outcomes were extracted for the two birds that conformed to matching, and the value of a single occurrence of shock per minute—in terms of negative food units—was estimated. A second experiment with concurrent chains provided a test of these parameter estimates. The close correspondence between predicted and obtained choice behavior found in Experiment II indicated that the estimates of outcome value were indeed reliable. Both experiments together support the contention that the effects on choice behavior of positive and aversive stimuli appear to be equal, though opposite in sign.
The ideal free model predicts that the distribution of organisms between resource sites should match the distribution of resources. The authors review tests of this model utilising a method of analysis derived from the psychological principle called the matching law. Reanalysis revealed that the distribution of organisms is consistently less extreme than the distribution of resources. This systematic deviation from the ideal free distribution means organisms underuse richer sites and overuse poorer sites. Deviations from the ideal free distribution may arise as a consequence of discrimination constraints, competitive interactions, competitive asymmetries and travel between sites. Ways in which the design of future tests may be improved are suggested. -from Authors
Suppose that two patches are available to a group of competing animals. Fretwell & Lucas (1970) considered how the animals should distribute themselves between these patches, and derived the Ideal Free Distribution. Sutherland & Parker (1985) extended the model of Fretwell & Lucas to include the case where animals differ in their competitive abilities. They showed that there are usually several ways in which an Ideal Free Distribution can be realized, and introduced the notion of the most probable realization. This paper gives a general analysis of the extended model of Sutherland & Parker. The most probable distribution of animals when the number of competing animals is large is derived. For this distribution the proportion of animals of a given competitive ability that are present on the more profitable patch increases with competitive ability. This has the consequence that the proportion of all animals on the more profitable patch is less than the proportion of all rewards that are delivered by this patch.
The joint effects of punishment and reinforcement on the pigeon's key-peck response were examined in three choice experiments conducted to compare predictions of Farley and Fantino's (1978) subtractive model with those made by Deluty's (1976) and Deluty and Church's (1978) model of punishment. In Experiment 1, the addition of equal punishment schedules to both alternatives of a concurrent reinforcement schedule enhanced the preference exhibited for the more frequent reinforcement alternative. Experiment 2 demonstrated decreases in the absolute response rate for each member of a concurrent reinforcement schedule when increasing frequencies of punishment were added to each alternative. Experiment 3 found that preference for the denser of two reinforcement schedules diminished when the absolute frequencies of reinforcement were increased by a constant factor and conditions of punishment for both alternatives were held constant. Diminished preferences were obtained regardless of whether the frequency of punishment associated with the denser reinforcement schedule was greater or less than that associated with the lean reinforcement alternative. The results from all three experiments uniquely supported Farley and Fantino's (1978) subtractive model of punishment and reinforcement.
In two experiments, pigeons' key pecking for food on concurrent variable-interval schedules was punished with electric shock according to concurrent variable-interval punishment schedules. With unequal frequencies of food but equal rates of punishment associated with the two keys and at several intensities of shock, the response and time allocation of all six pigeons overmatched the obtained relative frequency of food. The overmatching was predicted by a subtractive model of the interaction between punishment and positive reinforcement but not by two alternative models. Increases in the k and r(e) parameters of the generalized matching law could not account for the observed shifts in preference.
If the primary preference of all species is shared, but their secondary preferences are distinct, they are said to be organized centrifugally. The biological basis for centrifugal structure may be that habitats represent combinations of different environmental variables and, although the ideal combination is the same for many species in a guild, each species is adapted to tolerate relative deprivation of a different component of the mixture. The isolegs of centrifugal organization (lines of equal optimal behavior drawn in a state space) are modelled. Their principal distinguishing feature is their negative slope. Because of this, niche shifts in such communities should occur backwards; removing a competitor produces narrower niches in those left. Another feature which distinguishes it is its inability to generate a 'ghost of competition past': competition should always be a strong dynamical influence. Two species of gerbils in the Negev Desert, Israel, both exhibit negative isolegs. Other aspects of their habitat selection are consonant with a picture of centrifugal organization. -from Authors
A graphical theory of habitat selection is built in steps. The theory treats two species in an environment with two usable patch types in a matrix of unusable space. The first step assumes habitat selection is density independent and free of search costs. The second assumes density independence, and the third assumes neither. The first two steps produce results already known from earlier theories. The third, however, requires a new analytical device, the isoleg, which is a line in a two-dimensional-state space of the two species' densities. An isoleg is a set of points in such a density space, such that on one side of the set, individuals of a species optimize their foraging by being strict habitat selectors, whereas on the other side, they do so by using at least a bit of a poorer patch. The population dynamics of the competitors is analyzed using their isolegs. The isolegs allow us to deduce that the zero isoclines of the species are warped into nonlinear forms capable of producing competitive coexistence. It is shown that at the equilibrium point of this coexistence, there may be no overt competition remaining. The difficulties this presents to the field investigator are mentioned, and a modified definition of interspecific competition is suggested.
When fitness of a resource-limited animal depends only on that individual's share of the total resource in a habitat patch and individuals are free to move to the patch where their gains are highest, population density matches resource availability under the simple assumption that individual fitness increases with resource use. Previous theory on habitat matching required the stronger assumption that individual fitness was directly proportional to (rather than monotonically increasing with) resource use. The basic theory suggests conditions under which population density empirically indicates habitat quality. Extensions of this basic theory apply when individuals that are free to move among resource patches interact by interfering with each other's resource extraction or by competing unequally. Analysis of existing models of such ideal free competition yields conditions for a single general matching rule in which the logarithm of crowding is a linear function of the logarithm of resource abundance. Double logarithmic plots of empirical data on habitat use and habitat quality based on this rule furnish possible graphical indicators of the occurrence and intensity of competition in nature.
Habitat selection costs depend upon the scale of habitat. At the fine-grained microhabitat scale, cost is linked to optimal foraging, and habitat selection should be abandoned even though fitness is greater in one microhabitat than in another. At the coarse-grained macrohabitat scale, cost is linked to emigration, and habitat selection should often be maintained even though fitness may be less in the preferred macrohabitat than in others. Macrohabitat selection cost is easily incorporated into habitat selection theory and can be tested by linear regression techniques on isodars (lines of every point at which the fitness of individuals in one habitat equals that of individuals in another). The results of one recent survey of white-footed mice living in different macrohabitats are consistent with the predictions of emigration cost.