Density-dependent habitat selection in plants

Pea plants exhibit density-dependent habitat selection as they grow. We split the root of a young pea (Pisum sativum L.) so that half grew in one pot and half in an adjacent pot. The rest of the plant remained intact. This is a ‘fence-sitter plant’. Each root-half was exposed either to no competition in its pot or to competitor plants sharing its pot. There were one, two, three or five competitor plants. The total root biomass and the fitness (= dry weight of fruit) of the fence-sitter decreased only slightly and insignificantly in response to increased density of the competitor plants. The fitness of the competing plants decreased with density. The fence-sitter shifted its root system from the pot with competition to that free of competition in proportion to the number of competitors. The fence-sitter apparently invested in each of its two roots so that the ratio between the roots was similar to the ratio between the resources in the pots. This result is analogous to the habitat-matching rule of the ideal free distribution of populations (Fretwell, 1972). We suggest that plants invest in each of their roots until the uptake rate per unit root biomass is equal for all roots. This revised version was published online in July 2006 with corrections to the Cover Date.     

Density-dependent habitat selection in muskrats: a test of the ideal free distribution model

Summary Two predictions of the ideal free distribution model, a null hypothesis of habitat selection, were examined using free-ranging muskrats. We rejected the prediction that the proportion of the animals found in each of five habitats was independent of population size. Data on over-winter occupancy of muskrat dwellings tend also to refute the prediction of equal fitness reward among habitats. Habitat type and water-level had a profound effect on the suitability of a site for settlement. We concluded that the observed pattern of muskrat distribution followed more closely an ideal despotic distribution where some individuals benefited from a higher fitness because of resource monopolization. Current theories of density-dependent habitat selection, which assume an ideal free distribution, would not apply to muskrats and possibly to many other mammal species.     

The cost of habitat selection in prairie voles: an empirical assessment using isodar analysis

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.     

Environmental networks,compensating life histories and habitat selection by white-footed mice

Summary Analysis of 6 years' data on a population of free-living white-footed mice documents both phenotypic and environmental control of litter size. Litter size was positively correlated with maternal body size. Maternal size depended upon both seasonal and annual variation. Paradoxically, the proportion of small versus large litters varied among habitats independently of the effects of body size. The result is an influence of habitat on life history that yields patterns of reproduction and survival opposite to the predictions of demographic theory. The habitat producing the largest litters had a relatively high ratio of adult/juvenile survival. Litter size was small in the habitat where the adult/juvenile survival ratio was smallest. All of these anomalous patterns can be explained through density-dependent habitat selection by female white-footed mice. Life-history studies that ignore habitat and habitat selection may find spurious correlations among traits that result in serious misinterpretations about life history and its evolution.     

Density-dependent habitat selection: evaluating isoleg theory with a Lotka-Volterra model

Contemporary models of density-dependent habitat selection generally focus on long-term evolutionary consequences of intraspecific or interspecific competition and/or patterns of resource use in patchy environments. A primary goal of such studies often is to elucidate evolutionary stable strategies based on steady-state dynamics of population growth. In contrast, we developed a simulation model to explore short-term movements of interspecific competitors among fine-grained habitats of differing attributes, as might result from field manipulations of habitat quality or population densities. In this model, habitat quality is expressed in terms of mean individual fitness, represented by average per capita growth rate calculated according to the Lotka-Volterra equations describing interspecific competition. This model provides a mechanism for quantifying the effects of habitat quality, patterns of resource use and competition on distributions of individuals. Results demonstrate the heuristic value of this model in corroborating predictions derived from the ideal free distribution and isodar theory, and in generating isolegs to test the predictions of isoleg theory. Results indicate that small changes in model parameters have substantial impacts on patterns of habitat use and co-occurrence between species. The model identifies a variety of conditions under which isolegs for a given type of community organization deviate from predictions of contemporary isoleg theory, potentially expanding the universe of possible interspecific behaviors underlying the development of evolutionary stable strategies.     

Ontogenetic shifts in habitat use by the endangered Roanoke logperch (Percina rex)

1. Conservation of the federally endangered Roanoke logperch (Percina rex, Jordan and Evermann) necessitates protection of habitat that is critical for all age classes. We examined habitat use patterns of individual logperch to determine: (1) if age classes of logperch in the Nottoway and Roanoke Rivers exhibit habitat selectivity, (2) if age classes differ in habitat use, and (3) if ontogenetic patterns of habitat use differ between the Roanoke and Nottoway river populations. 2. In the summers of 2000 and 2001, we observed 17 young‐of‐year (YOY) logperch [<4 cm total length (TL)], 13 subadult logperch (4–8 cm TL), and 49 adult logperch (>8 cm TL) in the upper Roanoke River, and 40 subadult and 39 adult logperch in the Nottoway River, Virginia. 3. All size classes of Roanoke logperch demonstrated habitat selectivity and logperch used a wide range of habitats in the Roanoke and Nottoway rivers during ontogeny. Habitat use by logperch varied among age classes and between rivers. 4. In the Roanoke River, adult and subadult logperch primarily preferred run and riffle habitat, often over gravel substrate. Subadults were found in lower water velocities and slightly more embedded microhabitats than adults. YOY logperch were found in shallow, stagnant backwaters and secondary channels. In the Nottoway River, both adult and subadult logperch were found over sand and gravel in deep, low‐velocity pools and runs. Subadults were observed in slightly more silted, lower velocity habitat than adults. Shifts in habitat use were more distinct between age classes in the Roanoke River than the Nottoway River. 5. Successful conservation of this species will involve sound understanding of spatial variation in habitat use over logperch life history and preservation of the ecological processes that preserve required habitat mosaics.     

Do haddock select habitats to maximize condition?

Haddock Melanogrammus aeglefinus in the North sea increased their distributional range when more abundant, but this density dependent habitat selection (DDHS) explained only a small part of the year‐on‐year variation in distribution patterns. The condition of haddock was examined at 24 sites in the North Sea in August and September 2004 and related to their abundance, to examine if the ideal free distribution theory (IFD), which assumes that organisms select habitats that maximize their rate of food intake, can be used to explain this variation in large scale distribution patterns. At a given temperature, condition (hepato‐somatic index, I _H) was better at stations where haddock were most abundant. Therefore, haddock were not distributed perfectly according to the IFD in 2004. The positive correlation between abundance and I _H, however, indicated there was some habitat selection by haddock, as in the total absence of habitat selection no correlation between I _H and abundance, and no spatial variation in abundance was expected. DDHS may only explain a small part of the yearly variation in the distribution because haddock did not equalize and maximize their fitness at the scale of the North Sea. In addition, stable isotope analysis of muscle samples showed that haddock did not avoid competition for food when at high abundance by feeding at a lower or wider range of trophic levels.     

A generalized habitat matching rule

Summary 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.     

鸟类夜栖地选择研究进展

夜栖地可以为鸟类提供安全、舒适、温暖的夜间栖息场所。昼行性鸟类夜间视觉能力较差,对夜栖环境中潜在的威胁感知程度较低,导致其夜栖时常常处于被动和危险的环境中。鸟类通过选择适宜的夜栖地,进而达到充分利用夜栖地空间资源的目的。本文从鸟类夜栖地选择的行为适应性以及保温性、食物因子和安全性(隐蔽性)等3个影响因素,对国内外鸟类夜栖地选择相关研究进行综述,为今后该领域的研究提供更多思路。     

Two gerbils of the Negev: A long-term investigation of optimal habitat selection and its consequences

Optimal foraging theory has entered a new phase. It is not so much tested as used. It helps behavioural ecologists discover the nature of the information in an animals brain. It helps population ecologists reveal coefficients of interaction and their patterns of density-dependent variation. And it helps community ecologists examine niche relationships. In our studies on two species of Negev desert gerbil, we have taken advantage of the second and third of these functions. Both these gerbils prefer semi-stabilized dune habitat, and both altered their selective use of this habitat and stabilized sand according to experimental changes we made in their populations. Their changes in selectivity agree with a type of optimal foraging theory called isoleg theory. Isoleg theories provide examples of dipswitch theories – bundles of articulated qualitative predictions – that are easier to falsify than single qualitative predictions. By linking behaviour to population dynamics through isoleg theory, we were able to use the behaviour of the gerbils to reveal the shapes of their competitive isoclines. These have the peculiar non-linear shapes predicted by optimal foraging theory. Finally, when owl predation threatens, the behaviour of Gerbillus allenbyi reveals the shape of their victim isocline. As has long been predicted by predation theory and laboratory experiments, it is unimodal.     

Hybrid fitness, adaptation and evolutionary diversification: lessons learned from Louisiana Irises

Estimates of hybrid fitness have been used as either a platform for testing the potential role of natural hybridization in the evolution of species and species complexes or, alternatively, as a rationale for dismissing hybridization events as being of any evolutionary significance. From the time of Darwin's publication of The Origin, through the neo-Darwinian synthesis, to the present day, the observation of variability in hybrid fitness has remained a challenge for some models of speciation. Yet, Darwin and others have reported the elevated fitness of hybrid genotypes under certain environmental conditions. In modern scientific terminology, this observation reflects the fact that hybrid genotypes can demonstrate genotype × environment interactions. In the current review, we illustrate the development of one plant species complex, namely the Louisiana Irises, into a 'model system' for investigating hybrid fitness and the role of genetic exchange in adaptive evolution and diversification. In particular, we will argue that a multitude of approaches, involving both experimental and natural environments, and incorporating both manipulative analyses and surveys of natural populations, are necessary to adequately test for the evolutionary significance of introgressive hybridization. An appreciation of the variability of hybrid fitness leads to the conclusion that certain genetic signatures reflect adaptive evolution. Furthermore, tests of the frequency of allopatric versus sympatric/parapatric divergence (that is, divergence with ongoing gene flow) support hybrid genotypes as a mechanism of evolutionary diversification in numerous species complexes.     

Adaptation and habitat selection in the eco-evolutionary process

The struggle for existence occurs through the vital rates of population growth. This basic fact demonstrates the tight connection between ecology and evolution that defines the emerging field of eco-evolutionary dynamics. An effective synthesis of the interdependencies between ecology and evolution is grounded in six principles. The mechanics of evolution specifies the origin and rules governing traits and evolutionary strategies. Traits and evolutionary strategies achieve their selective value through their functional relationships with fitness. Function depends on the underlying structure of variation and the temporal, spatial and organizational scales of evolution. An understanding of how changes in traits and strategies occur requires conjoining ecological and evolutionary dynamics. Adaptation merges these five pillars to achieve a comprehensive understanding of ecological and evolutionary change. I demonstrate the value of this world-view with reference to the theory and practice of habitat selection. The theory allows us to assess evolutionarily stable strategies and states of habitat selection, and to draw the adaptive landscapes for habitat-selecting species. The landscapes can then be used to forecast future evolution under a variety of climate change and other scenarios.     

Movement between patches, unequal competitors and the ideal free distribution

Researchers have often commented on the ability of the original ideal free distribution (IFD) model to approximate observed animal distributions even though the critical assumption that competitors are of equal ability is usually violated. We provide an explanation by recognizing that animals will occasionally move between patches for reasons other than to simply maximize their resource payoffs, given perfect (i.e. ideal) information about the current payoff in each patch, and that these movements will continue to occur even after an equilibrium is reached. When such movements are incorporated into an unequal competitors IFD model, a single, stable distribution of each competitor type is predicted. This equilibrium will usually be characterized by under-matching of total competitive units relative to the distribution of resources (i.e. too few competitive units in the good patch). More importantly, it will often resemble the original, equal competitors IFD, in that total competitor numbers will come close to matching the distribution of resources. We argue that researchers claiming to have observed an IFD of equal competitors have actually observed this equilibrium distribution of unequal competitors. Our model predicts that the deviation from input-matching will usually be an under-matching of total competitor numbers relative to resources (i.e. too few competitors in the good patch). Examination of published data reveals that post-equilibrium movement between patches occurs frequently and, although the reported distributions are similar to those predicted by input-matching, under-matching is usually observed.     

Density-dependent habitat selection between maize cropfields and their borders in two rodent species (Akodon azarae and Calomys laucha) of Pampean agroecosystems

We studied habitat preferences and intra and interspecific density-dependent effects on habitat selection by Akodon azarae and Calomys laucha between maize fields and their adjacent borders, during different developmental stages of the crop. Akodon azarae detected quantitative differences between habitats, using preferentially borders throughout the year, while C. laucha perceived borders and cropfields as quantitatively similar during spring and summer and it detected borders as quantitatively better at the high density period (autumn and winter). These results support the prediction of differential habitat preferences as a model of community organisation at the low density period, while they are consistent with shared habitat preferences during autumn and winter when both species apparently coexist in the better habitat (border). Akodon azarae showed intraspecific density-dependent habitat selection throughout the year, except in spring, while habitat selection by C. laucha was density-dependent in spring, autumn and winter. The effect of interspecific density on habitat selection was detected in both habitats and changed seasonally. The effect of A. azarae over C. laucha by resources exploitation was detected in borders, while competitive effects of C. laucha over A. azarae was observed within cropfields. Both species were more affected by exploitation competition than interference, which was more common in borders than in maize fields. We conclude that seasonally have a profound effect in habitat selection of these species because it changes the intensity of intra and interspecific competition and affects different habitat preferences and basic suitability of habitats. This revised version was published online in November 2006 with corrections to the Cover Date.     

Measuring the benefit of habitat selection

We used a behavioral bioassay to estimate the advantages thattwo species of gerbils (Gerbillus allenbyi and G. pyramidum)experienced by preferring a semistabilized dune habitat overa stabilized sand habitat. We used the magnitude of foragingeffort by the gerbils to signal the difference between thetwo habitats. When they were foraging as much in stabilizedsand as in semistabilized dune, we inferred that these habitatswere providing equivalent rewards. We performed a series ofexperiments in two 1-ha field enclosures, each containing similarproportions of stabilized sand and semistabilized dune. Eachenclosure contained a population of only one of the species.By varying the amount of seeds added (either 0.5, 1, 2, or 3g of seeds in 18 seed trays) to each habitat and monitoringthe behavior of the gerbils, we were able to fit a curve thatreflected the change in habitat preference as a function ofseed addition rate. We were also able to show how much seedaddition had to be added to bring the two habitats into equaluse. Each species required only 13 g/ha/night to entirely offsetthe advantage of the semistabilized dune.     

Ecological principles of species distribution models: the habitat matching rule

Most species distribution models (SDMs) assume that habitats are closed, stable and without competition. In that environmental context, it is ecologically correct to assume that members of a species will be distributed in direct relation to the suitability of the habitat, that is, according to the so‐called habitat matching rule. This paper examines whether it is possible to maintain the assumption of the habitat matching rule in the following circumstances: (1) when habitats are connected and organisms can move between them, (2) when there are disturbances and seasonal cycles that generate instability, and (3) when there is inter‐specific and intra‐specific competition. Here I argue that it is possible as long as the following aspects are taken into account. In open habitats at equilibrium, in which habitat selection and competition operate, the habitat matching rule can be applied in some conditions, while competition tends to homogenize the species distribution in other environmental contexts. In the latter case, two methods can be used to incorporate these effects into SDMs: new parameters can be incorporated into the response functions, or the occurrence of proportions of categories of individuals (adult/young, male/female, or dominant/subordinate species in guilds) can be used instead of the occurrence of organisms. The habitat matching rule is not fulfilled in non‐equilibrium environments. The solution to this problem lies in the design of SDMs with two strategies that depend on scale. Locally, the disequilibrium can be encapsulated using average environmental conditions, with sufficiently large cells (in the case of metapopulations) and/or long enough sampling periods (in the case of seasonal cycles). At coarse scales, the use of presence‐only models can in some cases avoid the destabilizing effect of catastrophic historical processes. The matching law is a strong assumption of SDMs because it is based on population ecology theory and the principle of evolution by natural selection.     

Habitat matching: Alternatives and implications to populations and communities

Summary I evaluate habitat matching rules based on ideal distribution models of density-dependent habitat use. Recent approaches and the ideal free continuous input matching rule on which they depend, are restricted to only those habitats that are jointly occupied across the full range of population sizes. These assumptions may often be inappropriate to field applications of habitat matching. I develop alternatives that can be applied to a wide array of ideal forms of habitat selection, including the ideal free, continuous input example. Input matching can be distinguished from assumptions of consumer-resource models and preemptive habitat use by regressions of density between paired habitats (isodars). Isodars for continuous input models should be linear on a logarithmic scale, while those for consumer-resource models should be linear on an arithmetic scale. Pre-emptive isodars can be distinguished from the others by dramatic non-linearities at both low and high densities. Field data on white-footed mice support the consumer-resource theory. Implications of the rules for population regulation and community organization are highlighted by new models that specify how the fitness of pre-emptive habitat selectors should decline with increasing density. Strong non-linearities produced by comparisons between variable and homogeneous habitats produce reversing source-sink population regulation and a new form of cyclical community dynamics. Variable habitats act as a source of emigrants at low density and a sink for immigrants at high density. Subordinate species may occupy only the variable habitat at both low and high density.