Silver spoons in the rough: can environmental enrichment improve survival of hatchery Atlantic salmon Salmo salar in the wild?

This study tested the ‘silver spoon’ hypothesis which posits that individuals that develop under favourable conditions should enjoy a fitness advantage later in life because they are more likely to recognize and settle in high‐quality habitats. Atlantic salmon Salmo salar of two age classes (0+ and 1+ years) were reared in environmentally enriched or standard hatchery tanks for a short period (c. 10 weeks), were then released into a natural river and sampled on repeated occasions to test for silver‐spoon effects. Compared with controls, enriched fish had a 6·4% higher recapture rate and settled in higher velocity habitats when they were stocked as 0+ year fry, but not when they were stocked as 1+ year parr. The opportunity for selection was generally higher for environmentally enriched fish than for controls, and also higher for 0+ than for 1+ year fish. Selection favoured individuals with high condition factor, extensive fat reserves and longer than average pectoral fins in both age classes but favoured a small body size in 1+ year and a large body size in 0+ year releases. Stomach analysis showed that enriched fish ate more, and adapted quicker to natural prey than controls. These results provide support for silver‐spoon effects in fish and indicate that enrichment can improve post‐release performance in conservation programmes, but seemingly only if fish are not kept in captivity for too long.     

Someplace like home: Experience,habitat selection and conservation biology

Recent insights from habitat selection theory may help conservation managers encourage released animals to settle in appropriate habitats. By all measures, success rates for captive–release and translocation programs are low, and have shown few signs of improvement in recent years. We consider situations in which free-living dispersers prefer new habitats that contain stimuli comparable to those in their natal habitat, a phenomenon called natal habitat preference induction (NHPI). Theory predicts NHPI when dispersers experienced favorable conditions in their natal habitat, and have difficulty estimating the quality of unfamiliar habitats. NHPI is especially likely to occur when performance in a given habitat is enhanced if an animal developed in that same habitat type. Animals exhibiting NHPI are expected to rely on conspicuous cues that can be quickly and easily detected during search, and to prefer new habitats possessing cues that match those encountered in their natal habitat.A major obstacle to successful relocations is that newly released animals often reject the habitat near the release site and rapidly travel long distances away before settling. An NHPI perspective argues that long-distance movements away from release sites occur because releasees prefer to settle in familiar types of habitat, and reject novel areas lacking cues similar to those in their habitat of origin. Similarly, a preference by releasees for familiar cues may encourage them to seek out inappropriate, low quality habitats following release at a new location. We review evidence from a number of studies indicating that problems with habitat selection behavior compromise conservation efforts, and provide recommendations that may encourage animals to “feel more at home” in post-release habitats.     

Density‐dependent habitat selection predicts fitness and abundance in a small lizard

Density‐dependent habitat selection has been used to predict and explain patterns of abundance of species between habitats. Thermal quality, a density‐independent component of habitat suitability, is often the most important factor for habitat selection in ectotherms which comprise the vast majority of animal species. Ectotherms may reach high densities such that individual fitness is reduced in a habitat due to increased competition for finite resources. Therefore, density and thermal quality may present conflicting information about which habitat will provide the highest fitness reward and ectotherm habitat selection may be density‐independent. Using ornate tree lizards Urosaurus ornatus at 10 sites each straddling two adjacent habitats (wash and upland), we tested the hypothesis that habitat selection is density‐dependent even when thermal quality differs between habitats. We first tested that fitness proxies decline with density in each habitat, indicating density‐dependent effects on habitat suitability. We also confirmed that the two habitats vary in suitability (quantified by food abundance and thermal quality). Next, we tested the predictions that habitat selection depends on density with isodar analyses and that fitness proxies are equal in the two habitats within a site. We found that monthly survival rates decreased with density, and that the wash habitat had more prey and higher thermal quality than the upland habitat. Lizards preferred the habitat with more food and higher thermal quality, lizard densities in the two habitats were positively correlated, and fitness proxies of lizards did not differ between habitats. These patterns are consistent with density‐dependent habitat selection, despite differences in thermal quality between habitats. We expect that density‐dependent habitat selection is widespread in terrestrial ectotherms when densities are high and temperatures are close to their optimal performance range. In areas where thermal quality is low, however, we expect that depletable resources, such as food, become less limiting because assimilating resources is more difficult.     

Habitat selection reduces extinction of populations subject to Allee effects

Theoretical studies indicate that a single population under an Allee effect will decline to extinction if reduced below a particular threshold, but the existence of multiple local populations connected by random dispersal improves persistence of the global population. An additional process that can facilitate persistence is the existence of habitat selection by dispersers. Using analytic and simulation models of population change, I found that when habitat patches exhibiting Allee effects are connected by dispersing individuals, habitat selection by these dispersers increases the likelihood that patches persist at high densities, relative to results expected by random settlement. Populations exhibiting habitat selection also attain equilibrium more quickly than randomly dispersing populations. These effects are particularly important when Allee effects are large and more than two patches exist. Integrating habitat selection into population dynamics may help address why some studies have failed to find extinction thresholds in populations, despite well-known Allee effects in many species.     

Predation-imposed selection on threespine stickleback (Gasterosteus aculeatus) morphology: a test of the refuge use hypothesis

The transition from marine to freshwater life in the threespine stickleback (Gasterosteus aculeatus) is accompanied by complex morphological changes-including reduction in bony armor and change in body shape-but experimental evidence for the selective agents behind these evolutionary transitions is sparse. We investigated whether selection by predatory fish affects threespine stickleback morphology differentially when refuge is absent (pelagic lifestyle-ancestral condition) or present (benthic lifestyle-derived condition). Our results show that selection favors low numbers of lateral plates in habitats with refuge, whereas fully plated individuals have a selective advantage in habitats without refuge. We also found that a decrease in the length of the caudal peduncle increased survival probability, irrespective of habitat. The effect of spine lengths on survival was evident only in a multivariate analysis of selection, implying that it is essential to account for phenotypic and genetic correlations between traits before drawing conclusions about the effects of selection on single traits. Apart from uncovering targets and patterns of predator-induced selection on threespine stickleback morphology, our results provide direct evidence to support the hypothesis that differences in antipredator strategies in pelagic versus benthic sticklebacks could play a role in the repeated, independent cases of plate number reduction following freshwater colonization in this species.     

Differences in home-range sizes of a bird species in its original,refuge and substitution habitats: challenges to conservation in anthropogenic habitats

In the current context of the anthropocene, the original habitats of many species have been modified or destroyed. Animals may be forced to move from their original habitats, either to refuge habitats that are suboptimal natural habitats, or to substitution habitats that are anthropogenic. The quality of refuge habitats may be lower than that of the original ones, whereas substitution habitats may be of a similar or even better quality. Here, we test this hypothesis empirically, using the example of coastal populations of the bluethroat, Luscinina svecica namnetum. In a radio-tracking survey, we compared the home-range sizes (considered here a proxy of habitat quality) of the breeding males in their original (coastal saltmarshes), refuge (inland reedbeds) and substitution (coastal salinas) habitats. We found that home ranges are up to 15 times larger in the substitution habitat than in the original one, and intermediate in the refuge habitat, suggesting that substitution habitats have the lowest quality and original habitats the highest. To date, most studies and conservation programs related to this species have focused on its substitution habitats. This result challenges the interest of focusing on anthropogenic habitats when studying and conserving such a species, because such habitats may only be low-quality substitutes.     

Finding a place to live: conspecific attraction affects habitat selection in juvenile green and golden bell frogs

Conspecific attraction plays an important role in habitat selection of several taxa and can affect and determine distribution patterns of populations. The behaviour is largely studied and widespread among birds, but in amphibians, its occurrence seems limited to breeding habitats of adults and gregarious tadpoles. The Australian green and golden bell frogs (Litoria aurea) have suffered considerable shrinking of their original distribution in south-eastern Australia since the 1970s. Currently, with only about 40 populations remaining, the species is considered nationally threatened. In natural conditions, these frogs are aggregated in the landscape and do not seem to occupy all suitable ponds within the occurrence area. To date, studies focusing on the frogs’ habitat have failed in finding a general habitat feature that explains current or past occupancy. This led us to the hypothesis that social cues may play a key role in habitat selection in this species. Using two choice experiments, we tested the preference of juvenile green and golden bell frogs for habitats containing cues of conspecifics of similar size versus habitats without conspecific cues. Tested frogs did not show a preference for habitats containing only scent from conspecifics but did prefer habitats where conspecifics were present. Our results show that conspecific attraction is a determining factor in juvenile green and golden bell frog habitat selection. To our knowledge, this is the first time the behaviour is shown to occur in juvenile frogs in the habitat selection context. From a conservation management point of view, the behaviour may help to explain the failure of reintroductions to areas where the frogs have been extinct, and the non-occupation of suitable created habitats in areas where they still inhabit and develop appropriated management strategies.     

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.     

Emigration to new habitats by voles: the cost of dispersal paradox

We tested the hypothesis that dispersal and philopatry are components of a mixed evolutionarily stable strategy (ESS). The hypothesis predicts that fitness of dispersers should be equal to that of philopatric individuals. Alternatively, fitness of dispersers could be lower (the resident fitness hypothesis) or greater (the cost of dispersal hypothesis) than that of philopatric individuals. We compared fitness of individuals that moved to new habitats (emigrants) and those that remained within habitat boundaries (residents) in populations of the prairie vole, Microtus ochrogaster, and the meadow vole, M. pennsylvanicus. We established vole populations in four enclosures (). Within each enclosure, voles were free to move between four types of habitats that varied in the availability of supplemental food and the amount of vegetative cover. We analysed two fitness components: the survival rates of all individuals, and pregnancy rates of females. Our study showed that emigrants generally had greater fitness than residents and that the difference in fitness was habitat dependent (i.e. was greater when individuals were emigrating from low-quality habitats than from high-quality habitats). High-food, high-cover habitats were the only habitat types for which fitness of emigrants was lower than that of residents. Similar patterns occurred in both prairie voles and meadow voles. Our results support the cost of dispersal hypothesis.     

No Habitat Selection during Spring Migration at a Meso-Scale Range across Mosaic Landscapes: A Case Study with the Woodcock (Scolopax rusticola)

Success of migration in birds in part depends on habitat selection. Overall, it is still poorly known whether there is habitat selection amongst landbird migrants moving across landscapes. Europe is chiefly covered by agro-forestry mosaic landscapes, so migratory species associated to either agricultural landscapes or woodland habitats should theoretically find suitable stopover sites along migration. During migration from wintering to breeding quarters, woodcocks (Scolopax rusticola) tagged with PTT satellite-tracking transmitters were used to test for the hypothesis that migrants associated to agro-forest habitats have no habitat selection during migration, at a meso-scale level. Using a GIS platform we extracted at a meso-scale range habitat cover at stopover localities. Results obtained from comparisons of soil covers between points randomly selected and true stopover localities sites revealed, as expected, the species may not select for particular habitats at a meso-scale range, because the habitat (or habitats) required by the species can be found virtually everywhere on their migration route. However, those birds stopping over in places richer in cropland or mosaic habitats including both cropland and forest and with proportionally less closed forest stayed for longer than in areas with lower surfaces of cropland and mosaic and more closed forest. This suggests that areas rich in cropland or mosaic habitat were optimal.     

Resource selection,utilization and seasons influence spatial distribution of ungulates in the western Serengeti National Park

Understanding herbivore selection and utilization of vegetation types is fundamental to conservation of multispecies communities. We tested three hypotheses for how ungulate species select their habitats and how this changes with season: first, resources are distributed as a mosaic of patches so that ungulates are also distributed patchily; this distribution reflects habitat selection, which changes with season, the different ungulates behaving differently. Second, resources become scarcer in the dry season relative to those in the wet season. If interspecific competition prevails, then all species should show a contraction of habitats chosen. Third, if predation is limiting, competition will be minimal, and hence, habitat selection by herbivores will not differ between seasons. We used frequencies of occurrence in four common vegetation types in western Serengeti National Park to determine selection coefficients and utilization patterns and Chi‐square analysis to test the hypotheses. The results showed that selection changes differently in each species, agreeing with the first hypothesis. Herbivores did not all become more selective, as predicted by the competition hypothesis, nor did selection remain the same across seasons, as predicted by the predation hypothesis. These results can be useful in constructing habitat suitability maps for ungulate species with special conservation needs.     

Effects of individual condition and habitat quality on natal dispersal behaviour in a small rodent

1. Individuals should benefit from settling in high-quality habitats, but dispersers born under favourable conditions have a better physical condition and should therefore be more successful at settling in high-quality habitats. 2. We tested these predictions with root voles (Microtus oeconomus) by a manipulation of individual condition through litter-size enlargement and reduction during lactation combined with a manipulation of habitat quality through degradation of the vegetation cover. We accurately monitored movements of 149 juveniles during a settlement and breeding period of 3 months. 3. The litter size treatment had long-lasting effects on body size, life-history traits and home range size, but did not influence dispersal behaviour. 4. Different stages of dispersal were influenced by habitat quality. In low-quality patches, females dispersed earlier, spent more time prospecting their environment before settling, and settlers had a smaller adult body size than in high-quality patches. Preference and competition for high-quality patches is likely adaptive as it increased fitness both in terms of survival and reproduction. 5. We found no interactive effect of individual condition and habitat quality on natal dispersal and habitat selection. 6. These findings suggest that immediate conditions are more important determinants of dispersal decisions than conditions experienced early in life.     

Habitat selection by nocturnal passerine migrants en route: mechanisms and results

Habitat use and habitat selection are essential for successful stopovers. Passerine migrants use habitats in a clearly non-random manner, even if many species utilise a broader range of habitats during passage than during breeding or wintering. Habitat selection proceeds as a sequence of events: landfall; search/settling, including redistribution across habitats if necessary; and habitat exploitation, with all stages probably condition-dependent. This review is aimed at studying this sequence and also the factors which govern decision-making in nocturnal passerine migrants at all levels. In most cases, habitats are (pre)selected by migrants already at landfall using both visual and acoustic cues. After landfall, migrants start to perform direct sampling of habitats during which they may move constantly and perform fine-tuning of their habitat choice. Some species subsequently occupy a small home range in a particular (micro)habitats, whereas others continue to move broadly during the whole stopover period. An interaction of several factors shapes the use of habitats after landing, among which are migrants’ innate preferences and functional morphology, foraging strategies and food resource distribution, habitat carrying capacity and exposure to predators. The large-scale spatial context probably also plays a role which might be currently underestimated.     

Habitat light and dewlap color diversity in four species of Puerto Rican anoline lizards

Closely related species often have signals that differ dramatically in design. The evolution of such differences may be important in the process of speciation. Selection for signal detectability under different habitat conditions has been proposed as a mechanism leading to the evolution of signal diversity. We examined dewlap color in four closely related species of Anolis lizards that occupy habitats with different light conditions. Initially, we tested the hypothesis that lizards choose specific light conditions within each habitat in which to signal. We rejected this hypothesis for all four species. We next calculated the detectability of the dewlap color of all four species at display locations in each habitat. If selection for detectability under the different light conditions explained the divergence in signal design, the occupant of a given habitat was predicted to have the highest signal detectability in that habitat. However, the rank order of detectability of the four dewlap colors was nearly the same in all four habitats. We concluded that divergent selection for signal detectability does not, by itself, explain the evolution of dewlap color diversity. We hypothesize that the evolution of dewlap color diversity results from simultaneous selection for multiple functions of dewlap color.     

HABITAT SELECTION AS AN EVOLUTIONARY GAME

Under habitat selection, mobile foragers may not only possess behavioral flexibility that allows them to utilize habitats selectively or opportunistically, but they may also possess heritable traits that influence their performance within each habitat. A game theoretic model is developed that investigates this evolutionary dimension of habitat selection. The model follows that of Rosenzweig (1987b) and considers a patchy environment containing two distinct habitat types. Behaviorally, foragers may be selective or opportunistic; morphologically, foragers possess traits that represent a trade-off between performance in the two habitat types. Depending on the environment's structure, one of three types of communities emerges as the ESS: (1) a single generalist species that behaves opportunistically, (2) two species that are extreme specialists on habitat 1 and 2, respectively; behaviorally, these species are selective on their respective habitat types, and (3) one generalist species that behaves opportunistically and one specialist species that behaves selectively on its preferred habitat. Community (1) emerges when habitat selection is costly, community (2) emerges when habitat selection is cost-free, and community (3) emerges when the relative abundances or productivities of the two habitat types are lopsided.     

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.     

Natal habitat-biased dispersal in the Siberian flying squirrel

Theoretically, dispersers should target the habitat where prospects for fitness will be highest. Aiming for a habitat similar to the natal area (natal habitat-biased dispersal) has been hypothesized as a probable rule of thumb for dispersers, but has received very little empirical support to date. We investigated similarities between natal and post-dispersal settlement sites with radio-collared Siberian flying squirrels (Pteromys volans L.). Juveniles born in small patches and raised in nests close to patch edge settled in small patches and used nests close to edges after dispersal. In addition, post-dispersal use of dreys (versus cavities) was similar to that observed in natal sites. However, the quality of settlement habitat was unrelated to the quality of the natal site, which suggests that natal experience on average-quality habitats may not lead to ecological traps for flying squirrels. This study provides evidence that habitat selection at the landscape scale is influenced by habitat of natal area.     

Habitat segregation among mimetic ithomiine butterflies (Nymphalidae)

The extensive wing pattern diversity observed among sympatric unpalatable mimetic butterflies is difficult to explain. Diversity is a paradox because selection by predators is expected to drive local species to use the same aposematic patterns. Habitat segregation among mimicry complexes has been suggested as a hypothesis to explain how diversity could be maintained. However, very few studies have tested this hypothesis. To test whether mimicry complexes are associated with particular habitats, I sampled a diverse assemblage of ithomiine butterflies from eastern Ecuador comprising nine discrete mimicry complexes. Butterflies were sampled in four habitats varying along a gradient of succession. A total of 43 species and 902 individuals were sampled. Ithomiine species richness and abundance were lowest in open habitats, and habitat preferences were documented for many species. Mimicry complexes exhibited significant habitat differences supporting the role of habitat segregation in maintaining mimetic diversity. However, there was obvious overlap among mimicry complexes, particularly involving the two numerically dominant patterns at the site. The pattern of segregation appears to be driven by common species, with relatively little evidence that the distribution of rarer species matches that of the more abundant species. Thus, habitat segregation is likely to play a role in the evolution of mimetic diversity as a result of segregated abundant model species, but the effect is probably weak and other factors are also important.     

Habitat selection of three chrysomelid species associated with Rumex spp.

Summary Habitat selection of three chrysomelid beetles feeding on the Rumex plants was studied in relation to the ecological requirements of the three species and the habitat characteristics arising in their unstable and often unpredictable environment. The different ecological requirements and bionomic characteristics of the three chrysomelids were associated not only with their differing degrees of feeding specialization; they also influenced habitat selection. For example, the small body size and lower food demands of Mantura clavareaui, which is a feeding specialist, allowed it to exploit any habitat, irrespective of habitat size and duration. The other specialist, Gastrophysa atrocyanea, selected larger habitats because of the relatively brief lifespan of its adults after overwintering, its reduced mortality, and greater food demand. On the other hand, Galerucella vittaticollis, which is a multivoltine, general feeder, selected the habitat that lasted longer, rather than one that was larger. This species' life cycle is not synchronized with Rumex phenology, but its adults are long-lived and it has a flexible life cycle. The different patterns in habitat selection among these three chrysomelids prevented serious overlapping of their habitats; thus, the habitat overlapping of 2 or 3 species was 34.62%, that of all 3 species 3.85%. Most of the habitats utilized by Gastrophysa, however, overlapped those of the other two species, because Gastrophysa's habitat utilization range was the narrowest of the three. In Mantura, the degree of overlap with the other two species was lowest because this beetle had the broadest range of habitat utilization. On the other hand, Galerucella adjusted the temporal, rather than the spatial coordinates of its habitat usage when it occupied habitats where the other two specialists were also present. Galerucella, unlike the other two species, is multivoltine. Consequently, it was able to coexist in habitats occupied by the others by exploiting the host plants at times when the other species were numerically few, or even absent. No direct interspecific competition among adults of the three species was observed. Their different patterns of habitat selection created by their different responses to available food allowed them to coexist on Rumex without direct competition.