Role In Habitat Selection Research Articles

Do predation risk and food availability modify prey and mesopredator microhabitat selection in eelgrass (Zostera marina) habitat?

Positive relationships between organismal abundance and habitat structural complexity are common in seagrasses, but the role of habitat selection in structuring these relationships (as opposed to differential mortality among habitat treatments) has been little explored. Moreover, active habitat selection may be modified by factors such as perceived predation risk or food availability, and may differ between prey and predator species. We employed mesocosms to test whether two species that are abundant in southern California eelgrass (Zostera marina) habitat, the epifaunal grass shrimp Hippolyte californiensis and the mesopredatory juvenile giant kelpfish Heterostichus rostratus, a primary predator of grass shrimp, select habitat based on structural complexity (shoot density), and if habitat selectivity is modified by predation risk and food levels. We found that juvenile giant kelpfish displayed clear preference for high complexity artificial eelgrass habitat that was not altered by predation risk or food availability, spending the majority of their time in high complexity habitat even when predators were present in high complexity habitat and food was made available only in low complexity habitat. In contrast, grass shrimp exhibited no preference for high complexity habitat in mesocosms and appeared to primarily remain on whatever habitat was initially encountered. In field experiments within naturally occurring eelgrass habitat, however, grass shrimp remained in high complexity eelgrass but fled from low complexity eelgrass regardless of predator presence. Our research highlights the important role of active habitat selection in determining species distributions in seagrass beds, the strong and overriding influence of structural complexity on habitat selection, and the importance of comparing results from controlled laboratory experiments to those from naturally occurring habitat.

Social environment affects juvenile dispersal in great tits (Parus major)

1. Habitat selection can affect individual fitness, and therefore, individuals are expected to assess habitat quality of potential breeding sites before settlement. 2. We investigated the role of social environment on juvenile dispersal behaviour in the great tit (Parus major). Two main contradictory hypotheses can be formulated regarding social effects on juvenile dispersal as follows: (i) High fledgling density and sex ratio may enhance the intensity of local (kin) competition and, therefore, reduce individual survival chance, enhance emigration and reduce settlement ('repulsion' hypothesis) (ii) Alternatively, high fledgling density and sex ratio may signal high-quality habitat or lead to aggregation and thus increase individual survival chance, reduce emigration and enhance settlement ('attraction' hypothesis). 3. To disentangle positive from negative effects of high density and male-biased sex ratio on dispersal, we manipulated the social composition of the fledgling population in 12 semi-isolated nest-box areas (plots) via a change of fledgling density (low/high) as well as fledgling sex ratio (female-biased/balanced/male-biased) across 3 years. We then tested whether experimental variation in male and female fledgling densities affected variation in local survival, emigration and settlement of juveniles, and whether social effects on survival and dispersal support the 'repulsion' or 'attraction' hypothesis. 4. We found no experimental effects on local survival and emigration probabilities. However, consistent with the 'attraction' hypothesis, settlement was significantly and positively affected by local experimental sex ratio in each of the study years: both male and female juveniles avoided female-biased plots and settled more in plots that were balanced and male-biased the previous year. 5. Our study provides unprecedented experimental evidence that local sex ratio plays a causal role in habitat selection. We suggest that settlers avoid female-biased plots because a high proportion of females may reflect the absence or the low quality of local resources in the habitat. Alternatively, male territory acquisition may be facilitated by a high local density of 'candidate' males, and therefore, juveniles were less successful in settling in female-biased plots.

Migratory strategy of Bluethroats, Luscinia svecica, in eastern Fennoscandia. Part 2: response to acoustic markers and habitat selection at stopover

Response of bluethroats to the playback of species-specific song and the role of acoustic markers in habitat selection at autumn migratory stopovers were studied. The birds were mist-netted after the song playback and under the control conditions (no song). First captures and recaptures of birds were discussed. The song playback attracts bluethroats in the end of nocturnal migratory flights and the very beginning of a stopover and does not attract birds during stopover. Acoustic marker plays a significant role in habitat selection at the first stages of stopover: The tape-lured bluethroats concentrate around the sound source and then distribute over surrounding area. The possible reasons of response of migrating birds to the acoustic marker, as well as higher proportion of adults and lower proportion of fat birds among tape-lured bluethroats in comparison to the birds trapped under the control conditions have been discussed.

Habitat selection of rodents along a piñon–juniper woodland–savannah gradient

The biodiversity of pinon-juniper (P-J) woodlands of western North America has received increasing attention from conservationists because of concerns about woodland invasion into adjacent shrub-steppe communities. Small mammals contribute substantially to the faunal diversity of P-J woodlands, but the responses of small mammals to the inherent structural complexity of these woodlands have not been well described. We used occupancy modeling and ordination to describe habitat selection along a woodland-savannah gradient of 4 species of rodents, including 2 putative P-J woodland specialists, Peromyscus truei and Tamias dorsalis .W e accounted for the influences of season and prior conspecific capture on detectability. P. truei and T. dorsalis preferred old-growth woodland sites near rock outcrops with high canopy and bare-ground cover and low herb and shrub cover. Tamias minimus preferred savannah sites with high shrub and grass cover farther from rock outcrops. We found no evidence of resource selection along the gradient by Perognathus parvus. Our results provide insight into the role of habitat selection in facilitating coexistence among closely related species within P-J woodlands and suggest that management efforts that better discriminate between old, presettlement stands and young, invasive stands may enhance regional biodiversity. We demonstrate an effective application of occupancy modeling for describing habitat selection of cryptic rodents, which are particularly prone to imperfect detection. DOI: 10.1644/09-MAMM-A-060.1.

Comparing different environmental variables in predictive models of bird distribution

The response variable (often the presence of a species) in predictive habitat models relies on a set of environmental predictors. Among all known environmental predictors, vegetation has the most effect on species abundance and on their habitat preferences, due to the wide range of necessary resources that it provides for the survival of bird species. However, other predictors, in turn, affect bird distribution, and some-times they play a more important role in habitat selection, depending on the natural history and ecological needs of the bird species. In this regard, different analyses have been conducted to predict the distribution, and define habitat suitability (such as discriminant function analysis, General Linear Models, and ANOVA). In this study, all three analytical designs were used to investigate the relationship of seven bird species to the major environmental gradients in the study area, to find out the significance of each of these factors on habitat selection. GIS has been used to prepare spatial distributional data, and to overlay and calculate different aspects of the environmental factors. The results suggest that potential individual habitat patches play a small role compared to the landscape (entire corresponding habitat patches), when considering vegetation. The influence of built-up areas is significant for all the species, and the proximity to the sea shore is very significant for at least one of the species, however, it is not neutral for all other species.

Multivariate characterisation of the habitats of seven species of Malayan mudskippers (Gobiidae: Oxudercinae)

Mudskippers (Gobiidae: Oxudercinae) are amphibious gobies. The occurrence and habitats of seven sympatric species of Malayan mudskippers were recorded: Boleophthalmusboddarti, Boleophthalmuspectinirostris, Periophthalmodon schlosseri, Periophthalmus chrysospilos, Periophthalmus gracilis, Periophthalmus variabilis and Scartelaos histophorus. A total of 14 surveys were made at six localities along 120 km of the west coast of the Malay Peninsula in August 1996 and September 2006. A multivariate set of ordinal parameters were used to measure the quantity of environmental water in mudskippers’ habitats, and three guilds of species were discriminated. The resulting pattern mirrored the adaptive scenario depicted by physiological and anatomical studies of mudskippers’ terrestriality, although in one case a terminal taxon was less terrestrial than other more basal ones. Differentiation of trophic niches may also play a role in habitat selection by the herbivorous Boleophthalmus spp., which were only found in unvegetated, illuminated areas and on wet mud at low tide. With the more terrestrial guild, a significant negative co-occurrence suggested direct interspecific competition.

Microhabitat occupation and functional morphology of four species of sympatric agamid lizards in the Kyzylkum Desert, central Uzbekistán

We examined microhabitat occupation and functional morphology of four sympatric agamid lizards (Phrynocephalus helioscopus helioscopus, P. interscapularis, P. mystaceus galli and Trapelus sanguinolentus) at three sites in the arid zone of central Uzbekistan. At two sites located in sand dunes, substrate attributes played a key role in habitat selection by three syntopic species. At a third flat, stony site, P. helioscopus selected habitat non–randomly, tending to occur close to sparse, low vegetation. Syntopic taxa were separated in morphospace, and there was a trend for taxa with proportionally longer limbs to have faster field escape speeds. Field escape distances and predator avoidance tactics differed between species, with two main escape strategies (crypsis or sand–diving following an escape sprint). We caution that broad–scale threatening processes such as over–grazing and salinity may be having a detrimental effect on microhabitat features important to terrestrial reptiles in Uzbekistan.

Solar radiation plays a role in habitat selection by the sea star Pisaster ochraceus

In order to avoid intense abiotic stresses at low tide, motile animals in the intertidal zone must identify and move into 'safe' microhabitats before the tide drops. We experimentally investi- gated the role of solar radiation in the microhabitat selection of the sea star Pisaster ochraceus, an ecologically important predator in the Pacific Northwest. Specifically, we tested the response of P. ochraceus to UVB radiation (290 to 315 nm), UVA radiation (315 to 400 nm), and photosynthetically active radiation (PAR; 400 to 700 nm) to determine whether specific wavelength ranges prompted specific responses. Sea stars were exposed to ambient summer light levels in shallow water in out- door aquaria in which one-half of the tank was covered with a light-filtering material. We established 4 light filtration treatments: filter control (+UVB +UVA +PAR), no UVB (-UVB +UVA +PAR), no UVR (-UVB -UVA +PAR), and shade (-UVB -UVA -PAR), plus a no-filter control treatment (+UVB +UVA +PAR). P. ochraceus did not avoid UVB or UVA, but they did avoid PAR: individuals in tanks with shaded areas spent significantly more time under the filters than did individuals in the tanks with fil- ters that blocked UV radiation. In field surveys, 85% of emersed P. ochraceus encountered during low tide were located in areas completely shaded from both PAR and UV radiation (by rocks or by the intertidal kelp Hedophyllum sessile). This study demonstrates that the presence of PAR in shallow water conditions prompts P. ochraceus to seek out shaded microhabitats that have more benign low- tide abiotic conditions than exposed rock benches.

Modelling the recent and potential future spatial distribution of the Ring Ouzel (Turdus torquatus) and Blackbird (T. merula) in Switzerland

We present here a multiscale modelling approach to predict the current and future spatial distribution of Ring Ouzel (Turdus torquatus) and Blackbird (T. merula) in Switzerland. Species distribution models (SDMs) are applied on three different scales in order to analyse the scale-dependency of predictors that describe the species’ realised niche. While the models on the macro- and mesoscales (grid of 100 and 1 km2, respectively) cover the entire country, our small-scale models are based on a small set of territories. Ring Ouzels occur at altitudes above 1000 m a.s.l. only, while Blackbirds occur from the lowlands up to the timberline. Although both species coexist on the macro- and mesoscales, a direct niche overlap on territory scale is rare. Small-scale differences in vegetation cover and structure seem to play a dominant role in habitat selection. On the macroscale, however, we observed a high dependency on bioclimatic variables that mainly represent the altitudinal range and the related forest structure preferred by both species. Applying the models to climate change scenarios, we predict a decline of suitable habitat for the Ring Ouzel with a simultaneous median altitudinal shift of 440 m until 2070. In contrast, the Blackbird is predicted to benefit from higher temperatures and expand its range to higher elevations. Based on the species distribution models we (1) demonstrate the scale-dependency of environmental predictors, (2) quantify the scale-dependent habitat requirements of Blackbird and Ring Ouzel and (3) predict the altitudinal range shift of both species as related to climate change scenarios.

Acoustic information as a distant cue for habitat recognition by nocturnally migrating passerines during landfall

During migration, birds must fly over suboptimal habitats differing from those selected during breeding and wintering. Nocturnally migrating passerines need to assess the suitability of potential stopover habitats during landfall. Before actual landfall, distant cues may play a significant role in habitat selection. In this paper, we studied the possibility that migrant songbirds use acoustic information as distant cues for habitat selection when ceasing flight. We also investigated differences between habitat generalists and specialists in their use of acoustic cues by comparing the proportion of species killed at lighthouses with those captured in mist nets using song playback in both suitable habitats and suboptimal stopover habitats during spring and fall migratory seasons. Our observations showed that during twilight landfall, birds may respond to acoustic cues, especially when visual cues are reduced or absent. This was true for habitat specialists (Eurasian reed warblers and sedge warblers) whose songs are also more attractive to conspecifics and other birds of wetland habitats than to habitat generalists (pied flycatchers and redwings). Adult Eurasian reed warblers had a tendency to be more attracted by acoustic cues than juveniles. This finding suggests that previous experience may play a role in habitat recognition using acoustic stimuli. Key words: heterospecific attraction, lighthouses, song playback. [Behav Ecol]

A review of habitat selection by the swamp rat, Rattus lutreolus (Rodentia: Muridae)

Abstract This review clarifies important points on habitat selection by the Swamp Rat Rattus lutreolus (Rodentia: Muridae), a species that has been the subject of much research in Australia and has provided a useful model for understanding ecological and biological processes. It also provides an opportunity to cite important earlier research, not readily available through electronic search engines, thus bringing it into current literature to avoid its disappearance into Internet obscurity. We comment on some papers in the literature to correct errors detected and to emphasize the importance of due care in all aspects of a research project, including its reporting. We show that both floristic and structural components have been reported as important to an understanding of habitat and microhabitat selection by R. lutreolus and conclude that it is vegetation density that is of paramount importance. Female R. lutreolus are clearly dominant in driving microhabitat selection, occupying the ‘best’ or densest habitats with male R. lutreolus occupying the next best and Pseudomys or other species, where present occupying the remainder. This demonstrates the important role that intraspecific and interspecific competition play in determining habitat selection. Direct predation and the perception of predation risk may also play a role in habitat selection, again perceived to be pushing individuals towards denser vegetation, representing ‘better cover’. Whether these effects operate as bottom‐up or top‐down needs careful consideration. Climatic variables, such as ENSO‐affecting productivity, and related variables such as temperature and humidity may also play important roles in habitat selection, as can disturbance effects such as wildfire. The relative importance of all of these potential determining factors may vary from place to place, particularly when climatic clines are involved.

Habitat use, movement, and growth of young-of-the-year Fundulus spp. in southern New Jersey salt marshes: Comparisons based on tag/recapture

We examined habitat use, movement, and growth of young-of-the-year (YOY) Fundulus heteroclitus and Fundulus luciae with a tag/recapture experiment in tide-dominated salt marshes to determine if movements from Spartina marsh surface can account for the occurrence of larger, older individuals in other habitats. Evaluation of the tagging techniques in laboratory experiments with YOY F. heteroclitus (15–35 mm TL) found that coded wire tags were retained at least up to 77 days. The high rates of recapture in the field also indicate that the tagging approach generally worked well. Of a total of 5748 YOY F. heteroclitus (14–40 mm TL) and 133 YOY F. luciae (17–40 mm TL) tagged, 56.0% and 74.4% were recaptured, respectively. Most (44%) YOY F. heteroclitus recaptured occurred at or near (0–5 m) the release site, but some were captured up to 299 m away up to 166 days after tagging. By comparison, movement of F. luciae was very limited, with 99% of recaptures occurring at the exact site of release after up to 66 days at liberty. These different movement patterns by YOY Fundulus indicate that species-specific behavior plays an important role in habitat selection. In addition, it appears that dispersal of YOY F. heteroclitus can help to explain the occurrence of larger individuals of this species in Phragmites-dominated marshes even though there is little evidence of use of this habitat by small YOY.

Physiological ramifications of habitat selection in territorial male ovenbirds: consequences of landscape fragmentation.

Since boreal forest fragments are of lower quality than contiguous forest for breeding Ovenbirds (Seiurus aurocapillus), we predicted that competition for breeding sites in contiguous forest should lead to a greater prevalence of individuals in better condition in these habitats. We quantified male condition using morphological and hematological indices. Males in contiguous forest were larger than males in forest fragments and had higher hematocrits and mean corpuscular volumes, as well as a greater prevalence of polychromatic cells. These hematological indices are all positively associated with energy demands or stress, or both. Furthermore, the proportion of heterophils, a type of white blood cell positively associated with stress, decreased through the breeding season only for males in forest fragments. Total plasma protein and mass corrected for structural size did not differ between landscapes, suggesting that the nutritional status of males was similar between landscapes. All of these trends were independent of age. Overall, these results indicate that size of male Ovenbirds could be playing a role in habitat selection, but that defending territories in contiguous forest, where breeding success is higher and populations are denser, seems to result in greater energetic demands and a reduced immunological condition. These results demonstrate a physiological component to contrasting consequences associated with territory acquisition in birds.

Trade-offs between foraging and predation risk determine habitat use in a desert baboon population

This study explored how baboons,Papio cynocephalus ursinus, in a desert population trade off foraging and predation risk in patterns of habitat use. Four hypotheses were proposed and tested; that habitats are used on the basis of (1) availability, (2) foraging rewards (following the ideal free distribution), (3) foraging rewards modified by the constraints of nocturnal predation risk (partial trade-off) or (4) foraging rewards modified by the constraints of continuous predation risk (full trade-off). Food availability in each habitat was quantified through quadrat surveys describing stem density, plant part availability and nutrient availability (protein and energy) of key plant foods per unit area. Predation risk in each habitat was evaluated using measures describing the relative threat of attack and capture by both leopards,Panthera pardus, and lions,Panthera leo; attack risk was determined on the basis of habitat visibility and predator ambush distances, while capture risk was calculated using a simple model incorporating predator and prey velocities, habitat visibility and nearest refuge distance. Patterns of baboon behaviour were assessed for adult males and females in four groups through day-follows and instantaneous sampling. The baboons did not use habitats directly in proportion to their availability in the group's ranging area (in contrast to hypothesis 1). Nor did patterns of habitat use match those expected on the basis of the ideal free distribution, either during all activities (hypothesis 2) or during feeding (hypothesis 3). Instead, the baboons spent less time feeding in the high-risk food-rich habitat but more time feeding in the low-risk relatively food-poor habitat. Baboons also preferred low-risk habitats in other activities, where during resting and grooming all habitats were avoided except the safest (where key food species were absent). These results support hypothesis 4. Deviations from these patterns by a unimale group indicate that male reproductive strategies may also play a role in habitat selection.

Effects of forest fragmentation on a guild of wintering passerines: The role of habitat selection

This study analyses the winter colonization of an archipelago of 31 forests (0·1–350 ha) in central Spain by the guild of pariforms ( Parus, Aegithalos, Regulus, Sitta and Certhia). Two hypotheses are considered: (a) that birds with similar habitat preferences tend to disappear simultaneously with the reduction in forest size, leading to a ‘nested’ pattern of species distribution; or (b) that the species in the smallest forests are a random sample of those found in the larger ones. The results support hypothesis (a). The species that depend on relatively scarce resources, such as tree trunks and junipers Juniperus thurifera ( Sitta europaea, Certhia brachydactyla, Parus cristatus and P.ater) only occupied the largest forests. On the other hand, species that exploit abundant, ubiquitous resources, such as holm oak Quercus ilex foliage ( Regulus ignicapillus and Parus caeruleus), were distributed uniformly throughout all the fragments. These results emphasize the need for a better understanding of habitat selection by species when designing conservation strategies for fragmented populations.

Behavioural responses of competent lobster postlarvae to odor plumes

We determined the ability of competent laboratory-reared Homarus americanus postlarvae to use odor plumes as a potential mechanism of orientation during the habitat selection process at settlement. In a Y-maze apparatus, both swimming direction (upstream or downstream) and arm selection (control or experimental) were strongly influenced by the nature of the chemical stimulus tested. In control conditions (unconditioned water in both proximal arms of the experimental device), postlarvae swam predominantly upstream (61:39) but displayed no arm preference (48:52). When submitted to a metabolite solution of adult conspecifics or macroalgae, postlarvae swam consistently upstream (≥73%) and showed a significant preference for the arm providing these stimuli (≥69%). In contrast, postlarvae submitted to cunner (Tautogolabrus adspersus) metabolites swam predominantly downstream (68:32), and those that did swim upstream significantly avoided the experimental arm (30:70). In repeated trials with the same individuals, postlarvae behaved similarly in ≥80% of the assays, suggesting a low intra-individual variability in behavioural responses. It is concluded that distance chemoreception may play a role in habitat selection by lobster postlarvae at settlement.

Effects of predation-risk on habitat use by Himalayan Snowcocks

When given a choice, animals often prefer foraging habitats where predation risk is low, even if such habitats provide reduced foraging opportunities. We evaluated foraging rates of tame but free-ranging Himalayan Snowcocks (Tetraogallus himalayensis) in 16 types of alpine habitats. Foraging rate was highest on level or slightly-sloping terrain and where grasses were relatively abundant. We also observed 102 wild snowcocks and found they were most nervous about raptorial predators when on level or slightly-sloping terrain and in small coveys. Snowcocks face a dilemma: they are most vulnerable to raptors in areas where they can forage most efficiently. During summer snowcocks trade off higher foraging efficiency on level terrain for lower predation risk on steeper terrain. During winter, when raptor numbers are lower, snowcocks apparently revert to using level or slightly-sloping, high-efficiency foraging habitats. Risk of predation plays an important role in habitat selection and resource utilization by snowcocks.

The convergent trajectories of bird communities along ecological successions in european forests.

There is much more variation in the composition of bird communities in the earlier open and semi-open seral stages of ecological successions in forested landscapes of Europe than later on in preforested and forested climactic stages. The demonstration of this trend is achieved from the study of four habitat gradients, two in the mediterranean region (Provence and Corsica) and two in central Europe (Burgundy, France and Poland). A multivariate analysis has been used to illustrate the dynamics of communities along these succession. Displays of the results in bivariate space as well as an illustration of the distributional profiles of some of the most characteristic species show that: i) there is a discrimination between the two mediterranean gradients and the two medioeuropean ones and ii) each succession starts with a very distinct set of species and then the four gradients regularly converge in the last climactic stage where there is almost no discrimination between communities. These results are discussed in the light of the history of European biotas during the Pleistocene. The reason why there is more variation in species composition in the earlier seral stages than in the later forested stages are discussed according to current theories on the role of habitat selection on speciation processes.

Competition and the Role of Habitat Selection in Gerbillus Allenbyi and Meriones Tristrami: A Removal Experiment

In northern Israel, two seed—eating gerbiline rodents exhibit exclusive distributions. Gerbillus allenbyi is the only species inhabiting coastal sand dunes between Tel Aviv and Mt. Carmel. Meriones tristrami is also found in the same coastal region, but there it is restricted to nonsandy habitats. However, in the coastal area north of Mt. Carmel where G. allenbyi does not occur, M. tristrami inhabits dunes. This type of distribution is usually interpreted to result from competition. A perturbation experiment conducted in the narrow transition strip between soil and sand in an area where the above species are sympatric revealed no ongoing competition between the two species. It is suggested that strong interspecific competition occurring in the past resulted in genetically determined habitat selection in the present.

Attraction of Notropis lutrensis (Cyprinidae) to Water Conditioned by the Presence of Conspecifics

Responses of red shiners, Notropis lutrensis, to water conditioned by the presence of conspecifics were evaluated in laboratory preference chambers. Fish from the South Canadian River (Arkansas River drainage) and Lake Texoma (Red River drainage), tested under different conditions at different times of year, all exhibited a strong attraction to water previously occupied by conspecifics. Olfaction as a cue to habitat selection is discussed with respect to the known ecology of the species. Some fishes of the turbid streams of the Great Plains have highly developed olfactory senses (Moore 1950). Notropis lutrensis (red shiner), one of the most abundant Great Plains species, can use physico-chemical and auditory stimuli in selecting habitat or locating mates (Delco 1960, Matthews and Hill 1979), but its response to olfactory stimuli has not been reported previously. Our field studies in the South Canadian River near Norman, Oklahoma, suggested that odor might also provide this species a cue to habitat selection, resulting in their attraction to water preconditioned by the presence of conspecifics. In October 1976, we collected 19,182 N. Iutrensis in a ca. 600 m section of the South Canadian River. Of those, 90.7% were taken in two backwater pools (each ca. 20 x 5 m) located near the main river channel (Matthews and Hill 1979). In addition to these collections, repeated visits to the site indicated use of the backwaters by large numbers of red shiners throughout October. We hypothesized that the shiners concentrated in the backwaters, despite adequate water for greater dispersal, because those areas presented favorable physico-chemical conditions. The backwaters were relatively stable thermally, fluctuating 1-2?C daily, compared to fluctuation of up to 12?C daily at mainstream locations. Additionally, diurnal pH in the backwaters (8.2-8.5) was closest to the pH selected by N. lutrensis in the laboratory (Matthews and Hill 1979), and pH in the mainstream (8.7-9.5) approached values stressful to this species (Matthews and Hill 1977). During the day, sharp temperature and pH gradients, which shiners could have used as habitat cues, existed between the mainstream and backwaters. At night, by contrast, no marked gradients existed, yet the thousands of N. lutrensis remained in the backwaters, not dispersing into nearby standing-water pools of the river mainstream. This behavior suggested that some factor not detected by our measurements, e.g., conditioning of backwaters by the presence of conspecifics, played a role in habitat selection by the species. The crowding of thousands of individuals into small pools undoubtedly resulted in accumulation of odors of mucous, excretory products, or other exudates. The attraction of fishes to water having the odor of conspecifics is well documented (Kleerekoper 1969, Fine et al. 1977).