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.     

Foraging behaviour and habitat partitioning of two sympatric cormorants in Patagonia, Argentina

Radiotelemetry was used to assess the distribution and diving behaviour of Rock Shags Phalacrocorax magellanicus and Red-legged Cormorants Phalacrocorax gaimardi breeding in sympatry, and Rock Shags breeding in isolation. When breeding in sympatry there was little overlap in the foraging locations of the two species, with the highest densities of each species separated by 10 km. Red-legged Cormorants fed significantly closer to the breeding colony than did Rock Shags and undertook shorter foraging trips, making almost twice as many foraging trips per day as Rock Shags. Rock Shags breeding in isolation had a shorter foraging range than the birds breeding in sympatry with Red-legged Cormorants and foraging trip duration was significantly shorter. However, the number of feeding trips per day was similar between areas of sympatry and allopatry. Differences in the foraging ecology of Rock Shags in areas of sympatry and allopatry may be due to interspecific competition, which forces niche differentiation. The distance between foraging sites, the speed of movement of the prey, a species tendency to move into prey-depleted areas and the length of the breeding season (during which the birds are constrained to be in the same area) may play critical roles in determining the extent to which differential area use by competitors is a strategy that benefits both parties.     

High-use movement pathways and habitat selection by ungulates

The cumulative movements of large mammals are expressed in many areas as semi-permanent wildlife trails. The mapping of semi-permanent trail networks offers a direct approach to assess habitat selection of high-use movement routes at relatively fine spatial scales across a landscape. Here we examine an ungulate trail network in north-central Utah created and maintained by the repeated movements of mule deer (Odocoileus hemionus) and elk (Cervus elaphus). In a resource selection analysis using multivariable spatial regression analysis, we show that at a spatial scale of 70 m open and low cover and distance to water are important predictors of movement pathway density. We also demonstrate at a scale of 10 m that elk and deer movement pathways are less steep than adjacent terrain. The mapping of trail networks should be a particularly useful technique for examining functional connectivity among resource patches across a landscape and identifying important high-use movement routes.     

Unpredictability of vigilance in two sympatric Tibetan ungulates

Vigilance is important for anti-predation, and different animals adopt different vigilance strategies. Instantaneous and sequential randomness in vigilance behavior are two main principles for the classic Pulliam model (1973). Given this context, we studied the vigilance behaviors in two wild cloven-hoofed animals, the Tibetan antelope (Pantholops hodgsonii) and the Tibetan gazelle (Procapra picticaudata) on Qinghai–Tibet Plateau, to explore if the two randomness principles work across species. The results showed that the distribution of inter-scan intervals of both Tibetan antelope and Tibetan gazelle followed the negative exponential distribution; inter-scans of both Tibetan antelope and Tibetan gazelle were unrelated with their previous scan, and most sequences of inter-scan intervals could be considered as random organized or unpredictable. In conclusion, the vigilance patterns of Tibetan antelope and Tibetan gazelle followed instantaneous randomness and sequential randomness of Pulliam model. A random vigilance strategy might be the best choice for Tibetan ungulates, and how to distinguish the social vigilance from anti-predator vigilance is an important issue for future research.     

Density-dependent habitat selection of corridors in a fragmented landscape

This paper assesses the occurrence of density-dependent habitat selection in an urban fragmented landscape, composed of forest fragments (urban parks) connected by corridors (wooded streets), to test the hypothesis that as population density increased in the parks their suitability decreased and individuals entered alternative habitats, such as wooded streets. Density variation of six species was studied during two consecutive breeding seasons. Vegetation structure in wooded streets was significantly less complex than in urban parks, supporting the view that wooded streets were less suitable for breeding birds. Five species (Coal Tit Parus ater , Spotless Starling Sturnus unicolor , Serin Serinus serinus , Black-billed Magpie Pica pica and Woodpigeon Columba palumbus ) showed density- dependent habitat occupation of wooded streets, while the Common Blackbird Turdus merula did not. As park suitability decreased with rising densities, wooded streets became a profitable alternative in terms of foraging, breeding, or for moving between parks. However, the relationships varied both between and within species in different years. Such differences could have been caused by variable rates of human disturbance, renewal of resources and predation risks in wooded streets. More detailed studies are required to determine how birds perceive and regulate their population dynamics in fragments and associated corridors, particularly for species targeted for management.     

Static habitat partitioning and dynamic selection by sympatric young Atlantic salmon and brown trout in south-west England streams

Direct underwater observation of micro‐habitat use by 1838 young Atlantic salmon Salmo salar [mean L_T 7·9 ± 3.1(s.d.) cm, range 3·19] and 1227 brown trout Salmo trutta (L_T 10·9 ± 5·0 cm, range 3·56) showed both species were selective in habitat use, with differences between species and fish size. Atlantic salmon and brown trout selected relatively narrow ranges for the two micro‐habitat variables snout water velocity and height above bottom, but with differences between size‐classes. The smaller fishes <7 cm held positions in slower water closer to the bottom. On a larger scale, the Atlantic salmon more often used shallower stream areas, compared with brown trout. The larger parr preferred the deeper stream areas. Atlantic salmon used higher and slightly more variable mean water velocities than brown trout. Substrata used by the two species were similar. Finer substrata, although variable, were selected at the snout position, and differences were pronounced between size‐classes. On a meso‐habitat scale, brown trout were more frequently observed in slow pool‐glide habitats, while young Atlantic salmon favoured the faster high‐gradient meso‐habitats. Small juveniles <7 cm of both species were observed most frequently in riffle‐chute habitats. Atlantic salmon and brown trout segregated with respect to use of habitat, but considerable niche overlap between species indicated competitive interactions. In particular, for small fishes <7 cm of the two species, there was almost complete niche overlap for use of water depth, while they segregated with respect to water velocity. Habitat suitability indices developed for both species for mean water velocity and water depth, tended to have their optimum at lower values compared with previous studies in larger streams, with Atlantic salmon parr in the small streams occupying the same habitat as favoured by brown trout in larger streams. The data indicate both species may be flexible in their habitat selection depending on habitat availability. Species‐specific habitat overlap between streams may be complete. However, between‐species habitat partitioning remains similar.     

Density-dependent habitat selection: Testing the theory with fitness data

Summary According to density-dependent habitat selection theory, reproductive success should decline with increased density. Fitness should be similar between habitats if habitat selection follows an ideal free distribution; fitness should be dissimilar between habitats if habitat selection is modified by territorial behavior. I tested these assumptions by examining a variety of fitness estimates obtained from white-footed mice living in nest boxes in forest, forest edge and fencerow habitats in southwestern Ontario. As expected, mean litter size declined with increased population density. Litter sizes, adult longevity and the proportion of adult animals in breeding condition were not significantly different among the three habitats. The success at recruiting at least one offspring to the adult population and the number of recruits per litter were much greater in the forest than in either of the other two habitats. Fitness was thus unequal among habitats and the results confirm both assumptions of density-dependent habitat selection theory for territorial white-footed mice.     

Spatial distribution and habitat selection in coexisting species of mountain ungulates

One of the main objectives of community ecology is to understand the conditions allowing species to coexist, which requires identifying how co‐occurring species use and share space and resources. Species of the same trophic level, such as large herbivores, are of fundamental interest in that context because competition for resources is likely. Segregation in space or on some axes of the ecological niche are processes allowing coexistence, yet, both are seldom studied jointly. Based on annual spring censuses collected for 11 yr, we analysed the degree of overlap in spatial distribution among chamois Rupicapra rupicapra and mouflon Ovis gmelini musimon, 2 species of similar size that coexist on the same alpine pastures. We further investigated whether they differed in terms of habitat selection processes, and identified which environmental factors led to species being aggregated or segregated. The areas of intensive use for 2 species were more spatially aggregated than expected by chance. Habitat selection was studied using multivariate methods based on the niche concept, considering the presence of 1 species as an environmental variable for the other. Despite a large overlap in niches (88%), segregation was significant as chamois preferred meadows dominated by Sesleria and Carex sempervirens while mouflon selected meadows dominated by Carex ferruginea and avoided being close to areas affected by human activities. Importantly, habitat selection within each species was not affected by the presence of the other species. Coexistence between these 2 species and spatial overlap may be permitted because resource partitioning occurs at a fine temporal and/or spatial scale. We underscore that joint approaches of spatial and ecological processes are necessary to disclose the type of interaction (neutral, facilitation or competition) at play within a community.     

Assessing differential prey selection patterns between two sympatric large carnivores

Several conceptual models describing patterns of prey selection by predators have been proposed, but such models rarely have been tested empirically, particularly with terrestrial carnivores. We examined patterns of prey selection by sympatric wolves ( Canis lupus ) and cougars ( Puma concolor ) to determine i) if both predators selected disadvantaged prey disproportionately from the prey population, and ii) if the specific nature and intensity of prey selection differed according to disparity in hunting behavior between predator species. We documented prey characteristics and kill site attributes of predator kills during winters 1999–2001 in Idaho, and located 120 wolf-killed and 98 cougar-killed ungulates on our study site. Elk ( Cervus elephus ) were the primary prey for both predators, followed by mule deer ( Odocoileus hemionus ). Both predators preyed disproportionately on elk calves and old individuals; among mule deer, wolves appeared to select for fawns, whereas cougars killed primarily adults. Nutritional status of prey, as determined by percent femur marrow fat, was consistently poorer in wolf-killed prey. We found that wolf kills occurred in habitat that was more reflective of the entire study area than cougar kills, suggesting that the coursing hunting behavior of wolves likely operated on a larger spatial scale than did the ambush hunting strategy of cougars. We concluded that the disparity in prey selection and hunting habitat between predators probably was a function of predator-specific hunting behavior and capture success, where the longer prey chases and lower capture success of wolf packs mandated a stronger selection for disadvantaged prey. For cougars, prey selection seemed to be limited primarily by prey size, which could be a function of the solitary hunting behavior of this species and the risks associated with capturing prime-aged prey.     

Summer stream habitat partitioning by sympatric Arctic charr, Atlantic salmon and brown trout in two sub-arctic rivers

Summer habitat use by sympatric Arctic charr Salvelinus alpinus, young Atlantic salmon Salmo salar and brown trout Salmo trutta was studied by two methods, direct underwater observation and electrofishing, across a range of habitats in two sub-arctic rivers. More Arctic charr and fewer Atlantic salmon parr were observed by electrofishing in comparison to direct underwater observation, perhaps suggesting a more cryptic behaviour by Arctic charr. The three species segregated in habitat use. Arctic charr, as found by direct underwater observation, most frequently used slow (mean ±s .d . water velocity 7·2 ± 16·6 cm s⁻¹) or often stillwater and deep habitats (mean ±s .d . depth 170·1 ± 72·1 cm). The most frequently used mesohabitat type was a pool. Young Atlantic salmon favoured the faster flowing areas (mean ±s .d . water velocity 44·0 ± 16·8 cm s⁻¹ and depth 57·1 ± 19·0 cm), while brown trout occupied intermediate habitats (mean ±s .d . water velocity 33·1 ± 18·6 cm s⁻¹ and depth 50·2 ± 18·0 cm). Niche overlap was considerable. The Arctic charr observed were on average larger (total length) than Atlantic salmon and brown trout (mean ±s .d . 21·9 ± 8·0, 10·2 ± 3·1 and 13·4 ± 4·5 cm). Similar habitat segregation between Atlantic salmon and brown trout was found by electrofishing, but more fishes were observed in shallower habitats. Electrofishing suggested that Arctic charr occupied habitats similar to brown trout. These results, however, are biased because electrofishing was inefficient in the slow-deep habitat favoured by Arctic charr. Habitat use changed between day and night in a similar way for all three species. At night, fishes held positions closer to the bottom than in the day and were more often observed in shallower stream areas mostly with lower water velocities and finer substrata. The observed habitat segregation is probably the result of interference competition, but the influence of innate selective differences needs more study.     

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.     

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.     

Density-dependent habitat selection alters drivers of population distribution in northern Yellowstone elk

Although it is well established that density dependence drives changes in organismal abundance over time, relatively little is known about how density dependence affects variation in abundance over space. We tested the hypothesis that spatial trade-offs between food and safety can change the drivers of population distribution, caused by opposing patterns of density-dependent habitat selection (DDHS) that are predicted by the multidimensional ideal free distribution. We addressed this using winter aerial survey data of northern Yellowstone elk (Cervus canadensis) spanning four decades. Supporting our hypothesis, we found positive DDHS for food (herbaceous biomass) and negative DDHS for safety (openness and roughness), such that the primary driver of habitat selection switched from food to safety as elk density decreased from 9.3 to 2.0 elk/km². Our results demonstrate how population density can drive landscape-level shifts in population distribution, confounding habitat selection inference and prediction and potentially affecting community-level interactions.     

The scale-dependent impact of wolf predation risk on resource selection by three sympatric ungulates

Resource selection is a fundamental ecological process impacting population dynamics and ecosystem structure. Understanding which factors drive selection is vital for effective species- and landscape-level management. We used resource selection probability functions (RSPFs) to study the influence of two forms of wolf (Canis lupus) predation risk, snow conditions and habitat variables on white-tailed deer (Odocoileus virginianus), elk (Cervus elaphus) and moose (Alces alces) resource selection in central Ontario's mixed forest French River-Burwash ecosystem. Direct predation risk was defined as the frequency of a predator's occurrence across the landscape and indirect predation risk as landscape features associated with a higher risk of predation. Models were developed for two winters, each at two spatial scales, using a combination of GIS-derived and ground-measured data. Ungulate presence was determined from snow track transects in 64 16- and 128 1-km(2) resource units, and direct predation risk from GPS radio collar locations of four adjacent wolf packs. Ungulates did not select resources based on the avoidance of areas of direct predation risk at any scale, and instead exhibited selection patterns that tradeoff predation risk minimization with forage and/or mobility requirements. Elk did not avoid indirect predation risk, while both deer and moose exhibited inconsistent responses to this risk. Direct predation risk was more important to models than indirect predation risk but overall, abiotic topographical factors were most influential. These results indicate that wolf predation risk does not limit ungulate habitat use at the scales investigated and that responses to spatial sources of predation risk are complex, incorporating a variety of anti-predator behaviours. Moose resource selection was influenced less by snow conditions than cover type, particularly selection for dense forest, whereas deer showed the opposite pattern. Temporal and spatial scale influenced resource selection by all ungulate species, underlining the importance of incorporating scale into resource selection studies.     

Fine‐scale habitat selection by sympatric Canada lynx and bobcat

The Canada lynx (Lynx canadensis) and the bobcat (Lynx rufus) are closely related species with overlap at their range peripheries, but the factors that limit each species and the interactions between them are not well understood. Habitat selection is a hierarchical process, in which selection at higher orders (geographic range, home range) may constrain selection at lower orders (within the home range). Habitat selection at a very fine scale within the home range has been less studied for both lynx and bobcat compared to selection at broader spatiotemporal scales. To compare this fourth‐order habitat selection by the two species in an area of sympatry, we tracked lynx and bobcat during the winters of 2017 and 2018 on the north shore of Lake Huron, Ontario. We found that both lynx and bobcat selected shallower snow, higher snowshoe hare abundance, and higher amounts of coniferous forest at the fourth order. However, the two species were spatially segregated at the second order, and lynx were found in areas with deeper snow, more snowshoe hare, and more coniferous forest. Taken together, our findings demonstrate that the lynx and bobcat select different resources at the second order, assorting along an environmental gradient in the study area, and that competition is unlikely to be occurring between the two species at finer scales.     

Distinguishing trophic and habitat partitioning among sympatric populations of the estuarine fish Osmerus mordax Mitchill

Sympatric populations of rainbow smelt Osmerus mordax of the St Lawrence middle estuary exhibited distinct morphologies suitable for exploiting either pelagic or benthic habitats in the absence of consistent morphological features related to specific diets. The magnitude of morphological differentiation was elevated and constant over 3 years even though statistically significant differences were observed among samples within populations. Both populations were spatially segregated in the estuary but some population mixing occurred at low densities of rainbow smelt. Diet analysis revealed that both populations opportunistically feed on large macrozooplankton. An amended version of Schoener's D was developed and used to compare the magnitude of niche overlap. It revealed that the diet of the middle estuary rainbow smelt differed according to the area where samples were collected but no differences existed between populations. The exploitation of ecologically distinct habitats appeared to be effective in maintaining morphologically distinct sympatric populations within an estuarine setting in the absence of diet differentiation.