Head raising during foraging and pausing during intermittent locomotion as components of antipredator vigilance in chipmunks

Although many studies of vigilance examine head raising in foraging, grooming or resting animals, pauses during intermittent locomotion are rarely considered from the perspective of vigilance, and no studies have compared head raising and pausing in the same system. We videotaped central place foraging chipmunks, Tamias striatus, as they approached a patch, collected sunflower seeds, and left to return to their burrows. There was a strong similarity between head raising during foraging and pausing during intermittent locomotion. Chipmunks paused more frequently when moving towards the patch than when leaving the patch. Chipmunks in the patch raised their heads at an intermediate rate, which tended to decrease with time in the patch. Pauses and the duration of motionless periods during head raises were very short (∼0.4 s), and their frequency distributions were similar. Animals remained motionless during 22% of the time spent approaching the patch, 14% of the time spent in the patch and 7% of the time spent leaving the patch. Rates of pausing and head raising generally decreased with short-term familiarity (number of trips to the patch) and with long-term familiarity (proximity of the patch to the burrow). Trials with higher pause rates when approaching the patch had higher head-raising rates in the patch. Whether the focal individual was solitary, dominant or subordinate in a dyad, or competing with multiple chipmunks in the patch had no effect on pausing or head raising. In a separate experiment, exposure to a model hawk increased pause and head-raising rates. We conclude that head raising during foraging and pausing during locomotion serve a similar vigilance function, that this vigilance is directed towards detection of predators rather than conspecifics, and that time allocated to vigilance is sufficient to significantly reduce foraging rates and affect many space use and foraging decisions.     

How to climb a tree: lizards accelerate faster,but pause more,when escaping on vertical surfaces

Many species of lizards effectively traverse both two and three‐dimensional habitats. However, few studies have examined maximum locomotor performance on different inclines. Do maximum acceleration and velocity differ on a level and inclined surface? Do lizards pause more on an inclined surface? To address these questions, Sceloporus woodi lizards (N = 12) were run in the laboratory on a level trackway and a vertical tree trunk. This species is known to frequently utilize both vertical and horizontal aspects of its habitat. Average maximum acceleration on the vertical surface exceeded that on the level surface, although average maximum velocity exhibited the opposite pattern. The average number of pauses during level locomotion was lower compared to vertical locomotion. In addition, the average location of the first pause on the level surface was 0.51 m, which is farther than the average for vertical locomotion where the first pause was at 0.35 m. The combination of performance and pause data suggests that the relative lack of pausing during level locomotion allows individuals to reach higher maximum velocities on level surfaces because they accelerate over greater distances. The increased pausing when moving vertically could be a result of high energetic demands of vertical locomotion, or greater microhabitat complexity as a result of branching and/or refuges. The faster acceleration exhibited during vertical locomotion by S. woodi likely offsets the frequent pauses. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2011, 102 , 83–90.     

Behavioural thermoregulation of Orthopoms ornatus (Diplopoda: Spirostreptidae) in three desert habitats

Observations on feeding and locomotion of the desert millipede Orthopoms ornatus were made in three separate habitats during the summer feeding season. The habitats were Tornillo Flat in Big Bend National Park, Texas; the Jornada Validation Site of IBP/Desert Biome near Las Cruces, New Mexico; and the base of a volcanic escarpment near Albuquerque, New Mexico.
In each habitat diurnal or diel temperatures were monitored in a typical millipede microsite. Microsites included portions of a Larrea/Opuntia shrub complex (Tornillo Flat), an Ephedra shrub (Jornada Site), and a Salsola shrub (Albuquerque). Adjacent mammal-burrow temperatures were also monitored, as were soil-surface temperatures (two habitats) and air temperatures (one habitat).
Diurnal feeding and locomotion were greatest in early morning and late afternoon; nocturnal activity also occurred. Soil-surface activity generally ceased before surface temperatures reached 35^oC and began again when they cooled to about the same level. In the meantime, movement to burrows, to beneath rocks and vegetation, and to aerial portions of shrubs occurred. Additional behavioural thermoregulation was manifested by horizontal and vertical movements within shrubs, maximum air temperatures encountered being 35.5^oC.
Such behaviour is considered in light of recent studies of water balance and metabolism in this species.     

年龄、集群、生境及天气对鄱阳湖白鹤越冬期日间行为模式的影响

于2012年1—3月和2012年12—2013年3月,采用瞬时扫描法研究了鄱阳湖国家级自然保护区越冬白鹤(Grusleucogeranus)的昼间时间分配模式和日活动节律,分析了年龄、集群大小、生境类型和天气对白鹤越冬行为的影响。结果表明,白鹤越冬期的主要行为有觅食、警戒、理羽、游走、休息和争斗。其中,觅食是鄱阳湖越冬白鹤主要行为,占其昼间活动时间的82.94%;警戒占11.94%,理羽占3.52%,游走占1.88%,休息占0.15%,争斗占0.02%。白鹤在越冬期间的觅食、游走、休息和争斗行为存在明显的昼间节律性,其中游走、觅食和争斗行为在每天的上午和下午各存在1个高峰,休息行为在中午出现一个高峰;警戒和理羽行为则没有显著的昼间节律变化。年龄、集群大小、天气和生境对白鹤的越冬行为具有显著的影响:幼鹤取食的时间显著多于成鹤,而警戒时间则显著的小于成鹤;与家庭群中的个体相比,集体群中的个体取食和争斗时间显著增加,警戒时间显著减少;在天气晴朗时,白鹤的警戒时间显著大于阴天的警戒时间,取食时间则显著少于阴天时白鹤的取食时间;白鹤在大风天气显著增加理羽行为;在有雾的天气会显著增加取食时间,减少警戒时间。在草洲和浅水两种生境中,白鹤的游走和理羽行为存在显著差异,而觅食和警戒行为没有显著的差异,这可能源于草洲和浅水生境相互毗邻,并且具有相同程度的人为干扰。GLM分析结果表明,年龄和集群大小、年龄和生境、集群大小和天气、集群大小和生境、天气和生境对白鹤昼间时间分配模式的影响存在显著交互效应。     

Functions of Intermittent Locomotion in Mustached Tamarins (<Emphasis Type="Italic">Saguinus mystax</Emphasis>)

Many animals interrupt their moving with brief pauses, which appear to serve several different functions. We examined the function of such intermittent locomotion in wild living mustached tamarins (Saguinus mystax), small arboreal New World primates that form mixed-species groups with saddleback tamarins (Saguinus fuscicollis). We investigated how different environmental and social factors affect pausing during locomotion and used these data to infer the function of this behavior. As measures of intermittent locomotion, we used percentage of time spent pausing and pause rate. We considered 3 possible functions that are not mutually exclusive: increased endurance, route planning, and antipredator vigilance. Mustached tamarins spent on average (mean ± SE) 55.1 ± 1.0% of time pausing, which makes effective resource exploitation more time consuming and needs to be outweighed by correspondingly large benefits. Percentage of time spent pausing decreased in larger mixed-species groups vs. smaller mixed-species groups and decreased with height and in monkeys carrying infants. It was not affected by sex, age, spatial arrangement, or single-species group size. Pause rate increased in individuals traveling independently compared to those traveling in file, but was not affected by other factors. The group size effect in mixed-species groups lends support to the notion that pausing during locomotion is an antipredator tactic that can be reduced in the increased safety of larger groups, but other results suggest that additional functions, particularly route planning, are also of great importance. Benefits in terms of predator confusion and group movement coordination are also likely to play a role and remain a topic for further research.     

On the Relationship between Herbaceous Cover and Vigilance Activity of Degus (Octodon degus)

A number of studies demonstrate that plant cover provides prey animals with refuges to decrease vulnerability to predators. However, others suggest plant cover to visually obstruct detection of predators or conspecifics. We suggest these seemingly conflicting results can, to some extent, be resolved if overhead vs. lateral cover are distinguished. We recorded seasonal variation in vigilance activity of a natural population of degus (Octodon degus), a diurnal, semi‐subterranean and social rodent from central Chile. We used these data to determine whether cover provided by herbaceous vegetation is mostly obstructive. The height of herbaceous vegetation in the habitat of degus varied seasonally, and the ability of degus (estimated from human observers) to detect potential predators decreased when herbaceous vegetation was high. This effect was more important for degus using quadruped postures and when dealing with terrestrial simulated predators. Accordingly, degus adjusted the quality rather than the quantity of their vigilance activity: male and female degus allocated similarly more time to bipedal vigilance when the height of herbs was high. Such increase in bipedal vigilance seemed to occur at the expense of quadruped vigilance instead of foraging time. Collectively, these results support the hypothesis that cover of herbaceous vegetation is mostly obstructive to degus when active above ground, a finding that contrasts with previous evidence supporting that shrub cover provides refuges against predators. The differential effects of shrubs and herbs on degu vigilance are likely linked to differences in the costs and benefits associated with each cover type. For degus, shrubs may provide more overhead (protective) than lateral (obstructive) cover.     

Locomotion patterns in two South American gymnophthalmid lizards: Vanzosaura rubricauda and Procellosaurinus tetradactylus

We quantified gait and stride characteristics (velocity, frequency, stride length, stance and swing duration, and duty factor) in the bursts of locomotion of two small, intermittently moving, closely related South American gymnophthalmid lizards: Vanzosaura rubricauda and Procellosaurinus tetradactylus. They occur in different environments: V. rubricauda is widely distributed in open areas with various habitats and substrates, while P. tetradactylus is endemic to dunes in the semi-arid Brazilian Caatinga. Both use trot or walking trot characterised by a lateral sequence. For various substrates in a gradient of roughness (perspex, cardboard, sand, gravel), both species have low relative velocities in comparison with those reported for larger continuously moving lizards. To generate velocity, these animals increase stride frequency but decrease relative stride length. For these parameters, P. tetradactylus showed lower values than V. rubricauda. In their relative range of velocities, no significant differences in stride length and frequency were recorded for gravel. However, the slopes of a correlation between velocity and its components were lower in P. tetradactylus on cardboard, whereas on sand this was only observed for velocity and stride length. The data showed that the difference in rhythmic parameters between both species increased with the smoothness of the substrates. Moreover, P. tetradactylus shows a highly specialised locomotor strategy involving lower stride length and frequency for generating lower velocities than in V. rubricauda. This suggests the evolution of a central motor pattern generator to control slower limb movements and to produce fewer and longer pauses in intermittent locomotion.     

How do vigilance and feeding by common cranes Grus grus depend on age,habitat, and flock size?

Animals often spend less time vigilant and more time feeding when foraging in larger groups. This group-size effect does not, however, consider if larger groups differ systematically from smaller ones: Large groups could form in different habitats than small groups or be composed of a different mix of ages or classes than small groups. We examined how habitat differences and flock size and composition explain feeding and vigilance rates in common cranes Grus grus , wintering in holm oak Quercus ilex dehesas of Spain. Flock size and composition were related to habitat type in cranes: flocks formed in areas sown with cereal crops were larger than flocks formed in set aside areas. Vigilance rate depended on habitat but decreased with increasing flock size in a similar way across all habitats. Juveniles were less vigilant than adults and showed little change in vigilance with flock size. Vigilance increased and feeding time decreased over months from November through February. Our results show that vigilance is affected by habitat but that the group size effect on vigilance is not the product of differences between habitats in group size or composition.     

The Behavioral Ecology of Intermittent Locomotion

Most physiological and ecological approaches to animal locomotionare based on steady state assumptions, yet movements of manyanimals are interspersed with pauses lasting from millisecondsto minutes. Thus, pauses, along with changes in the durationand speed of moves, form part of a dynamic system of intermittentlocomotion by which animals adjust their locomotor behaviorto changing circumstances. Intermittent locomotion occurs ina wide array of organisms from protozoans to mammals. It isfound in aerial, aquatic and terrestrial locomotion and in manybehavioral contexts including search and pursuit of prey, matesearch, escape from predators, habitat assessment and generaltravel. In our survey, animals exhibiting intermittent locomotionpaused on average nearly 50% of their locomotion time (range6–94%). Although intermittent locomotion is usually expectedto increase energetic costs as a result of additional expenditurefor acceleration and deceleration, a variety of energetic benefitscan arise when forward movement continues during pauses. Endurancealso can be improved by partial recovery from fatigue duringpauses. Perceptual benefits can arise because pauses increasethe capacity of the sensory systems to detect relevant stimuli.Several processes, including velocity blur, relative motiondetection, foveation, attention and interference between sensorysystems are probably involved. In animals that do not pause,alternative mechanisms for stabilizing the perceptual fieldare often present. Because movement is an important cue forstimulus detection, pauses can also reduce unwanted detectionby an organism's predators or prey. Several models have attemptedto integrate energetic and perceptual processes, but many challengesremain. Future advances will require improved quantificationof the effects of speed on perception.     

Terrestrial Intermittent Exercise: Common Issues for Human Athletics and Comparative Animal Locomotion

The earliest studies of intermittent exercise physiology notedthat moving intermittently (i.e., alternating brief movementswith brief pauses) could transform a heavy workload into a submaximalone that can be tolerated and sustained. The brief pauses thatcharacterize intermittent locomotion permit at least partialrecovery from prior activity. This research provided the foundationfor the development of interval training and more recently forthe re-evaluation of steady-state paradigms for comparativeanimal locomotion. In this paper I review key concepts underlyingthe performance of repeated activity. I provide examples fromhuman athletics and training and comparative animal locomotion.To explore the limits of intermittent exercise performance,I examine the performance limits for continuous exercise andthe rate and extent of the recovery of performance capacityfollowing activity. While it is evident that altering locomotorbehavior (i.e., moving intermittently) can alter the capacityof an animal to perform work, mathematical models of intermittentexercise could predict strategies (i.e., exercise intensity,exercise duration, and pause duration) that will increase performancelimits for intermittent activity.     

Effects of Microhabitat‐Dependent Predation Risk on Vigilance during Intermittent Locomotion in Psammodromus algirus Lizards

Animals should be able to adjust their behavior by tracking changes in predation risk level continuously. Many animals show a pattern of intermittent locomotion with short pauses that may increase detection and vigilance of predators. These locomotor patterns may depend on the microhabitat structure, which affect predation risk levels. We examined in detail in the laboratory the characteristics of spontaneous locomotion, scanning behavior, and the escape performance of Psammodromus algirus lizards moving in two different microhabitats (leaf litter patches and open sand areas). Results showed that in leaf litter, lizards moved at slower speed and had shorter bursts of locomotion both in distance and duration, than in sand substrates. This locomotor pattern allowed lizards to increase scanning rate and total time spent in vigilance behavior. When lizards were forced to flee, they escaped to longer distances and during more time in open sand areas, but lizards were able to attain similar escape speed in the two substrates. Lizards may be able to compensate the cost of moving between different microhabitats with different predation risk by behaviorally changing their locomotor and vigilance patterns. However, complex interactions between the visibility of lizards to predators and the ability of lizards to detect predators, together with the need of attending simultaneously to other conflicting demands, may lead to apparently non‐intuitive solutions in locomotor patterns and the rate of vigilance behavior.     

Positional behavior of long-tailed macaques (Macaca fascicularis) in northern Sumatra

Although the majority of extant primates are described as "quadrupedal," there is little information available from natural habitats on the locomotor and postural behavior of arboreal primate quadrupeds that are not specialized for leaping. To clarify varieties of quadrupedal movement, a quantitative field study of the positional behavior of a highly arboreal cercopithecine, Macaca fascicularis, was conducted in northern Sumatra. At least 70% of locomotion in travel, foraging, and feeding was movement along continuous substrates by quadrupedalism and vertical climbing. Another 14-25% of locomotion was across substrates by pronograde clambering and vertical clambering. The highest frequency of clambering occurred in foraging for insects, and on the average smaller substrates were used in clambering than during quadrupedal movement. All postural behavior during foraging and feeding was above-substrate, largely sitting. Locomotion across substrates requires grasping branches of diverse orientations, sometimes displaced away from the animal's body. The relatively low frequency of across-substrate locomotion appears consistent with published analyses of cercopithecoid postcranial morphology, indicating specialization for stability of limb joints and use of limbs in parasagittal movements, but confirmation of this association awaits interspecific comparisons that make the distinction between along- and across-substrate forms of locomotion. It is suggested that pronograde clambering as defined in this study was likely a positional mode of considerable importance in the repertoire of Proconsul africanus and is a plausible early stage in the evolution of later hominoid morphology and locomotor behavior.     

Quantifying movement of the tropical Australian cubozoan <Emphasis Type="Italic">Chironex fleckeri</Emphasis> using acoustic telemetry

Cubomedusae are considered to have superior swimming abilities compared to other pelagic cnidarians, yet many of the theories describing such behaviours are based on anecdotal evidence, sting records or opportunistic sightings, rather than quantitative data. Acoustic telemetry was used to document the movements of adult Chironex fleckeri medusae within both coastal and estuarine habitats. The rate at which tagged medusae moved was influenced by an interaction between time period (day or night) and habitat (coastal or estuarine), with rates of travel being relatively similar during the day and night within the coastal habitat, but significantly greater at night than during the day within the estuarine habitat. Medusae in coastal habitats travelled at similar rates throughout all tidal states while estuarine medusae travelled at significantly faster rates towards the middle of the tide than at the low and high ebbs. Such movements occurred with, and independent of, tidally generated currents, but at increased current speeds, medusae tended to travel with the current. Data are also presented that show that large medusae may move from coastal to estuarine habitats. Guest editors: K. A. Pitt & J. E. Purcell Jellyfish Blooms: Causes, Consequences, and Recent Advances     

Influence of predation risk and plant structure on vigilance and intermittent locomotion in <Emphasis Type="Italic">Microcavia australis</Emphasis> (Rodentia,Caviidae)

The aim of this study was to analyze and compare vigilance behavior and intermittent locomotion at two sites (El Leoncito and Ñacuñán, Argentina) that differ in predation risk, plant structure, and plant resource availability. Subjects were lesser cavies (Microcavia australis), a social species that is semi-fossorial, diurnal, and native to South America. Continuous focal sampling was conducted during the day, at times of food shortage, food abundance, and reproduction from 2003 to 2005. The proportion of time spent vigilance was significantly higher at Ñacuñán, where vigilance peaked at midday and reached a minimum in the evening. This midday peak of vigilance at Ñacuñán was associated with a midday peak of danger from raptors as indicated by a raptor activity peak at that time. In contrast, both vigilance and predator activity at El Leoncito were constant through the day. Records of intermittent locomotion and number and duration of pauses in locomotion were significantly higher at El Leoncito, a difference that may have been due to the need for greater vigilance while moving across areas of less protective cover at this site.     

Integrating the Physiology, Mechanics and Behavior of Rapid Running Ghost Crabs: Slow and Steady Doesn't Always Win the Race

In 1979 Bliss predicted that, "land crabs are and will undoubtedlycontinue to be promising objects of scientific research." Studiesof rapid running ghost crabs support her contention and haveresulted in several general findings relating to locomotionand activity. 1) Energy exchange mechanisms during walking aregeneral and not restricted to quadrupedal and bipedal morphologies.2) "Equivalent gaits," such as trots and gallops, may existin 4-, 6- and 8-legged animals that differ greatly in leg andskeletal (i.e., exo- vs. endoskeletal) design. These findingssupport the hypothesis that terrestrial locomotion in many speciescan modeled by an inverted pendulum or spring-mass system. 3)An open circulatory system and chitin-covered gills do not necessarilylimit the rate at which oxygen consumption can be increasedor the factorial increase oxygen consumption over resting rates.4) Interspecific and intraspecific i.e., ontogenetic) scalingof sub-maximal oxygen consumption and maximal aerobic speedcan differ significantly. 5) Locomotion at speeds above themaximal aerobic speed requiring non-aerobic contributions maybe far more costly than can be predicted from aerobic costsalone. The cost transport may attain a minimum at less thanmaximum speed. 6) The speed which elicits maximal oxygen consumptionduring continuous exercise is attained at moderate walking speedsin crabs and probably other ectotherms. Speeds 15- to 20-foldfaster are possible, but cannot be sustained. 7) The low enduranceassociated with the low maximal oxygen consumption and maximalaerobic speed of ectotherms moving continuously can be increasedor decreased by altering locomotor behavior and moving intermittently.Ectotherms can locomote at high speeds and travel for considerabledistances or remain active for long periods by including restpauses. Alternatively, intense activity with extended exerciseperiods with short pause periods may actually reduce behavioralcapacity or work accomplished relative to continuous activityduring which the behavior is carried out at a lower intensitylevel without pauses.     

Comparative and behavioral analyses of preferred speed: Anolis lizards as a model system

I quantified the movement patterns of eight morphologically and ecologically diverse Caribbean Anolis lizard species in the field to address the following questions: (1) Do these eight species move at preferred speeds, and if so, what are these speeds? (2) What proportion of their maximum sprinting capacities do the anole species use when moving undisturbed? (3) What percentage of the time do lizards spend moving, and how far do they typically travel on a daily basis? (4) Have the preferred speeds of anoles coevolved with structural habitat use? Most of the distributions of speeds were highly skewed, with a preponderance of slow-speed locomotion (<20% of maximum capacity). Median speeds varied almost eightfold among species, from a low of 4.9 cm/s (3.0% of maximum) to a high of 38.0 cm/s (22.4% of maximum). For all eight species, at least 75% of their locomotor movements took place between 0% and 40% of maximum capacity. The eight species varied almost 15-fold in the percentage of time they spent moving, indicating that not all anole species are equally sedentary. Through usage of modern comparative methods, I showed that Anolis species that move slowly through their environments also tend to use narrow perch diameters and have large habitat breadths. These findings show how evolutionary approaches can be profitably integrated with physiological data to understand how species use their habitats.     

Locomotor behavior of wild orangutans (Pongo pygmaeus wurmbii) in disturbed peat swamp forest, Sabangau, Central Kalimantan, Indonesia

This study examined the locomotor behavior of wild Bornean orangutans (P. p. wurmbii) in an area of disturbed peat swamp forest (Sabangau Catchment, Indonesia) in relation to the height in the canopy, age-sex class, behavior (feeding or traveling), and the number of supports used to bear body mass. Backward elimination log-linear modeling was employed to expose the main influences on orangutan locomotion. Our results showed that the most important distinctions with regard to locomotion were between suspensory and compressive, or, orthograde (vertical trunk) and pronograde (horizontal trunk) behavior. Whether orangutans were traveling or feeding had the most important influence on locomotion whereby compressive locomotion had a strong association with feeding, suspensory locomotion had a strong association with travel in the peripheral strata using multiple supports, whereas vertical climb/descent and oscillation showed a strong association with travel on single supports in the core stratum. In contrast to theoretical predictions on positional behavior and body size, age-sex category had a limited influence on locomotion. The study revealed that torso orthograde suspension dominates orangutan locomotion, concurring with previous studies in dipterocarp forest. But, orangutans in the Sabangau exhibited substantially higher frequencies of oscillatory locomotion than observed at other sites, suggesting this behavior confers particular benefits for traversing the highly compliant arboreal environment typical of disturbed peat swamp forest. In addition, torso pronograde suspensory locomotion was observed at much lower levels than in the Sumatran species. Together these results highlight the necessity for further examination of differences between species, which control for habitat.     

Sexual segregation by Masai giraffes at two spatial scales

In this paper alternative explanations for observed patterns of sexual segregation by giraffes are examined at two spatial scales: within-habitats and within-landscape. Habitats are defined as recognizable plant associations and the landscape as the collection of all available habitat types. The study was conducted in Mikumi National Park, Tanzania. At the within-landscape level, all sex and age classes of giraffes exhibited high degrees of preference for riverine habitats. Sex differences in habitat selection were mostly due to females with young, who tended to select open floodplain habitats in which their vigilance time was lowest. Males, and females without young, preferred more heavily-wooded habitat. Habitat preferences were not related to observed habitat-specific forage intake rates for either males or females. Within habitats, male and female giraffes selected different feeding heights, males feeding higher in the canopy than females. Females showed a strong tendency to generalize with respect to feeding height. It is suggested that a sexual dimorphism–body size hypothesis provides a parsimonious explanation for the observed feeding height selection patterns, whereas a reproductive strategy hypothesis can explain sex-differences in habitat selection patterns within the landscape.     

Integrating demographic data and a mechanistic dispersal model to predict invasion spread of Rhododendron ponticum in different habitats

Understanding the invasion potential of a species in different habitat types within the non-native range is crucial in prioritising management and control efforts, and in the protection of vulnerable habitats through monitoring. Here, using the invasive shrub Rhododendron ponticum as a case study, we integrate information on both the demographics and spatial dynamics within an individual-based, spatially-explicit model to investigate invasion potential in different habitats. Firstly, empirical demographic data were used to establish relationships between demographic traits, such as height and fecundity, and habitat variables. The outputs from models fitted using a Generalised Linear Model approach were then incorporated into an individual-based simulation model of plant spread to investigate the invasion potential in different habitats using a factorial design of treatments. Plant height, and thus seed release height, was the main driver of invasion speed through an increase in dispersal potential, which resulted in the highest invasion speeds predicted for evergreen woodlands, and relatively low speeds for open habitats. Conversely, invasion density was driven by plant fecundity and seedling survival and not dispersal potential; the highest invasion densities were predicted for open habitats, with relatively low densities for evergreen habitats. Deciduous woodland had features resulting in intermediate invasion potential, both in terms of speed and density and may, therefore be the habitat that is most vulnerable to relatively rapid and dense invasion.     

贺兰山岩羊冬春季取食生境的比较

2003年11～12月和2004年4～6月,在贺兰山设定了25条固定样线,采用直接观察法对岩羊冬春季的取食生境选择进行了研究。结果表明,岩羊冬季对12种取食生境生态因子有选择性,偏爱选择位于山地疏林草原带,优势乔木为灰榆,乔木密度〈4株、高度4～6m,灌木密度〉5株、高度〉1．3m,食物质量〉50g,人为干扰距离〈500m,距裸岩距离〈2m的地方取食。而春季对11种取食生境生态因子有选择性,偏爱选择山地疏林草原带,优势乔木为灰榆,乔木密度〈4株、高度〈6m,灌木密度5～10株、高度1．3～1．7m,食物质量〉100g,海拔高度〈2000m,距水源距离〈500m,隐蔽级50％～75％的地点。冬春季岩羊对植被类型、地形特征、优势乔木、乔木密度、乔木高度、灌木密度、灌木距离、食物丰富度、坡向、坡度、距水源距离、人为干扰距离和隐蔽级的选择存在显著差异。主成分分析表明,冬季第1主成分的贡献率达24．493％,其中绝对值较大的权系数出现在植被类型、优势乔木、乔木高度、乔木距离、灌木密度、灌木高度、海拔高度、距水源距离和人为干扰距离等生态因子：春季第1主成分的贡献率达28．777％,其中绝对值较大的权系数出现在植被类型、乔木距离、灌木高度、灌木距离、食物丰富度、海拔高度和人为干扰距离等生态因子。随着北方地区冬春季食物数量和质量的剧烈变化,贺兰山岩羊对取食生境的利用对策也将发生一定程度的改变,与其他分布区的岩羊相比,贺兰山独特的地理位置和特殊生境使其在取食生境选择上存在很大差异。