移民,是否减轻了草场压力?

许多牧民在说:一旦进了城,每天一睁眼就要考虑水、电、煤气或牛粪的花销,吃饭的开销,这些花费要么还是从自家草场上来,要么就得有固定的工作和收入才能支撑。     

夏季草场缺水已经7年了

<正>"我和女儿、儿媳妇只好每天去山上的一条溪流背水"来到玛曲后连续几天一直是阴雨天气,湿润的气候和路上随处可见的溪流与湖泊,都让大家赞叹不已。我们在玛曲访问了第一户牧民:青培、才郎卓玛夫妇一家,这家人的帐篷驻扎在黄河附近的欧拉乡。男主人青培是个     

Seeing without being seen: a removal experiment with mixed flocks of Willow and Crested Tits Parus montanus and cristatus

This paper tests the hypothesis that foraging site selection reflects a trade-off between the various needs for concealment from predators, to find food, and for the individual to maintain some view of its surroundings. After removal of Crested Tits Parus cristatus (the dominant species in mixed flocks), Willow Tits P. montanus did not decrease their foraging heights as expected but remained in the most exposed parts of young pines. In contrast, after removal of Willow Tits, Crested Tits increased their foraging height from the sheltered lower canopy to sites previously occupied by Willow Tits. When flock size was reduced, the birds maintained the same high levels of vigilance without concealing themselves in dense vegetation. I suggest that flock members may benefit from foraging in sites that afford good anti-predator vigilance.     

The coordinated aerobatics of dunlin flocks

Flocks of dunlin (Calidris alpina) were filmed during rapid aerial manoeuvres. Two types of manoeuvres were identified: (a) course-change movements in which the entire flock altered course, and (b) rotation movements in which individual birds rotated on their sagittal axes without necessarily changing direction. Either manoeuvre could make a flock's overall appearance shift suddenly from light to dark (or vice versa) as the birds oriented first with their light ventral, then with their dark dorsal, plumage toward the observer. Slow-motion analysis revealed that the average time required by a majority of birds in camera view to change orientations during a manoeuvre was 196 ms (sd=137 ms). ‘Self-generated synchrony’ is a possible mechanism for this highly coordinated group behaviour.     

Early Plant Succession in Abandoned Pastures in Ecuador1

We compared early plant succession in four abandoned pastures of differing age since abandonment and a nearby secondary forest site in northwestern Ecuador. Two “Open” pastures had no tree canopy covering, and two “Guava” pastures had a well-developed canopy cover of Psidium guajava. No site had been seeded with pasture grasses. All pastures were compared in a chronological sequence; two were monitored for 18 months. Species richness was consistently higher in Guava sites than in Open sites and it continued to increase over time, whereas it remained static in Open sites. Species richness was highest in secondary forest. Recruitment of tree saplings in Guava sites was lower than in secondary forest; however, it was nearly absent in Open sites. The seed bank contained predominantly herbaceous species at all sites, and was highly dissimilar to aboveground vegetation. Dominance-diversity curves for Guava sites showed a more equitable distribution of species that increased over time. In contrast, dominance-diversity curves for Open sites were static and indicated dominance by a few aggressive species. Soil characteristics among sites were variable; however, a principal components analysis on soils isolated the older Open site from all others. The older Open site had the lowest species richness and was dominated by Baccharis trinervis, an aggressive shrub species. The site appears to be in a state of arrested succession and some form of restorative intervention may be necessary to initiate succession toward a forested condition. Succession in Guava sites appears headed toward secondary forest, whereas it does not in Open sites.     

Detection of prey flocks by predators

The model presented by Vine (1971) to demonstrate a possible advantage of flocking behaviour in reducing predation risk made the unrealistic assumption that flocking would not affect the maximum distance at which a predator could just detect that an animal or animals were present. If flocking does reduce this distance it might yield a compensating disadvantage to prey animals, or an advantage to the predator. While quantification of these possibilities remains problematic in general, empirical data suggests that detection range depends on a simple power function of the number of individuals in a tight circular flock and their physical size. It is concluded that small flocks are likely to be advantageous to the predator and quite possibly disadvantageous to the prey, although the “selfish” advantage of flocking is likely to remain high for prey animals in medium or large sized flocks.     

Spatial organization in flocks of domestic fowls

The movements of individual birds within flocks were recorded by marking a sample with coloured tags and photographing the flock at fixed intervals using eight time-lapse cine, cameras, each camera covering a field amounting to one-eighth of the total area. The birds were housed on deep litter in an area 13·4×5·2 m, and flock sizes of four to 600 were used they ranged in age from 9 weeks to adult. The films were analysed, the positions of the marked birds were plotted and the probability of the observed distributions occurring by chance were calculated. Previous workers have found that individual birds move over, only part of the total area; in our study the movements of individuals lay on a continuum ranging from apparent randomness at one extreme to clear non-randomness at the other. Moreover, in all experiments most birds were sighted in most camera fields, indicating that their movements were incompatible with the accepted definition of a delineated home range. These results were explained by postulating strain differences in aggressiveness and assigning an important role to illumination level. At very low light levels individuals may be able to move freely in a flock because they cannot be recognized as strangers and, in any case, individual recognition may be exceptional in large flocks. Even so, birds did differ in the extent to which they moved around and some did exhibit preferences for particular parts of the pen; such tendencies were consistent over a 4-week period in adults but not in juveniles.     

Some geometric aspects of house sparrow flocks

We analysed 50 movie films of house sparrows (Passer domesticus) foraging on an experimental grid. The location and orientation of each bird was recorded; sex of the individual was determined in about two of every three cases. Results revealed the following: (1) Flock size exhibited a weak inverse relationship to ambient temperature, though aggression was rare in the experimental patch. (2) The frequency of males in foraging groups exceeded the frequency of males in the local population. (3) Within a given flock size, nearest neighbour distances did not differ significantly between male-male and male-female pairs. However, average nearest neighbour distance was inversely related to flock size. (4) Solitaries oriented away from safety and toward a source of disturbance. Orientation of an individual within a larger group was more variable than that of a solitary, and the orientation of nearest neighbours indicated a significant tendency to keep each other in view.