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81.
82.
The role of secondary vegetation in restoring soil fertility during shifting cultivation in the tropics is well known. Yet
the effect of secondary succession on the spatial patterns of soil properties has received little attention. To determine
whether changes in the plant community as a result of shifting cultivation affect the scale of spatial dependence for biologically
important soil nutrients, we sampled three dry tropical forest stands in Campeche, Mexico. These stands represented a gradient
of cultivation history: one mature forest stand, a forest fallow that had undergone one cultivation-fallow cycle, and a forest
fallow that had undergone two cultivation-fallow cycles. We used an analysis of semivariance to quantify the scale and magnitude
of spatial dependence for organic matter content (OM), phosphorus (P), potassium (K), and aluminum (Al) in each stand. The
scale of spatial dependence varied with cultivation history, but the degree of spatial dependence did not differ among stands.
In the mature forest P and K were autocorrelated over distances >7.5 m. In the forest fallows 48–88% of the variation in soil
P and K was autocorrelated over distances up to 1.1–5.1 m. In contrast, the range of autocorrelation for Al (∼2.5 m) did not
differ among stands. We conclude that shifting cultivation changes the range of autocorrelation for biologically important
soil nutrients at a scale that may influence plant growth. The finer scaled pattern of soil nutrients in forest fallows is
likely to persist with continued shifting cultivation, since fallows are cleared every 3–15 years. 相似文献
83.
Armin Schafhauser Stefan Götz 《Palaeogeography, Palaeoclimatology, Palaeoecology》2007,251(2):210-221
Five sections of the Cardenas and Tabaco formations in east-central Mexico have been analyzed by means of bio-, Sr-isotope, and sequence stratigraphy, in order to evaluate their age as well as the timing of rudist decline.Ammonites [Pachydiscus (Pachydiscus) neubergicus (Hauer), Sphenodiscus pleurisepta (Conrad), Coahuilites sheltoni Böse] indicate an early Maastrichtian age for the topmost lower member of the Cardenas Formation and planktic foraminifera [e.g., Globotruncanita stuarti (de Lapparent), Archaeoglobigerina cretacea (d’Orbigny), Globotruncanella petaloidea (Gandolfi), Gansserina gansseri (Bolli), Globotruncana linneiana (d’Orbigny)] a late early Maastrichtian age for the middle member corresponding to the foraminiferal zones CF5 and CF6. Sr-isotope stratigraphic data yield an early late Maastrichtian age (66.93 Ma < 67.98 Ma < 68.96 Ma) for the last rudist assemblage in the topmost upper member of the Cardenas Formation, coinciding with the foraminiferal zone CF4.17 small-scale and 3 large-scale depositional cycles have been identified, which correspond to para- and depositional sequences. The progradational pattern of the large-scale cycles indicates an overall regression trend, which terminated in subaerial exposure of the area, indicated by paleosoils in the red beds of the Tabaco Formation. The correlation of the large-scale cycles with the global sea level charts indicate that eustatic sea level fall caused the regression and led to the exposure during the middle late Maastrichtian. This subaerial exposure resulted in the loss of habitat and thus the disappearance of rudists in east-central Mexico. 相似文献
84.
During 250 h of observation, a total of 20 episodes of self-anointing, that is, the application of scent-bearing material
onto the body, were recorded in a group of free-ranging Mexican spider monkeys (Ateles geoffroyi). The animals used the leaves of three species of plants (Brongniartia alamosana, Fabaceae; Cecropia obtusifolia, Cecropiaceae; and Apium graveolens, Umbelliferae) two of which have not been reported so far in this context in any New World primate species. The findings
that only two males displayed self-anointing, that only the sternal and axillary regions of the body were rubbed with the
mix of saliva and plant material, and a lack of correlation between the occurrence of self-anointing and time of day, season
of the year, ambient temperature or humidity do not fit the hypothesis that this behavior functions in repelling insects and/or
mitigating topical skin infections in this species. Rather, the data and the observation that the leaves of all three plant
species spread an intensive and aromatic odor when crushed, support the hypothesis that self-anointing in A. geoffroyi may play a role in the context of social communication, possibly for signaling of social status or to increase sexual attractiveness. 相似文献
85.
Abstract: Because of significant declines in mule deer (Odocoileus hemionus) populations across New Mexico, USA, we investigated survival of fawns in north-central New Mexico, USA. We captured 19 fawns, 34 fawns, and 47 fawns in 2002, 2003, and 2004, respectively, and used fawn morphological measurements, habitat characteristics, and adult female (hereafter “female”) condition to model preweaning fawn survival. Survival was 0.0, 0.12, 0.52 for 2002, 2003, and 2004, respectively, and was related to birth mass (χ12 = 9.5, P = 0.002), birth date (χ12= 8.4, P = 0.004), litter size (χ22 = 9.4, P = 0.009), female body fat (χ12 = 40.9, P < 0.001), annual precipitation (χ12 = 35.0, P < 0.001), summer precipitation (χ12= 37.5, P < 0.001), and winter precipitation (χ12 = 32.0, P < 0.001). Total ingesta-free body fat of females (β = 3.01, SE = 0.75; odds ratio = 20.19, 95% CI = 4.64-87.91) and birth mass of fawns (β = 1.188, SE = 0.428; odds ratio = 3.38, 95% CI = 1.42-7.59) were the best predictors of survival of individual fawns, although few of the logistic models differed in model selection criteria. Fawn survival in north-central New Mexico was driven by an interaction of total and seasonal precipitation and its effect on plant production, consequential effects on female nutrition, and ultimately, fawn birth attributes. Habitat conditions were so poor throughout north-central New Mexico during 2002 and 2003 (and likely during other drought yr) that, based upon birth attributes, few fawns could have survived regardless of proximate causes of mortality. In 2004, precipitation enhanced security cover, maternal body condition, birth attributes and, thus, survival of fawns. However, more habitat enhancements are needed to improve the nutritional quality of mule deer habitats in north-central New Mexico and further enhance maternal and fawn condition to recover mule deer populations in this region. 相似文献
86.
87.
Siphonophores are exclusively marine cnidaria and their predatory role in plankton food-webs is well recognised. In this study,
we analyse the structure and the spatial extent of siphonophore assemblages in relation to changes in freshwater outflows
and food availability in the southern Gulf of Mexico during a high (October) and a low (April) outflow periods. A total of
149 samples were collected using a 505 μm multiple closing net at 1–6 levels (0–100 m) of the water column, depending on the
bathymetry. Data on siphonophore species biovolumes (ml 100 m−3) were treated by means of the Bray-Curtis Dissimilarity Index, and two distinctive assemblages were identified: the ‘inner’
and the ‘outer’ assemblages, located over the inner and outer shelves. Temperature, salinity, zooplankton biomass, and siphonophore
species were included in a Principal Component Analysis (PCA) to identify the factors associated with each assemblage. Geographical
distribution of the assemblages practically remained the same during both seasons and its cross-shelf variability was stronger
than the vertical one. Seasonally, diversity values were higher in October, when the highest river discharges occur. Spatially,
the lowest diversity and mean siphonophore biovolumes values were registered in the ‘inner assemblage’, where the highest
and lowest salinity values were recorded. We suggest that even when extreme salinity values (>36.5 or <34) might depress siphonophore
populations in the coastal area, enough food availability in the concerned areas might mitigate the negative effect of salinity,
since a positive and significant (p < 0.05) relationship was found between siphonophores biovolume and zooplankton biomass. Bassia bassensis, Diphyes dispar and Enneanogum hyalinum, present in both assemblages during both seasons, were able to survive in a wide range of salinity values, following perhaps,
their prey. All the 23 species here registered were found in the ‘outer assemblage’; however, Abylopsis eschscholtzi, Chelophyes appendiculata and Diphyes bojani were more associated with the ‘outer’ group according to the PCA results. Enneagonum hyalinum was the only species frequently encountered and abundant in the ‘inner assemblage’ during both seasons and, supporting previous
observations, this species might be considered as an indicator of nearshore waters.
Handling editor: K. Martens 相似文献
88.
We studied the life-history of the Sierra Madre sparrow (Xenospiza baileyi) in a subalpine grassland-agriculture mosaic south of Mexico City. From March to early September 1999 we captured and color-banded
53 adults with mist-nets and mapped the breeding territories of 21 males. We again mapped territories in the same spot in
April and May 2000. The number of breeding territories was found to be the same in the two consecutive years and interannual
survivorship was found to be relatively high. Breeding territories were restricted to the bunchgrass-covered areas. We used
the density of territories and the amount of remaining habitat to estimate a total population size of 5,380–6,150 adults for
this species. Using this and other recent data, we recommend raising the Sierra Madre sparrow to the status of critically
endangered using BirdLife International criteria. 相似文献
89.
An analysis of the distribution patterns of 124 Mexican gymnosperm species was undertaken, in order to detect the Mexican
areas with high species richness and endemism, and with this information to propose areas for conservation. Our study includes
an analysis of species richness, endemism and distributional patterns of Mexican species of gymnosperms based on three different
area units (states, biogeographic provinces and grid-cells of 1° × 1° latitude/longitude). The richest areas in species and
endemism do not coincide; in this way, the Sierra Madre Oriental province, the state of Veracruz and a grid-cell located in
the state of Oaxaca were the areas with the highest number of species, whereas the Golfo de México province, the state of
Chiapas and a grid-cell located in this state were the richest areas in endemic species. A weighted endemism and corrected
weighted endemism indices were calculated, and those grid-cells with high values in both indices and with high species richness
were considered as hotspots; these grid-cells are mainly located in Southern and Central Mexico. 相似文献
90.