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ECOLOGÍA - MEXICO
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We examined the hypothesis that the procumbent growth habit of the rare, columnar cactus Stenocereus eruca is in part the result of a diminution of the mechanical properties of stem tissues by comparing the properties of S. eruca plants with those of the putatively closely related semi-erect shrub S. gummosus. Intact stems and surgically removed anatomically comparable regions of the stems of both species were tested in bending and tension to determine their Young's modulus and breaking stress. A computer program was used to evaluate the contribution of each region to the capacity of entire stems to resist bending forces. Our analyses indicate that the principal stiffening agent in the stems of both species is a peripheral tissue complex (= epidermis and collenchyma in the primary plant body) that has a significantly higher tensile breaking stress and greater extensibility for S. gummosus than that of S. eruca. Computer simulations indicate that the wood of either species contributes little to bending stiffness, except in very old portions of S. gummosus stems, because of its small volume and central location in the stem. These and other observations are interpreted to support the hypothesis that S. eruca evolved a procumbent growth habit as the result of manifold developmental alterations some of which reduced the capacity of tissues to support the weight of stems. Am. J. Bot., Vol. 90, No.5: 663-674. |
Polaskia chichipe, a columnar cactus, is cultivated for its edible fruits in central Mexico. This study analyzed whether artificial selection has modified its reproduction patterns and caused barriers to pollen exchange between wild, managed in situ, and cultivated populations. Anthesis was diurnal (16 h in winter, 10 h in spring) as well as partly nocturnal (12 h in winter, 3 h in spring), and flowers were pollinated by bees, hummingbirds, and hawk moths. Manual cross-pollination was 37–49% effective in all populations. Self-pollination was 12% successful in the wild, but twice as successful (22–27%) in managed and cultivated populations. Diurnal pollination was 35–55% effective in winter and 100% in spring. Nocturnal pollination was successful only in winter (15%). Crosses among individuals were more effective within populations than among populations, including populations under a similar management regimen. The least successful crosses were between wild and cultivated populations. Flowers were produced in all populations from January to March, but flowering peaks differed by 1 mo among wild, managed, and cultivated populations and by 2 mo between wild and cultivated populations. The latter interrupted pollen exchange in May. Seeds from managed and cultivated populations germinated faster than those from wild individuals. Domestication has seemingly favored self-compatible P. chichipe plants with higher fruit yield, a longer period of fruit production, and faster seed germination, attributes that have resulted in partial reproductive barriers between wild and manipulated populations. Am. J. Bot., Vol. 90, No. 4: 603-609. |
Plants with natural variation in their floral traits and reproductive ecology are ideal subjects for analyzing the effects of natural selection and other evolutionary forces on genetic structure of natural populations. Agave lechuguilla shows latitudinal changes in floral morphology, color, and nectar production along its distribution through north-central Mexico. Both the type and abundance of its pollinators also change with latitude. Using starch electrophoresis, we examined the levels and patterns of variation of 13 polymorphic allozyme loci in 11 populations of A. lechuguilla. The overall level of genetic variability was high (He = 0.394), but the levels of genetic variation had no geographic pattern. However, the southern populations exhibited an excess of heterozygotes in relation to expectations for Hardy-Weinberg equilibrium, whereas the northern populations had an excess of homozygotes. Total differentiation among populations was low ( = 0.083), although gene flow estimates (Nm) varied among groups of populations: southern populations had the lowest levels of genetic differentiation, suggesting high levels of gene flow; northern populations had greater levels of genetic differentiation ( = 0.115), suggesting low gene flow among them. The patterns and inferences of the genetic structure of the population at the molecular level is consistent with variation in floral traits and pollinator visitation rates across the range of the species. Am. J. Bot., Vol. 90, No. 5: 700-706. |
The evolutionary significance of stress-induced mutagenesis was evaluated by studying mutagenesis in aging colonies (MAC) of Escherichia coli natural isolates. A large fraction of isolates exhibited a strong MAC, and the high MAC variability reflected the diversity of selective pressures in ecological niches. MAC depends on starvation, oxygen, and RpoS and adenosine 3',5'-monophosphate regulons; thus it may be a by-product of genetic strategies for improving survival under stress. MAC could also be selected through beneficial mutations that it generates, as shown by computer modeling and the patterns of stress-inducible and constitutive mutagenesis. We suggest that irrespective of the causes of their emergence, stress-induced mutations participate in adaptive evolution. Science, Vol.300: 1404-1409. |
Previous studies have shown that the nectar-feeding bat, Leptonycteris curasoae, is the major pollinator of Pachycereus pringlei (cardon), a columnar cactus whose populations are either gynodioecious or trioecious in the Sonoran Desert. On the basis of evidence of pollinator limitation in females and low flower visitation rates, a hypothesis has been proposed to explain geographic variation in the breeding system of P. pringlei. The pollinator abundance hypothesis predicts high frequencies of males and females in areas where L. curasoae is abundant and high frequency of hermaphrodites in areas where this bat is uncommon. We test this hypothesis by describing the following: first, the relationship between sex frequencies and bat abundance as revealed by nocturnal pollen deposition on stigmas of female plants and, second, the extent of pollinator limitation for fruit set in females and hermaphrodites in four populations of P. pringlei from coastal Sonora, Mexico. We also describe geographic variation in the relative fitness of females compared with hermaphrodites and outcrossing rates and inbreeding depression of self-compatible hermaphrodites in these populations. Some of these parameters should vary with bat abundance according to this hypothesis. Sex-frequency distributions showed a clear latitudinal pattern, with gynodioecious populations in the southern range and trioecious populations in the northern range. Large differences in nocturnal pollen deposition in females and abundance of L. curasoae were detected among populations but not as predicted by the pollinator abundance hypothesis. Fruit set was greater in females than in hermaphrodites in different pollination treatments, and no evidence of pollinator limitation was detected in either sex class. In general, the relative fitness of females was greater than hermaphrodites in most populations. Outcrossing rates in hermaphrodites were close to 1.0 in each population. Field and experimental data indicated no evidence of inbreeding depression. Our results do not support the hypothesis that the sex-frequency distribution of P. pringlei is closely correlated with the abundance of L. curasoae but do demonstrate that females outperform hermaphrodites in gynodioecious and trioecious populations. Studies of the distribution of mtDNA variants are needed to understand geographic variation in the breeding system of P. pringlei. Int. J. Plant Sci., Vol. 164, No.3: 383-393. |
We analyze models of parental care, providing the first systematic comparison of the care given to young by one parent versus by two parents. In the Houston-Davies model of care, young always do better with two parents rather than with one parent. When one parent decides about its level of care before the other, then the young may do better with one parent when the costs of care for the parents are asymmetric. When the level of parental effort is reached by negotiation, there are cases in which young do better with one parent, even when costs are symmetric. The analysis suggests empirical ways to differentiate between different response rules. Behavioral Ecology, Vol. 14, No. 3: 301-310. |
In the tropical dry forest of the central Pacific coast of Mexico the pollination and reproductive success of the bombacaceous tree Ceiba grandiflora was negatively affected by habitat disruption. Two of the three bat species that function as effective pollinators for this species (Glossophaga soricina and Musonycteris harrisoni) visited flowers found in trees in disturbed habitats significantly less than trees found in undisturbed habitats. A similar pattern was observed for the effective bat pollinator, Leptonycteris curasoae; however the difference was not significant. The three nectarivorous bats that functioned as effective pollinators of C. grandiflora also visited flowers to exclusively feed on pollen by biting or pulling off an anther (see Fig. S1 of Electronic Supplementary Material). The number of pollen grains deposited on stigmas from flowers in undisturbed areas was significantly greater than from flowers in disturbed habitats. The greater visitation rate and the greater number of pollen grains deposited on flowers from trees in undisturbed forest resulted in a significantly greater fruit set for trees in these areas. Our study demonstrates the negative effect that habitat disruption has on bat pollinators in tropical dry forest ecosystems and documents the negative consequences for the plants they pollinate. Oecologia, Vol. 135, No.3: 400 - 406 p. |
The paper describes the Version 2 of the CO2FIX (CO2FIX V.2) model, a user-friendly tool for dynamically estimating the carbon sequestration potential of forest management, agroforesty and afforestation projects. CO2FIX V.2 is a multi-cohort ecosystem-level model based on carbon accounting of forest stands, including forest biomass, soils and products. Carbon stored in living biomass is estimated with a forest cohort model that allows for competition, natural mortality, logging, and mortality due to logging damage. Soil carbon is modeled using five stock pools, three for litter and two for humus. The dynamics of carbon stored in wood products is simulated with a set of pools for short-, medium- and long-lived products, and includes processing efficiency, re-use of by-products, recycling, and disposal forms. The CO2FIX V.2 model estimates total carbon balance of alternative management regimes in both even and uneven-aged forests, and thus has a wide applicability for both temperate and tropical conditions. Results for the model testing and validation in selected temperate and tropical forest management systems are presented and discussed. Ecological Modelling, Vol. 164, No. 2/3, p177, 23p. |
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Researchers concerned with sustainable management of forests in the tropics have argued that the road to improved stewardship of forest resources is the transfer of responsibility to the local communities who get their livelihoods from them. On the other hand, conservationists have declared that the only way to stem the tide of deforestation is to place as many tracts as possible under strict protection. In this context, Mexico presents a national laboratory for studying the social and ecological benefits of delivering forests to local people. As a little-noticed result of the Mexican Revolution in the second decade of the twentieth century, well over half of the forests of Mexico were placed in community-held lands. In historic struggles that passed through several phases, most of these communities have now gained substantial control over the use of their forests. Because of the substantial degree of social capital in rural forms of organization in Mexico, this control of forest resources has led to an estimated 290–479 community forest enterprises ( CFEs ), through which communities are producing timber on their own lands. New studies are beginning to suggest that important gains in both social and economic justice, good forest management, and biodiversity protection are resulting from the actions of these CFEs. As more forests globally are being devolved to local communities, it is important to carry out more research on the Mexican model of community forest management for timber production. Conservation Biology, Vol. 17, No. 3, p672, 6p. |
Studies of habitat fragmentation have been restricted primarily to anthropogenically-altered habitats, with most research conducted 60–90 years post-fragmentation. It is unclear whether patterns in older systems concur with results from these dynamic landscapes, and hence the long-term viability of populations inhabiting habitat fragments remains largely unexplored. I focused on resident birds in fragments of humid pine-oak forest in Oaxaca, southern Mexico, isolated over 5000 years ago by climate-change. Seventeen fragments, ranging from 2 ha to over 150,000 ha were sampled in 1997 and 1998 yielding 141 species, of which 60 residents were used for analysis. Avian assemblages exhibited a highly nested structure and, with several notable exceptions, assemblages of birds in low-richness fragments were predictable subsets of those in more diverse fragments. Patch-scale factors—area, shape, elevation, habitat diversity and fractal dimension of edge—all exerted strong univariate influence on avian richness but were so closely inter-related that none had a significant independent effect. Thus, larger fragments were more complex in shape, included higher peaks, supported more diverse forests, and contained higher diversities of resident species. In contrast, the landscape-scale index used—distance from nearest large fragment (>50,000 ha)—had little effect on richness. This was reinforced by species-level analyses—one species was significantly influenced by isolation, compared with 31 species that displayed significant minimum-area distributions, restricted to patches larger than a particular threshold value. In terms of autecology, vagility, relative abundance and elevational breadth were closely related to distribution—those species with greater mobility, higher abundances and broader elevational tolerances were consistently more widespread. I suggest that more abundant species were less prone to extinction initially, more ... Biological Conservation, Vol. 111, No. 3, p283, 21p. |
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The impact of volcanic eruptions on forest ecosystems can be investigated using dendrochronological records. While long-range effects are usually mediated by decreased air temperatures, resulting in frost rings or reduced maximum latewood density, local effects include abrupt suppression of radial growth, occasionally followed by greater than normal growth rates. Annual rings in Mexican mountain pine (Pinus hartwegii Lindl.) on Nevado de Colima, at the western end of the Mexican Neovolcanic Belt, indicate extremely low growth in 1913 and 1914, following the January 1913 Plinian eruption of Volcán de Fuego, 7.7 km to the south. That event, which is listed among the largest explosive eruptions since A.D. 1500, produced ashflow deposits up to 40 m thick and blanketed our study area on Nevado de Colima with a tephra fallout 15–30 cm deep. Radial growth reduction in 1913–14 was =30% in 73% of the sampled trees. We geostatistically investigated the ecological impact of the eruption by mapping the decrease in xylem increment and found no evidence of a spatial structure in growth reduction. Little information has been available to date on forest species as biological archives of past environments in the North American tropics, yet this historical case study suggests that treeline tropical sites hold valuable records of prehistoric phenomena, including volcanic eruptions. Quaternary Research, Vol. 59, No. 3, p293, 7p. |
The present research focused on the functional role of the phytoplankton of an economically important endorheic tropical lake from the perspective of algal growth bioassays. The algal growth potential of the lake water was compared for littoral and pelagic sites during the wet and dry seasons. Algal growth potentials at open waters reached minimum and maximum values following the seasonal alternation of dilution (by rain) and concentration (by evaporation) respectively. Conversely, at southern littoral stations high algal growth responses related to the availability of nutrients from point contamination sites. There was no such effect at nearby offshore sites or elsewhere in the lake which suggests filtration and competitive interactions for these nutrients between phytoplankton and littoral macrophytes. Nitrogen and phosphorus both acted consistently as limiting nutrients at open waters by colimitation. Nitrogen to phosphorus ratios seemed to approach equilibrium where limitation easily shifted to one element or the other temporally and spatially. Lakes & Reservoirs: Research & Management, Vol. 8, No. 2, p83, 11p. |
1. To assess the effectiveness of extrafloral nectaries (EFN) as a defensive mechanism of plants it is vital to use a multiple interactions approach and assess the contribution of all visiting species and their interactions. 2. The effect of EFN-visiting ants (Camponotus planatus, C.abdominalis, Conomyrma sp., Crematogaster brevispinosa, Forelius sp., Pseudomyrmex sp.) and wasps (Polistes instabilis, Polybia occidentalis) on the reproductive success (estimated as the number of reproductive structures) ofTurnera ulmifolia (Turneraceae) was experimentally evaluated. Herbivory effects were tested usingEuptoieta hegesia larvae (caterpillars), which is the main herbivore of this plant. The study was done in a coastal sand dune scrub in Veracruz, México. 3. Wasps and ants were selectively excluded using a two-factor design (Wasps, Ants) block design, both factors with two levels (absent, present). The response variables were an index of herbivory per branch and the number of buds, flowers, ripe and unripe fruit per plant, and the seed/fruit ratio per branch. 4. After a week of placing the larvae on experimental plants, they were significantly more frequent on plants where wasps and ants had been experimentally excluded. 5. Wasp presence was associated significantly with greater numbers of buds, flowers, ripe fruit and seeds. When acting separately, wasps and ants exerted a positive effect in decreasing herbivory levels and increasing the number of unripe fruit; when acting together, however, their effect was not additive. 6. This is the first demonstration of a positive effect on the plant by wasps associated with EFN. The ecological implication of this finding is that the function of EFN and the ultimate effects on a plant will probably depend on the array of organisms visiting its EFN. Functional Ecology, Vol. 17, No. 3, p417, 7p. |
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1. Many studies discuss the adaptive value of plant architecture, but few have actually measured architectural effects on plant growth and survival. In this study, sapling growth and survival are related to crown traits for two tree species, Trophis mexicana (Liebm.) Bur. and Pseudolmedia oxyphyllaria Donn. Sm., in the Los Tuxtlas lowland rainforest of Mexico. The traits investigated were crown width, crown depth, number of leaves, number of leaves per unit crown area (horizontal self-shading), and number of leaves per unit silhouette area (vertical self-shading). 2. Self-shading indices decreased with crown size, but were unaffected by the number of leaves per tree. Larger crowns thus had more diffuse foliage, with less self-shading. 3. The number of leaves had positive effects on growth and survival, while self-shading indices had no effect. This indicates that shaded leaves do not necessarily have negative carbon balances. 4. Negative effects of crown width on horizontal crown growth, and positive effects on vertical crown growth, suggest that saplings tend to grow towards a shape intermediate between the narrow and wide crown extremes. 5. Survival was positively correlated with crown width in Pseudolmedia, and with the number of leaves in Trophis. Apparently, dependence of survival on crown traits differed among species. 6. Crown traits affected plant growth and survival, but the hypothesis emerging from light-limited carbon acquisition was confounded by other factors, such as tree size and the inherent branching patterns. 7. Crown traits are good and rather simple predictors of future sapling growth and survival, and may help foresters to select potential crop trees. Functional Ecology, Vol. 17, No. 2, p194, 7p. |
Researchers concerned with sustainable management of forests in the tropics have argued that the road to improved stewardship of forest resources is the transfer of responsibility to the local communities who get their livelihoods from them. On the other hand, conservationists have declared that the only way to stem the tide of deforestation is to place as many tracts as possible under strict protection. In this context, Mexico presents a national laboratory for studying the social and ecological benefits of delivering forests to local people. As a little-noticed result of the Mexican Revolution in the second decade of the twentieth century, well over half of the forests of Mexico were placed in community-held lands. In historic struggles that passed through several phases, most of these communities have now gained substantial control over the use of their forests. Because of the substantial degree of social capital in rural forms of organization in Mexico, this control of forest resources has led to an estimated 290–479 community forest enterprises ( CFEs ), through which communities are producing timber on their own lands. New studies are beginning to suggest that important gains in both social and economic justice, good forest management, and biodiversity protection are resulting from the actions of these CFEs. As more forests globally are being devolved to local communities, it is important to carry out more research on the Mexican model of community forest management for timber production. Conservation Biology, Vol. 17, No. 3, p672, 6p. |
Results of earlier studies of Quaternary tephra–paleosol sequences in Central Mexico revealed contradictions between paleopedological and lacustrine records. To settle the contradictions, selected quantitative characteristics of mineral and organic components of the paleosols PT1–PT7 from the Nevado de Toluca sequence were studied as independent paleoclimate proxies. Mineralogical composition of sand and clay fractions allows assessment of weathering and secondary mineral formation, both being climate-dependent processes. Stable carbon isotope ratios of humus and phytolith morphology indicate paleovegetation. In all paleosols, the sand fractions consist mainly of unstable volcanogenic minerals, although the grains have weathering features. Short durations of pedogenesis are responsible for limited weathering status. The clay fraction is dominated by halloysite and dehydrated halloysite in units PT1–PT4, which suggests drier periods during paleosol formation, whereas major amounts of kaolinite in the units PT5–PT7 indicate extended humid pedogenesis. <f>d13C</f> values of PT2–PT4 humus are intermediate between those characteristic of C3 and C4 plants. Phytolith assemblages which include both forms of C3 (dominant) and C4 (minor quantities) grasses agree with carbon isotope data. These data indicate plant cover changes driven by wet/dry paleoclimate oscillations. The new results, showing drier stages during the formation of Late Pleistocene paleosols PT2 and PT3, correct the primary interpretation of these units as indicating only humid pedogenesis, and settles the contradiction between paleopedological and lacustrine records. Quaternary International, Vol. 106/107, p169, 16p. |
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Presented here is the first comprehensive late Quaternary alluvial stratigraphic record from desert grasslands in northern Mexico. Thc stratigraphic history was temporally constrained by radiocarbon dating, whereas the ecology of C4 plants was inferred by stable carbon isotopes from burled soils. Environmental relationships within the Casas Grandes and San Pedro River basins of the Juanaquena study area provide a picture of evolving landscapes where fluvial behavior is driven by climate change and ecosystem instability. Seven periods of flood-plain construction and six episodes of paleosol formation and channel erosion occurred during the last 14,000 [sup 14]C yr B.P. In the latest Pleistocene, channel erosion ensued in response to a general warming trend and landscape instability, although C3 plants still dominated in the alluvial valleys. In the Holocene when C4 plants began to dominate, channel crosion was typically initiated by a shift from relatively warmer and drier to cooler and wetter climates. The timing of erosion events exhibits a remarkable correlation to arroyo formation and paleosol formation of ephemeral streams in the North American southwest. In contrast to arroyos, the Casas Grandes and San Pedro Rivers created flood plains having mixed to bedload meanderbelts that persisted throughout the late Quaternary. However, bedrock valley constrictions accelerated flood-plain building, leading to periodic channel avulsion. Although intervals of landscape instability occurred numerous times in thc late Quaternary, fluvial style was largely governed by thc dominance of C4 grasslands, which appears to have maintained sufficient upland vegetative cover to mitigate flash flooding typical of arid environments. Geological Society of America Bulletin, Vol. 115, No. 5, p596, 11p. |
According to the desperado sibling hypothesis, chicks of obligately siblicidal species kill their junior broodmates as early as possible because junior broodmates face dire ecological prospects and are consequently predisposed to aggressively overthrow elder broodmates by all-out aggression. The agonistic behavior of junior chicks is virtually unknown because they die so young. We investigated the agonistic behavior of hypothetical desperados by fostering junior brown boobies (Sula leucogaster, an obligately siblicidal species) into nests of the blue-footed booby (S. nebouxii, a facultatively siblicidal species) containing an older singleton blue-foot chick. Controls were junior blue-footed boobies fostered into the same situation. Junior brown boobies were 7 times more aggressive than controls and most of them dominated their elder and larger nestmates. Four of nine brown booby juniors showed relentless aggression, delivering up to 711 pecks, bites and pushes (including "expulsion pushes") per hour, thereby overwhelming nestmates 90% heavier and permanently expelling one of them from the nest. Similarly, in natural broods of two surviving brown booby chicks, the losing chick was 13 times as aggressive as blue-foot subordinates, up to at least age 7 weeks. Contrast of the two species of booby suggests the evolution of agonistic roles within broods may be partly driven by selection on potential victims to express a level and type of aggressiveness appropriate to their status-related ecological prospects. Behavioral Ecology and Sociobiology, Vol. 53, No. 5: 287-296. |
The alkaloids present in the stem of three species of genus Neobuxbaumia Backeb. (Cactaceae), N. multiareolata (E.Y. Dawson) Bravo, Sheinvar & Sanchez-Mej., N. scoparia (Poselger) Backeb. and N. tetetzo ((J.M. Coult.) Backeb., were screened by gas chromatography-mass sepectrometry (GC-MS). In the three species analyzed the following tetrahydroisoquinolines were identified: anhalidine, carnegine and salsolidine, all in trace quantities. This paper is the first record of alkaloids in N. multiareolata, N. scoparia and N. tetetzo. Biochemical Systematics & Ecology, Vol. 31, No. 6, p581, 5p. |
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We explored, using a mathematical model, the changes in areole arrangement that have occurred during the evolution of stem succulence in different cactus life-forms. The study was based on field data. The model used two parameters: (a) areole density in the stem and (b) the divergence angle between successive areoles. The stem morphology in different cactus subfamilies is correlated to changes in one or both parameters. Gradual changes in stem compaction and in the divergence angle can explain the apparently complex evolutionary transition from primitive woody and leafy plants to the more evolved, stem-succulent morphologies of cacti. Journal of Arid Environments, Vol. 53, No. 4, p557. |
Examines the presence of disinfection byproducts and microorganisms in drinking water in Southeastern Mexico City, Mexico. Analysis of bacterial indicators, physiochemical parameters, and trihalomethane levels; Presence of indicator bacteria and enterobacteria; Implications of water treatment and distribution. Archives of Environmental Health, Vol. 58, No. 4, p233, 5p. |
1. Many studies discuss the adaptive value of plant architecture, but few have actually measured architectural effects on plant growth and survival. In this study, sapling growth and survival are related to crown traits for two tree species, Trophis mexicana (Liebm.) Bur. and Pseudolmedia oxyphyllaria Donn. Sm., in the Los Tuxtlas lowland rainforest of Mexico. The traits investigated were crown width, crown depth, number of leaves, number of leaves per unit crown area (horizontal self-shading), and number of leaves per unit silhouette area (vertical self-shading). 2. Self-shading indices decreased with crown size, but were unaffected by the number of leaves per tree. Larger crowns thus had more diffuse foliage, with less self-shading. 3. The number of leaves had positive effects on growth and survival, while self-shading indices had no effect. This indicates that shaded leaves do not necessarily have negative carbon balances. 4. Negative effects of crown width on horizontal crown growth, and positive effects on vertical crown growth, suggest that saplings tend to grow towards a shape intermediate between the narrow and wide crown extremes. 5. Survival was positively correlated with crown width in Pseudolmedia, and with the number of leaves in Trophis. Apparently, dependence of survival on crown traits differed among species. 6. Crown traits affected plant growth and survival, but the hypothesis emerging from light-limited carbon acquisition was confounded by other factors, such as tree size and the inherent branching patterns. 7. Crown traits are good and rather simple predictors of future sapling growth and survival, and may help foresters to select potential crop trees. Functional Ecology, Vol. 17, No. 2, p194, 7p. |
Forest-transition theory suggests that economic development eventually leads to forest recovery, but there is great uncertainty about the existence, the characteristics, and the mechanisms of forest transitions that might be occurring under current socioeconomic conditions. A case study in a small region of highland Mexico finds agricultural abandonment and forest regeneration, but also forest degradation due to woodcutting. A discussion locates those findings in the international political economy of agriculture and emigration in Mexico, which are substantially different from the conditions associated with forest recovery in developed countries. Although regional forest transitions are possible in marginal agricultural areas such as that studied here, their environmental implications depend on the evolution of social institutions coordinating rural people's environmental actions. Professional Geographer, Vol. 55, No. 2, p227, 11p. |
On-farm conservation is recognized as a key component of a comprehensive strategy to conserve crop genetic resources. A fundamental problem faced by any on-farm conservation project is the identification of crop populations on which efforts should be focused. This paper describes a method to identify a subset of landraces for further conservation efforts from a larger collection representing the diversity found in the Central Valleys of Oaxaca, Mexico. Mexico is a center of origin and diversity for maize (Zea mays L.). The 17 landraces selected from an initial collection of 152 satisfy two criteria. First, they represent the diversity present in the larger collection. Second, they appear to serve the interests of farmers in the region. Data for applying the method were elicited through participatory as well as conventional techniques. They incorporate the complementary perspectives of both men and women members of farm households, and of plant breeders and social scientists. Genetic Resources and Crop Evolution, Vol. 50, No. 4: 401-416. |
Surveys the lizard species along the transects of the desert shrubland-intertidal zone in Sonora, Mexico. Characterization of the Sonoran coast; Restriction of Cnemidophorus tigris to desert shrubland; Preference of Callisaurus draconoides for bare ground; Identification of less total lizard density and biomass; Influence of food resources on lizard density across the transition zone. Journal of Herpetology, Vol. 37, No. 2, p378, 5p. |
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Plants with natural variation in their floral traits and reproductive ecology are ideal subjects for analyzing the effects of natural selection and other evolutionary forces on genetic structure of natural populations. Agave lechuguilla shows latitudinal changes in floral morphology, color, and nectar production along its distribution through north-central Mexico. Both the type and abundance of its pollinators also change with latitude. Using starch electrophoresis, we examined the levels and patterns of variation of 13 polymorphic allozyme loci in 11 populations of A. lechuguilla. The overall level of genetic variability was high (H[sub e] = 0.394), but the levels of genetic variation had no geographic pattern. However, the southern populations exhibited an excess of heterozygotes in relation to expectations for Hardy-Weinberg equilibrium, whereas the northern populations had an excess of homozygotes. Total differentiation among populations was low (? = 0.083), although gene flow estimates (Nm) varied among groups of populations: southern populations had the lowest levels of genetic differentiation, suggesting high levels of gene flow; northern populations had greater levels of genetic differentiation (? = 0.115), suggesting low gene flow among them. The patterns and inferences of the genetic structure of the population at the molecular level is consistent with variation in floral traits and pollinator visitation rates across the range of the species. American Journal of Botany, Vol. 9, No. 5, p700, 7p. |
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We compare species richness of birds, fruit-feeding butterflies and ground-foraging ants along a coffee intensification gradient represented by a reduction in the number of species of shade trees and percentage of shade cover in coffee plantations. We sampled the three taxa in the same plots within the same period of time. Two sites were selected in the Soconusco region of the state of Chiapas, Mexico. Within each site four habitat types were selected and within each habitat type four points were randomly selected. The habitat types were forest, rustic coffee, diverse shade coffee, and intensive coffee (low density of shade). We found different responses of the three taxa along the intensification gradient. While ants and butterflies generally decrease in species richness with the decrease of shade cover, birds declined in one site but increased in the other. Ant species richness appears to be more resistant to habitat modification, while butterfly species richness appears to be more sensitive. Bird species richness was correlated with distance from forest fragments but not with habitat type, suggesting that scale and landscape structure may be important for more mobile taxa. For each of these taxa, the rustic plantation was the one that maintained species richness most similar to the forest. We found no correlation between the three taxa, suggesting that none of these taxa are good candidates as surrogates for each other. We discuss the implications of these results for the conservation of biodiversity in coffee plantations, in particular, the importance of distinguishing between different levels of shade, and the possibility that different taxa might be responding to habitat changes at different spatial scales. Biodiversity and Conservation,: Vol. 12, No.6: 1239-1252. |
Tamaulipan thornscrub (matorral) is a subtropical, semi-arid shrubland covering 200,000 km 2 of the coastal plain of the Gulf of Mexico in southern Texas and northeastern Mexico. The dynamics of this complex plant community are poorly understood. In here we look at the early stages of plant growth such as seed germination and seedling survival as they vary in time and environmental conditions thus contributing to a better understanding of the ecology of matorral. Spontaneous seed germination was investigated under different plant cover conditions that reflect local variations in plant cover currently influencing seedling establishment in semiarid northeastern Mexico. These conditions were: (i) dense thornscrub; (ii) border; and (iii) cleared thornscrub. Ten plots of one m square were used in each environment. Seed germination was higher in late summer and early autumn. A total of 3648 seedlings in 37 species were found to germinate in the three environments. Dense thornscrub and its border had the highest number of seedlings (1871 and 1107) and species (36 and 37), while cleared thornscrub had only 670 seedlings from 25 species. Grasses germinated similarly in all environments, but more forbs and woody species germinated under dense thornscrub. Forest Ecology and Management, Vol. 177, No. 1-3: 11-16. |
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