Region Of Southern Mexico Research Articles

Limnological comparison of pristine and impacted lakes from a tropical, high-altitude karst region in southern Mexico

ABSTRACT Climatic conditions strongly influence tropical karst lake limnology, but more information is required to understand how human impacts can modify their ecological patterns. The Lagunas de Montebello lake district, a tropical, high-altitude karst landscape, comprises 139 solution lakes surrounded by tropical rainforests. To investigate the limnological changes in the Lagunas de Montebello in the past 20 years, we selected 14 lakes for a comparative study: 4 impacted and 10 pristine. The impacted lakes are on the northwest (NW) plateau area fed by surface and underground waters, whereas the pristine lakes are on the southeast (SE) intermontane zone fed by underground waters. Impacted lakes receive nutrients and organic matter from agricultural and urban/domestic wastewater from point and nonpoint surface sources. Heavy tropical storms flood the plateau zone, interconnecting the lakes and facilitating pollutant dispersion among lakes. Pristine lakes remain isolated, and groundwater pollution is limited because most anthropogenic activities occur in the NW plateau zone. Most of the limnological variables measured differed between pristine and impacted lakes. Nutrients, chlorophyll a, total suspended solids, and particulate organic carbon concentrations were higher in the impacted lakes. The hydraulic connection between the Montebello Lakes facilitates the rapid dispersion of pollutants from one lake to another, threatening lakes that are still pristine. Although natural aquatic karst environments promote phosphorus precipitation, which leads to phosphorus limitation of primary production, anthropogenic additions of phosphorus, as well as nitrogen and organic matter, are leading to eutrophication and ecosystem degradation of lakes ecosystems in the Lagunas de Montebello lake district.

Beyond a generalized deagrarianization: Livelihood heterogeneity and its determinants in the Mixteca Alta, Mexico

In recent decades, rural livelihoods across the global South have increasingly turned away from farming. This process of “deagrarianization” is frequently seen as a uniform reaction to pervasive institutional and economic pressures against small-scale farming. Evidence at local scales, however, shows that households tend to adjust heterogeneously to such pressures, which challenges assumptions of uniform obstacles and/or motivations shaping deagrarianization. Studying this heterogeneity in livelihood adjustments and their drivers represents a key first step towards understanding the varying implications in terms of welfare and vulnerability for rural households. To that end, this paper investigates the manifestations and determinants of livelihood heterogeneity in the Mixteca Alta, a region in southern Mexico whose economy has shifted drastically away from agriculture over recent decades. Drawing on cluster analysis and machine learning applied to survey and secondary data, we show that this economic shift has actually unfolded in contrasting ways across households, with some diversifying across both agricultural and non-agricultural activities and others specializing in non-agricultural occupations. Much of this differentiation in livelihood strategies rests upon an unequal distribution of assets like land, education, and financial capital interacting with an uneven economic context in terms of road accessibility and aggregate poverty. By revealing the underlying conditions that enable and restrict occupational opportunities across households, our study emphasizes the need to move beyond one-size-fits-all interventions to enhance livelihood security in rural spaces.

Fungal diversity in shade-coffee plantations in Soconusco, Mexico.

BackgroundAs forested natural habitats disappear in the world, traditional, shade-coffee plantations offer an opportunity to conserve biodiversity and ecosystem services. Traditional coffee plantations maintain a diversity of tree species that provide shade for coffee bushes and, at the same time, are important repositories for plants and animals that inhabited the original cloud forest. However, there is still little information about shade-coffee plantation’s fungal diversity despite their relevance for ecosystem functioning as decomposers, symbionts and pathogens. Specifically, it is unknown if and what mycorrhizae-forming fungi can be found on the branches and trunks of coffee bushes and trees, which hold a diversity of epiphytes. Here, we evaluate fungal communities on specific plant microsites on both coffee bushes and shade trees. We investigate the ecological roles played by this diversity, with a special focus on mycorrhizae-forming fungi that may enable the establishment and development of epiphytic plants.MethodsWe collected 48 bark samples from coffee bushes and shade trees (coffee; tree), from four plant microsites (upper and lower trunks, branches and twigs), in two shade-coffee plantations in the Soconusco region in southern Mexico, at different altitudes. We obtained ITS amplicon sequences that served to estimate alpha and beta diversity, to assign taxonomy and to infer the potential ecological role played by the detected taxa.ResultsThe bark of shade trees and coffee bushes supported high fungal diversity (3,783 amplicon sequence variants). There were no strong associations between community species richness and collection site, plant type or microsite. However, we detected differences in beta diversity between collection sites. All trophic modes defined by FUNGuild database were represented in both plant types. However, when looking into guilds that involve mycorrhizae formation, the CLAM test suggests that coffee bushes are more likely to host taxa that may function as mycorrhizae.DiscussionWe detected high fungal diversity in shade-coffee plantations in Soconusco, Chiapas, possibly remnants of the original cloud forest ecosystem. Several mycorrhiza forming fungi guilds occur on the bark of coffee bushes and shade trees in this agroecosystem, with the potential of supporting epiphyte establishment and development. Thus, traditional coffee cultivation could be part of an integrated strategy for restoration and conservation of epiphytic populations. This is particularly relevant for conservation of threatened species of Orchidaceae that are highly dependent on mycorrhizae formation.

Harmonizing Definitions and Methods to Estimate Deforestation at the Lacandona Tropical Region in Southern Mexico

Deforestation is a major factor reducing natural habitats, leading to tropical ecosystems and biodiversity loss worldwide. The Lacandona region in southern Mexico holds one of the largest fragments of tropical rainforest in North America. We evaluated the deforestation of the Lacandona region harmonizing concepts and methodologies. An international (FAO definition), governmental (national definition), and regional definition of deforestation with applications at different scales were analyzed and harmonized with two classification methods (likelihood and spectral angle mapper (SAM)). We used 2015 and 2018 Landsat 8 images, and likelihood and SAM classifications were applied for FAO and regional definitions of deforestation. Overall, the best evaluated classifier in quantity was likelihood for 2015 and 2018 (kappa: 0.87 and 0.70, overall accuracy: 91.8 and 80.4%, and quantity disagreement: 4.1 and 10 %, respectively). The allocation disagreement only showed exchange between classes. Nevertheless, they did not show differences between classifiers, although 2015 had less disagreement than 2018: exchange, 4.1% for likelihood and SAM; shift: 0% for likelihood and SAM. Maps based on the regional definition of deforestation showed that the likelihood classification detected 11,441 ha less deforestation than SAM (40,538 and 51,979 ha, respectively). The FAO definition of deforestation showed that likelihood classification detected 11,914 ha less deforestation than SAM classification (37,152 and 49,066 ha, respectively). Further, the likelihood classification showed 3387 ha more of deforestation according to the regional definition than the FAO definition of deforestation (40,538 and 37,152 ha, respectively). SAM classification showed that the regional definition showed 2913 ha more deforestation than the FAO definition (51,979 and 49,066, respectively). We concluded that implementation of governmental programs in the Lacandona region requires estimations based on a careful selection of deforestation definitions and methods.

Serosurveillance and risk factors for Neospora caninum infection in water buffaloes (B. bubalis) from central and southern Mexico.

The water buffalo (B. bubalis) is an alternative to cattle ranching in several regions of southern Mexico. Here we report seroprevalence and risk factors associated with the protozoan parasite, Neospora caninum, in water buffaloes in six buffalo production units, in municipalities from central and southern Veracruz, Mexico. Antibodies to N. caninum were assessed in serum samples of 543 buffaloes by a commercial ELISA-kit, and 44.8% (243/543; 95% CI 40.5-49.0) were seropositive. Questionnaires were used to collect epidemiological data and to identify risk factors associated with N. caninum infection. Data analysis indicated that older buffaloes (≥7 year) exhibited the highest seroprevalence for neosporosis 62.3% (38/61; 95% CI 49.7-73.4) (P ≤ 0.05). Buffaloes that were in close contact with cattle had higher seroprevalence 47.6% (168/353; 95% CI 42.3-52.9) (P < 0.01) than those that were not in contact 36.8% (70/190; 95% CI 30.0-44.1). Our findings provide important information to implement preventive measures in the buffalo farms.

Water as a Hyperfact

Most entities studied by archaeologists share the same basic necessary conditions. They are limited spatiotemporal units which are continuous within a human frame of sensorial reference. These en­ tities cannot dissolve into their constituent parts without affecting their function, capacity, and mor­ phology. Further, they usually occupy one phys­ ical state at a time. The hyperfact, on the other hand, is vastly distributed, it can dissolve into most of its parts without affecting its “essence”, and it can be in several physical states at the same time. Water is a typical hyperfact, existing on multiple scales, from molecules to the hydrological cycle. In this text I show how local manifestations of this hyperfact can be found in ceramics, architectural features, agriculture, water management systems, and regional settlements of the Cochuah region in southern Mexico.

Fertilisation schemes based on organic amendments; decomposition and nutrient contribution of traditionally used species in an indigenous region of southern Mexico

ABSTRACT The use of organic soil amendments to sustain crop production provides a feasible option for low-income farmers. In La Montaña, in southern Mexico, farmers grow organic hibiscus (Hibiscus sabdariffa) as their main source of income, but yields are low. The aim of this study was to evaluate the decomposition and nutrient release of different species traditionally used by farmers and to make recommendations for their use as soil amendments. The decomposition of Mucuna pruriens, Calliandra houstoniana and stover of hibiscus (separately and as 1:1 mixtures) was evaluated using litterbags in the field, analysing dry weight and chemical components of the remaining litter at monthly intervals. Mucuna on its own and mixtures containing this species, decomposed on average 50% faster than the other amendments. Mucuna on its own released the highest amount of nitrogen (155 kg ha−1 year−1), which was in excess of that required by the hibiscus crop (45 kg nitrogen ha−1 year−1). To complement crop nutrition whilst minimising nitrogen losses and improving soil quality, the calliandra-mucuna mixture was proposed as the most promising amendment, because: (i) the nitrogen demand of the hibiscus crop was met (75.7 kg ha−1 year−1), (ii) the asynchrony of available nutrients was minimised as half of the mass of the amendment had decomposed by the time of the flowering stage of the hibiscus, and (iii) it contributed carbon and nutrients to the soil organic matter. This information can be used to promote agroforestry systems that include mucuna and calliandra trees in hibiscus plantations.

Sand deposits reveal great earthquakes and tsunamis at Mexican Pacific Coast

Globally, instrumentally based assessments of tsunamigenic potential of subduction zones have underestimated the magnitude and frequency of great events because of their short time record. Historical and sediment records of large earthquakes and tsunamis have expanded the temporal data and estimated size of these events. Instrumental records suggests that the Mexican Subduction earthquakes produce relatively small tsunamis, however historical records and now geologic evidence suggest that great earthquakes and tsunamis have whipped the Pacific coast of Mexico in the past. The sediment marks of centuries old-tsunamis validate historical records and indicate that large tsunamigenic earthquakes have shaken the Guerrero-Oaxaca region in southern Mexico and had an impact on a bigger stretch of the coast than previously suspected. We present the first geologic evidence of great tsunamis near the trench of a subduction zone previously underestimated as potential source for great earthquakes and tsunamis. Two sandy tsunami deposits extend over 1.5 km inland of the coast. The youngest tsunami deposit is associated with the 1787 great earthquake, M 8.6, producing a giant tsunami that poured over the coast flooding 500 km alongshore the Mexican Pacific coast and up to 6 km inland. The oldest event from a less historically documented event occurred in 1537. The 1787 earthquake, and tsunami and a probable predecessor in 1537, suggest a plausible recurrence interval of 250 years. We prove that the common believe that great tsunamis do not occur on the Mexican Pacific coast cannot be sustained.

A unique immune signature of serum cytokine and chemokine dynamics in patients with Zika virus infection from a tropical region in Southern Mexico

ObjectivesTo describe the kinetics of circulating cytokines and chemokines in humans with ZIKAV infection. MethodsSerum levels of different immune mediators in patients with ZIKAV infection were measured at distinct stages of the disease, as well as in culture supernatants from human monocytes infected with a clinical ZIKAV isolate. We also looked for clinical features associated with specific immune signatures among symptomatic patients. ResultsWe evaluated 23 ZIKAV-infected patients. Their mean age was 32 ± 8.3 years and 65% were female. ZIKAV patients showed elevated IL-9, IL-17A, and CXCL10 levels at acute stages of the disease. At day 28, levels of CCL4 and CCL5 were increased, whereas IL-1RA, CXCL8 and CCL2 were decreased. At baseline, IL-7 was increased among patients with headache, whereas CCL2, and CCL3 were decreased in patients with bleeding and rash, respectively. Our clinical ZIKAV isolate induced a broad immune response in monocytes that did not resemble the signature observed in ZIKAV patients. ConclusionsWe showed a unique immune signature in our cohort of ZIKAV-infected patients. Our study may provide valuable evidence helpful to identify immune correlates of protection against ZIKAV.

Regional Ecotourism Networks: Experiences and Lessons from Los Tuxtlas, Mexico

Ecotourism has the potential to foster human development in marginalised areas and, at the same time, promote environmental conservation. However, as studies have shown, ecotourism enterprises often struggle to be economically viable. In this context, cooperation and the establishment of networks on a regional scale have been proposed as promising strategies, which can benefit the enterprises in many ways, including information exchange, effective promotion, creation of competitive tourism products and representation of common political interests. Based on a mixed-method approach, the present investigation analyses to what extent this has been the case in the Los Tuxtlas region in Southern Mexico, where since the year 2000 several attempts have been made to form regional ecotourism networks. Mapping and the use of quantitative network analysis tools, based on the application of questionnaires, demonstrate the current structure of both formally established networks and the net of informal cooperation in the region. In combining these data with qualitative information obtained from interviews, insights are gained regarding the catalysts that initiate networking processes and the stakeholders involved, the main obstacles as well as costs and benefits of forming part of regional networks.

Dung beetles and their ecological functions in three agroforestry systems in the Lacandona rainforest of Mexico

There is growing interest in evaluating the impact that management intensity of agroecosystems has on animal communities and their ecological functions. Dung beetles are a highly used focal taxon for assessing the effects of anthropogenic disturbances and management practices on biodiversity. In the Lacandona rainforest region in southern Mexico, we quantified several metrics of the dung beetle community (number of species, number of individuals, total biomass, mean beetle size) and four of their ecological functions (dung removal, soil excavation, seed dispersal, seed exhumation) in conserved rainforest and three agroforestry systems with different management intensities: rustic cocoa, polyculture cocoa, and rubber monoculture. We also assessed the correlation between dung removal and the other functions, as well as the relationships between functions and community metrics. Land-use type affected the dung beetle communities as well as their functions, with negative effects on response variables in the most intensely managed agroecosystems (polyculture cocoa and rubber). Rustic cocoa had values similar to those of the conserved forest for all functions and community metrics, except the mean number of species per trap. Dung removal was correlated with the other ecological functions. The mean number of species per trap was significantly associated with all four functions. In our study region rustic cocoa plantations favor the maintenance of a high proportion of dung beetle species and maintain their ecological functions. Our findings corroborate that agroecosystems with less intense management may contribute to buffering the effects of landscape homogenization caused by more intensely managed agroecosystems, such as rubber plantations.

Regional differences in the composition of Fusarium Head Blight pathogens and mycotoxins associated with wheat in Mexico

Fusarium Head Blight (FHB) is a destructive disease of small grain cereals and a major food safety concern. Epidemics result in substantial yield losses, reduction in crop quality, and contamination of grains with trichothecenes and other mycotoxins. A number of different fusaria can cause FHB, and there are significant regional differences in the occurrence and prevalence of FHB pathogen species and their associated mycotoxins. Information on FHB pathogen and mycotoxin diversity in Mexico has been extremely limited, but is needed to improve disease and mycotoxin control efforts. To address this, we used a combination of DNA sequence-based methods and in-vitro toxin analyses to characterize FHB isolates collected from symptomatic wheat in Mexico during the 2013 and 2014 growing seasons. Among 116 Fusarium isolates, we identified five species complexes including nine named Fusarium species and 30 isolates representing unnamed or potentially novel species. Significant regional differences (P < 0.001) in pathogen composition were observed, with F. boothii accounting for >90% of isolates from the Mixteca region in southern Mexico, whereas F. avenaceum and related members of the F. tricinctum species complex (FTSC) accounted for nearly 75% of isolates from the Highlands region in Central Mexico. F. graminearum, which is the dominant FHB pathogen in other parts of North America, was not present among the isolates from Mexico. F. boothii isolates had the 15-acetyldeoxynivalenol toxin type, and some of the minor FHB species produced trichothecenes, such as nivalenol, T-2 toxin and diacetoxyscirpenol. None of the FTSC isolates tested was able to produce trichothecenes, but many produced chlamydosporol and enniatin B.

Weather regimes associated with summer rainfall variability over southern Mexico

ABSTRACTThis study focuses on the identification of weather regimes (WRs) over Mexico, the tropical eastern Pacific, Central America and the Caribbean, associated with seasonal precipitation over southern Mexico. A self‐organizing maps (SOM) analysis of sea level pressure, 850‐hPa horizontal winds and 925‐hPa‐specific humidity for the period 1997–2013 was carried out to identify the circulation patterns. This approach allowed the discrimination of wet and dry regimes, with clear and distinct features. Weather patterns exhibiting negative (positive) mean sea level pressure anomalies, southerly (northerly) winds, above‐ (below‐) average low‐level moisture and little (large) influence of the North Atlantic Subtropical High (NASH), resulted in above‐ (below‐) average precipitation over southern Mexico. The intra‐seasonal variability of the WRs and their associated rainfall is well captured by this methodology. In particular, the mid‐summer drought (MSD) during late July and early August, is clearly represented by a group of patterns, which evidence a strong influence of the NASH, strong easterly winds in the Caribbean Basin and reduced low‐level humidity, all factors that combine to induce below‐normal rainfall over southern Mexico. The analysis of the inter‐annual variability of the WRs suggests that year‐to‐year variations in their frequencies can impact summer rainfall in the regions of southern Mexico considered in this study. In particular, the analysis indicates that the dominant WR associated with the MSD exhibits a 3‐ to 4‐year modulation in its frequency of occurrence, which has not been previously reported in the literature. The main MSD pattern is more frequent during dry years and has a significant correlation with the Multivariate El Niño‐Southern Oscillation (ENSO) Index (MEI), indicating that MSD is stronger during ‘El Niño’ years. Also, WRs associated with negative (positive) rainfall anomalies showed positive (negative) correlations with the MEI, suggesting a possible modulation by ENSO phases.

Unique haplotypes of cacao trees as revealed by trnH-psbA chloroplast DNA.

Cacao trees have been cultivated in Mesoamerica for at least 4,000 years. In this study, we analyzed sequence variation in the chloroplast DNA trnH-psbA intergenic spacer from 28 cacao trees from different farms in the Soconusco region in southern Mexico. Genetic relationships were established by two analysis approaches based on geographic origin (five populations) and genetic origin (based on a previous study). We identified six polymorphic sites, including five insertion/deletion (indels) types and one transversion. The overall nucleotide diversity was low for both approaches (geographic = 0.0032 and genetic = 0.0038). Conversely, we obtained moderate to high haplotype diversity (0.66 and 0.80) with 10 and 12 haplotypes, respectively. The common haplotype (H1) for both networks included cacao trees from all geographic locations (geographic approach) and four genetic groups (genetic approach). This common haplotype (ancient) derived a set of intermediate haplotypes and singletons interconnected by one or two mutational steps, which suggested directional selection and event purification from the expansion of narrow populations. Cacao trees from Soconusco region were grouped into one cluster without any evidence of subclustering based on AMOVA (FST = 0) and SAMOVA (FST = 0.04393) results. One population (Mazatán) showed a high haplotype frequency; thus, this population could be considered an important reservoir of genetic material. The indels located in the trnH-psbA intergenic spacer of cacao trees could be useful as markers for the development of DNA barcoding.

Ensoulment, Entrapment, and Political Centralization

In this article, we examine the interplay of religion and politics during the later Formative period of Mesoamerica through a comparison of two regions of southern Mexico: the lower Río Verde Valley and the Valley of Oaxaca. Archaeological evidence shows that these regions had dramatically different later Formative histories. In the lower Verde, we find that religion constrained changes that could have stabilized political centralization. A crucial aspect limiting the creation of multicommunity authority and identities was the physical entrapment of the bones of ancestors, offerings, and divine beings within public buildings in local communities. In contrast, in the Valley of Oaxaca, we find that religion fostered developments that would eventually give rise to a politically centralized polity, with its seat of government at the hilltop city of Monte Albán. Both regions show that religion was not necessarily a unifying factor in social change, as has often been assumed, but instead could be a crucible of tension and conflict through which political innovations were produced. This comparative study leads us to considerations of broader historical factors that contribute to understandings of when religion can be constraining or enabling of political change.

Deregulating the social life of property: neoliberalism and the proliferation of normative dissonances in Mexico

In 1992, a neoliberal land reform was carried out in Mexico. One of its objectives was to open the ejido institution, created in 1915, to the market. Until then, ejido land was forbidden to be sold, rented or leased. Based on ethnographic research in an indigenous region of southern Mexico, the aim of this study is to clarify the normative divergences caused by neoliberal land reforms and to explore the meanings that privatization and commodification of communal ejido lands hold for indigenous peasants alive today. The main argument is that ejido residents have responded to the new Agrarian Law in diverse ways within the same ejido region. Some ejidatarios have conserved the principles of social justice and the moral peasant economy implicit in the abrogated agrarian legislation, and established de facto rights and obligations to govern access to ejidal plots, while other ejidatarios conceive land as an object of economic exchange. Hence, while the goal of Mexico's neoliberal reform was to eliminate divergences between the stipulations of Agrarian Law and people's acts with regard to the land, on the ground, its effects have been to widen conflicts and generate new ones between local customs and social practices.

Seed predation in a human-modified tropical landscape

Abstract:Contemporary defaunation of fragmented forests potentially alters patterns of seed predation and dispersal. Alternatively, the remaining fauna may compensate for missing animals, resulting in equivalent rates of seed dispersal and predation. In the Los Tuxtlas region of southern Mexico, populations of large terrestrial fruit-eating mammals are diminished or absent from many forest remnants. This study reports fruit removal and seed predation patterns of Poulsenia armata (Moraceae), in forest fragments and a continuous forest (LTBS). Contrary to expectation, we found no differences in seed predation (mean ± SD) between LTBS (7.2 ± 1.8 seeds per station) and forest fragments (5.6 ± 1.1). However more fruits were removed in the LTBS (11.4 ± 0.9 fruits per station) than in forest fragments (8.1 ± 0.8). Animal activity, recorded by camera traps, differed between animal guild with fewer seed dispersers in forest fragments (mean = 0.43 ± 0.02 photos wk−1) than in the LTBS (mean = 0.68 ± 0.05). Fruits and seeds attracted many species of mammal (n = 12) in both habitats, indicating substantial redundancy. Remnant forest patches in the Los Tuxtlas landscape retain great ecological value, both as refuges for mammals and habitats for trees, such as P. armata.

Edaphic and seasonal heterogeneity of seed banks in agricultural fields of a tropical dry forest region in southern Mexico

The slash-and-burn agriculture practiced across tropical dry regions results in the elimination of native vegetation. Upon field abandonment, the seed bank becomes a potentially important mechanism of natural regeneration at early successional stages. Soil properties and climate seasonality may affect seed bank characteristics, thus we analyzed the effects of these two factors on seed bank density and composition in agricultural fields of a seasonally dry tropical region of southern Mexico. Soil cores were collected for the rainy and the dry seasons in order to assess changes occurring in the seed bank from the time of harvest to the moment when succession could potentially start (the next rainy season). The 12 studied fields comprised three different soil types recognized by local inhabitants: sandy and stony, silty, and clayey soils, locally known as cascajo, black soil, and red soil, respectively. At each field 20 soil cores (8 cm diameter, 4.5 cm depth) were collected and mixed to form four pooled samples, which were placed in a greenhouse to induce germination. A total of 4,422 seedlings (2,291 seeds m-2) representing 40 species were recorded. The most abundant species were, in decreasing order, Melanthera nivea, Rhynchelytrum repens, Waltheria indica, Amaranthus scariosus, Digitaria bicornis , and Cenchrus pilosus . Herbs were the prevailing growth form (&gt; 80% of total richness). No clear pattern was observed in the seed bank related to soil type; however, seed bank characteristics tended to be associated with the time of use of the agricultural fields, a variable that was not controlled in the study. Seed bank species richness was significantly larger in the dry season, and although seed density showed a similar trend, it was not significant. The studied seed banks contain no elements of the regional primary tropical dry forest, which suggests that seed banks in deforested areas cannot guarantee their maintenance beyond forested areas.

Elevational patterns in the vascular flora of a highly diverse region in southern Mexico

We examined general and family-specific patterns of vascular plant richness along a large elevational gradient (0–3,670 m a.s.l.), assessed the continuity of these patterns and analysed their potential underlying causes in a high diversity region of the Sierra Madre del Sur, Oaxaca, Mexico. We used a vascular plant database constructed previously. The gradient was divided into 18 200-m elevation belts. To examine elevational patterns of richness, we used both observed and estimated (interpolated) species richness, as well as genus and family observed richness, for each belt. A generalised linear model (GLM) was used to assess the effect of altitude on area-corrected species richness (standard area = 100 km2), and a numerical classification of the elevational belts based on species richness was performed. Overall, richness at the three taxonomic levels decreased with elevation, but some individual families departed from this pattern. A sharp drop in species richness was observed at 1,800 m, and the dendrogram separated two elevational floristic groups at this elevation. The GLM revealed a significant negative effect of elevation on species richness. Despite this overall decreasing pattern for vascular plants along this extensive gradient, an examination of some family-specific patterns revealed the existence of other elevation–diversity relationships, indicating taxon-specific responses to elevation. The most noticeable discontinuity in species richness, at ca. 1,800 m, is likely related to a critical temperature isocline.

The effects of human land use on flow regime and water chemistry of headwater streams in the highlands of Chiapas

We studied the effects of land use changes on flow regime and water chemistry of headwater streams in the highlands of Chiapas, a region in southern Mexico that has experienced high rates of deforestation in the last decades. Samples for water chemistry were collected and discharge was measured between September 2007 and August 2008 at eight streams that differed in the land uses of their riparian and catchment areas, including streams draining protected forested areas. Streams with high forest cover (>70%) in their catchments maintained flow through the year. Streams draining more disturbed catchments exhibited reduced or no flow for 4 − 6 months during the dry season. Nitrate concentrations were lower at streams draining forested catchments while highest concentrations were measured where conventional agriculture covered a high proportion of the catchment and riparian zone. Highest phosphorus concentrations occurred at the catchment where poultry manure was applied as fertilizer. Differences between forest streams and those draining disturbed areas were correlated with the proportion of forest and agriculture in the riparian zone. Variation in stream variables among sampling dates was lower at the forest sites than at the more disturbed study streams. Conversion of forest into agriculture and urban areas is affecting flow regime and increasing nutrient concentrations, although the magnitude of the impacts are influenced by the type of agricultural practices and the alteration of the riparian zone.