Héctor Godínez-Alvarez

THE ROLE OF SEED DISPERSERS IN THE POPULATION DYNAMICS OF THE COLUMNAR CACTUS NEOBUXBAUMIA TETETZO

We examined the effect of frugivorous bats and birds on the population dynamics of the giant columnar cactus Neobuxbaumia tetetzo in the Tehuacan Valley, Mexico. Because successful seedling establishment occurs only beneath the canopies of shrubs and trees, we hypothesized that seed dispersal is a key process in the maintenance of its populations. We determined the dispersal effectiveness of different frugivores, considering the quantity and quality components of seed dispersal. We also evaluated the potential effects of each frugivore species on the finite rate of increase of N. tetetzo populations by modifying the fecundity values of a Lefkovitch matrix model. The bat Leptonycteris curasoae had the highest effectiveness whereas the bird Carpodacus mexicanus had the lowest. The estimated finite rates of increase calculated to evaluate the effects of frugivores on the population dynamics of the cactus differ marginally from unity, except when the effect of the bat L. curasoae was analyzed. Our results sug...

Global diversity and conservation priorities in the Cactaceae

The purpose of this paper was to analyze the diversity patterns of Cactaceae at a global scale, to identify those countries where conservation actions should be performed. In order to do this, the species richness and the number of endemic species for 34 American countries were determined. With these data, the relationship between the total number of species or the number of endemic species and the area of the countries were analyzed. In addition, a complementarity analysis was conducted to determine the most important countries for cactus conservation. Results showed that Mexico had the highest number of total and endemic species followed by Argentina, Bolivia, Brazil, and Peru, among others. There was a significant positive relationship between both, the total and endemic species, and the area of the countries. Despite this fact, the cactus diversity in Mexico, Argentina, Peru, Bolivia, Chile, and Costa Rica was higher than expected according to their area. Further, these countries also presented the highest proportions of endemic species. The complementarity analysis indicated that 24 countries are necessary to preserve all cactus species. However, 94% of all species could be preserved with only 10 countries. Considering the diversity patterns and the complementarity analysis, three important groups for cactus conservation were identified: (1) Mexico, Argentina, Peru, Bolivia, Chile, and Costa Rica, (2) Paraguay and Cuba, and (3) Brazil and USA. Conservation efforts should be focused on these countries in order to preserve cactus diversity.

Diversity and Conservation in the Cactus Family

Cacti are conspicuous elements of the Western hemisphere. They have long attracted attention due to their peculiar biology, and have maintained close relationships with local settlers. Cacti are used as food, fodder, medicinal and ornamental plants, and as a source of wood. The greatest diversity of this family is recorded in Mexico, with 586 species, followed by Brazil, Argentina, Bolivia and Peru. These countries are also the richest in endemic species. The underlying factors affecting diversity patterns are varied and include temperature, rainfall and geological history. Cacti are threatened by loss and degradation of habitat, and illegal collection. Though it is commonly stated that many species are endangered, most have not yet been evaluated. Due to the growing impact of human activities on biodiversity it is necessary to increase conservation actions to preserve this interesting group of plants, especially in South American countries where knowledge and conservation of cacti are still incipient.

Germination, survival and growth of three vascular plants on biological soil crusts from a Mexican tropical desert.

Information about the effects of biological soil crusts (BSC) on germination, seedling survival and growth of vascular plants is controversial because they can have positive, neutral or negative effects. This controversy may be because most studies conducted until now have just analysed one or two recruitment stages independently. To understand the BSC effects on vascular plants, it is necessary to consider each stage of the recruitment process and synthesise all this information. The goal of this study was twofold. First, we analyse germination, seedling survival and growth of three vascular plants (Agave marmorata, Prosopis laevigata and Neobuxbaumia tetetzo) on BSC (cyanobacteria and mixed crust) from a tropical desert region of south-central México. Second, we synthesise the information to determine the total effect of BSC on plant species performance. We conducted experiments under controlled conditions to evaluate the proportion of germinated seeds, proportion of surviving seedlings and seedling dry weight in BSC and bare soil. Results showed that BSC have different effects on germination, seedling survival and growth of plant species. Plant species performance was qualitatively higher on BSC than bare soil. The highest performance of A. marmorata and P. laevigata was observed on cyanobacteria and mixed crusts, respectively. The highest performance of N. tetetzo was on both crust types.

Demography of the columnar cactus Neobuxbaumia macrocephala :a comparative approach using population projection matrices

Neobuxbaumia macrocephala is a long-lived columnar cactus endemic to the Tehuacán-Cuicatlán Valley in south-central Mexico. This plant has a very restricted distribution and few recruitment events have been detected in its populations. In this study, we analyze the N. macrocephala demographic pattern using a projection matrix in order to determine the main limiting factors of this species. To accomplish this goal, we compare our results with those obtained for another species of the same genus, N. tetetzo. Considering that both species inhabit the same valley, we believe that this comparative study will offer insights into the main demographic limitations of N. macrocephala. Results showed that these species of columnar cacti have similar demographic patterns in which survival is the process with the highest relative contribution to λ, followed by growth and reproduction. Of all the life cycle stages, seeds and seedlings have the lowest survival probabilities due to a high mortality caused by seed predation and effects of direct solar radiation on germinated seeds. The estimated growth rates indicate that populations of these species of Neobuxbaumia are in a numerical equilibrium. With respect to reproduction, N. macrocephala produce a lower number of seeds per plant than N. tetetzo. This low level of sexual reproduction may decrease the probability of establishment of new individuals in N. macrocephala populations. It is suggested that pollen limitation and pre-dispersal seed predation could be some factors that limit the distribution and abundance of this columnar cactus.

Population Dynamics of Agave marmorata Roezl. under Two Contrasting Management Systems in Central Mexico

Population Dynamics ofAgave marmorataRoezl. under Two Contrasting Management Systems in Central Mexico. This paper evaluates the impacts of traditional management on the population dynamics of Agave marmorata, a multipurpose, useful species that is dominant in the Zapotitlán Salinas Valley, Puebla, Mexico. During 2002–2003 and 2003–2004, we constructed matrix models for two populations—one unmanaged, the other subject to plant extraction and cutting of flowering stalks. We also conducted prospective (elasticity) and retrospective (life table response experiments) analyses. Overall, the unmanaged population had higher finite rates of increase (λ) than the managed one. This variation in λ was the result of a decrease in the individual growth and fecundity in the managed population. Survival and growth were the demographic processes with the highest contribution to λ in the unmanaged population, while survival was the most important in the managed one. Our results suggest that management and plant extraction practices could be having negative effects on the population dynamics of this plant species. Recommendations are provided in order to promote positive effects on its vital rates and regeneration capacity.ResumoDinámica de poblaciones deAgave marmorataRoezl. en dos sistemas contrastantes de manejo en el Centro de México. Este trabajo tuvo como objetivo evaluar el impacto del manejo tradicional en la dinámica de poblaciones de Agave marmorata, una especie útil multipropósito dominante en el Valle de Zapotitlán Salinas, Puebla México. Durante 2002–2003 y 2003–2004, se construyó un modelo matricial para dos poblaciones, una no manejada y otra sujeta a extracción y corte de inflorescencias. También se llevaron a cabo análisis prospectivos (elasticidad) y retrospectivos (experimentos de respuesta de tablas de vida). En términos generales, la población no manejada tuvo tasas finitas de crecimiento (λ) más altas que la manejada. Esta variación de λ fue resultado de un decremento in el crecimiento y fecundidad individual en la población manejada. La sobrevivencia y el crecimiento fueron los procesos demográficos con la más alta contribución al valor de λ en la población no manejada, mientras que la sobrevivencia fue el más importante en la población manejada. Nuestros resultados sugieren que las prácticas de manejo y extracción pudieran tener efectos negativos en la dinámica poblacional de esta especie vegetal. Se proponen algunas recomendaciones con el objeto de promover efectos positivos en sus tasas vitales y capacidad de regeneración.

Seed banking in the columnar cactus Stenocereus stellatus : distribution, density and longevity of seeds

The soil seed bank is the reserve of viable seeds found in the soil. This reserve contributes to plant population persistence in unpredictable environments; thus, determining its presence is basic to understanding recruitment patterns and population dynamics. Studies of soil seed banks in the Cactaceae are scarce, although these plants are ecologically dominant in American arid and semi-arid environments. Most studies have inferred the presence of seed banks by analysing morphological seed traits or germination of seeds stored in the laboratory for different periods of time. Few studies have determined their presence through evaluation of distribution, density and longevity of seeds in the field. To fill this information gap, we determined the existence of, and studied, the soil seed bank of Stenocereus stellatus , a columnar cactus endemic to central Mexico. This study reports the evaluation of these characteristics in the field and discusses whether this species forms a soil seed bank. We found a higher number of seeds under shrubs than in areas lacking vegetation. Recently dispersed seeds did not germinate because they have primary dormancy. This dormancy was broken after 6 months of burial in the soil. Seeds buried for 10 months entered secondary dormancy and they were not viable at 24 months, probably because of pathogen attack. Considering dormancy and seed longevity, we suggest that S. stellatus has the potential to form a short-term persistent seed bank. However, this should be confirmed by conducting studies on other S. stellatus populations throughout their geographical distribution.

Are body size and volatile blends honest signals in orchid bees

Abstract Secondary sexual traits may convey reliable information about males’ ability to resist pathogens and that females may prefer those traits because their genes for resistance would be passed on to their offspring. In many insect species, large males have high mating success and can canalize more resources to the immune function than smaller males. In other species, males use pheromones to identify and attract conspecific mates, and thus, they might function as an honest indicator of a males condition. The males of orchid bees do not produce pheromones. They collect and store flower volatiles, which are mixed with the volatile blends from other sources, like fungi, sap and resins. These blends are displayed as perfumes during the courtship. In this study, we explored the relationship between inter‐individual variation in body size and blend composition with the males’ phenoloxidase (PO) content in Euglossa imperialis. PO content is a common measure of insect immune response because melanine, its derived molecule, encapsulates parasites and pathogens. Body size and blend composition were related to bees’ phenolic PO content. The inter‐individual variation in body size and tibial contents could indicate differences among males in their skills to gain access to some compounds. The females may evaluate their potential mates through these compounds because some of them are reliable indicators of the males’ capacity to resist infections and parasites.

Diversity of Ants Inhabiting a Mosaic of Environmental Conditions in a Semi-Desert of Central Mexico

Abstract. Biotic and abiotic factors affect the species richness and abundance of ants inhabiting arid and semiarid zones. We evaluated how soil surface temperature, texture, percentage of surface coarse fragments, and vegetation affected the composition, diversity, and distribution of ants inhabiting a mosaic of environmental conditions in a semiarid zone in central Mexico. We captured 22 ant species at five sites studied, with the top of Cutac hill (1,750 m altitude) and Terrace 2 (1,376 m altitude) the richest sites with 10 species each. Based on redundancy analyses, the percentage of clay followed by the percentage of coarse fragments at the surface of the soil were the variables most related to the composition of ants in these communities. Percentage of clay ranged from 16 to 26 and was positively related to abundance of Pheidole sp. 1 and negatively related to abundance of Dorymyrmex bureni Trager (R2 = 0.76, P = 0.04; R2 = 0.86, P = 0.02, respectively), whereas percentage of soil surface coarse fragments ranged from 0.8 to 59 and were negatively correlated to Pheidole sp. 1 (R2 = 0.76, P = 0.05), but positively correlated to abundance of Brachymyrmex sp. (R2 = 0.86, P = 0.06). When ants were analyzed by functional group, vegetative structure (ranging from 109 to 246 hits with vegetation) and percentage of coarse fragments in the soil were most correlated with their distribution. Although results of the redundancy analyses were not statistically significant, preliminary analysis of variables might explain the composition and distribution of an ant community. We suggested that soil characteristics might be important for explaining ant distribution and community composition.

Patterns of Arthropod Diversity in Contrasting Habitats of El Pedregal de San Angel, a Preserve in Mexico City

Abstract. We studied the community of arthropods inhabiting the ecological preserve El Pedregal de San Angel, in Mexico City. We estimated morphospecies richness, abundance, and diversity patterns in two contrasting habitats, open and closed, during four seasons of the year. Because closed-habitat sites have greater net primary productivity than open habitats, we predicted arthropod diversity would be greater in closed than in open habitats. Morphospecies richness of Acari and Arachnida was similar in both types of habitats throughout the year, whereas richness of some orders of Insecta such as Coleoptera, Diptera, Hemiptera, and Hymenoptera was greatest in open habitats. Mites were the most abundant arthropods and were always more abundant in open than closed habitats. Diversity of arthropods was greater in closed habitats during fall and spring, suggesting that habitat characteristics related to primary productivity might affect this community. We suggest that studies of patterns of arthropod diversity and factors associated with the maintenance of such diversity should be considered in the preservation of these organisms and of their natural habitats.