Numa P. Pavón

Reanálisis de la diversidad alfa: alternativas para interpretar y comparar información sobre comunidades ecológicas.

El indice de entropia de Shannon y otras medidas de complejidad se utilizan frecuentemente para evaluar la diversidad de especies en comunidades ecologicas, aun cuando su comprension es dificil y sus valores no son comparables. En este trabajo se muestra que los numeros efectivos de especies (medidas de diversidad verdadera) permiten obtener una interpretacion intuitiva y facilmente comparable de la diversidad de especies. Se ejemplifica su uso reanalizando los datos de 4 trabajos publicados en la Revista Mexicana de Biodiversidad (realizados en distintos ecosistemas y regiones de Mexico, con distinta resolucion taxonomica y enfocados en distintos grupos biologicos). Se utilizan modelos de estimacion en los que se considera que las muestras son representaciones incompletas de las comunidades. Se explica tambien la manera en que las medidas de diversidad de distinto orden incorporan a las especies segun su abundancia en la comunidad. Los resultados obtenidos pueden resultar de especial interes cuando los valores de diversidad se utilizan para proponer medidas para el manejo de recursos y la conservacion biologica.

Sexual dimorphism and reproductive cycle in the arboreal spiny lizard Sceloporus formosus Wiegmann (Squamata: Phrynosomatidae) from central Oaxaca, México

Es bien conocido que la variacion geografica ocurre en las caracteristicas de historias de vida entre poblaciones de especies de lagartijas. Las variaciones en las caracteristicas de historias de vida encontradas en algunos estudios son posiblemente adaptaciones a las presiones del ambiente. En el suroeste de Mexico, habita la especie endemica, Sceloporus formosus la cual ha sido poco estudiada en sus caracteristicas reproductivas a traves de su intervalo de distribucion geografica. En este estudio se usaron especimenes provenientes de colecciones cientificas para describir el ciclo reproductivo de machos y hembras y dimorfismo sexual de S. formosus. En otras especies viviparas de altitudes elevadas del genero Sceloporus muestran dimorfismo, donde los machos son mas grandes que las hembras en muchas caracteristicas morfologicas; debido a esto, esperabamos que S. formosus mostrara dimorfismo sexual. Sin embargo, no encontramos dimorfismo sexual. Los machos alcanzaron la madurez sexual a menor talla (47 mm) en longitud hocico-cloaca (LHC) que las hembras (50 mm). En cuanto a la masa del higado no hubo diferencias significativas entre sexos y meses. Sin embargo, los cuerpos grasos en las hembras fueron mayores que en machos. En los machos, no hubo relaciones significativas entre log10-volumen testicular y el log10-LHC. El volumen testicular se correlaciono positivamente con la temperatura y este fue mas pequeno durante octubre, noviembre y diciembre. Para las hembras, no hubo efectos significativos del mes sobre el volumen de la gonada. La vitelogenesis ocurrio de abril a noviembre. Los foliculos vitelogenicos y el desarrollo embrionario se correlacionaron significativamente con la precipitacion (Correlacion de Pearson, r = 0.80,n=10,P = 0.0081) y el fotoperiodo (Correlacion de Pearson, r = 0.72, n = 10, P < 0.0001). El tamano de la carnada basado con el numero de embriones, se correlaciono con la LHC (Correlacion de Pearson, r = 0.82, n = 10, P = 0.0034). La masa relativa de la carnada no se correlaciono con la LHC de las hembras (Correlacion de Pearson, r = 0.43, n = 10, P = 0.2110). Este patron reproductivo otonal encontrado es tipico de especies viviparas que habitan altitudes elevadas, donde la temperatura es baja, la precipitacion y la humedad son altas. Lo que sugiere que la convergencia en los ciclos reproductivos puede estar asociada con un cambio a las condiciones ambientales imperantes en altitudes elevadas.

Morphometric variation in island and mainland populations of two lizard species from the Pacific Coast of Mexico

BackgroundBody size exerts a strong influence on the physiology, morphology, ecology, and evolution of other life history traits in vertebrates. We compared the morphometry and allometry of two lizard species (Anolis nebulosus and Aspidoscelis lineattissima) occurring on mainland and island populations on the Pacific Coast of Mexico in order to understand the effect of an insular environment on body size and other morphometric structures.ResultsResults showed that both males and females of A. nebulosus from San Pancho Island were larger in body size than those from the mainland. Moreover, males of A. lineattissima from Cocinas Island exhibited larger forms of most measured morphometric traits than those from the mainland, whereas females from both island and mainland populations did not differ in body size or in other morphometric traits analyzed. Multivariate allometric coefficients of males and females of A. nebulosus from island and mainland populations showed a lower percentage of positive allometries than in A. lineattissima, probably because the former species is highly sedentary. Island populations of both species exhibit male-biased sexual dimorphisms in body size and size-adjusted morphometric traits. In contrast to the mainland population, morphometric comparisons of body size-adjusted traits showed that male A. lineattissima were larger than females only in head length, head width, forearm length, and tibia length, whereas in A. nebulosus, sexual dimorphism was observed just in HL.ConclusionsThis study supports the hypothesis (island rule) that vertebrates on islands are larger than those of conspecifics on the mainland. In addition, sexual dimorphism observed between males and females of both species and populations could be associated with allometric growth (positive or negative) from some morphometric structures, as well as differences in the growth rates of these organisms.

Sex ratio, size distribution and nitrogen resorption in the dioecious tree species Bursera morelensis (Burseraceae)

Dioecious plant populations have generally been considered to maintain sex ratios of 1:1 (Allen & Antos 1993, Carroll & Mulcahy 1993, Charnov 1982). The sex ratio does, however, change if the plants are growing in a stressful environment, often being male-biased under these conditions (Ortiz et al. 1998, 2002). This pattern has been explained by differences in how the sexes resolve their respective energy requirements (Obeso et al. 1998). Females generally assign more resources to reproduction in comparison to growth and maintenance than do males, and they are smaller and longer-lived (Allen & Antos 1993, Nicotra 1999, Obeso et al. 1998, Willson 1983). Thus their higher reproductive costs take a toll on females, modifying the population sex ratio in favour of males (Allen & Antos 1993, Lovett-Doust & Lovett-Doust 1988). However, in order to determine the reproductive cost it is necessary to consider the compensatory mechanisms (Obeso 2002). These reduce the reproductive cost and include plastic response in terms of plant architecture and plant physiology. For example, nutrient resorption is a process that may help decrease dependence on nutrient intake, thereby constituting an important nutrient economy mechanism that can also reduce the reproductive cost if a significant proportion of the nutrients had been reassigned to reproduction (Killingbeck 1986, Obeso 2002). For the above-mentioned, we postulate the following hypothesis: dioecious plant populations in a stressful environment maintain their sex ratio 1:1, provided that the compensatory mechanisms are significantly more efficient in the females than males. We test this prediction in Bursera morelensis (Burseraceae), a dioecious deciduous trees which dominates a tropical dry forest in a

Effects of seed ingestion by the lesser long-nosed bat Leptonycteris yerbabuenae on the germination of the giant cactus Isolatocereus dumortieri

Abstract Bats have been considered efficient seed dispersers of columnar cactus (Isolatocereus dumortieri). However, to date, very little is known about the effect of seed ingestion by bats on seed germination. We carried out some germination assays for seeds of the cactus I. dumortieri obtained directly from fruits and fecal clumps of the endangered lesser long-nosed bat (Leptonycteris yerbabuenae) in a semiarid zone, within the Biosphere Reserve Barranca of Metztitlán in Central México. Additionally, we evaluated seed deposition patterns by bats. We found that 61.7% of the removed seeds were dispersed in fecal clumps, whereas the remaining seeds were spitted as single seeds. We also found that ingestion diminished seed germination by 65%. Under these conditions, seeds deposited individually around the mother plant may be of importance for local restoration and have the function of reforesting the locality, whereas ingested seeds can reach long distances and colonize new areas.

FINE ROOT BIOMASS AND PRODUCTION IN A SEMIARID MEXICAN SHRUBLAND

Abstract I describe the vertical distribution of root biomass and root productivity in a xeric shrubland of central Mexico. The effect of the soil nitrogen on root production was evaluated with a fertilization experiment. The highest percentage of roots was found at a depth of 40 to 60 cm. Monthly variation in root biomass for diameter class was significant. Fine and very fine roots were the most important fraction in terms of biomass and production. The highest production of fine root occurred in native soil versus the other treatments.

Litter fauna communities and litter decomposition in a selectively logged and an unmanaged pine-oak forest in Mexico

Se analizo el efecto del manejo silvicola en la abundancia, riqueza y diversidad de la fauna de invertebrados, asi como en la tasa de descomposicion de la hojarasca en un bosque templado de pino (Pinus spp.) y encino (Quercus spp.) del centro de Mexico. Se comparo un bosque manejado (tala selectiva) y otro sin manejo, empleando un experimento de bolsas de malla plastica (litterbags). El bosque sin manejo tuvo mayor abundancia, riqueza y diversidad de morfoespecies de invertebrados que el bosque donde se aplico tala selectiva. Se encontraron diferencias significativas en la hojarasca remanente y en la tasa de descomposicion entre bosques: la hojarasca en el bosque sin manejo se descompuso mas rapidamente que en el que se aplica tala selectiva. La hojarasca remanente correlaciono de forma negativa con la riqueza de invertebrados en el bosque manejado y con la riqueza y diversidad en el bosque sin manejo. Estos resultados indican que a mayor diversidad de fauna, mas hojarasca puede descomponerse. El estudio muestra que las practicas silviculturales pueden afectar los patrones de diversidad de invertebrados del suelo y los procesos de descomposicion de los que forman parte. Se propone como medida de conservacion dejar fragmentos de bosque sin manejo en localidades donde se aplica tala selectiva para mantener la diversidad y los procesos ecosistemicos en estos bosques templados

DIVERSITY OF ARTHROPODS PREYED UPON BY THE CARNIVOROUS PLANT PINGUICULA MORANENSIS (LENTIBULARIACEAE) IN A TEMPERATE FOREST OF CENTRAL MEXICO.

Abstract Diversity of arthropods preyed upon by the carnivorous plant Pinguicula moranensis was analyzed during 2006 within a pine-oak (Pinus-Quercus) forest in the Sierra de Pachuca, Hidalgo, Mexico. A total of 570 individuals of nine orders was collected. Hemiptera was recorded as prey of this plant for the first time. The most prevalent order was Diptera, with 53.6% of total prey; within this order, Sciaridae accounted for 64.7% of prey. The month with the highest values for alpha diversity was July (H  =  1.32). Greatest turnover of orders was June–November and that of families of Diptera was July–August. Total beta diversity was greater in families of Diptera, with a value of 1.29, while that of orders of arthropods was 0.53.

Water and carbon storage capacity in Isolatocereus dumortieri (Cactaceae) in an intertropical semiarid zone in Mexico

In the biosphere reserve Barranca de Metztitlan in Mexico, there is an extensive area with a semiarid scrub. The dominant species is the cactus Isolatocereus dumortieri. This is a key species in the ecosystem, because many species of birds, bats and insects are feeding from its nectar, pollen and fruits during the dry season thanks to their capacity to store water and carbon. However, there is no information about their potential to store water and carbon. The purpose of this study was to estimate the ability of I. dumortieri to store water and carbon. Water content per plant was estimated as 537.64 ± 71.59 L during the dry season and 692.24 ± 92.18 L during the wet season. In the same way the carbon stored per cactus was 16.75 kg ± 7.07 corresponding to 1.25 kg C m2. The results shows the importance of I. dumortieri in maintaining the ecosystem services of the scrub vegetation.