Resumen. El índice de entropía de Shannon y otras medidas de complejidad se utilizan frecuentemente para evaluar la diversidad de especies en comunidades ecológicas, aun cuando su comprensión es difícil y sus valores no son comparables. En este trabajo se muestra que los números efectivos de especies (medidas de diversidad verdadera) permiten obtener una interpretación intuitiva y fácilmente 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 México, con distinta resolución taxonómica y enfocados en distintos grupos biológicos). Se utilizan modelos de estimación en los que se considera que las muestras son representaciones incompletas de las comunidades. Se explica también la manera en que las medidas de diversidad de distinto orden incorporan a las especies según su abundancia en la comunidad. Los resultados obtenidos pueden resultar de especial interés cuando los valores de diversidad se utilizan para proponer medidas para el manejo de recursos y la conservación biológica. Palabras clave: diversidad verdadera, especies equivalentes, especies igualmente comunes, índice de Shannon, riqueza, equidad, dominancia, abundancia.

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Abstract. Plant life‐form abundance along a 600 m altitudinal gradient (1600–2200 m a.s.l.) in the semi‐arid valley of Zapotitlán, México was correlated with soil characteristics and climatic variables. One mixed soil sample was taken and analysed for each of six elevations, temperature was estimated using a terrestrial thermal gradient, and precipitation using a linear regression between total annual precipitation and the elevation of the weather stations in the valley. Rosettes, microphanerophytes, therophytes and nanophanerophytes were well represented throughout the gradient. Columnar cacti were restricted to the 1600–1800 m range, and geophytes to the 1700–1800 m range. In general, abundance of life forms was inversely associated with altitude. Multiple regression analysis did not show parameters to significantly explain the abundance of rosettes, nanophanerophytes, epiphytes, geophytes and hemiparasites; altitude and nitrogen proved significant for columnar cacti, succulents and chamaephytes; altitude, pH, electrical conductivity and nitrogen were significant for globose cacti; pH was significant for therophytes; and altitude was significant for microphanerophytes.

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It is well known that geographic variation occurs in life history characteristics among populations of lizards. Variations in life histories found in some studies are possibly adaptations to environmental pressures. In southwestern México, the reproductive characteristics of the endemic species Sceloporus formosus have been poorly studied throughout its geographic range. We used specimens from scientific collections to describe male and female reproductive cycles and sexual dimorphism of S. formosus. Other viviparous Sceloporus from high elevations show sexual dimorphism, with males being larger in many morphological characteristics than females. We therefore expected that S. formosus would show sexual dimorphism. However, we found no sexual dimorphism. Males reached sexual maturity at a smaller (47 mm) snout-vent length (SVL) than females (50 mm). There were no significant differences between sexes and months in liver mass. However, fat body mass in females was greater than in males. For males, there was significant relationship between log 10testis volume and log 10-SVL. Testis volume was positively correlated with temperature and was smallest during October, November, and December. For females, there was no significant effect of month on gonad volume. Vitellogenesis occurred from April to November. Vitellogenic follicles and embryonic development were significantly correlated with precipitation

Background: Body 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. Results: Results 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. Conclusions: This 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.

The contribution of cacti and shrubs to root biomass and fine-root production was described in a semiarid Mexican scrub. Both life forms were evaluated for fine-root production variation in relation to changes of nitrogen in the soil, with a fertilization experiment. Cacti represented 78% of the total mean root biomass (660 ± 70 g·m–2 (mean ± SE)) in the complete soil profile (50 cm in depth). In both life forms, root biomass was higher near the surface of the soil. Roots <3 mm in diameter represented 92.5% for cactus root biomass and 69.4% for shrubs. Monthly root biomass varied significantly between months, and significant differences were obtained between plant life forms. Fine and very fine root production was estimated as 3.76 Mg·ha–1·year–1, and cactus contribution to total root production was 81.2%. Significant differences were obtained between life forms. It was clear that a low concentration in the soil nitrogen diminishes fine-root production, supporting the hypothesis that in arid ecosystems nitrogen is a limiting factor for primary production.