Dioon edule Lindl. (Zamiaceae) is a cycad endemic to Mexico, that occurs as one species D. edule and the geographical variety D. edule var. angustifolium (Miq.) Miq. Dioon edule has a north to south distribution in eastern Mexico. In this study, we analysed 14 allozymic loci in eight populations of D. edule from its total distribution range by sampling all known populations. Patterns of diversity and genetic variability, within and among populations, were obtained. The mean number of alleles per locus ( A ) was 1.44 and the percentage of polymorphic loci was relatively high ( P  = 54.78). The mean observed ( H O ) and expected heterozygosity ( H E ) were 0.27 and 0.24, respectively. F ‐statistics revealed an excess of heterozygous genotypes, locally and globally ( F  = −0.17 and f  = −0.27, respectively). The genetic variation explained by differences among populations was only 7.5%. We also detected a negative relationship between genetic diversity and latitude. On average, the gene flow between population pairs was relatively high ( Nm  = 2.98); furthermore, gene flow between population pairs was significantly correlated with geographical distances ( r  = −0.38, P  = 0.025). Therefore, patterns of genetic diversity in D. edule appear to be associated with the post‐Pleistocene spread of the species, from its southerly (origin) to its northerly range (derived populations, including its central distribution). The biogeographical and evolutionary aspects of the results of this study are discussed. We recognize Dioon angustifolium Miq. for the northernmost disjunct populations. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 80 , 457–467

The results indicated that the populations evaluated have high genetic variability, compared with other endemic and geographically narrowly distributed plant species, in areas with high levels of environmental heterogeneity (e.g. tropical forests). The conservation implications of the results are discussed, and in this regard it is proposed that A. aromatica should be considered as an indicator species with economic potential. It is suggested that sustainable management practices should be implemented and that the areas where the species is distributed should be declared a natural reserve to ensure the species conservation.

The demographic dynamics of three populations of Dioon edule Lindl. (Zamiaceae) were studied in a fragmented landscape using projection matrix modelling. Compared with other plant species, D. edule behaves like a tree life-form species. Density and spatial distribution patterns varied among populations according to models for animal-dispersed tree species. In all scenarios, l was most sensitive to changes in abundance of adult plants. The elasticity reproductive component (F) for the three populations was zero and stasis values (L) were higher, this being a function of the permanence of non-reproductive individuals. It was detected that disturbance influences the population dynamics of D. edule as a function of adult plant persistence. This observation suggests that the conservation of adult plants is critical for D. edule and perhaps for all cycads species. Adult plant decapitation should be halted at the 'Monte Oscuro' population, subjected to sustainable management since 1990, if higher seed production is needed in rural nurseries.

The cycad Ceratozamia mirandae is endemic to Chiapas, Mexico. Demographic studies were made in two of its populations in the Sepultura Biosphere Reserve under different conservation conditions; in the nucleus zone ''Tres Picos'' (conserved) and buffer zone ''La Sombra'' (disturbed and under management). Spatial distribution of C. mirandae was aggregated, showed a clumped local distribution on shallow soils on steep slopes and male and female cones appear to be synchronous in both populations. The population structure was of type I (Bongers) for both sites. Individuals between the sites showed differences in growth pattern. The oldest plants (80-90 cm tall) were estimated to be about 490 years at ''La Sombra''. The finite growth rate (k) in the buffer zone population showed a tendency for decrease whilst in the nucleus zone this estimate remained stable. The highest elasticity values lied in the transition of the first three classes of the ''La Sombra'' population, in ''Tres Picos'' this corresponded to adult plants between 20 and 30 cm tall. Given the above, it is proposed that in the nucleus zone, reproductive adults should be of highest conservation priority, whereas in the buffer zone seedling reintroduction should be carried out regularly until the population increases. We recommend an IUCN Red List category of Vulnerable (VU C, 2a), largely due to difficult-to-control destructive annual forest fires that occur in this Reserve.

Dioon caputoi is a long-lived cycad known from only four populations that range in size from 50 to 120, mostly adult individuals. Dioon caputoi has the most narrow geographical range of all Dioon spp. (less than 10 km), existing completely within the boundaries of the Tehuacán-Cuicatlán Biosphere Reserve, Mexico. Negative inbreeding values were found in all four populations (FIT = -0.242) and within subpopulations (FIS = -0.379). Only c. 10% of the total genetic variation was partitioned among populations (FST = 0.099). We also found that most mean values of genetic variation (A = 1.91 ± 0.12; P = 78.9 ± 10.2; HE = 0.35 ± 0.01) are within the range reported for other Dioon species with larger populations and with wider geographical ranges. These results support recent findings that rare plant species maintain high levels of genetic diversity. The heterozygote excess found at all loci is discussed in detail from a neutral evolutionary perspective, leaving arguments as working hypotheses for further research.