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The role of three thorny legume species as nurse plants and competitive relationships with columnar cacti were evaluated in a semiarid enclave of the Venezuelan Andes. Abundance and size of three columnar cacti species (Stenocereus griseus, Cereus repandus and Pilosocereus tillianus) under isolated shrubs of three thorny legumes species (Prosopis juliflora, Acacia farnesiana and A. macracantha) were recorded and compared with open areas. Using size -distance data we inferred the intensity of intra-and interspecific competition between both life forms in a ''cardonal'' (xeric zone) and ''espinar'' (mesic zone) of the enclave. Sixty-one columnar cacti were found beneath the three thorny legume species (92%), while only 5 cactus individuals were found in open areas (8%). Comparison of observed and expected number of cactus individuals shows that S. griseus and C. repandus are significantly associated with isolated shrubs of A. farnesiana and A. macracantha. Although P. juliflora contributed more than 40% of the total legume plant cover, the number of columnar cacti under its isolated shrubs was significantly lower than would be expected by chance. In all, 19 of 21 possible plant -plant combinations between and within columnar cacti and legume shrubs were recorded (espinar: 18, cardonal: 13, common combinations: 12). In both zones, intra-and interspecific combinations among columnar cacti species were relatively high (positive correlation between the sum of neighbour plant sizes and the distances separating them). Our results strongly suggest, at least for case of S. griseus and C. repandus, that these columnar cacti species require nurse plants for their establishment. The results on interference need still support from further research. We discuss the effects of plant -plant positive interactions on natural regeneration of these columnar cacti.

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Distribution patterns of palms, particularly species richness along latitudinal and altitudinal gradients in Bolivia, are still unknown. A species distribution model was used to identify areas with potentially high palm species richness and endemism. Two models were used: the environmental niche model and a deductive model. A map of potential palm species richness in Bolivia was prepared using 1 km 2 pixels. Two areas are predicted to concentrate most of the potential palm species richness of Bolivia: the southwestern Amazonian moist forest, and the Yungas Andean forest of Bolivia. The potential distribution of Bolivia's endemic palm species appeared to be associated to the eastern side of the Andes. Most palms have Amazonian phytogeographic influences, followed by Andean. The areas identified with greatest potential species diversity coincided with observed patterns at a continental level. The greatest palm richness of northern and northwestern Bolivia may be related to the climatic conditions of these zones. Bolivia represents the southern and western distribution limit of many palm species in terms of continental phytogeographic patterns.Keywords: Arecaceae; Bolivia; phytogeography; modelling distribution. ResumenAún se desconocen los patrones de distribución en un contexto geográfico de las palmeras nativas de Bolivia que incluyan el análisis del decremento de la riqueza de especies respecto a la latitud y altitud. Se aplicó el enfoque de modelaje de distribución de especies para identificar áreas con riqueza potencial de palmeras y concentración de las especies endémicas. Se aplicaron dos métodos de modelaje para la distribución de palmeras: el modelo de nicho ambiental y el deductivo. El mapa de riqueza potencial de palmeras bolivianas fue preparado en base al número de especies en cada unidad de muestreo (un pixel de 1 km 2 ). Se predijeron dos ecorregiones que concentrarían la mayor riqueza de palmeras de Bolivia: el sudoeste del bosque húmedo amazónico y los Yungas, en las laderas orientales de los Andes. La distribución potencial de las palmeras endémicas podría estar asociada a la zona andina oriental del país. La mayor parte de las palmeras estaría influenciada por afinidades amazónicas, seguida por las andinas. Estas áreas con la mayor riqueza de especies de palmeras coinciden con patrones continentales que caracterizan a este grupo de plantas. La mayor riqueza de palmeras al N y NO de Bolivia podría relacionarse a las condiciones climáticas. En Bolivia se representa la distribución más sureña y occidental de muchas especies de palmeras en términos de patrones fitogeográficos continentales.Palabras clave: Arecaceae; Bolivia; fitogeografía; modelos de distribución.

The fast expansion of human population around La Paz, Bolivia (3,200–4,100 m.a.s.l.) triggered new suburban settlements in nearby areas in valleys and mountain feet. The white mesquite, Prosopis alba Griseb. (Leguminosae), is a resource (originally used by native communities) that is strongly affected by changes in land use. A gradient in the level of disturbance is found moving away from the La Paz city toward less altitude areas. The main objective of this study was to characterize genetically three P. alba populations with different levels of human disturbance located at different altitudes in Bolivia, in order to provide some guidelines for management and conservation of these species. Based on 10 SSR loci, the populations showed high level of genetic diversity in comparison with other forest species. The population less disturbed and situated at the lowest altitude was the most variable (H e  = 0.51–0.42), whereas the less variable was the most disturbed and situated at the highest altitude. Heterozygote excess was observed in all populations. Most of genetic diversity (99%) is contained within populations. Genetic differentiation among populations is low (1%), suggesting low gene flow among populations. No evidence of recent bottlenecks events was detected. The estimates of the effective population size were low in all populations. The results are in agreement with the hypothesis that genetic diversity is reduced by the impact of anthropic disturbance in the population located at higher altitude in comparison with the lightly disturbed situated at lower altitude and farther from urban settlements.

Spatial associations between columnar cacti and mimosoid shrubs were evaluated in an Andean semidesert enclave. An analysis on the capacity of other deciduous shrub species and medium-sized ephemeral plants to modify the spatial distribution of cacti was also included. The number of Stenocereus griseus (Haw.) F. Buxb., Cereus repandus (L.) Backeb. and Pilosocereus tillianus Gruber & Schaftzl that grow below the canopies of perennial plants were assessed and compared with open areas. Comparison of observed and expected number of cactus individuals shows a positive spatial association between S. griseus and Prosopis juliflora DC. Moreover, abundance of C. repandus and P. tillianus under the canopies of mimosoid shrubs were not statistically different from what was expected by chance. Positive spatial association between S. griseus and Capparis odoratissima Jacq., Cassia emarginata L., Cordia curassavica (Jacq.) Roem. & Schult., Croton rhamnifolius H.B.K. and Jatropha gossypifolia L. was also found. Evidence suggests that positive columnar cacti-shrub spatial association may occur either in the cactus thickets or in the thornscrubs. Due to the clumped distributions of S. griseus beneath P. juliflora and other perennial plants, we here postulate that S. griseus is the primary nursed columnar cactus of the enclave. The presence of cacti in open areas suggests that facilitation may be less important for establishment of columnar cacti species in this Andean semidesert landscape than in other temperate and tropical semiarid zones. We here discuss the importance of the nurse syndrome phenomenon for recruitment of S. griseus and for the conservation of this Andean semiarid environment.

Understanding large-scale patterns of beta-and alpha diversity is essential for ecoregional conservation planning. Using the 110,000 km 2 shallow basin of the Beni savannas of northeastern Bolivia, we examined the spatial patterns of biological diversity of four taxonomic groups (bats, palms, reptiles, and amphibians). Since the Beni savannas are shaped by cycles of drought and flood, and farming, which, together with topography, create a mosaic of sub-ecoregions from Amazonian forests to aquatic vegetation, we hypothesize that this landscape heterogeneity results in a high turnover in species composition across sub-ecoregions. We compiled species lists for each taxon and modeled the potential distribution of each species using the algorithm MaxEnt. For those species whose modeled potential distribution produced incongruous results, we built presence-absence maps. Using the potential distribution maps, we estimated the lists of species for each subecoregion. In order to establish present similarities in species composition between subecoregions, we constructed dendrograms using cluster analysis of presence-absence matrices. The sub-ecoregions associated with the Moxos (southern part of the Beni savannas) were richer than the sub-ecoregions composing the Beni Cerrado (northern part of the Beni savannas). Centers of species richness were detected in the savanna-type subecoregions (bats and reptiles) or associated with the Várzea forests (palms and amphibians). A south-north gradient in the pattern of distribution of four taxonomic groups was also recorded. The results suggest that the patterns of biological diversity partially respond to the mosaic arrangement of the landscape. Future exercises on conservation planning will