Juan Traba

Seed size, shape and persistence in dry Mediterranean grass and scrublands

Seed size and shape, measured as the variance of the three main dimensions, have been proposed as good indicators for predicting seed persistence. We tested whether these variables were robust predictors of seed persistence in the soil for 58 abundant herbaceous species, primarily annuals, in grass and scrubland of central Spain. Seed persistence was estimated from data on germinable seed banks, while seed weight and shape were measured using fresh seeds collected in the study area. There was a significant tendency for species with persistent seeds to have smaller seeds than species with transient seeds. Seed shape was not, however, related to persistence and we did not find any clear seed weight/shape threshold for predicting persistence. The binary logistic model of seed bank type as a function of seed weight was significant and explained 67% of total variability. Supplementary information about dormancy, environmental conditions of habitat, predation and attack by pathogens has to be used to elaborate more accurate general predictive models of seed persistence.

From what depth do seeds emerge? A soil seed bank experiment with Mediterranean grassland species

Seed germination and emergence are influenced by the position of seeds in the soil bank profile. Mediterranean grasslands are heavily dependent on seed banks, as these systems are mainly composed of annual species. Seed bank germination experiments in a greenhouse were conducted to analyse the role played by burial depth on seed bank dynamics in annual Mediterranean grasslands. Specifically, they addressed two objectives: (1) to assess the ability of seeds in the shallow layer of the soil bank to emerge when they are buried at different depths, and (2) to ascertain the ability of seeds from deep layers to germinate and emerge to the surface. The study also produced a depth profile of species and seeds. The results show that: (1) all species (100%) and the majority of viable seeds (98.9%) are situated in the first centimetre, with a significant fall in the number of species and seeds in the soil bank as depth increases; (2) for the majority of species (92%) and seeds (85.4%) in the shallow bank, the emergence percentage declines significantly with burial depth; and (3) seeds that are present in deep layers need to rise to the surface in order to produce seedlings. In conclusion, the function of the seed bank in Mediterranean grasslands depends on the number of species and seeds in it, but also on the seed position in the profile and vertical movements that enable them to reach the surface.

The fate of seeds in Mediterranean soil seed banks in relation to their traits

Abstract Question: Is there any change in seed density and species richness in Mediterranean surface soil banks during summer? Are there any relationships between these summer variations and seed traits (weight, length and shape), without and with controlling the phylogenetic effect? Location: Central Spain. Methods: Samples of the surface soil seed bank were collected in two Mediterranean systems, grassland and scrubland, at two points in the year: immediately after the summer production peak and immediately prior to the autumn germination peak. We used Canonical Correspondence Analysis ordination to check for changes in floristic composition and ANOVAs to check for changes in seed density and species richness between summer and autumn samples. We used multiple regression analysis to analyse the relationship between summer variations in soil seed density and form traits, with and without controlling phylogenetic relations. Results: Soil seed density dropped significantly during the summer in the two systems (28% in grasslands, repeated measures ANOVA test; F = 58.19, P < 0.01; 72% in scrublands, repeated measures ANOVA test; F = 75.67, P < 0.001). Species richness, however, only dropped significantly in the scrubland (32%; repeated measures ANOVA test; F = 9.17, P < 0.05). Variation in the floristic composition of the shallow banks was only significant in the scrubland. Changes in shallow bank density were related significantly to seed morphology features, with greater drops in species with heavier seeds in grasslands and species with longer seeds in scrubland. Conclusions: Our results show a substantial loss of seeds in the uppermost soil layer during the summer period between the point of peak production and the autumn germination peak. This drop was clearer in the scrubland than in the grassland. Longer and/or heavier seeds underwent the greatest declines in density at the end of summer, indicating a more intense effect of post-dispersal predation on large-seeded species. Abbreviations: CSA = Cross-species analysis; PIA = Phylogenetically independent analysis; PIC = Phylogenetically independent contrast.

Is the European population of Dupont's Lark Chersophilus duponti adequately estimated?

Capsule Our estimate using mapping is less than previously reported using transects.

Niche segregation between wild and domestic herbivores in Chilean Patagonia.

Competition arises when two co-occuring species share a limiting resource. Potential for competition is higher when species have coexisted for a short time, as it is the case for herbivores and livestock introduced in natural systems. Sheep, introduced in the late 19th century in Patagonia, bear a great resemblance in size and diet to the guanaco, the main native herbivore in Patagonia. In such circumstances, it could be expected that the two species compete and one of them could be displaced. We investigated spatial overlap and habitat selection by coexisting sheep and guanaco in winter and in summer. Additionally, we studied habitat selection of the guanaco in a control situation free from sheep, both in summer and winter. We also determined overlap between species in areas with different intensity of use (named preferred and marginal areas) in order to further detect the potential level of competition in the case of overlapping. Guanaco and sheep showed significantly different habitat preferences through all seasons, in spite of their spatial overlap at landscape scale. Additionally, the habitat used by guanaco was similar regardless of the presence or absence of livestock, which further indicates that sheep is not displacing guanaco where they coexist. These results suggest that habitat segregation between guanaco and sheep is due to a differential habitat selection and not to a competitive displacement process. Therefore, the potential for competition is considered low, contrary to what has been previously observed, although this could be a density-dependent result.

Density and habitat use at different spatial scales of a guanaco population (Lama guanicoe) in the Monte desert of Argentina

Abstract The first density estimates of a peripheral guanaco population and its habitat use at different spatial scales are presented for a protected area of Monte desert, Argentina. Transects were surveyed in the wet and dry seasons of 2005. All guanaco herds observed during systematic surveys using roads and tracks were GPS located and their habitat use was identified. Herd size differed significantly between the dry and wet seasons. Population densities differed between wet (0.10–0.12 individuals/km2) and dry seasons (0.60–0.75 individuals/km2). The population estimates ranged from 75 individuals (dry season) to 388 individuals (wet season). Guanacos showed differential habitat use, the first determinant being abiotic factors, such as topography, soil characteristics or microclimate conditions, animals being detected in rougher rocky substrata in the dry season and in open flat terrain in the wet season, followed by a mesoscale selection defined by plant communities. At the latter scale, guanaco preferentially used mixed creosote bushland and saltbush more intensively during the wet season, and open scrub and columnar-cactus slopes in the dry season. The estimated population of this protected area was small but its population density was within the range of other populations and was relatively high for this dry and unproductive area.

Abiotic gradients drive floristic composition and structure of plant communities in the Monte Desert

La definicion de comunidades discretas de plantas en zonas deserticas es complejo debido tanto a su homogeneidad a gran escala como a su heterogeneidad a pequena escala, lo que acaba generando dificultades para la toma de decisiones de conservacion. En este trabajo analizamos las comunidades de plantas del sector mas arido del Desierto del Monte en funcion de su estructura y composicion floristica. Se han utilizado tambien variables ambientales estimadas a pequena escala como la pendiente o la superficie de roca, suelo desnudo y hojarasca, asi como variables que operan a mayor escala como la diversidad de especies, la composicion floristica y la similitud dentro y entre localidades. Los analisis de composicion floristica resaltaron la dificultad para segregar diferentes comunidades debido al elevado nivel de heterogeneidad interna y de solapamiento floristico entre las distintas localidades estudiadas. Solo la comunidad situada en el extremo del gradiente de humedad edafica, el algarrobal, se segrego del resto. La ordenacion realizada sobre las variables estructurales en funcion del tipo de sustrato y de la cobertura arborea y de matorral segrego mejor las diferentes comunidades. Nuestros resultados muestran la dificultad para diferenciar comunidades de plantas en desiertos templados, sugiriendo la existencia de ensamblajes de especies relativamente estables en los extremos de los gradientes y de una gran heterogeneidad dentro y entre localidades. Las comunidades de plantas, por tanto, no pueden ser definidas unicamente por variables floristicas, sino que es preciso incluir informacion ambiental.

Habitat selection and density-dependent relationships in spatial occupancy by male Little Bustards Tetrax tetrax.

This study analyses habitat selection by male Little Bustards Tetrax tetrax of a central Spanish population within their display sites during two consecutive breeding seasons. The areas of sexual display sites were determined by applying minimum convex polygons (MCP) to locations of individual males throughout the 2001 and 2002 breeding seasons. Habitat selection was addressed by means of a compositional analysis. The effects of variables related to phenology and to the density of conspecifics were examined, assuming an ideal free distribution of organisms. The spatial distribution of display areas was also analysed. During both years male Little Bustards showed a significant preference for old and same-year fallows, these offering both shelter and food. Contrary to what was expected, individuals did not select lower quality habitats when conspecific density increased or when areas were occupied earlier, suggesting that the population was not saturated. Display sites were not clumped, showing that in this population the Little Bustard may not employ a lek mating system, even though certain requirements of an exploded lek system — the inclusion in territories of particular habitat types that females could potentially use — were met. Our results emphasise the importance of agrarian substrates in determining occupation by this species, a finding with direct implications for the conservation of populations given that the agrarian landscape is on a sufficiently large scale for management measures to be introduced.

Not Only Habitat But Also Sex: Factors Affecting Spatial Distribution of Little Bustard Tetrax tetrax Families

Abstract Species distribution patterns are determined not only by habitat preferences but also by biotic factors. Particularly, the presence of conspecifics may yield different types of benefits and costs for the individuals involved. This study analyses the spatial distribution of Little Bustard families during the breeding season in relation to the distribution of male core areas in three Spanish populations of the species. A compositional analysis is used to evaluate habitat selection and the habitat types most preferred by females with offspring. Spatial analyses were performed to evaluate the proximity of Little Bustard families to male core areas and male displaying sites. The habitat selection pattern was similar between regions, with semi-permanent vegetation and stubbles as most preferred habitats while others, such as ploughed land, were avoided. Families were located closer to male core areas than expected by chance and were spatially associated to male displaying sites. Little Bustard females may obtain different benefits from this spatial association such as access to more food resources and chances for second matings after a clutch loss. Also in stubbles, the capture of prey by chicks would be easier and semi-permanent habitats would serve as shelter. These results highlight the importance of intraspecific interactions in the definition of the habitat selection pattern of females and families. This study shows how spatial point pattern analysis may be a useful tool for integrating landscape and behavioural ecology.

Lack of feral livestock interference with native guanaco during the dry season in a South American desert

Analyzing coexistence of exotic and native ungulates in arid areas is important from both a theoretical and a species conservation perspective. We assess the habitat use patterns and possible interference between guanaco (Lama guanicoe) and feral livestock (donkey and cattle) in arid environments of South America. To determine habitat use and niche overlap between exotic and native ungulate species, ten sites with different habitats and six natural waterholes were selected. Plots (20 at each site, ten around each waterhole) were randomly set up and characterized by environmental variables and relative use by cattle, donkey and guanaco through faecal pellet counts. Aggregation, niche breadth and niche overlap of the three herbivores were analyzed at habitat level (mesoscale). A direct redundancy analysis was used to examine the relationships between abundance of herbivore faeces and environmental variables at microscale. Mesoscale analyses showed (i) an extensive use of the area by all three species, with guanaco having the highest niche breadth followed by donkey and cattle and (ii) a large, broad guanaco–donkey and donkey–cattle habitat overlap. However, results at a finer scale showed high spatial aggregation of feral livestock species and an independent use of territory by guanacos. This study is the first to provide information about habitat partitioning between guanacos and feral livestock in the hyper-arid Monte Desert biome and points to an apparent lack of negative effects on the native ungulate.