The demographic costs of nectar production in the desert perennial Prosopis glandulosa (Mimosoideae): a modular approach

Nectar production in angiosperms is considered to represent a reproductive cost, and has been associated with a decrease in fruit set or an overall decrease in the energetic budget of the plant. Populations of Prosopis glandulosa var. torreyana (honey mesquite) are a suitable system to evaluate the demographic costs of nectar production, as populations are composed of a 1:1 proportion of nectarful to nectarless individuals. The study was carried out in a population of 404 individuals of Prosopis g1andulosa var. torreyana found in an area with differing water availability in the Southern Chihuahuan Desert. The possible costs of nectar production were assessed on 1212 shoots of the honey mesquite that were tagged in 1994 and followed until 1998. We used two methods of analysis to describe the effect of nectar production on modular population dynamics: matrix analysis and log-xlinear models. Water availability and the varying environmental conditions affected plant growth, but nectar production did not have an effect on the demographic parameters we measured. The values of λ did not differ between nectar morphs and the only important effects we detected were the year to year variation in precipitation and microclimate differences at each site. Furthermore, the elasticity of each demographic process (growth, fecundity, retrogression and stasis) between nectar morphs did not differ. The log-linear models suggested a similar pattern but could discriminate the importance of each factor (nectar morph, year and site) on module fate. We were not able to detect a demographic cost of nectar production in the honey mesquite. The absence of a demographic response could be due to the negligible cost of producing nectar for this species or that the resources allocated for growth are different from those allocated for reproduction. Our results suggest that the modular fates of mesquites are mainly determined by environmental factors. This revised version was published online in August 2006 with corrections to the Cover Date.     

From plains to inselbergs: species in special habitats as indicators for climate change?

Aim This study proposes a process to select plant species that would provide suitable candidates for monitoring climate change impacts in areas where complete biological inventories are lacking. Location Inselberg floras of nine inselberg landscapes (i.e. isolated mountains) in the arid Desert and Nama Karoo biome in Namibia were analysed to develop a selection process. Methods Data from detailed field surveys were summarized to determine species only occurring on inselberg habitats (i.e. inselberg specialists). Detrended correspondence analysis and a step‐by‐step selection process based on species distribution patterns were used to determine species occurring on inselbergs beyond their zonal distribution ranges. Results The systematic selection process initially identified 88 plant species. Based on field observations and published sources eliminating species (1) with a wide distribution elsewhere, (2) distribution influenced by local effects and (3) for which their status of knowledge of distribution was clearly inadequate, this list was further reduced to 25 species. This included southern species occurring on inselbergs likely beyond their zonal distribution, such as Adenolobus garipensis, Aloe dichotoma and Euphorbia gummifera, as well as savanna and escarpment species at their western zonal distribution edge (e.g. Cordia sinensis, Commiphora glaucescens and Moringa ovalifolia). Main conclusions The step‐by‐step selection process proposed in this study to assist with the selection of indicators for climate change provides an objective tool in areas where biodiversity coverage is not adequate and little is known about physiology, growth and reproductive patterns of individual species. As such it introduces a method for preliminary screening of species, but will require further input based on field observations and expert knowledge.     

Plant distribution and dispersal mechanisms at the Hassayampa River Preserve, Arizona, USA

This study focuses on the relationships between plant dispersal syndromes and plant distributions at the community scale. Species composition and cover are reported from 29 10 × 20-m vegetation plots along five topographic cross-sections in the riparian zone of the Hassayampa River Preserve, Arizona. We find that spatial patterns of dispersal guilds vary within the flood plain of this semiarid region river. Our main results are: (1) wind-dispersed species are fairly evenly distributed at all elevations and distances from the river, whereas cover of animal-dispersed species increases with elevation above, and at greater distances from, the river; (2) wind-dispersed species are proportionally more abundant in the pioneer Populus–Salix community, whereas plants in the late-seral Prosopis community are predominantly animal-dispersed; (3) most of the species classified as obligate-wetland and facultative-wetland are wind-dispersed, whereas facultative-upland and obligate-upland species are mostly animal-dispersed; and (4) there are significantly fewer wind-dispersed species in areas of high total vegetation cover. These results may reflect successional patterns resulting from periodic flooding. Low areas close to the river flood more frequently and with greater intensity than areas farther from the river. Many pioneer species that establish in disturbed areas are wind-dispersed. Over successional time, pioneer species cede to more drought tolerant species that are predominantly animal-dispersed.     

Calcareous nannofossil biostratigraphy and paleoecology of the Cretaceous–Tertiary transition in the central eastern desert of Egypt

The Gebel Qreiya and nearby Wadi Hamama sections of the central Eastern Desert are among the most complete K/T boundary sequences known from Egypt. The two sections were analyzed spanning an interval from l.83 Myr below to about 3 Myr above the K/T boundary. A 1-cm-thick red clay layer at the K/T boundary at Gebel Qreiya contains an Ir anomaly of 5.4 ppb. The high-resolution study and well-preserved nannoflora provide good age control and the first quantitative records of calcareous nannofossil assemblages for paleoecological interpretations across the K/T transition in Egypt. Four zones (Micula murus, Micula prinsii, NP1, and NP2) were distinguished and correlated with other nannofossil and planktonic foraminiferal zonations that are broadly applicable for the eastern Tethys region. Latest Maastrichtian assemblages are abundant and diverse, though Cretaceous species richness progressively decreased across the K/T boundary. Dominant species include Arkhangelskiella cymbiformis, Micula decussata and Watznaueria barnesae, with high abundance of dissolution-resistant M. decussata reflecting periods of high environmental stress. Thoracosphaera blooms mark the K/T boundary and are followed by an acme of the opportunistic survivor Braarudosphaera bigelowii, the first appearance of the new Tertiary species Cruciplacolithus primus, and an acme of Coccolithus cavus/pelagicus. These successive abundance peaks provide the basis for subdivision of the Early Danian Zones NP1 and NP2 into five subzones. Correlation of selected nannofossil taxa from the Egyptian sections with those from various onshore marine and deep-sea sections provides insights into their paleoenvironmental and paleoecological affinities.     

Plant communities of a Central Namib inselberg landscape

Abstract. Three granite inselbergs and six dolerite dykes and their surroundings were investigated in the Central Namib, at the interface between the Namib Desert and Nama Karoo biomes. The main objectives of this study included a phytoso‐ciological interpretation of the described plant communities, explanation of the correlation of the communities with environmental variables and quantification of the relative contribution of different types of variables to structuring plant communities. Nine grassland and shrubland plant communities were recognized, largely organized according to general habitat, elevation, size of inselberg and geology. Soil properties, often thought to play an important role in arid environments, showed no clear patterns in the level of analyses used in this study. Other environmental parameters of importance in arid mountain habitats, such as slope aspect and angle, also played a minor role. The main implications of the study are: 1. Central Namib inselbergs, particularly granite domes, harbour diverse plant communities, often with species from neighbouring higher rainfall areas, and are thus of high conservation value. 2. The poor contribution of environmental variables in this study, which are conventionally used in field studies of plant community – environment relationships, may demand a critical review of additional parameters to be included when analysing plant community – environment relations in arid environments. In particular between‐season variation, phytogeographic aspects and the heterogeneity of microhabitats, often contained within a plant community, need to be taken into account.     

Classification and ordination of plant communities of the Naukluft Mountains,Namibia

Abstract. To determine the relationship between vegetation and environmental parameters in a desert‐savanna ecotone, vegetation in the Naukluft Mountains of Namibia (southern Africa) was described and analysed in terms of environmental parameters determining the observed patterns. 12 plant communities, largely characterized by low shrubs, were described on the basis of a classification. Through ordination altitude, clay, potassium content of the substrate and, to a lesser extent, habitats were identified as underlying environmental factors determining the composition of these plant communities. The edaphic parameters showed some correlation with subsurface geology, but microclimatic conditions, water availability and level of disturbance appeared to override the effect of nutrient composition provided by different substrate. Communities of conservation importance, such as plant communities composed of Karoo shrubs, are prevalent on the mountain plateau, presenting an isolated outpost of Nama Karoo vegetation. The transitional status between desert and savanna, remnant patches of Nama Karoo and the high habitat diversity mark the importance of this high altitude area for biodiversity conservation.     

Utilization of food resources by small and medium‐sized mammals in the Monte Desert biome,Argentina

Theoretical models of species coexistence between desert mammals have generally been based on a combination of food and microhabitat selection by granivorous rodents. Although these models are applicable in various deserts of the world, they cannot explain resource use by mammals in Neotropical deserts. The present study examines diet composition in a mammal assemblage in the Monte desert, Argentina. The results show that two main strategies are used by these mammals: medium‐sized species (hystricognath rodents: Dolichotis patagonum, Lagostomus maximus, Microcavia australis and Galea musteloides; and an exotic lagomorph: Lepus europaeus) are herbivores, whereas small‐sized species (a marsupial: Thylamys pusillus; and sigmodontine rodents: Graomys griseoflavus, Akodon molinae, Calomys musculinus, Eligmodontia typus) are omnivorous. Small mammals also show a tendency towards granivory (C. musculinus), insectivory (A. molinae and T. pusillus) and folivory (G. griseoflavus).     

Thermal responses and temperature tolerance of a dessert ant-lion larva

1. 1.Cueta trivirgata larvae construct pits in the dry Kuiseb River bed in the Namib Desert.

2. 2.An art, Ocymyrmex robustior comprises 65.4% of the biomass of prey consumed by the ant-lions.

3. 3.O. robustior is active between surface temperatures of 27–68°C.

4. 4.Ant-lions tolerate high body temperatures (LD₅₀ = 53.4°C).

5. 5.By exploiting the pit microclimate and by digging below the surface during extreme thermal loads, ant-lions can capture prey at surface temperatures of 13–63°C.

6. 6.These behavioural and physiological adaptations enable ant-lions to maximize the duration of vigilance and hence prey capture success.