Absorption cytophotometry: Evaluation of three methods for the determination of DNA in feulgen stained nuclei

Summary Three methods for the evaluation of the relative amount of DNA per nucleus by absorption cytophotometry are compared. A combination of the two-wavelength method (Patau, 1952; Ornstein, 1952), the one-wavelength two-area method (Garcia, 1965) and the determination of transmission through nuclear plugs, is proposed in order to estimate systematic errors made by absorption cytophotometry.     

Patchiness and disturbance: plant community responses to porcupine diggings in the central Negev

We tested the hypothesis that diversity and productivity of herbaceous plant communities in disturbed soil are related to the physical and biological heterogeneity of the landscape Our study was earned out on vegetation responses in porcupine diggings on a rocky slope in the central Negev desert in Israel We measured aboveground bio-mass and plant density per species in 150 porcupine diggings (15 cm deep and 15 to 20 cm wide) and in equally sized adjacent control samples in the undisturbed soil matrix We calculated mean annual biomass production, plant density and species richness for 10 sample areas along the slope In addition, we divided the plants into groups according to propagule size and dispersal mode We denoted two types of landscape heterogeneity, which we called physical and biological patchiness Physical patchiness was measured as the ratio of bare rock to soil surface Biological patchiness was the area of the soil covered by shrubs with associated soil mound and under-story relative to the total soil surface We also measured disturbance density, as the long term (17 yr) average density of newly made porcupine diggings We found that 1) the physical patchiness explained 30% of the variation of biological patchiness along the slope, while 2) the patterns of disturbance intensity and biological patchiness were similar (R-=0 386) 3) Biomass, density and species richness were significantly higher in diggings than m the soil matrix 4) Plant density in the matrix, but not m the diggings, was significantly correlated with physical patchiness, 5) species richness in diggings was significantly correlated with biological patchiness, but 6) biomass production in diggings and matrix was not affected by either physical or biological patchiness of the landscape 7) Disturbance density did not affect vegetation responses in diggings and matrix 8) A shift in the plant communities in the matrix towards plants with smaller seeds was associated with increasing physical patchiness, while m diggings there was an opposite shift 9) The proportion of wind dispersers was higher in diggings than outside, while the proportion of runoff dispersers was lower, 10) the densities of runoff dispersers in diggings and matrix were positively correlated with physical and biological patchiness 11) Physical and biological patchiness formed the two major gradients of species composition, explaining 30 and 25% respectively We conclude that the network of physical and biological patchiness and soil disturbance are important in the redistribution of resources and seeds, which control plant biomass, density, species richness and diversity The bare rock surface is the main source for runoff flow with associated soil, organic matter and nutrients The understory vegetation of shrubs provides seeds for creating and maintaining diversity The soil matrix absorbs runoff flow, and disturbances absorb runoff and trap seeds Thus, differences in landscape heterogeneity and their effects on resource and seed movement interact in controlling plant community productivity and diversity in the landscape     

Life histories of two desert species of the bulbous genus Bellevalia

Summary Annual biomass increment and biomass partitioning to leaves, roots and reproduction, and biomass storage in the below-ground bulb was measured in plants of two species of the geophytic genus Bellevalia grown outdoors at three levels of soil moisture. The differences between the species were in accordance with the hypothesis that plants of more arid environments should rely more on internal reserves than plants of more productive environments. In Bellevalia desertorum, a shallow rooted species of the most arid habitats in the Central Negev, leaf and root development during outgrowth at the beginning of winter was rather variable, and followed soil moisture availability to a certain degree. A small portion of its biomass budget was committed to seed production, which varied little among the irrigation regimes. The rest of the biomass was stored in the bulb. The amount of biomass devoted annually to reproduction was mainly determined by the amount of reserves already present in the bulb. In contrast, in B. eigii, which grows in the more productive wadis with its bulb at a depth of 15 to 30 cm, leaf and root growth was not only determined by water availability, but also by initial bulb mass. This resulted in a greater potential relative growth rate than in B. desertorum, but also in a greater risk of accumulating less biomass than it spent in root and leaf construction under poor soil moisture conditions. In this species, reproductive biomass and seed yield were proportional to current biomass gain and, in contrast to B. desertorum, independent of initial bulb mass, provided that the bulb was large enough to initiate flowering.     

The germination of mucilaginous seeds of t Plantago coronopus,Reboudia pinnata,and t Carrichtera annua on cyanobacterial soil crust from the Negev Desert

We studied the effect of intact, crushed or sterile cyanobacterial soil crust from the Negev Desert highlands of Israel as substrates for the germination of seeds of three annual plant species from local populations that produce mucilaginous seeds t Plantago coronopus, Reboudia pinnata and t Carrichtera annua. Mucilaginous seeds of these species were wetted on local intact cyanobacterial soil crust which inhibited their germination in comparison with their germination on filter paper. However, the percentage of germination of each species differed on sterile and crushed soil crust after 72 h of wetting. Germination of t Plantago coronopus seeds was inhibited mechanically and biologically on intact soil crust, since it was significantly higher on both crushed and sterile soil crust. Germination of t Reboudia pinnata seeds was slightly mechanically and biologically inhibited on intact soil crust, but germination on live crushed crust was higher than on sterile crushed crust. By contrast, germination of t Carrichtera annua seeds was not mechanically inhibited by live soil crust but there was significant biological acceleration of germination on intact soil crust relative to crushed soil crust. Each of the three species exhibited different requirements for germination. Germination of t P. coronopus is promoted by disturbances and removing the crust components (including its biological and structural effects). t R. pinnata and t C. annua germination is less affected by crust disturbance but is generally inhibited after removing the live components of the cyanobacterial crust.These species differences suggest different functions of the mucilage of the seed coat for the three species.     

The dynamics of abundance and incidence of annual plant species during colonization in a desert

We studied colonization of annual plants in small-scale disturbances and undisturbed soil for four years in the northern Negev desert. The experiment consisted of 24 patches of I m²: eight undisturbed patches, eight 30 cm deep pits and eight 20 cm high mounds. Disturbance removed the seed bank from the pits and mounds. Rainfall was average during 1992 and 1993 (163.5 and 157.0 mm), very low in 1994 (97.5 mm) and very high in 1995 (283.0 mm). For all ca 100 species in the assemblage, we measured abundance, as the average local density in patches occupied by the species, and incidence, the proportion of patches occupied. Abundance and incidence were positively correlated in the four years of the study (R^2-= 0.71, 0.56. 0. 56 and 0.64). The relationship became steeper during the course of colonization. A minority of species colonized all patches rapidly and increased exponentially in abundance. These high incidence and abundance species responded little or not at all to variations in rainfall. Most of the species experienced frequent local extinctions and colonizations in few patches. These low abundance and low to medium incidence species responded strongly to rainfall by fluctuations in abundance. Differences per species between abundance and/or incidence in pits, mounds and matrix were evident in many species along the entire relationship. These were mostly in favor of pits. We propose that the positions and trajectories of the individual species in the abundance-incidence phase plane signify constraints on population growth along the abundance axis, and on colonization and population persistence along the incidence axis. Since species with lower incidence and abundance respond strongly to rainfall variation, we conclude that their local populations and spatial distribution are to a large extent site-limited. High-abundance and high-incidence species have few or weak constraints and experience density-independent population growth, which is only seed-limited. The causes of site limitation and its effect on population growth and on eotonization and extinction processes differ for different species. In a number of species, abundance and incidence may be limited by dispersal, in conjunction with dispersal mode and patch-specific seed capture. Based on our case study we suggest that the trajectories of the species in the abundance-incidence phase plane are a useful tool for investigating assemblage dynamics.     

Linking Community and Ecosystem Processes: The Role of Minor Species

The link between species and ecosystem functioning is a central issue in ecology. In natural plant communities, the dominant species determine most of the productivity-related processes but what is the function of minor species? A recent hypothesis suggests that after disturbance, minor species facilitate the recruitment and abundance of dominants during re-colonization, thus indirectly determining ecosystem function. We tested this hypothesis using a long-term dataset of annual plant communities in a semiarid shrubland by comparing plant density and biomass from plots in which all vegetation had been removed; plots from which only the dominant (the annual grass Stipa capensis) had been removed, and control plots. In the absence of vegetation, the dominant failed to re-establish during the following growing season. After being removed the dominant re-established similarly to the controls. An ant exclosure experiment excluded the possibility that this was due to seed predation. In an experiment with individual dispersal units of S. capensis, we demonstrated the mechanism by which minor species can control the dominant’s abundance. Minor species indirectly govern ecosystem processes by providing structures facilitating seed soil penetration and thus recruitment of the dominant.     

Integration of ecosystem engineering and trophic effects of herbivores

Herbivores affect vegetation in a variety of ways, involving both trophic and ecosystem engineering interactions, but the study of these different interaction types has rarely been integrated. The aim of this study was to investigate both the trophic and engineering effects of herbivores on plant communities in the Negev desert, Israel, and to promote an integrative approach to the study of herbivore effects in ecosystems. First, we summarise previous studies of the Indian crested porcupine (Hystrix indica), which show that in digging for food, porcupines excavate soil pits, which accumulate resources and seeds resulting in marked changes in plant species richness, density and biomass. By contrast, their trophic effect, via consumption of bulbs, has little impact on populations of perennial plants. Second, we present an empirical study of the trophic and ecosystem engineering effects of harvester ants (Messor spp.). An exclusion experiment, using barriers to restrict ant access, failed to reveal any significant effect of seed collection by harvester ants on plant species incidence (proportional occurrence in samples) or abundance (number of individuals). However, we show that vegetation on nest mounds of M. ebeninus differs in plant density, species richness and biomass from that on undisturbed soil. An analysis of incidence and abundance responses of individual plant species suggests that the observed differences in vegetation resulted from multiple interacting mechanisms.
The case studies highlight that many interactions between herbivores and plant communities can occur simultaneously, and that ecosystem engineering and trophic processes can be closely associated, resulting from single actions of herbivores. We propose a conceptual framework that classifies the range of possible trophic and engineering interactions between herbivores and plant communities with respect to the level of association between trophic and engineering effects. The framework is presented as an aid to the design and interpretation of studies of interactions between herbivores and plant communities, and promotes integrative research into the roles of herbivores in ecosystems.