Why is the world green? The interactions of top–down and bottom–up processes in terrestrial vegetation ecology

A classic question in plant ecology is “why is the world green?” That is, if plants are food for animals why do not animals eat all the available food – changing a ‘green world’ into a ‘brown world’. We first reviewed this question in 2009 and now revisit our arguments in the light of new data and new thinking. Here we argue that (1) the top–down bottom–up dichotomy is probably too simple for understanding a complex system – such as vegetation – rich in feedback processes. (2) Nevertheless it appears that bottom–up processes are generally more important for maintaining the presence of some sort of vegetation while top–down control process are generally more important in determining the type of vegetation at a site. (3) Although this review mainly takes a qualitative and experimental approach to the question, we also argue that simple well-known mathematical models from population ecology can be very informative in thinking about the types of explanations for the green world phenomenon, and demonstrating that it is rarely a simple choice between one form of control or another.     

The new eubacterium Roseomonas baikalica sp. nov. isolated from core samples collected by deep-hole drilling of the bottom of Lake Baĭkal

Microbiological analysis of samples of sedimentary rocks from various eras of the geological history of the Baikal rift has enabled us to isolate a large number of microorganisms that can be classified into new, previously undescribed species. The present work deals with the identification and study of the morphological, biochemical, and physiological properties of one such strain, Che 82, isolated from sample C-29 of 3.4-3.5 Ma-old sedimentary rocks taken at a drilling depth of 146.74 m. As a result of our investigations, strain Che 82 is described as a new bacterial species, Roseomonas baikalica sp. nov., belonging to the genus Roseomonas within the family Methylobacteriaceae, class Alphaproteobacteria.     

Introduction: Who's at the bottom? Examining claims about racial hierarchy

Why do claims about racial hierarchy matter? The question whether some groups are worse off than others is highly pertinent at a time when there is growing recognition of multiple forms of racisms and racial oppression. It is widely accepted that racial hierarchies are still with us today, and this concept is peppered throughout writings on “race” and racisms, but, what, exactly, are racial hierarchies, how do racial hierarchies continue to matter, and in what ways do they operate? This special issue, which focuses on the USA and Britain, also addresses the following questions: Does the concept of racial hierarchy aid us in illuminating racial inequalities and the differential experiences of groups in Western multi-ethnic societies such as the USA and Britain? What sorts of criteria are used in arguments about the place of groups along racial hierarchies? What are the political implications of claims made about racial hierarchies?     

Vertical distribution and abundance of gymnamoebae (Rhizopoda) in bottom sediments of the brackish water Nivå Bay (Baltic Sea,The Sound)

The sandy sediments of Niv? Bay (Baltic Sea, The Sound, Denmark) are often covered with the mats of sulphur bacteria and are temporarily anoxic. The vertical distribution and abundance of naked amoebae species in three sediment cores from this bay were studied. Amoebae were most abundant and diverse in the upper 1 cm of sediment, and their number and diversity decreased with increasing depth into the sediment. Amoebae were recovered from both upper oxygenated and deep anoxic layers of sediments. The species composition and abundance of amoebae was very heterogeneous, even at spatial scales of several centimeters, suggesting the existence of microhabitats selectively occupied by particular species. All species found were recorded from aerobic cultures and some of these amoebae occur in both the aerobic and anaerobic layers of the sediment. Minimal possible number of amoebae in the sediments, estimated for the first time as areal abundance integrated for depth was: core 1 -597 cm(-2); core 2 -1,110 cm(-2); core 3 -1,430 cm(-2). These abundances are probably best regarded as "potential" abundances of amoebae hidden in the sediments, as the question of the ratio between active and resting amoebae remains open.     

Proteome, transcriptome and genome: top down or bottom up analysis?

Biological systems are comprised of protein components found at a wide variety of abundances from millions of molecules of a single species per cell to less than one copy per cell. Because of this wide range of concentrations, measurement or a full accounting of each system is presently unavailable. Conventional separation and analytical methods (two-dimensional gel electrophoresis and mass spectrometry) allow identification and quantitation of many of the most abundant gene products (top down methods); and the majority of gene products, which are found at low abundance, can be neither identified nor measured in complex mixtures at present. The gene products that are found at low levels can be characterized and their properties analyzed by preparing ordered gene libraries of limited complexity from mRNA. When such preparations are expressed in cell free systems and analyzed by two-dimensional gel electrophoresis, the features of the gene products are available for analysis. This 'bottom up' approach allows identification of gene product properties so that analytical procedures can be devised and applied to complex mixtures.     

Distinct or similar? Soft bottom polychaete diversity in Arctic and Antarctic glacial fjords

The main aim of this study was to compare the polychaete communities in two similar polar areas: an Arctic fjord, Hornsund (Svalbard) and an Antarctic fjord, Ezcurra Inlet (South Shetlands). This is the first attempt to compare Arctic and Antarctic diversity based on fully comparable datasets. Forty van Veen grab samples were collected in each fjord: twenty replicates were taken in each of two fjord areas characterized by a different level of glacial disturbance—in the inner (glacial bay) and outer (fjord mouth) region of both fjords, from depths of about 100 m in 2005 (Hornsund) and in 2007 (Ezcurra Inlet). In the glacial bays, species richness and diversity were significantly higher in Ezcurra Inlet than in Hornsund due to higher rate of glacial disturbance in the latter one. In the outer areas, species richness was similar in both fjords, although diversity values were higher in Ezcurra Inlet. Polychaete species richness in the habitats characterized by similar level of disturbance (outer areas of the fjords) was the same in both polar regions. At this small scale, where community drivers are very similar, the species richness seems to be independent from the local or regional species pool.     

The significance of depth and substratum incline for the structure of a hard bottom sublittoral community in glacial Kongsfjorden (Svalbard, Arctic)—an underwater imagery approach

In order to reveal the structure of the sparsely known deeper sublittoral hard bottom communities of glacial Kongsfjorden, the macroepibenthos from six depth zones (30–200?m) was analysed. A total of 180 still images derived from 6-h video recorded at the Kongsfjordneset remotely operated vehicle station were assessed quantitatively. Overall 27 mainly suspension-feeding species/taxa were observed. Of these, two-thirds have an arcto-boreal distribution, while the remainder are cosmopolitan. The overall mean epibenthos abundance was 33 ind. m^?2 with maximum values at 150?m depth (97.9 ind. m^?2). The majority of the taxa inhabited the entire depth range. Encrusting red algae, an unidentified sponge and the sea anemone Urticina eques, characterized the assemblage of the shallow zone. The sea anemones Hormathia spp. were important below 30?m, the Serpulid polychaete Protula tubularia was characteristic for the community below 50?m and the demosponge Haliclona sp. was a key taxon between 100 and 200?m depth. Cluster analysis and non-metrical multidimensional scaling based on abundance data showed differences between the assemblages along the bathymetric gradient, but only in the shallower depths in relation to the substratum surface incline. As surface and tidal current impacts attenuate with increasing depth, there is a gradual trend from robust key species towards more fragile ones (i.e. P. tubularia), in line with the ‘Physical control hypothesis’.     

Woody plant phylogenetic diversity mediates bottom–up control of arthropod biomass in species-rich forests

Global change is predicted to cause non-random species loss in plant communities, with consequences for ecosystem functioning. However, beyond the simple effects of plant species richness, little is known about how plant diversity and its loss influence higher trophic levels, which are crucial to the functioning of many species-rich ecosystems. We analyzed to what extent woody plant phylogenetic diversity and species richness contribute to explaining the biomass and abundance of herbivorous and predatory arthropods in a species-rich forest in subtropical China. The biomass and abundance of leaf-chewing herbivores, and the biomass dispersion of herbivores within plots, increased with woody plant phylogenetic diversity. Woody plant species richness had much weaker effects on arthropods, but interacted with plant phylogenetic diversity to negatively affect the ratio of predator to herbivore biomass. Overall, our results point to a strong bottom–up control of functionally important herbivores mediated particularly by plant phylogenetic diversity, but do not support the general expectation that top–down predator effects increase with plant diversity. The observed effects appear to be driven primarily by increasing resource diversity rather than diversity-dependent primary productivity, as the latter did not affect arthropods. The strong effects of plant phylogenetic diversity and the overall weaker effects of plant species richness show that the diversity-dependence of ecosystem processes and interactions across trophic levels can depend fundamentally on non-random species associations. This has important implications for the regulation of ecosystem functions via trophic interaction pathways and for the way species loss may impact these pathways in species-rich forests.     

Structuring factors in a marine soft bottom community during eutrophication—an experiment with radio-labelled phytodetritus

The quantitative development and uptake of radio-labelled phytodetritus in benthic macro- and meiofauna was studied in a 5-month experiment in two mesocosms, one of which received a single large nutrient (N and P) addition, while the other served as control. In reponse to the 12-fold increase in phytoplankton biomass noted after 2 weeks and the resulting enhanced accumulation of fresh phytodetritus, the abundance and biomass of the polychaetes Mediomastus ambiseta and Polydora ligni and the mud anemone Cerianthiopsis americanus increased significantly in the enriched tank. The abundances of P. ligni and M. ambiseta increased 37-fold and 12-fold, respectively, within the first two months of the experiment. No other macrofaunal or meiofaunal taxa showed any consistent positive quantitative response to the increased input of phytodetritus. In the control tank no considerable change in the benthic community structure was noted. The measurements of radio-label uptake within the benthic fauna showed that the quantitatively most successful species utilized fresh phytodetritus highly. However, a high degree of utilization of fresh detritus was also shown by taxa that did not respond quantitatively within the 5 month of the experiment, and almost all taxa showed a preference for fresh detritus over older organic material. Within the benthic meiofauna, kinorhynchs and especially foraminiferans showed a remarkably low preference for fresh detritus. A budget calculation comparing the total amounts of labelled organic carbon bound in animal tissue and in the sediment indicated that at any time at least 75% of this carbon was available for assimilation by deposit feeders. These results suggest that factors other than the availability of food, such as competition for space by a few opportunistic macrofauna species, limited the response of other species within this benthic community to the increased input of phytodetritus.     

Basin-scale internal waves in the bottom boundary layer of ice-covered Lake Müggelsee, Germany

We performed high-resolution temperature measurements under ice cover in Lake Müggelsee, Germany, during the winter of 2005–2006. Intense seiche-like temperature oscillations developing after the ice-on have been encountered in a thin water layer above the sediments. The oscillations were initiated immediately after lake freezing by the release of the potential energy of the thermocline slope and existed for several weeks without appreciable external forcing. The oscillations were associated with a basin-scale internal waves existing in the lower stratified part of the water column. The weakness of the density stratification under ice ensured the long wave periods, exceeding the period of geostrophic inertial oscillations at the lake’s latitude. As a result, two frequency peaks were present in the oscillations corresponding to two rotational waves, one of Kelvin-wave type and another of Poincaré type wave. The rotational character provided long dissipation times of the waves and allowed the oscillations to persist in lake several weeks. Temperature measurements in the upper several centimeters of the sediment demonstrated that oscillations of the near-bottom temperature produced vertical density instability and pore-water convection in the upper sediments.     

The message of ‘The Other North Sea’

This paper explains the objectives of organising the symposium ‘The Other North Sea’. A major goal is the submission of the conclusions and content of this symposium to the North Sea Task Force, an initiative of the Second Conference on the Protection of the North Sea (London, 1987) which is responsible for preparing a Quality Status Report on the North Sea by mid-1993. The conclusions and recommendations of the symposium are summarised and presented to the North Sea Task Force in connection with the preparations for the 1993 Quality Status Report. The paper stresses the need to subdivide the North Sea into areas of ecological entity and underlines the advantages of using diatom and meiofauna assemblages as indicators for present and past environmental actors in sediment conditions. A plea is also made for the promotion of research to improve understanding of disturbance of sensory perception and behaviour as an ecologically relevant early warning signal. The major role of fisheries in inducing changes of seabird and harbour porpoise populations is highlighted.     

The New Health Practitioner — The Past as Prologue

The training and utilization of New Health Practitioners (NHP''s) has moved within a decade from a small tentative beginning in the mid-1960''s to a major health manpower policy issue. Its discussion transcends a number of important and emotionally charged areas including the role of various professions in providing health care services, the renaissance of primary care, the geographic and economic maldistribution of medical services, and the cost of care. It is valuable to highlight what is known, what is professed and what needs to be studied further about this issue.     

The Weaker Sex? The Propensity for Male-Biased Piglet Mortality

For the most part solutions to farm animal welfare issues, such as piglet mortality, are likely to lie within the scientific disciplines of environmental design and genetic selection, however understanding the ecological basis of some of the complex dynamics observed between parent and offspring could make a valuable contribution. One interesting, and often discussed, aspect of mortality is the propensity for it to be sex-biased. This study investigated whether known physiological and behavioural indicators of piglet survival differed between the sexes and whether life history strategies (often reported in wild or feral populations) relating to parental investment were being displayed in a domestic population of pigs. Sex ratio (proportion of males (males/males+females)) at birth was 0.54 and sex allocation (maternal investment measured as piglet birth weight/litter weight) was statistically significantly male-biased at 0.55 (t₃₅ = 2.51 P = 0.017), suggesting that sows invested more in sons than daughters during gestation. Despite this investment in birth weight, a known survival indicator, total pre-weaning male mortality was statistically significantly higher than female mortality (12% vs. 7% respectively z = 2.06 P = 0.040). Males tended to suffer from crushing by the sow more than females and statistically significantly more males died from disease-related causes. Although males were born on average heavier, with higher body mass index and ponderal index, these differences were not sustained. In addition male piglets showed impaired thermoregulation compared to females. These results suggest male-biased mortality exists despite greater initial maternal investment, and therefore reflects the greater susceptibility of this sex to causal mortality factors. Life history strategies are being displayed by a domestic population of pigs with sows in this study displaying a form of parental optimism by allocating greater resources at birth to males and providing an over-supply of this more vulnerable sex in expectation of sex-biased mortality.     

When does fishing lead to more fish? Community consequences of bottom trawl fisheries in demersal food webs

Bottom trawls are a globally used fishing gear that physically disturb the seabed and kill non-target organisms, including those that are food for the targeted fish species. There are indications that ensuing changes to the benthic invertebrate community may increase the availability of food and promote growth and even fisheries yield of target fish species. If and how this occurs is the subject of ongoing debate, with evidence both in favour and against. We model the effects of trawling on a simple ecosystem of benthivorous fish and two food populations (benthos), susceptible and resistant to trawling. We show that the ecosystem response to trawling depends on whether the abundance of benthos is top-down or bottom-up controlled. Fishing may result in higher fish abundance, higher (maximum sustainable) yield and increased persistence of fish when the benthos which is the best-quality fish food is also more resistant to trawling. These positive effects occur in bottom-up controlled systems and systems with limited impact of fish feeding on benthos, resembling bottom-up control. Fishing leads to lower yields and fish persistence in all configurations where susceptible benthos are more profitable prey. Our results highlight the importance of mechanistic ecosystem knowledge as a requirement for successful management.     

Upwelling-like bottom intrusion enhances the pelagic–benthic coupling by a fish predator in a coastal food web

Upwelling regions where nutrients are transported from deep to surface waters are among the most productive in the oceans. Although it is well known that the upwelling affects fishery production through bottom-up trophic cascading, it remains unexplored how temporal variation in its intensity alters overall trophic energy flows within a focal food web. In the present study, we demonstrate that inter-annual variation in the intensity of upwelling-like bottom intrusion alters food web properties in coastal waters of the Uwa Sea by focusing on the levels of δ¹³C and δ¹⁵N for a demersal fish predator, Acropoma japonicum. This approach integrates information on prey–predator interactions. In the season following a stratification period when pelagic productivity is limited by nutrient availability, A. japonicum showed lower levels of δ¹³C in years with high bottom intrusion intensity than in those with low intensity. One possible cause for this isotopic depletion is that the bottom intrusion-induced nutrient supply enhances pelagic productivity and consequently facilitates a foraging shift by A. japonicum from ordinary benthic prey to supplementary pelagic prey with a lower δ¹³C. In conclusion, the increased intensity of bottom intrusion results in coupling of two major trophic energy flows, pelagic and benthic food chains, through the demersal predator’s foraging shift.