Seasonal shifts in host usage in Uroleucon gravicorne (Homoptera: Aphididae) and implications for the evolution of host alternation in aphids

ABSTRACT.

• 1 Uroleucon gravicome (Patch) feeds on annual Erigeron species and on perennial Solidago species.
• 2 To test whether Solidago and Erigeron are analogous to winter and summer hosts of typical host-alternating aphids, reproductive performance, host preferences, and host associations were measured for E.strigosus, S.juncea and S.nemoralis during spring, summer and autumn.
• 3 Caged individuals can reproduce on both genera throughout the season, though colonies are rare on Solidago during summer.
• 4 Developmental rate, adult weight, and fecundity decline between May and August on all hosts; however, the drop is least on E.strigosus, intermediate on S.nemoralis, and greatest on S.juncea.
• 5 Throughout the season, reproductive performance is at least as great on Erigeron as on Solidago, the difference being least in spring.
• 6 Soluble nitrogen content shows the same seasonal trends as reproductive performance, declining in all hosts, but declining least in E.strigosus.
• 7 Preference tests and field records show that U.gravicorne moves from Solidago to Erigeron when the first alatae mature in late spring, feeds on Erigeron during the summer, and, in late summer, returns to Solidago where overwintering eggs are laid.
• 8 Solidago is not nutritionally superior during spring or autumn; its use during those seasons may result from the need for good oviposition sites.

     

Linking above- and belowground responses to global change at community and ecosystem scales

Cryptic belowground organisms are difficult to observe and their responses to global changes are not well understood. Nevertheless, there is reason to believe that interactions among above- and belowground communities may mediate ecosystem responses to global change. We used grassland mesocosms to manipulate the abundance of one important group of soil organisms, arbuscular mycorrhizal (AM) fungi, and to study community and ecosystem responses to CO₂ and N enrichment. Responses of plants, AM fungi, phospholipid fatty acids and community-level physiological profiles were measured after two growing seasons. Ecosystem responses were examined by measuring net primary production (NPP), evapotranspiration, total soil organic matter (SOM), and extractable mineral N. Structural equation modeling was used to examine the causal relationships among treatments and response variables. We found that while CO₂ and N tended to directly impact ecosystem functions (evapotranspiration and NPP, respectively), AM fungi indirectly impacted ecosystem functions by influencing the community composition of plants and other root fungi, soil fungi and soil bacteria. We found that the mycotrophic status of the dominant plant species in the mesocosms determined whether the presence of AM fungi increased or decreased NPP. Mycotrophic grasses dominated the mesocosm communities during the first growing season, and the mycorrhizal treatments had the highest NPP. In contrast, nonmycotrophic forbs were dominant during the second growing season and the mycorrhizal treatments had the lowest NPP. The composition of the plant community strongly influenced soil N, and the community composition of soil organisms strongly influenced SOM accumulation in the mesocosms. These results show how linkages between above- and belowground communities can determine ecosystem responses to global change.     

Meiotic Chromosome Pairing, Isozyme Analyses and Ferritin Expression in a Red Clover Mutant Capable of Somatic Embryogenesis

Red clover genotypes capable of regenerating plantletsin vitrofromnon-meristem-derived callus are rare. A previous study identifieda pair of near isogenic lines which were derived from a singleseed but differed in regenerative ability. The callus-derivedplants of this clone were highly regenerative when reintroducedto culture whereas the epicotyl-derived plants produced non-regenerativecallus cultures. The objective of the present study was to observemeiotic chromosome pairing and to compare isozyme profiles andferritin gene expression in regenerative and non-regenerativeplants and cultures from the clone. Meiotic cells exhibitednormal homologous chromosome pairing. Starch gel zymograms fromglasshouse-grown regenerative (F49R) and non-regenerative (F49M)plants failed to show somaclonal variation for alcohol dehydrogenase,glutamate dehydrogenase, esterase or peroxidase. Isoelectricfocusing of callus cultures from regenerative and non-regenerativeplants revealed that regeneration was accompanied by a reductionin staining intensity and numbers of peroxidase bands comparedto non-regenerative cultures. A unique cathodic peroxidase band(pI 7.6) was associated with non-regenerative cultures. Ferritinexpression was greater in callus than in fresh petiole tissue.Ferritin expression remained high in non-regenerative calluscultures but declined in regenerative cultures as regenerationprogressed.Copyright 1999 Annals of Botany Company Trifolium pratenseL., red clover, ferritin, isozymes, meiosis, peroxidase, somatic embryogenesis.     

Inter‐regional comparison of land‐use effects on stream metabolism

1. Rates of whole‐system metabolism (production and respiration) are fundamental indicators of ecosystem structure and function. Although first‐order, proximal controls are well understood, assessments of the interactions between proximal controls and distal controls, such as land use and geographic region, are lacking. Thus, the influence of land use on stream metabolism across geographic regions is unknown. Further, there is limited understanding of how land use may alter variability in ecosystem metabolism across regions. 2. Stream metabolism was measured in nine streams in each of eight regions (n = 72) across the United States and Puerto Rico. In each region, three streams were selected from a range of three land uses: agriculturally influenced, urban‐influenced, and reference streams. Stream metabolism was estimated from diel changes in dissolved oxygen concentrations in each stream reach with correction for reaeration and groundwater input. 3. Gross primary production (GPP) was highest in regions with little riparian vegetation (sagebrush steppe in Wyoming, desert shrub in Arizona/New Mexico) and lowest in forested regions (North Carolina, Oregon). In contrast, ecosystem respiration (ER) varied both within and among regions. Reference streams had significantly lower rates of GPP than urban or agriculturally influenced streams. 4. GPP was positively correlated with photosynthetically active radiation and autotrophic biomass. Multiple regression models compared using Akaike’s information criterion (AIC) indicated GPP increased with water column ammonium and the fraction of the catchment in urban and reference land‐use categories. Multiple regression models also identified velocity, temperature, nitrate, ammonium, dissolved organic carbon, GPP, coarse benthic organic matter, fine benthic organic matter and the fraction of all land‐use categories in the catchment as regulators of ER. 5. Structural equation modelling indicated significant distal as well as proximal control pathways including a direct effect of land‐use on GPP as well as SRP, DIN, and PAR effects on GPP; GPP effects on autotrophic biomass, organic matter, and ER; and organic matter effects on ER. 6. Overall, consideration of the data separated by land‐use categories showed reduced inter‐regional variability in rates of metabolism, indicating that the influence of agricultural and urban land use can obscure regional differences in stream metabolism.     

Effects of Visible and Ultraviolet Light on Carotene-deficient Mutants of Crypthecodinium cohnii

Synopsis.
The ability of carotenoids to protect a heterotrophic dinoflagellate, Crypthecodinium cohnii , from photodynamic damage by sunlight in the presence of an exogenous dye was demonstrated. Wild-type C. cohnii and 2 carotcnoid-deficient mutants were plated on agar-solidified media and exposed to natural sunlight. The wild-type strain, which synthesizes γ–carotene and β–carotene, had the lowest mortality. Mutant car 17 which accumulates mainly §–carotene had an intermediate mortality rate while the albino mutant, car 3, which contains only phytoene, lost viability most rapidly. Wild-type cells treated with diphenylaminc, a carotenogenic inhibitor, were killed at the same rate as mutant car 3. The survival rate of mutants on exposure to sunlight was dependent upon the chromophore length of the accumulated carotenoids. The 3 strains showed no difference in rate of mortality when exposed to ultraviolet light. Protection from sunlight by accumulation of carotenes may be an important ecological factor for this species whose natural habitat is tidepools.     

Lion density and population structure in the Selous Game Reserve: evaluation of hunting quotas and offtake

In 1992, tourist hunting in the Selous Game Reserve generated 1.28 million dollars for the Tanzanian government, of which 0·96 million dollars were returned to wildlife conservation. Lions ( Panthera leo ) are one of three critical species for tourist hunting, consistently generating 12%–13% of hunting revenue from 1988 to 1992. Because of their ecological and economic importance (and intrinsic value), it is important that lion quotas be set so that offtake is sustainable. The population density of lions in Selous ranges from 0·08 to 0·13 adults km⁻², comparable to unhunted ecosystems. The adult sex ratio (36–41% male) and the ratio of cubs to adults (29% cubs) are similar to those of unhunted populations. The ratio of lions to hyaenas is lower in heavily hunted areas (0·17 lions/hyena) than in unhunted areas (0·43 lions/hyena). Hunting levels between 1989 and 1994 took 2·7–4·3% of adult males annually, which is sustainable. The current quota is 10–16% of the adult male population, which exceeds natural mortality rates for male lions. To remain stable if the quota was filled, the population would have to compensate via increased fecundity, increased juvenile survival, or an altered sex‐ratio. Compensation occurs in Selous by producing (or raising) more male than female cubs (66–81% of juveniles are male). Only 28% of the Selous quota was filled in 1992. The percentage of quota filled (both in Selous and nationwide) has dropped since 1988 as quotas have increased. The current intensity of lion hunting in Selous is sustainable, but the quota cannot be filled sustainably.     

Finding a Place for the Commonplace: Hurricane Katrina, Communities, and Preservation Law

The aftermath of Hurricane Katrina drew attention to commonplace landscape markers that create for a community a sense of place— that connection between people and places crucial to a sense of corporate and individual identity and heritage. There is a legal context for sense of place within extant federal preservation legislation. Nevertheless, many such markers with special meanings for residents have been overlooked in federal documentation, the cornerstone of which is the National Register of Historic Places. Grassroots efforts and national media coverage have helped forge a niche for sense of place within the recovery plans and policy emerging in the affected region. However, it is unclear whether this will carry over into practice. In terms of long-term policy shifts, remedying the shortcomings highlighted by Katrina may require changes to the National Historic Preservation Act and its associated guidelines and regulations, or it may entail a new approach altogether.     

Harvester ants modify seed rain using nest vegetation and granivory

1. In annual plant communities, the seed bank determines each generation's potential emergence and any factor that influences it could alter community structure. Harvester ants are common seed predators that may cause seed sources and seed rain composition to vary. 2. In semi‐arid shrubland of the Negev desert of Israel, we hypothesised that the positive effects of dense vegetation on nests of the harvester ant Messor ebeninus Sant. would outweigh reduction by granivory. 3. To test whether nests were seed sources, we removed vegetation on 10 of 20 sampled nests prior to seeding. We tested the interaction between seed sources and granivory by locating seed traps at three locations: nests, foraging trails, and unvisited areas. Trapped seeds were recorded during spring and summer dispersal. Plant recruitment from the altered seed bank was recorded the following spring. 4. During April, seeds were more abundant near nests than the other locations and at uncut compared with cut nests. Foraging trails had fewer species and higher equitability compared with nests or unvisited areas. May and June to August had highest species richness at nests, but richness of cut and uncut nests were similar. Ordination of plant recruitment showed slightly lower vegetation community variation by diminished species in the cut seed bank. 5. While intact nest vegetation initially had high abundance of locally deposited seeds, it apparently blocked wind‐dispersed seeds later in the summer. Conversely, granivory reduced seed species richness only on trails and decreased dominance structure by preferential removal of some species.     

FLUXNET and modelling the global carbon cycle

Measurements of the net CO₂ flux between terrestrial ecosystems and the atmosphere using the eddy covariance technique have the potential to underpin our interpretation of regional CO₂ source–sink patterns, CO₂ flux responses to forcings, and predictions of the future terrestrial C balance. Information contained in FLUXNET eddy covariance data has multiple uses for the development and application of global carbon models, including evaluation/validation, calibration, process parameterization, and data assimilation. This paper reviews examples of these uses, compares global estimates of the dynamics of the global carbon cycle, and suggests ways of improving the utility of such data for global carbon modelling. Net ecosystem exchange of CO₂ (NEE) predicted by different terrestrial biosphere models compares favourably with FLUXNET observations at diurnal and seasonal timescales. However, complete model validation, particularly over the full annual cycle, requires information on the balance between assimilation and decomposition processes, information not readily available for most FLUXNET sites. Site history, when known, can greatly help constrain the model‐data comparison. Flux measurements made over four vegetation types were used to calibrate the land‐surface scheme of the Goddard Institute for Space Studies global climate model, significantly improving simulated climate and demonstrating the utility of diurnal FLUXNET data for climate modelling. Land‐surface temperatures in many regions cool due to higher canopy conductances and latent heat fluxes, and the spatial distribution of CO₂ uptake provides a significant additional constraint on the realism of simulated surface fluxes. FLUXNET data are used to calibrate a global production efficiency model (PEM). This model is forced by satellite‐measured absorbed radiation and suggests that global net primary production (NPP) increased 6.2% over 1982–1999. Good agreement is found between global trends in NPP estimated by the PEM and a dynamic global vegetation model (DGVM), and between the DGVM and estimates of global NEE derived from a global inversion of atmospheric CO₂ measurements. Combining the PEM, DGVM, and inversion results suggests that CO₂ fertilization is playing a major role in current increases in NPP, with lesser impacts from increasing N deposition and growing season length. Both the PEM and the inversion identify the Amazon basin as a key region for the current net terrestrial CO₂ uptake (i.e. 33% of global NEE), as well as its interannual variability. The inversion's global NEE estimate of −1.2 Pg [C] yr⁻¹ for 1982–1995 is compatible with the PEM‐ and DGVM‐predicted trends in NPP. There is, thus, a convergence in understanding derived from process‐based models, remote‐sensing‐based observations, and inversion of atmospheric data. Future advances in field measurement techniques, including eddy covariance (particularly concerning the problem of night‐time fluxes in dense canopies and of advection or flow distortion over complex terrain), will result in improved constraints on land‐atmosphere CO₂ fluxes and the rigorous attribution of mechanisms to the current terrestrial net CO₂ uptake and its spatial and temporal heterogeneity. Global ecosystem models play a fundamental role in linking information derived from FLUXNET measurements to atmospheric CO₂ variability. A number of recommendations concerning FLUXNET data are made, including a request for more comprehensive site data (particularly historical information), more measurements in undisturbed ecosystems, and the systematic provision of error estimates. The greatest value of current FLUXNET data for global carbon cycle modelling is in evaluating process representations, rather than in providing an unbiased estimate of net CO₂ exchange.