Indirect versus direct effects of grasses on growth of a cactus (Opuntia fragilis): insect herbivory versus competition

Variation in plant performance between microhabitats is usually attributed to direct mechanisms, such as plant physiological tolerances or competitive interactions. However, indirect mechanisms, such as differences in herbivore pressure mediated by microhabitat differences, could create the same pattern of variation. In this study, we investigated the effect of insect herbivore pressure on the growth of the grassland cactus Opuntia fragilis under different regimes of grassland canopy cover. Our purpose was to establish the extent to which canopy cover plays a direct, competitive role versus an indirect, mediatory role in cactus growth. We manipulated aboveground microhabitat, specifically the cover of adjacent grasses. The three treatments were: (1) open canopy, with grass pinned down away from the cactus; (2) shaded canopy, with a partial mesh cage staked over the cactus; and (3) ambient grass canopy. We measured seasonal plant growth and recorded changes in insect herbivore occurrence and damage in relation to cover. Cactus growth, defined as the change in number of live cladodes, was higher in the open than under either treatment where the plant was more shaded (P<0.05). However, allocation to new growth, measured as the proportion of new segments (cladodes) in a patch, did not differ among cover treatments. Thus, the hypothesis that physiological constraints, or competition for light, limited cactus performance in grass is rejected. Instead, we found that both cladode mortality, caused by the larvae of a cactus moth borer (Melitara dentata), and occurrence of the moth were lower in the open microhabitat than in either shaded microhabitat. Thus, higher net growth in the open, unshaded treatment, rather than representing a release from competition for light with grasses, was better explained as an indirect effect of grass cover on the activity and impact of the cactus moth. These results show that indirect effects can lead to a misinterpretation of experimental data on direct effects. These data also contribute to an improved understanding of mixed results in the biological control of weedy cacti. Clearly, future evaluations of the relative importance of physiology, competition, and insect herbivory in plant performance must be environmentally explicit.     

Interactions among juveniles of two freshwater crayfish species and a predatory fish

Two freshwater crayfish species, Astacus astacus L. and Pacifastacus leniusculus Dana, co-occur in some Swedish lakes. Observational studies indicate that the introduced, North American species P. leniusculus may gradually replace the native A. astacus, but the mechanism behind the replacement is not known. This study examined the direct effects of interspecific competition between the crayfish, and indirect effects of competitive interactions and fish (European perch, Perca fluviatilis L.) predation. Three different experiments with young-of-the-year (YOY) crayfish were performed. P. leniusculus was strongly dominant over similar-sized A. astacus in interference competition for shelter in a laboratory experiment. However, in a 35-day experiment in outdoor pools, A. astacus growth and survival were about equally affected by interactions with conspecifics and P. leniusculus. In contrast, P. leniusculus was significantly more affected by intraspecific competition than by competition with A. astacus, suggesting asymmetric competition between the two species. The presence of perch in outdoor ponds with mixed-species groups of the two crayfish species resulted in considerably higher predation rates on A. astacus than on P. leniusculus. Both species showed strong antipredator responses to perch by increasing refuge use. I suggest that higher perch predation rates on A. astacus originate from P. leniusculus being the superior species in interspecific competition for shelter. Because of displacement from refuges, A. astacus individuals become more exposed to the predator. This indirect effect of interactions among the two cray-fish species and the predator may be important in the observed in situ replacement of A. astacus by P. leniusculus.     

Effects of scale on detecting interactions between Coleophora and Eriocrania leaf-miners

Abstract.

• 1 The possibility of interactions between leaf-miners in the genus Eriocrania and Coleophora serratella L. on birch (Betula pendula Ehrh. and B.pubescens Roth.) was studied via: (i) co-occurrence patterns on random samples of leaves; (ii) palatability of Eriocraniidae-mined leaves to C.serratella, in laboratory and field preference tests.
• 2 In the field, C.serratella and Eriocrania spp. fed on common leaves less frequently than expected by chance. This result is consistent with, and could be explained by, the avoidance of mined leaves shown by C.serratella in most of the feeding trials.
• 3 Avoidance was not observed at the between-tree level, i.e. the number of leaves mined by each type of insect was unrelated. However, when tree species was taken into account, an inverse distribution was observed: mine abundance was higher for Eriocrania spp. on B.pubescens, whereas for C.serratella it was higher on B.pendula.
• 4 B.pubescens leaves mined by Eriocrania spp. had consistently higher phenolic contents and were better at precipitating proteins than unmined leaves. These induced chemical changes could explain the reduced preference shown by C.serrate1la for Eriocrania mined leaves.
• 5 We speculate about the evolution of adult host-plant selection shown by these insects in response to potential larval interactions.

     

Resource limitation and the lethal and sublethal effects of a viral pathogen in the Indian meal moth, Plodia interpunctella

Abstract.

• 1 The effects of resource limitation and the lethal and sublethal effects of a granulosis virus on a lepidopteran host, the Indian meal moth, Plodia interpunctella, were examined.
• 2 The food quality was manipulated by the addition of an inert bulking agent (methyl cellulose) which caused the size, development rate and fecundity of the moths to be reduced.
• 3 The resource quality had no effect on the mortality due to the virus. In contrast, sublethal effects of the virus on pupal weight were more apparent under conditions of resource limitation.
• 4 Considerable variation between the sublethal effects after challenge with different doses of the virus was found. The balance between deleterious sublethal effects of the virus and the selection of more robust individuals by the bioassays is proposed as a mechanism to explain this variation.
• 5 Implications for the dynamics of insect hosts and their pathogens are discussed.

     

Effects of Ca(II) ions on Mn(II) dynamics in chick glia and rat astrocytes: Potential regulation of glutamine synthetase

Previous studies have demonstrated that in glia and astrocytes Mn(II) is distributed with ca. 30–40% in the cytoplasm, 60–70% in mitochondria. Ca(II) ions were observed to alter both the flux rates and distribution of Mn(II) ions in primary cultues of chick glia and rat astrocytes. External (influxing) Ca(II) ions had the greatest effect on Mn(II) uptake and efflux, compared to internal (effluxing) or internal-external equilibrated Ca(II) ions. External (influxing) Ca(II) ions inhibited the net rate and extent of Mn(II) uptake but enhanced Mn(II) efflux from mitochondria. These observations differ from Ca(II)–Mn(II) effects previously reported with brain (neuronal) mitochondria. Overall, increased cytoplasmic Ca(II) acts to block Mn(II) uptake and enhance Mn(II) release by mitochondria, which serve to increase the cytoplasmic concentration of free Mn(II). A hypothesis is presented involving external L-glutamate acting through membrane receptors to mobilize cell Ca(II), which in turn causes mitochondrial Mn(II) to be released. Because the concentration of free cytoplasmic Mn(II) is poised near the K_d for Mn(II) with glutamine synthetase, a slight increase in cytoplasmic Mn(II) will directly enhance the activity of glutamine synthetase, which catalyzes removal of neurotoxic glutamate and ammonia.     

Effects of aluminium on growth and cation uptake in seedlings of Eucalyptus mannifera and Pinus radiata

This experiment was designed to examine the effects of aluminium (Al) on the growth of Pinus radiata (D. Don) and Eucalyptus mannifera subsp. mannifera (Mudie) seedlings in culture solutions in a glasshouse to help explain the failure of radiata pine trees on some acid, low fertility soils in Australia on which the native eucalypts flourish. Aluminium (Al) in culture solution increased the growth of roots and shoots of seedlings of both species but while growth of the eucalypt continued to increase with increases in Al to 2.222 μM, growth of the pine was largest at 370 μM Al. In addition to total root length, specific root length (length per unit dry weight), a measure of fineness of the root, increased in the eucalypt seedlings as the substrate Al increased. Growth of the shoots and roots of the pine in the absence of any added Al was extremely poor suggesting that Al, in low concentrations, may be an essential element or ameliorate some other factors in solution culture at low pH. Root and shoot concentrations of K increased with increasing Al, whilst Ca and Mg Concentrations decreased and Mn concentrations were unaffected in both species. Tissue Ca and Mg concentrations were 2 to 3 times higher in the eucalypt seedlings than the pine at all levels of added Al due to greater uptake of these elements by the eucalypt. In contrast, at the highest concentration of Al in the medium, shoot Al concentrations were lower in the cucalypt than in the pine due to a greater proportion of Al being retained in the eucalypt roots. These differences between the seedlings in terms of root growth and tissue cation concentrations may help explain the ability of eucalypt species to maintain vigorous growth on acid soils high in Al and low in Ca and P, where growth of the pines failed.     

Nutrient fluxes in the shifting cultivation system of south-west Côte d'Ivoire

At two sites, one with a 4-year-old (4-Y) secondary vegetation and the other with a 20-year-old (20-Y) vegetation, the influence of burning slashed vegetation on crop performance was studied during three seasons. In the first season after clearing, also the influence on weed growth was studied. At both sites, burning significantly decreased the number of weed seedlings. The lowest number of seedlings was found on the burnt plots of the 20-Y site. Burning increased yield and nutrient uptake significantly in the first and second season after clearing. In the third season after burning, only at the 4-Y site a significantly higher yield and nutrient uptake were found. At the 20-Y site the effect had disappeared. Calculations of efficiency of utilization of absorbed N, P and K indicated that P was the least available nutrient, also after burning. At both sites three consecutive crops absorbed approximately 40% of P applied in ash, while the cumulative recovery of K was at least 36% at the 4-Y site and at least 59% at the 20-Y site. On non-burnt plots, yields were not lower in the third season than in the first season after clearing, thus indicating that the inherent soil fertility did not decrease. Hence, yield decline on the burnt plots could be ascribed to ash depletion. It was concluded that in the local shifting cultivation system, the combination of ash depletion and infestation of weeds are the main reasons for abandoning the fields.     

Estimating transpiration from 6-year-old Eucalyptus grandis trees: development of a canopy conductance model and comparison with independent sap flux measurements

Daily patterns of stomatal conductance (g_s), xylem pressure potential (P) and canopy microclimatic variables were recorded on 11 sample days as part of a one-year study of the water use of Eucalyptus grandis Hill ex Maiden in the eastern Transvaal, South Africa. Measured g_s was found to be largely controlled by quantum flux density (Q) and ambient vapour pressure deficit (D). Canopy conductance (g_c) was determined for hourly intervals using g_s measurements and leaf areas in four different canopy levels. A simple model was constructed to allow the prediction of g_c and transpiration from Q, D and season of year. The model was used to estimate transpiration rates from 10 trees in a later study of similarly-aged E. grandis trees, in which sap flow in each tree was measured using the heat pulse velocity (HPV) technique. Five of the trees were monitored on a summer day and five on a winter day. Correspondence between HPV sap flow and modelled transpiration was good for the summertime comparisons, but measured winter-time sap flow rates were underestimated by the model, especially under conditions of high sap flow. The discrepancy is believed to result from having insufficient data from the conductance study to describe the response of g_s to relatively high D in winter. Marked variation in transpiration per unit leaf area indicates that a relatively large number of trees must be sampled for the HPV technique to be used to obtain a mean rate for an entire stand in winter.     

NMR imaging of root water distribution in intact Vicia faba L plants in elevated atmospheric CO2

The effect of elevated atmospheric CO₂ on water distribution in the intact roots of Vicia faba L. bean seedlings grown in natural soil was studied noninvasively with proton (¹H) nuclear magnetic resonance (NMR) imaging. Exposure of 24-d-old plants to atmospheric CO₂-enriched air at 650 cm³ m^?3 produced significant increases in water imaged in upper roots, hypogeal cotyledons and lower stems in response to a short-term drying-stress cycle. Above ground, drying produced negligible stem shrinkage and stomatal resistance was unchanged. In contrast, the same drying cycle caused significant depletion of water imaged in the same upper root structures in control plants subject to ambient CO₂ (350 m³ m^?3), and stem shrinkage and increased stomatal resistance. The results suggest that inhibition of transpiration caused by elevated CO₂ does not necessarily result in attenuation of water transport from lower root structures. Inhibition of water loss from upper roots and lower stem in elevated CO₂ environments may be a mitigating factor in assessing deleterious effects of greenhouse changes on crops during periods of dry climate.