Coastal insect herbivore communities are affected more by local environmental conditions than by plant genotype

Abstract.

• 1 We compared the effects of plant genotype and local environment on population densities of a community of coastal insect herbivores in west-central Florida. Reciprocal transplants of four genotypes of three species of coastal plants, Borrichia frutescens, Iva frutescens and Limbricata, were made in July 1992 between a series of off-shore islands.
• 2 For each plant species, phytophagous insects with a wide range of feeding modes including gall-makers, stem borers, leaf miners and sap suckers were affected more by local environment than by plant genotype. Whereas host genotype had a significant effect on the population densities of gall-makers on B.frutescens in the spring of 1993, no significant effect on the denrities of any other insect species was found and the effect on the gall-makers on Borrichia disappeared in the summer, 1 year after the transplants had been made. In our study, local environment had by far the greatest effect on insect population densities among islands. This is an unusual result because in other studies over 80% of the insect species examined have been affected by plant genotype (Karban, 1992). This result is consistent with that reported by Stiling (1994), who censused populations of two phytophagous insects on reciprocal transplantr ot Borrichia in north Florida.
• 3 Local environment also had an effect on insect population densities within islands. This result contrasts with similar studies performed in north Florida (Stiling, 1994), where population densities did not differ within areas, and underlies how some biotic processes may change with in the same community even over relatively small changes in species range.

     

Variability in temperature regulation of CO2 fluxes and N mineralization from five Hawaiian soils: implications for a changing climate

We examined the possibility that microbial adaptation to temperature could affect rates of CO₂, N₂O and CH₄ release from soils. Laboratory incubations were used to determine the functional relationship between temperature and CO₂, N₂O and CH₄ fluxes for five soils collected across an elevational range in Hawaii. Initial rates of CO2 production and net N mineralization increased exponentially from 15 °C to 55 °C; initial rates of CH₄ and N₂O release were more complex. No optimum temperature (in which rates decline at higher and lower temperatures) was apparent for any of the gases, but respiration declined with time at higher temperatures, suggesting rapid depletion of readily available substrate. Mean Q₁₀S for respiration varied from 1.4 to 2.0, a typical range for tropical soils. The functional relationship between CO₂ production and temperature was consistent among all five soils, despite the substantial differences in mean annual temperature, soils, and land-use among the sites. Temperature responses of N₂O and CH₄ fluxes did not follow simple Q₁₀ relationships suggesting that temperature functions developed for CO₂ release from heterotrophic respiration cannot be simply extrapolated. Expanding this study to tropical heterotrophic respiration, the flux is more sensitive to changes in Q₁₀ than to changes in temperature on a per unit basis: the partial derivative with respect to temperature is 2.4 Gt C ·° C^?1 with respect to Q₁₀, it is 3.5 Gt C · Q₁₀ unit^?1. Therefore, what appears to be minor variability might still produce substantial uncertainty in regional estimates of gas exchange.     

Influence of ultraviolet-B radiation on the drift response of stream invertebrates

1. Recent studies have shown that ultraviolet (UV) radiation (280–400 nm) has increased by ≈ 8% in temperate regions over the past decade, but little effort has been devoted to understanding the ecological effects on temperate ecosystems. This research examined the effects of artificial ultraviolet-B (UVB; 280–320 nm) radiation on the drift response of immature stream insects in laboratory microcosms.
2. Two experiments involved natural populations of stream invertebrates, collected from the Cache la Poudre River (September 1994) and the Arkansas River (October 1995) in Colorado. UVB lamps were turned on from 10.00 to 14.00 h each day, and drifting animals were collected on days 1, 3, 5 and 7 during the exposure period. Levels of artificial UVB used in these experiments were similar to levels that stream organisms experience during clear, mid-day conditions at Fort Collins, Colorado (longitude 105°30'; latitude 40°35').
3. Drift was significantly higher in microcosms exposed to UVB than in controls and was dominated by Baetis sp. (Ephemeroptera: Baetidae), Trichoptera (caddisflies) and Simulium sp. (Diptera: Simuliidae). The increased drift of some stream invertebrates in UVB-exposed streams may be a behavioural response and/or a result of injury.
4. Stream organisms may be particularly sensitive to predicted increases in UV radiation, because streams are generally shallow with clear water. As a result of this potential sensitivity, we recommend that research be directed to understanding the ecological effects of UV radiation on these habitats.     

Carbon dioxide-induced changes in beech foliage cause female beech weevil larvae to feed in a compensatory manner

The phenology of Fagus sylvatica was unaffected by exposure to an atmosphere of elevated CO₂ (600 μL L^-1) after two years of fumigation. Non-significant changes in nitrogen and phenolic content of the leaves decreased the nutritional status of beech for female larvae in elevated CO₂ such that they responded by eating in a compensatory manner; males were unaffected. Rates of development, mortality and adult biomass of Rhynchaenus fagi were no different from those in ambient CO₂ concentrations (355 μL L ^-1). It is possible that, with the changes in leaf chemistry affecting the females, fecundity will be altered, with important consequences for populations of beech weevil.     

Neurophysiological and Behavioral Changes in Non-Target Wildlife Exposed to Organophosphate and Carbamate Pesticides: Thermoregulation, Food Consumption, and Reproduction

Originally utilized or developed as human poisons, anticholinesterasecompounds are among the most widely used pesticides in the worldand non-target wildlife are frequently exposed. Because thesecompounds primarily act by inhibiting acetylcholinesterase atsynapses within the central and peripheral nervous systems,the potential for altering physiological and behavioral responsesessential for survival and reproduction in exposed animals isgreat. We review the effects of acute but sublethal exposureto organophosphates and carbamates on thermoregulation (hypothermia),food consumption (anorexia and altered foraging behavior), andreproduction (altered hormone levels, reductions in clutch andlitter size, and alterations in reproductive behavior), andthe mechanisms believed to cause them. We believe these arethe direct toxic effects most likely to reduce populations offree-living birds and mammals within treated areas. Data fromstudies of captive birds and laboratory mammals and free-livingindividuals given controlled dosages are included with an emphasison information published within the last 5 years. The limitationsof existing data for determining the biological and regulatorysignificance of these effects are discussed.     

Duration and benefits of biparental brood care in the dung beetle Onthophagus vacca (Coleoptera: Scarabaeidae)

Abstract.

• 1 Single males, single females or pairs of dung beetles, Onthophagus vacca, were released on artificial small (100 g) or large (1000 g) dung pats in the laboratory. Emigrating beetles were trapped at 12 h intervals, and the number and size of the brood chambers were recorded after each replicate.
• 2 Emigration of males was delayed if females were present in the same dung pats, whereas emigration times of females were independent of the presence or absence of males.
• 3 A residency of 60 h proved to be a threshold value. Females emigrating before this time did not breed, whereas those emigrating later had built at least two brood chambers.
• 4 Females paired with males built more brood chambers than single females.
• 5 The reproductive success of pairs was not influenced by the size of the dung pats.

     

The relative importance of temperature and diet to larval development and adult size of the winter stonefly, Soyedina carolinensis (Piecoptera: Nemouridae)

SUMMARY. 1. Soyedina carolinensis Claassen, a leaf shredding stonefly, was reared in a series of three laboratory experiments from early instar to adult on different species of deciduous leaves and at various constant and fluctuating temperature regimes.
2. Experiment 1, which involved rearing larvae on fourteen different leaf diets at ambient stream temperatures, showed that diet significantly affected larval growth and adult size but did not affect overall developmental time.
3. Experiment 2, which involved rearing larvae on five different leaf diets at each of three fluctuating temperature regimes (viz ambient White Clay Creek (WCC), ambient WCC+3°C, and ambient WCC+6°C), showed that: (i) adding 6°C to the normal temperature regime of WCC was lethal to 99% of the larvae regardless of diet; and (ii) warming WCC by 3°C did not affect developmental time but did significantly reduce adult size relative to adults reared at WCC temperatures on certain diets.
4. Experiment 3, which involved rearing larvae on five different leaf diets at each of five constant temperatures (viz 5, 10, 15, 20, 25°C), showed that: (i) temperature significantly affected the mortality, growth, and development time of larvae whereas diet only affected larval growth and mortality; (ii) temperatures at or near 10°C yielded maximum larval growth and survival for most diets; (iii) at 5°C, larval mortality was high and growth was low resulting in a few small adults for most diets; (iv) larval mortality was at or near 100% at 15°C regardless of diet; and (v) no larvae survived at 20 and 25°C.     

The spatial pattern of air seeding thresholds in mature sugar maple trees

Air seeding threshold (P_a) of xylem vessels from current year growth rings were measured along the vertical axis of mature sugar maple trees (Acer saccharum Marsh.), with sampling points in primary leaf veins, petioles, 1-, 3-, and 7-year-old branches, large branches, the trunk and roots. The air seeding threshold was taken as the pressure required to force nitrogen gas through intervessel pit membranes. Although all measurements were made on wood produced in the same year, P_a varied between different regions of A. saccharum, with distal organs such as leaves and petioles having lower P_a than basal regions. Mean (SE) P_a ranged from 1.0 (± 0.1) MPa in primary leaf veins to 4.8 (± 0.1) MPa in the main trunk. Roots exhibited a P_a of 2.8 (± 0.2) MPa, lower than all other regions of the tree except leaf veins and petioles. Mean xylem vessel diameter increased basipetally, with the widest vessels occurring in the trunk and roots. Within the shoot, wider vessels had greater air seeding thresholds, contrasting with trends previously reported. However, further experimentation revealed that differences in P_a between regions of the stem were driven by the presence of primary xylem conduits, rather than differences in vessel diameter. In 1-year-old branches, P_a was significantly lower in primary xylem vessels than in adjacent secondary xylem vessels. This explained the lower values of P_a measured in petioles and leaf veins, which possessed a greater ratio of primary xylem to secondary xylem than other regions. The difference in P_a between primary and secondary xylem was attributed to the greater area of primary cell wall (pit membrane) exposed in primary xylem conduits with helical or annular thickening.