Facilitation of periphyton production by tadpole grazing: functional differences between species

1. This study examined how interactions between resources that vary in edibility, and herbivores that vary in ability to acquire resources, control primary productivity. In a northern California river, grazing on Cladophora glomerata , a relatively inedible filamentous green alga, and its more nutritious epiphytic diatoms, was manipulated by exposing cobbles to tadpoles ( Rana boylii or Hyla regilla ) or excluding tadpoles.
2. Rana indirectly facilitated Cladophora by removing diatoms, whereas Hyla did not significantly change biomass relative to controls. Algal ash-free dry mass on cobbles in Rana treatments was 65 and 72% greater than on controls in two years of investigation (1991 and 1993). Rana decreased epiphytic diatom biovolume by 56% and detritus by 87%.
3. Because nitrogen excretion rates of Hyla and Rana were similar, the differences in effect between the two species were probably due to their roles as consumers rather than as recyclers.
4. The net effect of Rana on periphyton was a 10% increase in areal specific primary productivity (mg O₂ h^–1 m^–2); Hyla caused an 18% decrease. Rana decreased biomass-specific productivity (mg O₂ h^–1 g^–1) 44%; Hyla had no effect.
5. In tadpole exclosures, grazers such as baetid mayfly larvae (mostly Centroptilum sp.) were 4.7 (1991) and 1.8 (1993) times more abundant, and midge larvae (Chironomidae) were 2.5 (1991) and 2 (1993) times more abundant than in Rana enclosures. Invertebrate assemblages in Hyla enclosures, however, were similar to exclosures. Few predatory insects and fish colonized Rana enclosures. Path analyses indicated that Rana affected macroinvertebrates via both interference and exploitation of epiphytic diatoms.     

Humidity, nectar and insect visits to flowers, with special reference to Crataegus, Tilia and Echium

Abstract. 1. This paper describes field observations on diel changes in the nectars of Crataegus, Tilia and Echium in relation to microclimate and insect visits.
2. Nectar concentration is highly correlated with ambient relative humidity, but the concentrative properties of the nectars differ from those of pure sugar solutions in ways that could be accounted for by microclimate or chemical effects.
3. The microclimate inside flowers may influence the rate of equilibration of nectar with the relative humidity of the air, or the equilibrium concentration itself.
4. The vapour pressure relationships of nectars may be influenced by the presence of components with low r/P values (that is, solutes in which solution show a relatively large lowering of vapour pressure, P , for a small change in refraction, r ). Such components might be released by micro-organisms in the nectar.
5. The quantity of sugar per flower depends on the relative rates of secretion and reabsorption, among other things. The flowers we studied showed evidence for morning and evening peaks of secretion, and in Crataegus substantial quantities of sugar were 'sequestered' around midday.
6. The pattern of visits by bumblebees to the flowers of Tilia and Echium can be related to the changing concentration of sugar in the nectar; caloric reward was probably not limiting for bumblebees visiting Tilia.     

Evidence of a sex-linked diapause response in Papilio glaucus subspecies and their hybrids

ABSTRACT. Pupal diapause in the eastern tiger swallowtail. Papilio glaucus L., is obligatory or facultative depending on subspecies. The southern subspecies, P. g. glaucus has a facultative diapause, induced by a larval exposure to short photoperiods. In the northern subspecies. P.g. canadensis , diapause is obligatory, occurring in each generation. F*** hybrids inherit the diapause response of their father or respond differently from either parent. The incidence of diapause in hybrids was associated with sex, indicating that the diapause response may be determined by an X-linked gene.     

Isolation and characterization of eight polymorphic microsatellite loci in the rainforest canopy tree,Syzygium sayeri (Myrtaceae)

Eight polymorphic microsatellite loci were isolated from the rainforest canopy tree, Syzygium sayeri, in order to study parentage and subsequently pollen dispersal among individuals in wild populations. Screening of one natural population (n = 64) mapped in a 500 × 500 m area at Cape Tribulation, north Queensland, Australia, yielded two to 11 alleles per locus with observed levels of heterozygosity ranging from 0.07 to 0.70. One locus was significantly out of Hardy–Weinberg equilibrium after Bonferroni correction for multiple tests. These loci should provide a useful tool in further understanding the dispersal patterns of this species.     

Estimation of rates of predation on tsetse

The levels of natural predation of puparial and adult tsetse flies, Glossina pallidipes Austen, were investigated at Nguruman, Kenya, during January 1989. Puparial experiments involved the production, by individually tubed female flies, of naturally deposited, unhandled puparia in soil cores; handled puparia were obtained from groups of caged flies kept in the normal way. Equal numbers of handled and unhandled puparia were planted out at different densities (1, 2, 4 or 8 per linear metre) in fifty-one natural puparial sites in four major vegetation types. After 10 days puparia were recovered using a soil corer and sieving system. Average predation rates (adjusted for the displacement of puparia by vertebrate activity at the puparial sites) were 9.4% and 7.8% for the two types of puparia during the experiment, equivalent to an average loss of 23.7% of all puparia during a normal 30-day developmental period. Maximum potential predation rates of adult flies were investigated by pinning freshly killed adults at densities of 1, 2, 4 or 8 per m3 to natural vegetation and scoring the results after 24 h. 70% of flies were attacked during this time, by a variety of predators, thought to include both vertebrates and invertebrates. No density dependence was detected in the experiments, either because natural puparial densities were too low for it to occur at this stage of the life cycle or because adult predation levels were too high for it to be detected. Present results are compared and contrasted with previous results for this and another species of tsetse. Calculations of the life-time fertilities of female tsetse suggest that the levels of puparial predation revealed by the present experiments are entirely realistic. Behaviour of the adult flies allows them to escape most of the considerable predation pressure under which they live. How they do so remains a mystery.