Positive effects of native shrubs on three specially protected cacti species in Durango,México

The spatial association between the specially protected cacti Coryphanta durangensis, Echinocereus longisetus and Peniocereus greggii and potential nurse plants was evaluated, as was their relative position to the sun under the crown of the latter in the southern Chihuahuan Desert. The soil temperature under potential nurse plants was lower than under direct sunlight. There was more nitrogen and organic matter in the soil under Prosopis laevigata trees than in soil under direct sunlight. There were 68 plants of C. durangensis, 59 plants of E. longisetus and 157 of P. greggii. Only one individual of C. durangensis and one of E. longisetus grew outside the shade of the crown of other plants. Echinocereus longisetus was not associated with any particular species, but grew more often than expected by chance in the northern segment of the crown (i.e. the area most shaded in the afternoon). Coryphanta durangensis and P. greggii grew more often under P. laevigata than expected by chance. The effect of other plants, and P. laevigata in particular, on the facilitation of growth of protected cacti species should be considered in management plans of the Chihuahuan Desert, where mesquite (P. laevigata) is often harvested for charcoal production.     

Variable strength of top‐down effects in Nothofagus forests: bird predation and insect herbivory during an ENSO event

Abstract Predators are thought to play a key role in controlling herbivory, thus having positive indirect effects on plants. However, evidence for terrestrial trophic cascades is still fragmentary, perhaps due to variation in top‐down forces created by environmental heterogeneity. We examined the magnitude of predation effects on foliar damage by chewing insects and mean leaf size, by excluding birds from saplings in ‘dry’ and ‘wet’Nothofagus pumilio forests in the northern Patagonian Andes, Argentina. The experiment lasted 2 years encompassing a severe drought during the La Niña phase of a strong El Niño/Southern Oscillation event, which was followed by unusually high background folivory levels. Insect damage was consistently higher in wet than in dry forest saplings. In the drought year (1999), bird exclusion increased folivory rates in both forests but did not affect tree leaf size. In the ensuing season (2000), leaf damage was generally twice as high as in the drought year. As a result, bird exclusion not only increased the extent of folivory but also significantly decreased sapling leaf size. The latter effect was stronger in the wet forest, suggesting compensation of leaf area loss by dry forest saplings. Overall, the magnitude of predator indirect effects depended on the response variable measured. Insectivorous birds were more effective at reducing folivory than at facilitating leaf area growth. Our results indicate that bird‐initiated trophic cascades protect N. pumilio saplings from insect damage even during years with above‐normal herbivory, and also support the view that large‐scale climatic events influence the strength of trophic cascades.     

Isolation and characterization of polymorphic microsatellite markers for peacock wrasse (Symphodus tinca)

Eight polymorphic microsatellite loci were isolated and characterized for the peacock wrasse (Symphodus tinca), a labrid fish inhabiting the Mediterranean and Black seas. Characterization of 35 individuals from the western Mediterranean indicated a relatively high allelic diversity (mean = 12.4, range 9–17), and observed heterozygosity ranging from 0.65 to 0.91. We found no evidence of linkage disequilibrium between pairs of loci. Two loci showed significant departure from Hardy–Weinberg equilibrium. These polymorphic markers can be useful in most basic population genetic applications.     

Resistance management strategies in malaria vector mosquito control. Baseline data for a large-scale field trial against Anopheles albimanus in Mexico

Abstract. A high level of DDT resistance and low levels of resistance to organophosphorus, carbamate and pyrethroid insecticides were detected by discriminating dose assays in field populations of Anopheles albimanus in Chiapas, southern Mexico, prior to a large-scale resistance management project described by Hemingway et al. (1997) . Biochemical assays showed that the DDT resistance was caused by elevated levels of glutathione S-transferase (GST) activity leading to increased rates of metabolism of DDT to DDE. The numbers of individuals with elevated GST and DDT resistance were well correlated, suggesting that this is the only major DDT resistance mechanism in this population. The carbamate resistance in this population is conferred by an altered acetylcholinesterase (AChE) -based resistance mechanism. The level of resistance observed in the bioassays correlates with the frequency of individuals homozygous for the altered AChE allele. This suggests that the level of resistance conferred by this mechanism in its heterozygous state is below the level of detection by the WHO carbamate discriminating dosage bioassay. The low levels of organophosphate (OP) and pyrethroid resistance could be conferred by either the elevated esterase or monooxygenase enzymes. The esterases were elevated only with the substrate pNPA, and are unlikely to be causing broad spectrum OP resistance. The altered AChE mechanism may also be contributing to the OP but not the pyrethroid resistance. Significant differences in resistance gene frequencies were obtained from the F₁ mosquitoes resulting from adults obtained by different collection methods. This may be caused by different insecticide selection pressures on the insects immediately prior to collection, or may be an indication that the indoor- and outdoor-resting A. albimanus collections are not from a randomly mating single population. The underlying genetic variability of the populations is currently being investigated by molecular methods.     

Complexity of leaf miner–parasitoid food webs declines with canopy height in Patagonian beech forests

1. Consumer–resource species interactions form complex, dynamic networks, which may exhibit structural heterogeneity at various scales. This study set out to address whether host–parasitoid food web size and topology vary across forest canopy strata, and to what extent foliar resources and species abundances account for vertical patterns in network structure. 2. The vertical stratification of leaf miner–parasitoid food webs was examined in two monotypic beech (Nothofagus pumilio) forests in northern Patagonia, Argentina. Quantitative food webs were constructed for separate canopy layers by sampling foliage from three tree‐height classes at 0.5–1, 2–3 and 5–6 m above ground. 3. Leaf miner abundance per unit leaf mass and foliar damage (%) did not differ across strata, although foliage quality and quantity increased from the understorey to the upper canopy. Parasitism rates and food web complexity decreased with canopy height, as reflected by reduced linkage richness, linkage density, mean interaction strength, and host vulnerability. 4. Null model analyses revealed that food web metrics, especially in the upper canopy, were often lower than expected when compared with randomly structured networks. Overall, these patterns held for two forests differing in vertical structure and in dominant miner morphotype and parasitoid species. 5. These results suggest that vertical declines in network complexity may be driven by the parasitoids' limited functional response to host abundance and dispersal from pupation sites in the forest floor. A broader constraint on food web structure seemed to be imposed by host–parasitoid trait matching, a reflection of large‐scale assembly processes.