Michel De Garine-Wichatitsky

Correlations between leaf toughness and phenolics among species in contrasting environments of Australia and New Caledonia.

BACKGROUND AND AIMSnPlants are likely to invest in multiple defences, given the variety of sources of biotic and abiotic damage to which they are exposed. However, little is known about syndromes of defence across plant species and how these differ in contrasting environments. Here an investigation is made into the association between carbon-based chemical and mechanical defences, predicting that species that invest heavily in mechanical defence of leaves will invest less in chemical defence.nnnMETHODSnA combination of published and unpublished data is used to test whether species with tougher leaves have lower concentrations of phenolics, using 125 species from four regions of Australia and the Pacific island of New Caledonia, in evergreen vegetation ranging from temperate shrubland and woodland to tropical shrubland and rainforest. Foliar toughness was measured as work-to-shear and specific work-to-shear (work-to-shear per unit leaf thickness). Phenolics were measured as total phenolics and by protein precipitation (an estimate of tannin activity) per leaf dry mass.nnnKEY RESULTSnContrary to prediction, phenolic concentrations were not negatively correlated with either measure of leaf toughness when examined across all species, within regions or within any plant community. Instead, measures of toughness (particularly work-to-shear) and phenolics were often positively correlated in shrubland and rainforest (but not dry forest) in New Caledonia, with a similar trend suggested for shrubland in south-western Australia. The common feature of these sites was low concentrations of soil nutrients, with evidence of P limitation.nnnCONCLUSIONSnPositive correlations between toughness and phenolics in vegetation on infertile soils suggest that additive investment in carbon-based mechanical and chemical defences is advantageous and cost-effective in these nutrient-deficient environments where carbohydrate may be in surplus.

Sclerophylly in two contrasting tropical environments: low nutrients vs. low rainfall

The defining characteristics of sclerophylly are mechanical (e.g., hardness, toughness, stiffness), but little is known about how they vary in contrasting environments and contribute to the adaptiveness of sclerophylly. Here we investigate how the degree and nature of sclerophylly in terms of leaf mechanics differ between vegetation of two contrasting stressful environments, maquis on nutrient-deficient, moist sites and dry forest on moderate-nutrient, drier sites. We measured toughness, strength, and stiffness at the level of the whole leaf (structural) and per unit thickness (material). Leaves of maquis plants were on average structurally stiffer, stronger, and tougher than those in dry forest. There was little difference in material properties between habitats, and leaf thickness was the main contributor to differences in structural mechanical properties between habitats. Flexural stiffness varied most among species and habitats, correlating strongly with leaf mass per area and thickness. We suggest that having thicker leaves allows efficient packaging of biomass to reduce branching costs in sunny but stressful environments, with subsequent impacts on structural mechanical properties. Sclerophylly is probably a complex phenomenon, however, with its mechanical constitution arising from both evolved mechanical properties that confer protection or resistance to stress and nonadaptive mechanical consequences of adaptation to stressful environments.

Effects of tsetse targets on mammals and birds in Kasungu National Park, Malawi

Possible effects on wildlife of targets baited with an attractive odour (acetone), impregnated with deltamethrin and used to control tsetse flies (Glossina morsitans morsitans), were investigated in the Kasungu National Park, Malawi. Mammals and birds were censused simultaneously along transects in Miombo woodland or Dambo grassland/mixed woodland, with and without targets. Mammals were also monitored by surveys of their spoor on experimentally cleared plots (5 m in diameter) and a target relocation experiment was conducted when the targets were removed from the test transects and transferred to the controls. Significantly fewer small antelopes (e.g. common Duiker Sylvicapra grimmia) were detected in plots along transects with targets (tests) than along control transects. The presence of targets affected the frequency of occurrence of antelopes, suids and large herbivores in the experimental plots, but small carnivores, monkeys, rodents and hares were unaffected. A conclusion of the relocation experiment was that the deltamethrin-impregnated cloth was responsible for the observed effects and not the acetone. A total number of 23 species of birds meeting a criterion for their abundance in the areas surveyed were selected for detailed analyses. Lower indices in the test areas than in the controls were recorded for 15 of these 23 species in the Dambos and for 10 of 21 species in the Miombo woodland. Matched paired comparisons revealed significantly lower numbers in the test areas than in the controls in both habitats only for black-headed Oriole Oriolus larvatus, but for Grey Lourie Corythaixoides concolor, little bee-eater Merops pusillus, fork-tailed Drongo Dicrurus adsimilis and combined data on three species of sunbirds in the Dambos. The black-eyed bulbul Pycnonotus barbatus had significantly lower numbers in the tests than in the controls in Miombo woodland. The numbers of three species of dove were higher in the tests than in the controls in both habitats, significantly so for the Cape turtle dove Streptopelia capicola, and numbers of the Flappet Lark Mirafra rufocinnamomea were significantly higher in the tests than in the controls in Dambos. The results are discussed in the light of previous studies on environmental effects of tsetse control, including effects of tsetse targets on pollinators especially non-target horseflies.

Effect of the association of cattle and rusa deer #Cervus timorensis russa# on populations of cattle ticks (#Boophilus microplus#)

Abstract: The wild population of rusa deer (Cervus timorensis russa) in New Caledonia (South Pacific) is nearly as large as the cattle population. The cattle tick is widespread and occurs all year round. Opinions are divided on the role of deer in the biological cycle of the tick: i) Do they maintain a sustainable tick population that is secondarily available for cattle? ii) Do they decrease the infestation of the environment by collecting larvae on the pasture, but preventing their development to the engorged female stage? or iii) Do they contribute to both situations? An experiment was conducted in three groups of pastures, each seeded with 450 000 larvae/ha and allowed to be grazed only by cattle, only by deer, and by a mixed herd of deer and cattle (deer representing 30% of the biomass), at approximately the same stocking rate (470‐510 kg/ha). After 15 months of exposure, the tick burden per weight unit of host was 42 ticks/kg for the steers‐only herd and 0.01/kg for the deer‐only herd. The steers in the “mixed group” harbored 7 times fewer ticks (6.2/kg) than the cattle‐only group, and the deer in the “mixed group,” 130 times more (1.3/kg) than the deer‐only group. Five emergency acaricide treatments had to be applied in the cattle‐only group, but none in the other groups. The long‐term sustainability of a viable tick population on deer as well as the potential benefit resulting from the association of deer and susceptible cattle in the tick control of cattle are highlighted.

Adult Tick Burdens and Habitat Use of Sympatric Wild and Domestic Ungulates in a Mixed Ranch in Zimbabwe

Abstract: Ticks do not usually infest sympatric hosts species according to their availability in a given environment, and it has been suggested that habitat use by hosts is a major determinant of tick burdens. The knowledge of such infestation patterns and their relationship with host habitat use is important for the control of the vectors of some major stock diseases in Africa, particularly in the context of mixed game/cattle ranching. In a ranch of Zimbabwe, we monitored the number of adult ticks found on cattle and wild ungulates. Tick burdens were measured weekly during one year on 12 heifers of an experimental herd (no acaricide used), and on wild ungulates occasionally shot for meat. Adult ticks were not evenly distributed among wild hosts, and infestation patterns corresponded to observations made by several authors in similar conditions. However, these infestation patterns could not be related to habitat use by ungulates, which had been previously monitored by road transect at the scale of the ranch, as these authors found a high niche overlap and no habitat segregation between ungulate species. In an attempt to relate habitat use by Brahman and Simmental heifers with the number of adult ticks collected during one day of grazing, we followed the heifers and recorded their position and activity (one or two days per week; each recording session was 7 h 30 min on average, for a total of 940 hours of survey). No correlation was found between the number of ticks collected and the distance (or time spent) traveled in each vegetation type or the number of grooming episodes. The possible role of other behavioral and physiological parameters is discussed, and the results are compared with those found for other tick‐host associations.