Water Relations of Tropical Epiphytes: III. EVIDENCE FOR CRASSULACEAN ACID METABOLISM

Five tropical epiphytes were examined for evidence of CrassulaceanAcid Metabolism (CAM), namely the orchids Eria velutina Lindl.,Dendrobium tortile Lindl. and Dendrobium crumenatum Sw., andthe ferns Pyrrosia adnascens (Forst.) Ching and Pyrrosia angustata(Sw.) Ching, family Polypodiaceae. Diurnal variations in leaftitratable acidity, diffusive conductance and water potentialwere measured at various levels of water stress. The three orchidsshowed typical CAM behaviour, namely large diurnal fluctuationsin leaf acidity, day-time closure and night opening of stomataand a very slow decline in water potential under stress. Theferns showed some evidence of CAM, but this was not as well-developedas had been reported for two other tropical epiphytic membersof the same family. Key words: Crassulacean acid metabolism, Tropical epiphytes, Water stress     

The natural regulation of buffalo populations in East Africa: III. Population trends and mortality*

Analysis is carried out on the population data obtained from censuses and estimates of fertility and recruitment for the years 1965–1972, From the analysis, adult mortality is the only detectable reduction which acts as a negative feedback on the population, and is the only one needed to regulate the population in a way similar to the observed population trends. From a frequency distribution of ages at death, an approximate composite life table is constructed. This shows that males and females have similar age-specific mortalities until old age, but then males survive relatively better. Information on the causes of mortality indicate that the regulating adult mortality is caused in part by undernutrition, which in turn is due to food limitation rather than to social and physiological factors. Predation causes only a small part of the annual adult mortality, and its effect is swamped by other factors. Diseases play an important part as a primary factor in juvenile mortality but not in adult mortality because of the development of immunity. However, both disease and predation are important as secondary agents killing adults already weakened by moderate undernutrition. It is suggested that they play an essential role by hastening the population's response to changes in the food supply, and hence dampening oscillations that might develop in population and resource.     

European rabbit survival and recruitment are linked to epidemiological and environmental conditions in their exotic range

The European rabbit, Oryctolagus cuniculus, is threatened within its native range, yet it is a highly successful colonizing pest species across its worldwide introduced range, causing large economic losses and widespread environmental degradation. To date, there has been no long‐term empirical evidence documenting the relative roles of climatic, epidemiological and biological factors in limiting life‐history determinants of rabbit range and abundance. Using 12 years of capture–mark–recapture data from their exotic range in Australia, we constructed candidate Cormack–Jolly–Seber models to test the influence of environmental, competition and disease conditions on rabbit survival and recruitment. Our results show that: (i) population‐level disease infection rate has the largest overall impact on rabbit survival, explaining 80% of variance in survival rates; (ii) environmental as well as epidemiological conditions constrain rabbit survival, especially for younger animals; (iii) temporal variation in rabbit kitten recruitment patterns are best described by a combination of climate, competition and disease settings (accounting for 68% of variance), while temperature alone has a strong negative influence on kitten recruitment; and (iv) recruitment responds positively to rabbit haemorrhagic disease, but negatively to myxomatosis – the former, probably being mediated through a disease driven effect on intraspecific competition for food. A strengthened understanding of climate change impacts on rabbit range and abundance can be achieved by accounting explicitly for potential synergisms between disease dynamics and climate. In this analysis, we provide the first step towards such an attempt for this important mammal species. Integrated approaches of this kind are essential for future forecasts of rabbit range and abundance, offsetting the conservation threat faced by O. cuniculus in its native range, and achieving effective management in exotic habitats.     

Water balance in the arborescent palm, Sabal palmetto. I. Stem structure, tissue water release properties and leaf epidermal conductance

The functional importance of water storage in the arborescent palm, Sabal palmetto, was investigated by observing aboveground water content, pressure-volume curve parameters of leaf and stem tissue and leaf epidermal conductance rates. The ratio of the amount of water stored within the stem to the leaf area (kg m^?2) increased linearly with plant height. Pressure-volume curves for leaf and stem parenchyma differed markedly; leaves lost turgor at 0.90 relative water content and –3.81 MPa, while the turgor loss point for stem parenchyma occurred at 0–64 relative water content and ?0.96 MPa. Specific capacitance (change in relative water content per change in tissue water potential) of stem parenchyma tissue was 84 times higher than that of leaves, while the bulk modulus of elasticity was 346 times lower. Leaf epidermal conductance rates were extremely low (0.32–0.56 mmol m^?2 s^?1) suggesting that S. palmetto are able to strongly restrict foliar water loss rates. Structurally, stems of S. palmetto appear to be well suited to act as a water storage reservoir, and coupled with the ability to restrict water loss from leaf surfaces, may play an important role in tree survival during periods of low water availability.     

Water Content-Potential Relationship in Soya Bean: Changes in Component Potentials for Mature and Immature Leaves under Field Conditions

Changes in turgor and osmotic potentials of soya bean leaves(Glycine max.) with changes in water content were measured throughouta season using the pressure-volume technique. Two distinct reponsesto water loss were found. When water was expressed from leavesin the pressure chamber their osmotic behavior was describedby a concentration effect based on the osmotic volume. The osmoticfraction of the total water content averaged 0·72 and0·84 for mature and immature leaves, respectively. Thechanges in turgor pressure in the chamber were described bya volumetric modulus of elasticity which increased linearlywith turgor pressure. The changes in total potential at highturgor pressures were almost exclusively due to changes in turgordue to the high modulus (high tissue rigidity) in that range.Responses were different, however, for leaves drying in thefield. For these, the osmotic changes were always large anddominated by solute adjustment. Diurnal changes in osmotic potentialwere as much as 5 bars (500 kPa), or around 50 per cent, andwere about the same magnitude as the changes in turgor pressurefor both mature and immature leaves. The elastic modulus atthe time of sampling showed the normal turgor dependence forimmature leaves but for mature leaves the initial modulus wasapparently constant at about 180 bars. The different behaviourin the pressure bomb and the field is interpreted in terms ofa rate dependence for turgor and osmotic response to water loss.     

Leaf miners: The hidden herbivores

Leaf mining is a form of endophagous herbivory in which insect larvae live and feed within leaf tissue. In this review we discuss aspects of leaf miner ecology, and the current evidence for three hypotheses relating to the evolution of this feeding guild. We also present a summary of the literature coverage relating to these herbivores, which have been relatively poorly studied compared with insects that feed externally such as sap suckers and leaf chewers. The majority of published studies concern leaf miners from the northern hemisphere, with a general focus on those species considered to be agricultural, forestry or horticultural pests. In a more detailed case study, we examine aspects of leaf miner ecology of Australian species. At least 114 species have been recorded as leaf miners in Australia in four orders: Coleoptera, Lepidoptera, Diptera and Hymenoptera. Lepidoptera and Diptera are the most speciose orders of Australian leaf miners; Hymenoptera are represented by a single endemic genus and half of all coleopteran miners are species introduced for biological control. Both the known number of leaf‐mining species in Australia and the known number of hosts have increased in recent years following new targeted surveys. Leaf miners in Australia occur in many habitats and feed on a wide variety of host plants in at least 60 families although most individual species are monophagous. Although much of the research on leaf miners in Australia has focused on species that are commercially important pests or biological control agents, studies on fundamental aspects of leaf miner ecology are increasing. We identify a number of research questions aimed at better understanding the ecology of leaf miners in Australia and elsewhere.