Edge effects and extinction proneness in a herpetofauna from Madagascar

Edge effects are thought to play a key role in fragmented habitats. It is often assumed that edge-avoiding species are more prone to local extinction than non-edge-avoiding species, but there are few data to support this assumption. Also, few data are available on the effects of edges on some groups, and there is little understanding of seasonal changes in edge effect intensity. To better understand the role that edge effects play in fragmented tropical forests, we assessed the distribution of reptiles and amphibians in six littoral rainforest fragments in southeastern Madagascar in 1999 (dry season) and 2000 (wet season). Using randomization tests, we found all three types of edge responses: edge-avoiders, interior-avoiders, and omnipresent species. However, edge responses varied considerably among seasons. For example, some species (e.g., Mantidactylus bicalcaratus, Phelsuma quadriocellata) were strong edge-avoiders in the dry season, but showed no preference for edge or interior habitats during the wet season. Also, edge-avoiding species tended to be more extinction-prone than non-edge-avoiding species. Abiotic data documented significantly higher temperatures and dew point temperatures near edges. Wind speed also tended to be higher and relative humidity tended to be lower near edges. Our results indicate that many amphibians and reptiles respond to altered microclimates near edges and these responses have a strong seasonal component. For many species, edge sensitivity does seem to be correlated with extinction vulnerability and therefore should be of primary consideration in plans to conserve biodiversity in fragmented tropical forests.     

Intracellular pH control in Dictyostelium discoideum: A P-NMR analysis

Phosphorus metabolites and intracellular pH have been examined in the slime mold Dictyostelium discoideum by non-destructive ³¹P-NMR measurements. In a spectrum from a suspension of aerobic amoebae, the major peaks are inorganic phosphate, nucleotide di- and triphosphates. In the corresponding perchloric acid extract, resonances originating from purine and pyrimidine nucleotides are resolved. Adenine nucleotides are the most abundant components, but the other nucleotides are present in significant amounts. In a spectrum from intact spores in a dormant state, only inorganic phosphate and polyphosphates are detected and nucleotides are no longer present in large amounts.Of particular importance is the ability to observe separately in aerobic amoebae the resonance of inorganic phosphate localized in two different cell compartments: the cytosol and the mitochondria. The cytosolic pH and mitochondrial pH have been measured as 6.7 and 7.7, respectively, on the basis of intracellular inorganic phosphate chemical shifts. They are essentially unaffected over a large range of external pH and they are not modified transiently or permanently during the initiation of the developmental program of the organism. A weak acid, such as propionate, which modifies the progression of differentiation by favoring prestalk cells, perturbs intracellular pH gradients by selectively decreasing mitochondrial pH without any effect on cytosolic pH.     

Cytosolic acidification as a signal mediating hyperosmotic stress responses in Dictyostelium discoideum

Background  

Dictyostelium cells exhibit an unusual response to hyperosmolarity that is distinct from the response in other organisms investigated: instead of accumulating compatible osmolytes as it has been described for a wide range of organisms, Dictyostelium cells rearrange their cytoskeleton and thereby build up a rigid network which is believed to constitute the major osmoprotective mechanism in this organism. To gain more insight into the osmoregulation of this amoeba, we investigated physiological processes affected under hyperosmotic conditions in Dictyostelium.