Predicting climate-induced range shifts: model differences and model reliability

Predicted changes in the global climate are likely to cause large shifts in the geographic ranges of many plant and animal species. To date, predictions of future range shifts have relied on a variety of modeling approaches with different levels of model accuracy. Using a common data set, we investigated the potential implications of alternative modeling approaches for conclusions about future range shifts and extinctions. Our common data set entailed the current ranges of 100 randomly selected mammal species found in the western hemisphere. Using these range maps, we compared six methods for modeling predicted future ranges. Predicted future distributions differed markedly across the alternative modeling approaches, which in turn resulted in estimates of extinction rates that ranged between 0% and 7%, depending on which model was used. Random forest predictors, a model‐averaging approach, consistently outperformed the other techniques (correctly predicting >99% of current absences and 86% of current presences). We conclude that the types of models used in a study can have dramatic effects on predicted range shifts and extinction rates; and that model‐averaging approaches appear to have the greatest potential for predicting range shifts in the face of climate change.     

Decomposition of leaf litter exposed to simulated acid rain in a buffered lotic system

SUMMARY. 1. Exposure to simulated acid rain resulted in changes in the chemical content of riparian vegetation and terrestrial leaf litter and had significant effects on leaf litter decay rates in a well-buffered lotic ecosystem.
2. Foliar nitrogen and phosphorus decreased with decreasing pH of the simulated acid rain and microbial activity was greater on leaf litter exposed to rain of pi I 5.4 than on leaf litter exposed to pH 3.0 or pH 4.0.
3. Detritivore numbers and biomass were significantly higher on leaf litter exposed to pH 5.4, probably due to the greater palatability of the leaf substrate.
4. Decay rates of leaf litter processed in the summer months were significantly lower than decay rates of leaf litter processed during the autumn/winter due to reduction in nitrogen content and microbial respiration.     

Neural Generation of the Peristaltic and Non-Peristaltic Heartbeat Coordination Modes in the Leech, Hirudo Medicinalis

The bilateral paired heart tubes of the leech Hirudo medicinalisare controlled, via excitatory synapses, by a set of bilaterallypaired segmental heart motor neurons (HE cells) which are inturn controlled, via inhibitory synapses, by a set of bilaterallypaired segmental heart interneurons (HN cells). The HE cellsproduce rhythmic impulse bursts because their inherent steadydischarge is periodically inhibited by the HN cells, most ofwhich produce impulse bursts endogenously. The known synapticinteractions among the HN cells and HE cells can account wellfor the observed behavior of the hearts. The HE cells are coordinatedby the HN cells such that the segmental heart tube sectionson one side constrict in a caudorostral sequence (peristalsis),while the segmental heart tube sections on the other side constrictnearly synchronously (non-peristalsis). This difference in thecoordination modes of the two hearts is not permanent; reciprocalcoordination mode transitions occur every 10–50 heartbeatcycles. Only one member of HN(5) cell pair (the HN cells ofthe fifth segmental ganglion) is rhythmically active at a time,the other being completely inactive. By coordinating the frontand rear HN cells the active HN(5) cell produces non-peristalsisipsilaterally and peristalsis contralaterally. Reciprocal changesin the activity-inactivity pattern of the HN(5) cell pair areresponsible for the reciprocal changes in the coordination mode.     

Modelling animal populations in changing landscapes

Models of Mobile Animal Populations (MAP models) simulate long-term land use changes, population trends and patterns of biological diversity on landscapes of 10³-10⁵ ha. MAP models can incorporate information about past land-use patterns and management practices and can project future patterns based on management plans. We illustrate this approach with an example of how implementation of a U.S. Forest Service management plan at the Savannah River Site in South Carolina, U.S.A., might influence population trends of Bachman's Sparrow Aimophila aestivalis, a relatively rare and declining species in southeastern pine forests. In this case, a management plan, largely designed to improve conditions for an endangered species, Red-cockaded Woodpecker Picoides borealis, may have a negative impact, at least in the short term, on another species of management concern, Bachman's Sparrow.
In a parallel processing version of the MAP models, a single landscape that would ordinarily be too large or detailed to be simulated on a single computer is subdivided into a number of smaller landscapes, and each landscape is simulated in parallel, either on a single multi-tasking machine or on a group of networked machines. With this approach we are attempting to determine just how large a landscape must be before the dynamics of a population within it are more or less independent of factors beyond the landscape boundaries.     

A Freeze-Fracture Study of the Cortex of Xenopus laevis Eggs

The organization of the cortex of Xenopus laevis eggs was investigated by freeze-fracture electron microscopy. The cortical endoplasmic reticulum (CER) formed a network surrounding and interconnecting the cortical granules. It formed junctions with the plasma membrane and was confluent with the ER in subcortical regions. Intramembranous particles (IMP₁) were only present in the P face of the CER, the E face being apparently devoid of pits and particles. Arrays of densely packed IMP₁, having a mean diameter of 17 nm, were restricted to the microvillar region of the plasma membrane. The cortical granule membrane also contained IMP₁ (mean diameter, 21 nm) that were sparsely and randomly distributed. Several types of cortical granule seemed to exist based on an analysis of the distribution of the different IMP sizes.