The interfaces between pinon-juniper canopies and grasslands in the southwestern USA present an opportunity to use the modern theory of spatial phase transitions as a formal characterization of ecotone structure. The theory requires an estimation of a critical value of tree cover at which the woodlands switch abruptly from a fragmented collection of small patches of trees to a network of connected canopies. Presumably, this transition is associated with critical environmental conditions that regulate the ecologies of trees vs. grasses. We developed and tested a new method to estimate the critical cover value of woodlands on complex terrain. The method was based on multiscale assessments of the associations between local tree coverage and two types of patch edge. Tests on artificial gradient percolation maps revealed an interaction between the type of edge used ("hull edge," which is based on only the orthogonal connections between canopy-occupied cells, vs. "accessible edge," which is based on both orthogonal and diagonal connections beween canopy-occupied cells) and the neighborhood rule used to define a cluster (von Neumann 5-cell or Moore 9-cell). When applied to digitized, geographically referenced aerial photographs, the method indicated that areas :56.6 ha exhibited the theoretical critical value of 59.3% cover predicted for square lattices and the 5-cell neighborhood. Construction of both edge types on a given map can reveal locations of steep environmental gradients that may be buffered against modest climate fluctuations. The edges can be used in the calibration of independent variables to predict tree cover. The agreement between the expected and observed critical densities will motivate extensions of phase transition theory to studies of ecotones in real landscapes. PLATE l. Density dependence of the accessible edges of artificial and empirical percolation clusters. (A) Gradient percolation map of occupied cells (green) with accessible edge of spanning cluster (red line); probabilities within each column vary from p = 0 (left) to p = 1.0 (right). (B, C, D) Subsets of occupied cells (red) for which density in the surrounding region equals 0.5928 :!:: 5% of cells occupied in windows of length 5, 15, and 50 cells, respectively. Accessible-edge cells that intersected locations at critical density (yellow) were compared to those that did not intersect (white). Magnification of C (inset) shows details. (E) 39.3-ha map of pinon-juniper woodland (green) and grassland (black) with accessible edge of largest cluster (red). (F, G, H) Subsets of tree cells (red) for which density measured with windows of length 12.7, 38.1, and 127 m was within 5% of Pc = 0.5928. 807 PLATE 2. Association between the hull and various local densities (proportions of cells occupied). (A) Classified image showing trees (green) and grassland (black). (B, C, D) Hulls (white) of tree clusters and the number of scales (windows of size 0.006 ha :s L :s 2.48 ha) at which local densities of 0.50, 0.5928, and 0.70 (each ::' :: 1 %), respective...

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Aquatic systems are increasingly exposed to multiple stressors from anthropogenic sources. These stressors can vary in the consistency and magnitude of responses they elicit in biota and in how the presence of additional stressors modifies their effects. Understanding how the biological environment and temporal dynamics influence responses to stressors, and how stressors interact, is important to predicting their effects in the natural environment. We examined temporal variability in responses of an experimental estuarine food web to elevated trace elements and nutrients, as well as non-additive effects of the combination of these two stressors. Experiments were conducted four times during spring through autumn 1996 in 20 l-m 3 mesocosms. We measured a range of system-, population-, and individual-level parameters to quantify responses of phytoplankton, bacterioplankton, heterotrophic nanoflagellates, copepods, fish, and benthic invertebrates to trace element and nutrient additions.The response to trace element additions was more variable both temporally and among phytoplankton and higher trophic level taxa than was the response to nutrient additions. Most taxa increased, either significantly or showed a trend toward increasing, in response to nutrient additions in all four mesocosm runs. In contrast, the direction as well as the magnitude of responses to trace element additions varied considerably among taxa and experimental runs. Two distinct types of nutrientϫtrace element interactions were important. First, temporal dynamics of nutrient ratios appeared to affect the temporal pattern of toxicity of trace elements to phytoplankton. Second, in the June mesocosm run when trace element additions reduced production, abundance, or growth of many organisms, these reductions were often proportionately greater in nutrient addition tanks than where no nutrients were added. Our results suggest that considerable temporal and taxonomic variation in responses to trace element loadings are likely to be seen in field settings even under constant loadings to the system and that trace elements may mask the magnitude of the response to high nutrient loadings in eutrophic systems. More generally, the presence of multiple stressors may either increase or dampen the temporal and spatial variability seen in aquatic systems, depending on the interactions among stressors and the influence of background environmental conditions and sensitive species on the expression of stressor effects.Stressors vary considerably in the specificity of their effects; some stressors may affect nearly all organisms within AcknowledgmentsWe thank B. Albright, D. Butera, L. Cole, D. Connell, R. DeKorsey, T. Huber, A. Imirie, S. Sellner, J. Smallwood, M. Ward, M. Weinstein, T. Wiegner, and W. Yates for their help in running experiments and data analysis; S. Nixon, S. Grainger, and E. Buckley for suggestions on mesocosm design and unpublished data from their seagrass mesocosm experiments; M. Bundy for advice on copepod feeding strategies; E. Perry f...

For most older adults, memory performance depends on the time of day, with performance being optimal early in the morning and declining during the late afternoon hours. In the present study, we asked whether this decline could be ameliorated by a simple stimulant, caffeine. Adults over the age of 65 who considered themselves "morning types" were tested twice over an interval of 5 to 11 days, once in the morning and once in the late afternoon. Participants ingested either coffee with caffeine or decaffeinated coffee at both sessions. Participants who ingested decaffeinated coffee showed a significant decline in memory performance from morning to afternoon. In contrast, those who ingested caffeine showed no decline in performance from morning to afternoon. The results suggest that time-of-day effects may be mediated by nonspecific changes in level of arousal.

Interest in the possibility of sexual selection in plants has focused primarily on competition among pollen donors based on the speed of pollen-tube growth. However, when pollen arrives on stigmas, there is the opportunity for both races for access to ovules (exploitation competition) and interference with the germination and growth of pollen from other donors (interference competition). We considered whether this second form of competition might occur among pollen grains of wild radish in two experiments. In the first, interference likely occurred because the amount of pollen germination was less in mixed-donor than in single-donor pollinations. This result was duplicated in a second experiment, which also showed that interference occurred only when pollen grains from different donors were in direct contact with each other. In addition, in the second experiment, the opportunity for interference affected the frequency of seeds sired by different pollen donors. Because pollen loads are often mixed in nature, interference competition among pollen grains may be important in the ecology and evolution of plant reproduction.

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